[Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
Hello,
I largely don't know what I'm doing or even trying to do, it is a voyage into
the unknownbutif I go...
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
class Foo x y | x - y, y - x
instance Foo Integer Integer
That seems to workand my head seems to say...your created some sort of
binary relation between 2 types...and made Integer,Integer a member of it...
Something like that anyway
Then I go
data Bar
instance Foo Bar x
Error!but I'm think I understand thisI can't claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my functional
dependencies, because Bar,Integer is now a member of Foo..
Bad programmer...
So how I naively go
class NotAnInteger a
instance (NotAnInteger x) = Foo Bar x
I haven't declared integer to be NotAnIntegerso (in a closed
world)this would seem to exclude the contradictionbut...
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12
So
i)I clearly don't understand something about the type
system.
ii) I don't know how to restrict type variables in instance
declarationsi.e. how do I use the notion of Foo across different
combinations of types, without them colliding.
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Re: [Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
The constraint on an instance never influences which instance is
selected. So as far as instance selection goes, 'instance Foo x' and
'instance C x = Foo x' are the same. The constraint is only checked
after the instance is selected.
Erik
On Fri, Jul 5, 2013 at 2:43 PM, Nicholls, Mark nicholls.m...@vimn.com wrote:
Hello,
I largely don’t know what I’m doing or even trying to do, it is a voyage
into the unknown….but….if I go…
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
class Foo x y | x - y, y - x
instance Foo Integer Integer
That seems to work….and my head seems to say…your created some sort of
binary relation between 2 types…and made Integer,Integer a member of it…
Something like that anyway….
Then I go….
data Bar
instance Foo Bar x
Error!but I’m think I understand this….I can’t claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my
functional dependencies, because Bar,Integer is now a member of Foo..
Bad programmer…….
So how I naively go….
class NotAnInteger a
instance (NotAnInteger x) = Foo Bar x
I haven’t declared integer to be “NotAnInteger”….so (in a closed
world)….this would seem to exclude the contradiction….but…
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12
So
i)I clearly don’t understand something about the type
system.
ii) I don’t know how to restrict type variables in
instance declarations….i.e. how do I use the notion of “Foo” across
different combinations of types, without them colliding.
CONFIDENTIALITY NOTICE
This e-mail (and any attached files) is confidential and protected by
copyright (and other intellectual property rights). If you are not the
intended recipient please e-mail the sender and then delete the email and
any attached files immediately. Any further use or dissemination is
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While MTV Networks Europe has taken steps to ensure that this email and any
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message and any attachments are virus free and do not affect your systems /
data.
Communicating by email is not 100% secure and carries risks such as delay,
data corruption, non-delivery, wrongful interception and unauthorised
amendment. If you communicate with us by e-mail, you acknowledge and assume
these risks, and you agree to take appropriate measures to minimise these
risks when e-mailing us.
MTV Networks International, MTV Networks UK Ireland, Greenhouse,
Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions
International, Be Viacom, Viacom International Media Networks and VIMN and
Comedy Central are all trading names of MTV Networks Europe. MTV Networks
Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks
Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent,
London, NW1 8TT.
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Re: [Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
You're running into the open worldassumption--anybody could come along
and make Integer part of your NotAnInteger class, and there's nothing you
can do to stop them. This is a design tradeoff for typeclasses: typeclass
instances are always global and are exported to all other modules you use.
This means you cannot ensure a type is *not* part of a typeclass. (Or, at
the very least, you can't convince GHC of this fact.)
For more information about this, take a look at the following StackOverflow
question:
http://stackoverflow.com/questions/8728596/explicitly-import-instances
On Jul 5, 2013 8:47 AM, Nicholls, Mark nicholls.m...@vimn.com wrote:
Hello,
** **
I largely don’t know what I’m doing or even trying to do, it is a voyage
into the unknown….but….if I go…
** **
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
** **
class Foo x y | x - y, y - x
instance Foo Integer Integer
** **
That seems to work….and my head seems to say…your created some sort of
binary relation between 2 types…and made Integer,Integer a member of it…
** **
Something like that anyway….
** **
Then I go….
** **
data Bar
** **
instance Foo Bar x
** **
Error!but I’m think I understand this….I can’t claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my
functional dependencies, because Bar,Integer is now a member of Foo..***
*
** **
Bad programmer…….
** **
** **
So how I naively go….
** **
** **
class NotAnInteger a
** **
instance (NotAnInteger x) = Foo Bar x
** **
I haven’t declared integer to be “NotAnInteger”….so (in a closed
world)….this would seem to exclude the contradiction….but…
** **
** **
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12**
**
** **
So
**i)**I clearly don’t understand something about the
type system.
**ii) **I don’t know how to restrict type variables in
instance declarations….i.e. how do I use the notion of “Foo” across
different combinations of types, without them colliding.
** **
** **
** **
** **
** **
** **
** **
CONFIDENTIALITY NOTICE
This e-mail (and any attached files) is confidential and protected by
copyright (and other intellectual property rights). If you are not the
intended recipient please e-mail the sender and then delete the email and
any attached files immediately. Any further use or dissemination is
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While MTV Networks Europe has taken steps to ensure that this email and
any attachments are virus free, it is your responsibility to ensure that
this message and any attachments are virus free and do not affect your
systems / data.
Communicating by email is not 100% secure and carries risks such as delay,
data corruption, non-delivery, wrongful interception and unauthorised
amendment. If you communicate with us by e-mail, you acknowledge and assume
these risks, and you agree to take appropriate measures to minimise these
risks when e-mailing us.
MTV Networks International, MTV Networks UK Ireland, Greenhouse,
Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions
International, Be Viacom, Viacom International Media Networks and VIMN and
Comedy Central are all trading names of MTV Networks Europe. MTV Networks
Europe is a partnership between MTV Networks Europe Inc. and Viacom
Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley
Crescent, London, NW1 8TT.
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Re: [Haskell-cafe] newbie question about Functional dependencies conflict between instance declarations:.....
Ah
So it isn't a closed world
So how do I stop my instances clashing?
The x in
instance Foo Bar x
is never intended to be Integer.
Mark Nicholls | Lead broadcast corporate architect, Programmes Development
- Viacom International Media Networks
A: 17-29 Hawley Crescent London NW1 8TT | e:
nicholls.m...@vimn.commailto:m...@vimn.com T: +44 (0)203 580 2223
[Description: cid:image001.png@01CD488D.9204D030]
From: Tikhon Jelvis [mailto:tik...@jelv.is]
Sent: 05 July 2013 2:08 PM
To: Nicholls, Mark
Cc: haskell-cafe
Subject: Re: [Haskell-cafe] newbie question about Functional dependencies
conflict between instance declarations:.
You're running into the open worldassumption--anybody could come along and
make Integer part of your NotAnInteger class, and there's nothing you can do to
stop them. This is a design tradeoff for typeclasses: typeclass instances are
always global and are exported to all other modules you use. This means you
cannot ensure a type is *not* part of a typeclass. (Or, at the very least, you
can't convince GHC of this fact.)
For more information about this, take a look at the following StackOverflow
question: http://stackoverflow.com/questions/8728596/explicitly-import-instances
On Jul 5, 2013 8:47 AM, Nicholls, Mark
nicholls.m...@vimn.commailto:nicholls.m...@vimn.com wrote:
Hello,
I largely don't know what I'm doing or even trying to do, it is a voyage into
the unknownbutif I go...
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
class Foo x y | x - y, y - x
instance Foo Integer Integer
That seems to workand my head seems to say...your created some sort of
binary relation between 2 types...and made Integer,Integer a member of it...
Something like that anyway
Then I go
data Bar
instance Foo Bar x
Error!but I'm think I understand thisI can't claim that Bar,x is a
member of Foo and Integer,Integer is member of Foo and preserve my functional
dependencies, because Bar,Integer is now a member of Foo..
Bad programmer...
So how I naively go
class NotAnInteger a
instance (NotAnInteger x) = Foo Bar x
I haven't declared integer to be NotAnIntegerso (in a closed
world)this would seem to exclude the contradictionbut...
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1.lhs:13:12
So
i)I clearly don't understand something about the type
system.
ii) I don't know how to restrict type variables in instance
declarationsi.e. how do I use the notion of Foo across different
combinations of types, without them colliding.
CONFIDENTIALITY NOTICE
This e-mail (and any attached files) is confidential and protected by copyright
(and other intellectual property rights). If you are not the intended recipient
please e-mail the sender and then delete the email and any attached files
immediately. Any further use or dissemination is prohibited.
While MTV Networks Europe has taken steps to ensure that this email and any
attachments are virus free, it is your responsibility to ensure that this
message and any attachments are virus free and do not affect your systems /
data.
Communicating by email is not 100% secure and carries risks such as delay, data
corruption, non-delivery, wrongful interception and unauthorised amendment. If
you communicate with us by e-mail, you acknowledge and assume these risks, and
you agree to take appropriate measures to minimise these risks when e-mailing
us.
MTV Networks International, MTV Networks UK Ireland, Greenhouse, Nickelodeon
Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be
Viacom, Viacom International Media Networks and VIMN and Comedy Central are all
trading names of MTV Networks Europe. MTV Networks Europe is a partnership
between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for
service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.
___
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Communicating by email is not 100
[Haskell-cafe] Newbie Question
Hello everybody! I am quite new to haskell programming. I am trying to make a gui for a drumming project made in euterpea and I want to do something like this : b - button f [text := Run! , on command := (test mambo_orig)] test mambo_orig is executed from ghci but I want It to be able to execute each time the user presses the button. Hope I didn't mess it up in the explaination, Giannis Chionidis ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question
On Fri, 31 Dec 2010 12:32:06 +0100, Chionidis Ioannis j...@echidna-band.com wrote: Hello everybody! I am quite new to haskell programming. I am trying to make a gui for a drumming project made in euterpea and I want to do something like this : b - button f [text := Run! , on command := (test mambo_orig)] test mambo_orig is executed from ghci but I want It to be able to execute each time the user presses the button. Hope I didn't mess it up in the explaination, Giannis Chionidis This should work as expected, but GHCi does not play nice with wxHaskell; the problem is being worked on. Try compiling your program. Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html -- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question about using WinGHCi
Hello -
I just a day or so ago downloaded Hakell and am playing around with it, and
I came upon this problem with WinGHCi:
I am able to enter a multi-line do statement that works if I use brackets
and semi-colon like so:
Prelude :{
Prelude| let main2 = do {
Prelude| putStrLn Please enter your name: ;
Prelude| name - getLine;
Prelude| putStrLn (Hello, ++ name ++ , how are you?) }
Prelude| :}
Prelude main2
Please enter your name:
CT
Hello, CT, how are you?
Prelude
Note there is no indentation. This makes sense to me because the :{ :}
just take all the lines in between and make it one line.
But it seems to me to be IMPOSSIBLE to input this into WinGHCi using
indentation like most of the code samples seem to do.
Should I just be satisfied that it works using brackets/ semi-colons? Or is
there something obvious that I am missing that allows for indentation?
Thanks in advance. It's issues lik this that keep me up into the depths of
night.
Best Regards,
Chris
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Re: [Haskell-cafe] Newbie question about using WinGHCi
On 1 July 2010 16:04, Christopher Tauss ctau...@gmail.com wrote:
Hello -
I just a day or so ago downloaded Hakell and am playing around with it, and
I came upon this problem with WinGHCi:
I am able to enter a multi-line do statement that works if I use brackets
and semi-colon like so:
Prelude :{
Prelude| let main2 = do {
Prelude| putStrLn Please enter your name: ;
Prelude| name - getLine;
Prelude| putStrLn (Hello, ++ name ++ , how are you?) }
Prelude| :}
Prelude main2
Please enter your name:
CT
Hello, CT, how are you?
Prelude
Note there is no indentation. This makes sense to me because the :{ :}
just take all the lines in between and make it one line.
But it seems to me to be IMPOSSIBLE to input this into WinGHCi using
indentation like most of the code samples seem to do.
Should I just be satisfied that it works using brackets/ semi-colons? Or is
there something obvious that I am missing that allows for indentation?
Typically, ghci (including WinGHCI) and Hugs are used to evaluate and
experiment with code, rather than writing it. Write your actual code
in a file and load it with :load (or :l for short) into ghci.
In this sense, they aren't real REPLs in that you can't define new
data types, classes, etc. in them (you can write functions with a let
statement, but it gets cumbersome for long functions).
It might be better off thinking of the prompt as being individual
lines in a big do-block for a specialised version of the IO monad (in
the sense that normally just entering 5+4 wouldn't typecheck let
alone print the result, etc.).
Thanks in advance. It's issues lik this that keep me up into the depths of
night.
Hopefully you can now get to sleep ;-)
--
Ivan Lazar Miljenovic
ivan.miljeno...@gmail.com
IvanMiljenovic.wordpress.com
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Re: [Haskell-cafe] newbie question how to pass data
Hi If you are working with characteristic functions (Point - Bool or Point - Colour...) the common way to do this is to manufacture a Num instance for functions. This gives you syntax overloading of the (+, -, *) operators. Similarly you might want to overload (or have to overload) Floating, Fractional... Examples using this technique are Jerzy Karczmarczuk's Clastic, Conal Elliott's Vertigo, Tangible Values, Pan etc. To overload Num you have to define Show and Eq instances for functions as well. Something along the lines of this is adequate: type CF = (Double,Double) - Bool instance Show CF where show _ = function instance Eq CF where (==) _ _ = error No Eq on Characteristic functions instance Num CF where f + g = \pt - f pt + g pt -- ... negate f = \(x,y) - f (negate x, negate y) -- ... rest follows this pattern, Floating, Fractional similar If you characteristic function is Point - Bool then you also need a Num instance for Bool. All that said, I think your formulation of func above is slightly wrong to fit this style. Its forming a function (- Point) to point rather than a characteristic function Point - Bool. Best wishes Stephen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie question how to pass data
Great job Stephen. Thank for explaining . I got it to work. On Tue, Apr 20, 2010 at 9:21 AM, Stephen Tetley stephen.tet...@gmail.comwrote: Hi If you are working with characteristic functions (Point - Bool or Point - Colour...) the common way to do this is to manufacture a Num instance for functions. This gives you syntax overloading of the (+, -, *) operators. Similarly you might want to overload (or have to overload) Floating, Fractional... Examples using this technique are Jerzy Karczmarczuk's Clastic, Conal Elliott's Vertigo, Tangible Values, Pan etc. To overload Num you have to define Show and Eq instances for functions as well. Something along the lines of this is adequate: type CF = (Double,Double) - Bool instance Show CF where show _ = function instance Eq CF where (==) _ _ = error No Eq on Characteristic functions instance Num CF where f + g = \pt - f pt + g pt -- ... negate f = \(x,y) - f (negate x, negate y) -- ... rest follows this pattern, Floating, Fractional similar If you characteristic function is Point - Bool then you also need a Num instance for Bool. All that said, I think your formulation of func above is slightly wrong to fit this style. Its forming a function (- Point) to point rather than a characteristic function Point - Bool. Best wishes Stephen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Mujtaba Ali Alboori ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] newbie question how to pass data
Hello I am sorry for the silly question. I have a function as the following func:: ((Float,Float) -Bool) - Float - ((Float,Float) - Bool) I am trying to make calculation in this type ((Float,Float) -Bool) with Float and then pass the information to ((Float,Float) - Bool) Thank again appreciated. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie question how to pass data
Can you at least give an example of how you intend to use this func? Since you do not describe it's behaviour, it is very hard to make a useful comment (at least for me) Best, On 19 April 2010 16:54, Mujtaba Boori mujtaba.bo...@gmail.com wrote: Hello I am sorry for the silly question. I have a function as the following func:: ((Float,Float) -Bool) - Float - ((Float,Float) - Bool) I am trying to make calculation in this type ((Float,Float) -Bool) with Float and then pass the information to ((Float,Float) - Bool) Thank again appreciated. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Ozgur Akgun ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie question how to pass data
sorry ok I am trying to make these calculation func (x,y) s dg =((x*(cos dg) - y*(sin dg)),(x*(sin dg) - y*(cos dg))) This work for type (Float - Float) but how can make it work with ((Float - Float) - Bool) because my main function that I want use with. it takes (Float,Float) -Bool) I need to return the same type ((Float,Float) -Bool) so it could be used with other function. On Mon, Apr 19, 2010 at 5:54 PM, Ozgur Akgun ozgurak...@gmail.com wrote: Can you at least give an example of how you intend to use this func? Since you do not describe it's behaviour, it is very hard to make a useful comment (at least for me) Best, On 19 April 2010 16:54, Mujtaba Boori mujtaba.bo...@gmail.com wrote: Hello I am sorry for the silly question. I have a function as the following func:: ((Float,Float) -Bool) - Float - ((Float,Float) - Bool) I am trying to make calculation in this type ((Float,Float) -Bool) with Float and then pass the information to ((Float,Float) - Bool) Thank again appreciated. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Ozgur Akgun -- Mujtaba Ali Alboori ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie question how to pass data
First of all, your function func (x,y) s dg =((x*(cos dg) - y*(sin dg)),(x*(sin dg) - y*(cos dg))) does NOT work for type (Float - Float), unless you mean that that is the type of the unused parameter s. Also, your desired type ((Float - Float) - Bool) itself looks suspicious. It must accept any function (without something to apply it to) and arbitrarily return True or False. How will you decide which? I suspect you need another parameter for this function. Second, on the off chance you are trying to calculate the position on a circle scaled then rotated an angle dg from (x,y), that new position is f (x,y) s dg = (s*(x*(cos dg) - y*(sin dg)),s*(x*(sin dg) + y*(cos dg))) in which case you are missing the s and the last minus sign in your formula should be a plus sign. If so, this can be evaluated with greater clarity (and probably accuracy) in polar coordinates: g (x,y) s dg = (r * cos a, r * sin a) where r = s * sqrt (x^2 + y^2) a = atan2 y x + dg Third, if you did not need the scale, I would use an underscore to make that clear: h (x,y) _ dg = (r * cos a, r * sin a) where r = sqrt (x^2 + y^2) a = atan2 y x + dg That's all the observations I can make unless you describe the problem more clearly. Sorry. Dan Mujtaba Boori wrote: sorry ok I am trying to make these calculation func (x,y) s dg =((x*(cos dg) - y*(sin dg)),(x*(sin dg) - y*(cos dg))) This work for type (Float - Float) but how can make it work with ((Float - Float) - Bool) because my main function that I want use with. it takes (Float,Float) -Bool) I need to return the same type ((Float,Float) -Bool) so it could be used with other function. On Mon, Apr 19, 2010 at 5:54 PM, Ozgur Akgun ozgurak...@gmail.com mailto:ozgurak...@gmail.com wrote: Can you at least give an example of how you intend to use this func? Since you do not describe it's behaviour, it is very hard to make a useful comment (at least for me) Best, On 19 April 2010 16:54, Mujtaba Boori mujtaba.bo...@gmail.com mailto:mujtaba.bo...@gmail.com wrote: Hello I am sorry for the silly question. I have a function as the following func:: ((Float,Float) -Bool) - Float - ((Float,Float) - Bool) I am trying to make calculation in this type ((Float,Float) -Bool) with Float and then pass the information to ((Float,Float) - Bool) Thank again appreciated. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Ozgur Akgun -- Mujtaba Ali Alboori ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie question how to pass data
Thanks Dan. Great help but my problem has not solved yet This doesn't work for type ((Float - Float)-Bool) to make it easier ignore the rotation and suppose I want just multiplay with whatever (x ,y) and return the result to this type ((Float - Float)-Bool) note this type is shorten and replace by t Type Point = (Float, Float) Type Bitmap = Point - Bool so the function type actually func :: Bitmap - Float - Bitmap I want to take Bitmap do some calculation on Bitmap the return it as Bitmap. GHCi response for Dan method is this Couldn't match expected type `Bitmap' against inferred type `(a, b)' so it is missing a Bool. hopefully it is clear . On Mon, Apr 19, 2010 at 7:02 PM, Dan Weston weston...@imageworks.comwrote: First of all, your function func (x,y) s dg =((x*(cos dg) - y*(sin dg)),(x*(sin dg) - y*(cos dg))) does NOT work for type (Float - Float), unless you mean that that is the type of the unused parameter s. Also, your desired type ((Float - Float) - Bool) itself looks suspicious. It must accept any function (without something to apply it to) and arbitrarily return True or False. How will you decide which? I suspect you need another parameter for this function. Second, on the off chance you are trying to calculate the position on a circle scaled then rotated an angle dg from (x,y), that new position is f (x,y) s dg = (s*(x*(cos dg) - y*(sin dg)),s*(x*(sin dg) + y*(cos dg))) in which case you are missing the s and the last minus sign in your formula should be a plus sign. If so, this can be evaluated with greater clarity (and probably accuracy) in polar coordinates: g (x,y) s dg = (r * cos a, r * sin a) where r = s * sqrt (x^2 + y^2) a = atan2 y x + dg Third, if you did not need the scale, I would use an underscore to make that clear: h (x,y) _ dg = (r * cos a, r * sin a) where r = sqrt (x^2 + y^2) a = atan2 y x + dg That's all the observations I can make unless you describe the problem more clearly. Sorry. Dan Mujtaba Boori wrote: sorry ok I am trying to make these calculation func (x,y) s dg =((x*(cos dg) - y*(sin dg)),(x*(sin dg) - y*(cos dg))) This work for type (Float - Float) but how can make it work with ((Float - Float) - Bool) because my main function that I want use with. it takes (Float,Float) -Bool) I need to return the same type ((Float,Float) -Bool) so it could be used with other function. On Mon, Apr 19, 2010 at 5:54 PM, Ozgur Akgun ozgurak...@gmail.commailto: ozgurak...@gmail.com wrote: Can you at least give an example of how you intend to use this func? Since you do not describe it's behaviour, it is very hard to make a useful comment (at least for me) Best, On 19 April 2010 16:54, Mujtaba Boori mujtaba.bo...@gmail.com mailto:mujtaba.bo...@gmail.com wrote: Hello I am sorry for the silly question. I have a function as the following func:: ((Float,Float) -Bool) - Float - ((Float,Float) - Bool) I am trying to make calculation in this type ((Float,Float) -Bool) with Float and then pass the information to ((Float,Float) - Bool) Thank again appreciated. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Ozgur Akgun -- Mujtaba Ali Alboori -- Mujtaba Ali Alboori ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question about Parsec
Hey everybody
I've been playing around with Parsec a little bit lately. I like it a
lot, but now I've hit a bit of a challenge. Suppose I have to parse a
variable length string representing a time interval. Depending on how
many fields there are, the time is either interpreted as seconds,
minutes and seconds or hours, minutes and seconds.
For example:
... 31 ... would be parsed as 31 seconds.
... 05:31 ... would be parsed as 5 minutes and 31 seconds.
... 01:05:31 ... would be parsed as 1 hour, 5 minutes and 31 seconds.
I've come up with the following solution using optionMaybe to deal with
the problem:
data ElapsedTime = ElapsedTime {
hours :: Int,
minutes :: Int,
seconds :: Int
} deriving (Show, Eq, Ord)
p_elapsed_time :: CharParser () ElapsedTime
p_elapsed_time = toElapsedTime $ (optionMaybe p_Int)
* (optionMaybe (char ':' * p_Int))
* (optionMaybe (char ':' * p_Int *
skipSpaces))
where toElapsedTime Nothing Nothing Nothing = ElapsedTime 0 0 0
toElapsedTime (Just s) Nothing Nothing = ElapsedTime 0 0 s
toElapsedTime (Just m) (Just s) Nothing = ElapsedTime 0 m s
toElapsedTime (Just h) (Just m) (Just s) = ElapsedTime h m s
Where p_Int simply parses a sequence of digits as an Int and skipSpaces
does just that.
This works correctly, but it also feels kinda clumsy. For one the
compiler rightly complains about non-exhaustive pattern matches in the
definition of the toElapsedTime function, although I believe that's
negligible in that particular case.
Is there a better i.e. more elegant way to tackle such a problem?
regards,
david
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Re: [Haskell-cafe] Newbie question about Parsec
Am Montag 18 Januar 2010 19:09:23 schrieb david fries:
Hey everybody
I've been playing around with Parsec a little bit lately. I like it a
lot, but now I've hit a bit of a challenge. Suppose I have to parse a
variable length string representing a time interval. Depending on how
many fields there are, the time is either interpreted as seconds,
minutes and seconds or hours, minutes and seconds.
For example:
... 31 ... would be parsed as 31 seconds.
... 05:31 ... would be parsed as 5 minutes and 31 seconds.
... 01:05:31 ... would be parsed as 1 hour, 5 minutes and 31 seconds.
I've come up with the following solution using optionMaybe to deal with
the problem:
data ElapsedTime = ElapsedTime {
hours :: Int,
minutes :: Int,
seconds :: Int
} deriving (Show, Eq, Ord)
p_elapsed_time :: CharParser () ElapsedTime
p_elapsed_time = toElapsedTime $ (optionMaybe p_Int)
* (optionMaybe (char ':' * p_Int))
* (optionMaybe (char ':' * p_Int *
skipSpaces))
where toElapsedTime Nothing Nothing Nothing = ElapsedTime 0 0 0
toElapsedTime (Just s) Nothing Nothing = ElapsedTime 0 0 s
toElapsedTime (Just m) (Just s) Nothing = ElapsedTime 0 m s
toElapsedTime (Just h) (Just m) (Just s) = ElapsedTime h m s
p_elapsed_time = toElapsedTime $ sepBy p_int (char ':')
where
toElapsedTime (h:m:s:_) = ElapsedTime h m s
toElapsedTime [m,s] = ElapsedTime 0 m s
toElapsedTime [s] = ElapsedTime 0 0 s
toElapsedTime []= ElapsedTime 0 0 0
You can replace the first pattern for toElapsedTime with [h,m,s] and add a
failure case if sepBy p_int (char ':') parses more than three ints (or
write a combinator that parses up to n results of p separated by sep).
Or
p_elapsed_time = do
s - p_int
et - cont_elapsed_time (ElapsedTime 0 0 s)
cont_elapsed_time et@(ElapsedTime h m s) = do
char ':'
n - p_int
cont_elapsed_time (ElapsedTime m s n)
| return et
Where p_Int simply parses a sequence of digits as an Int and skipSpaces
does just that.
This works correctly, but it also feels kinda clumsy. For one the
compiler rightly complains about non-exhaustive pattern matches in the
definition of the toElapsedTime function, although I believe that's
negligible in that particular case.
Is there a better i.e. more elegant way to tackle such a problem?
regards,
david
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Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
On 5/30/08, Martin Blais [EMAIL PROTECTED] wrote: Dear Philip, could you point your virtual finger towards a reference/paper/book/any-bleeping-thing that would help this simple beginner understand why it doesn't work in this case? I'm trying to picture why a read function that terminates the program would be useful anywhere. In fact, any library function other than something like UNIX's exit or kill which terminates my program is not really welcome in any of my computer programs, but then again, I haven't yet been illuminated by the genie of pure functional languages. A reference would be awesome. As others have pointed out, the reason why it doesn't work is due to lazy evaluation; the exception is hidden in a pure value and isn't triggered until some other bit of code causes the pure value to be evaluated. You can force the exception to be generated in your catch block by using seq (or $!, which is implemented using seq). This forces the evaluation to happen at that point, although there are a lot of gotchas involved; for Int it works fine, though. Your question, then, requires asking why would you want to hide an exception in a pure value? The answer to that is quite interesting, but here's a simpler example that might enlighten you: head :: [a] - a head (x:_) = x head _ = error head: empty list Just as you are required to prove before calling head that you aren't passing an empty list, for your program to be total, you should prove before calling read that the string parses properly. If you can't provide that proof (because, in this case, the string is provided by the user), you should be using another function for parsing. Don suggested reads to implement maybeRead, which seems like a great idea to me. -- ryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
Hi The best thing to do is bypass read and use 'reads' to define your own safe read. maybeRead :: Read a = String - Maybe a maybeRead s = case reads s of [(x, )] - Just x _ - Nothing Or just use the Safe package: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/safe http://hackage.haskell.org/packages/archive/safe/0.2/doc/html/Safe.html#v%3AreadMay Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
Allright, this is a definitely a newbie question. I'm learning Haskell and going through the exercises in the beautiful Hutton book, and one of them requires for me to write a loop that queries a line from the user (stdin), looping until the user enters a valid integer (at least that's how I want to implement the interface to the exercise). I have tried tons of variants and modifications of code, and I can't find the way to implement this. Here is what my emacs buffer is at now:: import Text.Read import System.IO import qualified Control.Exception as C getNum :: IO Int getNum = do line - (hGetLine stdin) x - (C.catch (do return (read line :: Int)) (\e - getNum)) return x main = do x - getNum putStr ((show x) ++ \n) Now, I've tried the Prelude's catch, the Control.Exception catch, I've tried moving it at the top of getnum, I've tried without catch, I've tried a ton of other shtuff, so much that I'm starting to think that Emacs is going to run out of electrons soon. I asked some half-newbie friends who are insanely enthousiastic about Haskell and they can't do it either (I'm starting to think that those enthousiastic friends are dating a beautiful girl with 3 PhDs, but she has a 2-inch thick green and gooey wart smack on her nose and they're so blindly in love that they can't admit that she does). I've asked some university profs and they sidetrack my question by saying I shouldn't do I/O so early. Can anyone here restore my faith in the Haskell section of humanity? 1. How do I catch the exception that is raised from read? 2. Where do I find the appropriate information I need in order to fix this? I'm probably just not searching in the right place. (Yes, I've seen the GHC docs, and it doesn't help, maybe I'm missing some background info.) 3. Please do not tell me I should solve the problem differently. Here is the problem I'm trying to solve, and nothing else: Write a program that reads a line from the user, looping the query until the line contains a valid integer. It shouldn't be too hard i think. The best answer would be a two-liner code example that'll make me feel even more stupid than I already do. Thanks in advance. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
On Fri, May 30, 2008 at 5:28 PM, Martin Blais [EMAIL PROTECTED] wrote: Allright, this is a definitely a newbie question. I'm learning Haskell and going through the exercises in the beautiful Hutton book, and one of them requires for me to write a loop that queries a line from the user (stdin), looping until the user enters a valid integer (at least that's how I want to implement the interface to the exercise). I have tried tons of variants and modifications of code, and I can't find the way to implement this. Here is what my emacs buffer is at now:: import Text.Read import System.IO import qualified Control.Exception as C getNum :: IO Int getNum = do line - (hGetLine stdin) x - (C.catch (do return (read line :: Int)) (\e - getNum)) return x main = do x - getNum putStr ((show x) ++ \n) Now, I've tried the Prelude's catch, the Control.Exception catch, I've tried moving it at the top of getnum, I've tried without catch, I've tried a ton of other shtuff, so much that I'm starting to think that Emacs is going to run out of electrons soon. I asked some half-newbie friends who are insanely enthousiastic about Haskell and they can't do it either (I'm starting to think that those enthousiastic friends are dating a beautiful girl with 3 PhDs, but she has a 2-inch thick green and gooey wart smack on her nose and they're so blindly in love that they can't admit that she does). I've asked some university profs and they sidetrack my question by saying I shouldn't do I/O so early. Can anyone here restore my faith in the Haskell section of humanity? 1. How do I catch the exception that is raised from read? I think you want readIO, which yields a computation in the IO monad, so it can be caught. 2. Where do I find the appropriate information I need in order to fix this? I'm probably just not searching in the right place. (Yes, I've seen the GHC docs, and it doesn't help, maybe I'm missing some background info.) In this particular case, I am not sure where you'd find this information. It's not very intuitive to a beginner why read doesn't work in this case. 3. Please do not tell me I should solve the problem differently. Here is the problem I'm trying to solve, and nothing else: Write a program that reads a line from the user, looping the query until the line contains a valid integer. It shouldn't be too hard i think. The best answer would be a two-liner code example that'll make me feel even more stupid than I already do. Thanks in advance. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
blais: Allright, this is a definitely a newbie question. I'm learning Haskell and going through the exercises in the beautiful Hutton book, and one of them requires for me to write a loop that queries a line from the user (stdin), looping until the user enters a valid integer (at least that's how I want to implement the interface to the exercise). 1. How do I catch the exception that is raised from read? The best thing to do is bypass read and use 'reads' to define your own safe read. maybeRead :: Read a = String - Maybe a maybeRead s = case reads s of [(x, )] - Just x _ - Nothing For example, yielding: import System.IO main = do x - getNum print x getNum :: IO Integer getNum = do n - getLine case maybeRead n of Nothing - getNum Just n - return n 2. Where do I find the appropriate information I need in order to fix this? I'm probably just not searching in the right place. (Yes, I've seen the GHC docs, and it doesn't help, maybe I'm missing some background info.) I think Control.Exception.catch should be fine here. 3. Please do not tell me I should solve the problem differently. Here is the problem I'm trying to solve, and nothing else: Write a program that reads a line from the user, looping the query until the line contains a valid integer. It shouldn't be too hard i think. The best answer would be a two-liner code example that'll make me feel even more stupid than I already do. Of course, it's easy. You can have fun now abstracting out the loop form in getNum using say, MaybeT or friends. But a loop is simple and easy. If you want to write it with explict exception handling of the read parse failure, it's more tedious, as you need to ensure the read exception is thrown within the body of the enclosing catch. For example, import System.IO import qualified Control.Exception as C main = do x - getNum print x getNum :: IO Integer getNum = do y - maybeRead case y of Nothing - getNum Just n - return n maybeRead :: Read a = IO (Maybe a) maybeRead = C.catch (do x - getLine let n = read x n `seq` return (Just n)) -- ensure any exception is thrown here (const (return Nothing)) -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
philip.weaver: 1. How do I catch the exception that is raised from read? I think you want readIO, which yields a computation in the IO monad, so it can be caught. Ah, that's a third option, sequence the effect using readIO, import System.IO import qualified Control.Exception as C main = do x - getNum print x getNum :: IO Integer getNum = do y - maybeRead case y of Nothing - getNum Just n - return n maybeRead :: Read a = IO (Maybe a) maybeRead = C.catch (do x - getLine n - readIO x return (Just n)) -- ensure any exception is thrown here (const (return Nothing)) -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
On Fri, 30 May 2008 16:54:18 -0700, Philip Weaver [EMAIL PROTECTED] said: 1. How do I catch the exception that is raised from read? I think you want readIO, which yields a computation in the IO monad, so it can be caught. Holy schmoly, there it is, words of wisdom, written as clearly as can be, from the docs: The readIO function is similar to read except that it signals parse failure to the IO monad instead of terminating the program. I'll be prosternating on the floor towards my web browser for the next four hours. Thank you very much Philip. (And thank you Don for the verbose examples.) 2. Where do I find the appropriate information I need in order to fix this? I'm probably just not searching in the right place. (Yes, I've seen the GHC docs, and it doesn't help, maybe I'm missing some background info.) In this particular case, I am not sure where you'd find this information. It's not very intuitive to a beginner why read doesn't work in this case. Dear Philip, could you point your virtual finger towards a reference/paper/book/any-bleeping-thing that would help this simple beginner understand why it doesn't work in this case? I'm trying to picture why a read function that terminates the program would be useful anywhere. In fact, any library function other than something like UNIX's exit or kill which terminates my program is not really welcome in any of my computer programs, but then again, I haven't yet been illuminated by the genie of pure functional languages. A reference would be awesome. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
On Fri, May 30, 2008 at 5:14 PM, Martin Blais [EMAIL PROTECTED] wrote: On Fri, 30 May 2008 16:54:18 -0700, Philip Weaver [EMAIL PROTECTED] said: 1. How do I catch the exception that is raised from read? I think you want readIO, which yields a computation in the IO monad, so it can be caught. Holy schmoly, there it is, words of wisdom, written as clearly as can be, from the docs: The readIO function is similar to read except that it signals parse failure to the IO monad instead of terminating the program. I'll be prosternating on the floor towards my web browser for the next four hours. Thank you very much Philip. (And thank you Don for the verbose examples.) 2. Where do I find the appropriate information I need in order to fix this? I'm probably just not searching in the right place. (Yes, I've seen the GHC docs, and it doesn't help, maybe I'm missing some background info.) In this particular case, I am not sure where you'd find this information. It's not very intuitive to a beginner why read doesn't work in this case. Dear Philip, could you point your virtual finger towards a reference/paper/book/any-bleeping-thing that would help this simple beginner understand why it doesn't work in this case? I'm trying to picture why a read function that terminates the program would be useful anywhere. In fact, any library function other than something like UNIX's exit or kill which terminates my program is not really welcome in any of my computer programs, but then again, I haven't yet been illuminated by the genie of pure functional languages. A reference would be awesome. Sorry, I wouldn't know where to point you, other than stating the simple rule that you can't catch exceptions in pure code. Others may be able to enlighten you better. By the way, the example that Dons gave may look more verbose, but (when possible) it's a probably a better idea to capture failure in a Maybe than in IO. I gave you readIO because it fit in to the exception handling that you were trying to do, and because you said you didn't want anyone to tell you you were doing things wrong :). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Newbie question] -- Looping stdin until condition is met
On Fri, 30 May 2008 17:19:54 -0700, Philip Weaver [EMAIL PROTECTED] said: Dear Philip, could you point your virtual finger towards a reference/paper/book/any-bleeping-thing that would help this simple beginner understand why it doesn't work in this case? I'm trying to picture why a read function that terminates the program would be useful anywhere. In fact, any library function other than something like UNIX's exit or kill which terminates my program is not really welcome in any of my computer programs, but then again, I haven't yet been illuminated by the genie of pure functional languages. A reference would be awesome. Sorry, I wouldn't know where to point you, other than stating the simple rule that you can't catch exceptions in pure code. Others may be able to enlighten you better. By the way, the example that Dons gave may look more verbose, but (when possible) it's a probably a better idea to capture failure in a Maybe than in IO. I gave you readIO because it fit in to the exception handling that you were trying to do, and because you said you didn't want anyone to tell you you were doing things wrong :). Here is a private reply from another user, which is more explanatory, the problem was that the read function wasn't getting called at a point where it could have been caught (I'll have to look into that into more detail): All you need is a little strictness, x - (C.catch (return $! read line :: Int) (\e - getNum)) works. Another option is using evaluate instead of return. The problem is that (read line :: Int) is not evaluated until it is needed, that is when it's going to be printed, but then it's too late to catch the exception. Some general remarks: hGetLine stdin === getLine do x - action return x is the same as action ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question: Using Haskell Functions in a C Program
On Thu, May 8, 2008 at 5:09 PM, Philip Müller [EMAIL PROTECTED] wrote: Thanks for all the answers. I'm testing this right now and simples cases work as expected. However from what I've read it seems it'll get ugly once I try to pass a C array to a Haskell function. Well, maybe arrays in C have been ugly before trying to pass them to Haskell functions ;) To elaborate a bit about the C program, it's a small game using OpenGL for output and mouse and keyboard for input. The SDL package (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/SDL) is quite nice for input handling and window making and whatnot. I have heard it is a bitch to get working on Windows, though. The OpenGL package (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OpenGL) is good, but can be a real pain to get going because the documentation layout is terrible. To toot my own horn, if your graphics are simple enough (specifically, 2D), you can use graphics-drawingcombinators (http://hackage.haskell.org/cgi-bin/hackage-scripts/package/graphics-drawingcombinators), which has a simple interface, for output. It depends on SDL, so above caveat applies. But once you learn the pattern, I/O is basically a transliteration from C. The reason I struggle with such C-like libraries in Haskell is that I spend all my time trying to clean them up and make them more Haskell-like. I definitely consider FFI more of a pain (when you want to do anything sophisticated) than the OpenGL package, though. Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie Question: Using Haskell Functions in a C Program
Hi, I'm in the process of writing a C program, but I can't stop thinking about how some functions would be much nicer implemented in Haskell. Is there a way to write some of the functions in Haskell and then use them in my C code via some kind of interface? BTW yes, I have been thinking about writing the whole program in Haskell, but I just don't have the level of experience in Haskell programming for that, since it's really heavy on IO. Thanks in advance for the answers! Regards Philip ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question: Using Haskell Functions in a C Program
Hello Philip, Friday, May 9, 2008, 2:17:41 AM, you wrote: Is there a way to write some of the functions in Haskell and then use them in my C code via some kind of interface? http://haskell.org/haskellwiki/IO_inside#Interfacing_with_foreign_evil_.28under_development.29 and then entries 1,6,7 in http://haskell.org/haskellwiki/IO_inside#Further_reading -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question: Using Haskell Functions in a C Program
mail:
Is there a way to write some of the functions in Haskell and then use
them in my C code via some kind of interface?
Using C just for IO is a bit weird -- perhaps you could illustrate
the kind of IO you're doing? Learning how to do IO in Haskell is
a much safer solution that linking the Haskell runtime into your
C program.
That said, this is done by using 'foreign export' declarations
in your Haskell code, then linking the compiled Haskell objects
into your C code, as follows:
We define the fibonacci function in Haskell:
{-# LANGUAGE ForeignFunctionInterface #-}
module Safe where
import Foreign.C.Types
fibonacci :: Int - Int
fibonacci n = fibs !! n
where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)
fibonacci_hs :: CInt - CInt
fibonacci_hs = fromIntegral . fibonacci . fromIntegral
foreign export ccall fibonacci_hs :: CInt - CInt
And call it from C:
#include A_stub.h
#include stdio.h
int main(int argc, char *argv[]) {
int i;
hs_init(argc, argv);
i = fibonacci_hs(42);
printf(Fibonacci: %d\n, i);
hs_exit();
return 0;
}
Now, first compile the Haskell file:
$ ghc -c -O A.hs
Which creates some *.c and *.h headers, which you import into
your C program. Now compile your C code with ghc (!), passing
the Haskell objects on the command line:
$ ghc -optc-O test.c A.o A_stub.o -o test
How run your C code:
$ ./test
Fibonacci: 267914296
And that's it.
-- Don
P.S. Its easier to learn how to do IO in Haskell :)
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Re: [Haskell-cafe] Newbie Question: Using Haskell Functions in a C Program
Thanks for all the answers. I'm testing this right now and simples cases work as expected. However from what I've read it seems it'll get ugly once I try to pass a C array to a Haskell function. Well, maybe arrays in C have been ugly before trying to pass them to Haskell functions ;) To elaborate a bit about the C program, it's a small game using OpenGL for output and mouse and keyboard for input. I just don't know how to do that in Haskell - my knowledge is quite basic in nature ;) Regards Philip ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question: Using Haskell Functions in a C Program
mail: Thanks for all the answers. I'm testing this right now and simples cases work as expected. However from what I've read it seems it'll get ugly once I try to pass a C array to a Haskell function. Right, passing arrays back and forth is going to get tiring. Well, maybe arrays in C have been ugly before trying to pass them to Haskell functions ;) To elaborate a bit about the C program, it's a small game using OpenGL for output and mouse and keyboard for input. I just don't know how to do that in Haskell - my knowledge is quite basic in nature ;) Oh, then you'll want to use the Haskell OpenGL bindings. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/OpenGL There's a few OpenGL-based games in Haskell, here: http://haskell.org/haskellwiki/Applications_and_libraries/Games and a section of blog articles on using the OpenGL bindings: http://haskell.org/haskellwiki/Blog_articles/Graphics#OpenGL -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Newbie Question: Using Haskell Functions in a C Program
Hello Philip, Friday, May 9, 2008, 3:09:33 AM, you wrote: Thanks for all the answers. I'm testing this right now and simples cases work as expected. However from what I've read it seems it'll get ugly once I try to pass a C array to a Haskell function. http://haskell.org/haskellwiki/Modern_array_libraries read about foreign arrays -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Newbie question: mutually exclusive strict / lazy
Yes, sorry, GHC's strictness analyzer. What I meant with this email is that I guess that for a strictness analyzer, the information that a function is strict in an argument *independent from the other arguments* would not be good enough in itself for optimization, it would be better to also use the dependencies between the arguments (as in the case of the if.then.else). It seems one can indicate in GHC that an argument is strict using annotiations, but I don't see a way of specifying these dependencies (maybe this does not make sense, and this is all newbie nonsense). Of course, with whole program optimization this would not be necessary, but if the compiler just sees the function signature, he must assume that a lazy argument is always lazy, independent of the value of other strict arguments no? Cheers, Peter From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Lennart Augustsson Sent: maandag 11 februari 2008 0:28 To: Peter Verswyvelen Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Newbie question: mutually exclusive strict / lazy I'm not sure what you mean by the strictness analyzer. GHC's strictness analyzer? I don't know, but I would hope so since it was done already in 1980 by Alan Mycroft. -- Lennart On Sat, Feb 9, 2008 at 4:33 PM, Peter Verswyvelen [EMAIL PROTECTED] wrote: Consider the function cond x y z = if x then y else z I guess we can certainly say cond is strict in x. But what about y and z? If x is true, then cond is strict in y If x is false, then cond is strict in z So we can't really say cond is lazy nor strict in its second or third argument. Of course, this is the case for many more functions, but in the case of the if-then-else primitive, does the strictness analyzer make use of this mutually exclusive strictness fact? Cheers, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: mutually exclusive strict / lazy
2008/2/11, Peter Verswyvelen [EMAIL PROTECTED]: Yes, sorry, GHC's strictness analyzer. What I meant with this email is that I guess that for a strictness analyzer, the information that a function is strict in an argument *independent from the other arguments* would not be good enough in itself for optimization, it would be better to also use the dependencies between the arguments (as in the case of the if…then…else). It seems one can indicate in GHC that an argument is strict using annotiations, but I don't see a way of specifying these dependencies (maybe this does not make sense, and this is all newbie nonsense). Of course, with whole program optimization this would not be necessary, but if the compiler just sees the function signature, he must assume that a lazy argument is always lazy, independent of the value of other strict arguments no? It may not always be the case, but, here, for your particular example, what you need is an inline followed by a reduction (dunno which). Reminder: cond x y z = if x then y else z The translation in core, is this: cond x y z = case x of True - y False - z So, suppose we know at some call site that x is True. So, the call cond x e1 e2 -- e1 and e2 are arbitrary expressions is equivalent to: cond True e1 e2 An inline replaces the call by this: cond x e1 e2 = case True of True - e1 False - e2 In this case, the compiler can easily determine at compile time the selected branch. Therefore, this case expression is replaced by the correct branch: e1 I would be surprised if GHC doesn't already perform this kind of optimization [1,2]. So, no need for a fancy strictness analyser for this code. About more complicated cases, I'm clueless, thought. Cheers, Loup [1] http://research.microsoft.com/~simonpj/Papers/inlining/ [2] http://citeseer.ist.psu.edu/jones91unboxed.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question: mutually exclusive strict / lazy
Consider the function cond x y z = if x then y else z I guess we can certainly say cond is strict in x. But what about y and z? If x is true, then cond is strict in y If x is false, then cond is strict in z So we can't really say cond is lazy nor strict in its second or third argument. Of course, this is the case for many more functions, but in the case of the if-then-else primitive, does the strictness analyzer make use of this mutually exclusive strictness fact? Cheers, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: mutually exclusive strict / lazy
Am Samstag, 9. Februar 2008 17:33 schrieb Peter Verswyvelen: Consider the function cond x y z = if x then y else z I guess we can certainly say cond is strict in x. But what about y and z? If x is true, then cond is strict in y If x is false, then cond is strict in z So we can't really say cond is lazy nor strict in its second or third argument. Of course, this is the case for many more functions, but in the case of the if-then-else primitive, does the strictness analyzer make use of this mutually exclusive strictness fact? Cheers, Peter Hope I remember correctly... A function is strict in an argument, if whenever that argument is _|_, the result is _|_, regardless of possible other arguments. Since if True then 0 else _|_ == 0, if-then-else is nonstrict in the third argument, similarly if False then _|_ else 0 == 0, so if-then-else is nonstrict in the second argument. Cheers, Daniel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question
Hi, I try to undestand why this code dosen't work f :: (Num a)=Integer-a f i = i Integer is an instance of Num, so why does this code produce error: Couldn't match expected type 'a' againsta inferred type 'Integer' ... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question
2008/1/21 Alexander Seliverstov [EMAIL PROTECTED]: Hi, I try to undestand why this code dosen't work f :: (Num a)=Integer-a f i = i Integer is an instance of Num, so why does this code produce error: Couldn't match expected type 'a' againsta inferred type 'Integer' ... But the type of this function says that it can return *any* instance of Num -- that is, the caller gets to choose which particular instance of Num they want. This function can only ever return an Integer. There is actually a function of this type, however; it's called fromIntegral. It works because it is a member of the Num type class. -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question
How does caller choose which particular instance of Num they want? In object-oriented language If function return type is an interface it means that it can return any implementation of this interface, but caller can't choose which particular inplementation they want. What the difference between haskell class and interface in object-oriented languge such Java or C#? 2008/1/21, Brent Yorgey [EMAIL PROTECTED]: 2008/1/21 Alexander Seliverstov [EMAIL PROTECTED]: Hi, I try to undestand why this code dosen't work f :: (Num a)=Integer-a f i = i Integer is an instance of Num, so why does this code produce error: Couldn't match expected type 'a' againsta inferred type 'Integer' ... But the type of this function says that it can return *any* instance of Num -- that is, the caller gets to choose which particular instance of Num they want. This function can only ever return an Integer. There is actually a function of this type, however; it's called fromIntegral. It works because it is a member of the Num type class. -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Newbie question
Hello Alexander, Monday, January 21, 2008, 7:36:18 PM, you wrote: How does caller choose which particular instance of Num they want? In object-oriented language If function return type is an interface it means that it can return any implementation of this interface, but caller can't choose which particular inplementation they want. but type class isn't an interface! it's just like interface in one concrete area - it includes method specifications, but not includes data fields the type that should ìó returned by function is passed by means of so-called dictionary and which type should be returned is defined by type inference process. for example main = print (length [] + f 1) here f should return Int because length return Int and you can't add values of different types (without explicit type conversion). you should also read something about two-way type inference but i don't know any good source please note that in modern OOP languages (latest C# versions, C++ 0x) support for *one-way* type inference was only added, i.e. they only can deduce type of expression from types of operands, while Haskell deduces types in both directions -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
Henning Thielemann writes: Caching is not the default, but you can easily code this by yourself: Define an array and initialize it with all function values. Because of lazy evaluation the function values are computed only when they are requested and then they persist in the array. One should add this most simple case to the Wiki. A posteriori thought, when I reread that... This is a =part= of the story. Whatever you do with the initial calls, if there is no automatic memoization, further calls will be executed normally. The user has to replace his/her calls with the elements of the memo-array. Suppose (sorry for the awful Fibonacci again...) we define fibs = [fib n | n-[0 ..]] fib 0 = 0 fib 1 = 1 fib n = fib(n-1) + fib(n-2) If you try to get fibs!!1000 you will die before anyway. The solution is obviously to replace the recursive definition of fib by fib n = fibs!!(n-1) + fibs!!(n-2) This works well. I had a similar problem in physics, perturbation theory offers often some quite intricate, knotty recurrencies, and the memoization offers a solution for the worse than exponential complexity. But I had to use trees, 2_dim lists of lists, etc. in order to sort the order of the creation of the results appropriately. So, I agree wholeheartly with the statement that memoization is not a blind automaton, it should be used consciously, and adapted to concrete needs. Jerzy Karczmarczuk ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On 12 Jan 2008, at 23:16, Hugh Perkins wrote: On Jan 12, 2008 10:54 PM, Henning Thielemann [EMAIL PROTECTED] wrote: On Sat, 12 Jan 2008, Hugh Perkins wrote: I guess that Haskell's referential transparence means the answers to the isPerfectSquare will be cached, ie automatically memoized? (not sure if is correct term?) http://www.haskell.org/haskellwiki/Memoization Interesting... but I dont understand... I thought that referential transparence meant that once the answer to a function has been calculated once, it will always be the same, and that the interpreter can, and will, cache this answer? So, if I call f( 20 ) once, for some, arbitrary, f, it will have to go away and calculate f(20), but if I call it multiple times, it will just return the value it already calculated? No, Memorisation has it's costs too... Suppose you wanted to computer map f [1..100]? Each time f was called, your program would look up a table of on average 50 results for f. That doesn't sound very efficient if f is a simple function. Now suppose you're running a program for several hours -- imagine how large your table would become, and how slow your lookup would be. What you can do however, is introduced constants. Constants are evaluated once and only once, so using them, you can tell the compiler exactly what should be memorized. Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
(While writing this message GMail told me I was too late to answer the question. Oh well, as I already typed it, let's send =) On Jan 12, 2008 9:16 PM, Hugh Perkins [EMAIL PROTECTED] wrote: Interesting... but I dont understand... I thought that referential transparence meant that once the answer to a function has been calculated once, it will always be the same, and that the interpreter can, and will, cache this answer? It *can*, but all Haskell implementations I know do not. The reason is very simple, it would need a veery large amount of memory, and sometimes searching to see if the answer was already calculated could be worse than recalculating it (think of (+1) or perhaps (null)). Polimorphic functions would also complicate the matter, as multiple different caches would be needed. So, if I call f( 20 ) once, for some, arbitrary, f, it will have to go away and calculate f(20), but if I call it multiple times, it will just return the value it already calculated? If you do something like let x = f 20 in x + x it *probably* will be calculated only once (although it could be calculated twice). But in the (bad) implementaion of fibonacci below, fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2) when calculating (fib 40), (fib 38) will be calculated twice, (fib 37) will be calculated thrice, etc. -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On Sun, 13 Jan 2008, Hugh Perkins wrote: On Jan 12, 2008 10:54 PM, Henning Thielemann [EMAIL PROTECTED] wrote: On Sat, 12 Jan 2008, Hugh Perkins wrote: I guess that Haskell's referential transparence means the answers to the isPerfectSquare will be cached, ie automatically memoized? (not sure if is correct term?) http://www.haskell.org/haskellwiki/Memoization Interesting... but I dont understand... I thought that referential transparence meant that once the answer to a function has been calculated once, it will always be the same, and that the interpreter can, and will, cache this answer? Caching is not the default, but you can easily code this by yourself: Define an array and initialize it with all function values. Because of lazy evaluation the function values are computed only when they are requested and then they persist in the array. One should add this most simple case to the Wiki. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On Jan 12, 2008, at 18:16 , Hugh Perkins wrote: On Jan 12, 2008 10:54 PM, Henning Thielemann [EMAIL PROTECTED] wrote: On Sat, 12 Jan 2008, Hugh Perkins wrote: I guess that Haskell's referential transparence means the answers to the isPerfectSquare will be cached, ie automatically memoized? (not sure if is correct term?) http://www.haskell.org/haskellwiki/Memoization Interesting... but I dont understand... I thought that referential transparence meant that once the answer to a function has been calculated once, it will always be the same, and that the interpreter can, and will, cache this answer? It *can* cache the answer, if it so chooses... but that often turns out to be a pessimization, as it caches values that are only used once or twice. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On Jan 12, 2008 10:54 PM, Henning Thielemann [EMAIL PROTECTED] wrote: On Sat, 12 Jan 2008, Hugh Perkins wrote: I guess that Haskell's referential transparence means the answers to the isPerfectSquare will be cached, ie automatically memoized? (not sure if is correct term?) http://www.haskell.org/haskellwiki/Memoization Interesting... but I dont understand... I thought that referential transparence meant that once the answer to a function has been calculated once, it will always be the same, and that the interpreter can, and will, cache this answer? So, if I call f( 20 ) once, for some, arbitrary, f, it will have to go away and calculate f(20), but if I call it multiple times, it will just return the value it already calculated? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On 1/12/08, Henning Thielemann [EMAIL PROTECTED] wrote: Caching is not the default, but you can easily code this by yourself: Define an array and initialize it with all function values. Because of lazy evaluation the function values are computed only when they are requested and then they persist in the array. But how can I implement memoization for a more complicated function? For example, perhaps I want to memoize f :: String - Int - Double - String - Bool In Python, it's pretty easy to memoize this. How can I do it in Haskell? I suspect the only way would involve changing the function signature to use IO or ST. It would be nice if I could just tell the compiler I command you to memoize this function, and have it produce the required code automatically. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On 12 Jan 2008, at 3:30 PM, David Benbennick wrote: On 1/12/08, Henning Thielemann [EMAIL PROTECTED] wrote: Caching is not the default, but you can easily code this by yourself: Define an array and initialize it with all function values. Because of lazy evaluation the function values are computed only when they are requested and then they persist in the array. But how can I implement memoization for a more complicated function? For example, perhaps I want to memoize f :: String - Int - Double - String - Bool In Python, it's pretty easy to memoize this. How can I do it in Haskell? I suspect the only way would involve changing the function signature to use IO or ST. It would be nice if I could just tell the compiler I command you to memoize this function, and have it produce the required code automatically. You can cache anything using mutable hash tables, as you know, and googling will find you `function's in Haskell that do this for you. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On Sat, 12 Jan 2008, David Benbennick wrote: On 1/12/08, Henning Thielemann [EMAIL PROTECTED] wrote: Caching is not the default, but you can easily code this by yourself: Define an array and initialize it with all function values. Because of lazy evaluation the function values are computed only when they are requested and then they persist in the array. But how can I implement memoization for a more complicated function? For example, perhaps I want to memoize f :: String - Int - Double - String - Bool There was a long thread about a sophisticated technique called blue prints, which allows you to use binary search trees as memoizing structure. http://www.haskell.org/pipermail/haskell-cafe/2006-September/018204.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [newbie question] Memoization automatic in Haskell?
On Jan 12, 2008 11:30 PM, David Benbennick [EMAIL PROTECTED] wrote: On 1/12/08, Henning Thielemann [EMAIL PROTECTED] wrote: Caching is not the default, but you can easily code this by yourself: Define an array and initialize it with all function values. Because of lazy evaluation the function values are computed only when they are requested and then they persist in the array. But how can I implement memoization for a more complicated function? For example, perhaps I want to memoize f :: String - Int - Double - String - Bool In Python, it's pretty easy to memoize this. How can I do it in Haskell? I suspect the only way would involve changing the function signature to use IO or ST. No, that is one way to do it, and probably the easiest to think about. Because its conceptually pure, I wouldn't be opposed to wrapping it in unsafePerformIO (but that can be, well, unsafe if you do it wrong :-) But there is a way to do it if you demand to be a purist, but only if you can code a data structure representing all values of a type. Doing this for a particular type is one of my favorite ways to spend a half hour when I'm bored :-) For an obvious case, but to illustrate the point, I'll do Bool. data BoolCache a = BC a a bools :: BoolCache Bool bools = BC True False lookupBool :: BoolCache a - Bool - a lookupBool (BC t f) True = t lookupBool (BC t f) False = f memoBool :: (Bool - a) - (Bool - a) memoBool f = lookupBool (fmap f bools) The pattern is the same for any type. You can do it for types with infinitely many members, like Integer, but it's trickier (but it's the same pattern, just a trickier data structure). The Integer case is scattered here and there online. I haven't found any other cases online, but I've implemented a few. It would be nice if I could just tell the compiler I command you to memoize this function, and have it produce the required code automatically. Tru dat! But it's not clear what the best way for the compiler writer to do that is. For example, if I know the access patterns of the function, I can design the aforementioned data structure to favor those. Also, not every type admits memoization, for example functions. But I can certainly envisage a library providing: class Memo a where memo :: (a - b) - (a - b) For a bunch of different types. Hmm, one probably already exists, actually... Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question: the need for parentheses
Hi, I have this function that sums up a list of numbers: suma [] = 0 suma (h:t) = h + suma t However, why do I need the parenthes in the second clause? What does the compiler think I'm trying to do when I type suma [] = 0 suma h:t = h + suma t Thanks! :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: the need for parentheses
Hi suma [] = 0 suma (h:t) = h + suma t suma [] = 0 suma h:t = h + suma t Infix operators bind less tightly than function application, so the compiler sees: (suma h) : t = h + (suma t) Hence the compiler gets confused. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: the need for parentheses
Fernando Rodriguez wrote: Hi, I have this function that sums up a list of numbers: suma [] = 0 suma (h:t) = h + suma t However, why do I need the parenthes in the second clause? What does the compiler think I'm trying to do when I type suma [] = 0 suma h:t = h + suma t Definitions are parsed in very much the same way as expressions. In principle they don't need to be, but it makes the language more uniform if they are. If you wrote suma h:t as an expression, it would mean (suma h) : t, because function application binds tighter than any infix operator. So, as a definition: suma h : t = h + suma t looks to the compiler like an attempt to redefine the operator ':', except that the left parameter is suma h, which looks like a function application, and that's not allowed in a definition. [it also doesn't make sense because ':' is actually a constructor not any old operator. But that turns out not to be the key problem here] Note that you can define operators directly infix, e.g.: a * b = multiply a b ...and if I try to do that with something like 'suma' on the left... suma a * b = multiply a b ...I get the same error (parse error in pattern, in GHC). Hope that helps, Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: the need for parentheses
However, why do I need the parenthes in the second clause? What does the compiler think I'm trying to do when I type suma [] = 0 suma h:t = h + suma t (suma h):t = h + suma t ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question related to list evaluation
Hi, I am new to functional and lazy programming languages ( that's correct, my life has been pretty pathetic so far) and am not able to understand GHC's behaviour for a particular function. Can someone help me please? I am trying to write a function which would compare two strings (from reverse) and return the position of first mismatch. This is part of the right-to-left scan of bayer-moore algorithm.. and it is essential for me to do it from reverse. Since my goal is to learn haskell, I am not using Data.ByteString. My function is as follows: matchReverse :: String - String -Int-(Bool,Int) matchReverse [] [] pos = (True,pos) matchReverse _ [] pos = (False,pos) matchReverse [] _ pos = (False,pos) matchReverse (x:xs) (y:ys) pos = let (matched,pos) = matchReverse xs ys (pos +1) in if matched then ((x==y),pos) else (False,pos) The behaviour I expected in four scenarios is as below: 1.matchReverse kapilash kapilash 0 --should return (True,0) 2.matchReverse kapilash kapilast 0 --should return (False,8) 3.matchReverse str1 str2 0 --should return (False,0) 4.matchReverse str1 str1 0 --should return (True,0) where str1 and str2 are defined as below: str1 = replicate 1000 'a' str2 = 'b':(replicate 999 'a') what confounds me is that it is able to identify the first element of the tuple in ALL the cases. Invoking fst on the each of the four calls instantly returns the expected value.(even for the cases 3 and 4 where, there are thousand elements) But it seems to go into an infinite loop while calculating the 'snd' of the tuple. Even for strings containing just one element each. can someone throw some light on this please? Why does it go into an infinite loop? Many thanks Kapilash -- I drink I am thunk. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question related to list evaluation
You have used the name 'pos' twice, for both the parameter and the returned value of the recursive call. The reason this results in an infinite loop is that in code like let x = x + 1 Haskell treats both xs to be references to the same thing, so evaluates: x = x + 1 = (x + 1) + 1 = ((x + 1) + 1) + 1 ... which results in the infinite loop. On 06/01/2008, Sai Hemanth K [EMAIL PROTECTED] wrote: Hi, I am new to functional and lazy programming languages ( that's correct, my life has been pretty pathetic so far) and am not able to understand GHC's behaviour for a particular function. Can someone help me please? I am trying to write a function which would compare two strings (from reverse) and return the position of first mismatch. This is part of the right-to-left scan of bayer-moore algorithm.. and it is essential for me to do it from reverse. Since my goal is to learn haskell, I am not using Data.ByteString. My function is as follows: matchReverse :: String - String -Int-(Bool,Int) matchReverse [] [] pos = (True,pos) matchReverse _ [] pos = (False,pos) matchReverse [] _ pos = (False,pos) matchReverse (x:xs) (y:ys) pos = let (matched,pos) = matchReverse xs ys (pos +1) in if matched then ((x==y),pos) else (False,pos) The behaviour I expected in four scenarios is as below: 1.matchReverse kapilash kapilash 0 --should return (True,0) 2.matchReverse kapilash kapilast 0 --should return (False,8) 3.matchReverse str1 str2 0 --should return (False,0) 4.matchReverse str1 str1 0 --should return (True,0) where str1 and str2 are defined as below: str1 = replicate 1000 'a' str2 = 'b':(replicate 999 'a') what confounds me is that it is able to identify the first element of the tuple in ALL the cases. Invoking fst on the each of the four calls instantly returns the expected value.(even for the cases 3 and 4 where, there are thousand elements) But it seems to go into an infinite loop while calculating the 'snd' of the tuple. Even for strings containing just one element each. can someone throw some light on this please? Why does it go into an infinite loop? Many thanks Kapilash -- I drink I am thunk. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question related to list evaluation
On 6 Jan 2008, at 10:34 AM, Sai Hemanth K wrote: Hi, I am new to functional and lazy programming languages ( that's correct, my life has been pretty pathetic so far) and am not able to understand GHC's behaviour for a particular function. Can someone help me please? I am trying to write a function which would compare two strings (from reverse) and return the position of first mismatch. This is part of the right-to-left scan of bayer-moore algorithm.. and it is essential for me to do it from reverse. Since my goal is to learn haskell, I am not using Data.ByteString. My function is as follows: matchReverse :: String - String -Int-(Bool,Int) matchReverse [] [] pos = (True,pos) matchReverse _ [] pos = (False,pos) matchReverse [] _ pos = (False,pos) matchReverse (x:xs) (y:ys) pos = let (matched,pos) = matchReverse xs ys (pos +1) in if matched then ((x==y),pos) else (False,pos) let is always recursive in Haskell, so this is a recursive definition of pos. To break the recursion, use matchReverse (x:xs) (y:ys) pos = let (matched, pos') = matchReverse xs ys (pos + 1) in if matched then ((x==y), pos') else (False, pos') The behaviour I expected in four scenarios is as below: 1.matchReverse kapilash kapilash 0 --should return (True,0) 2.matchReverse kapilash kapilast 0 --should return (False,8) 3.matchReverse str1 str2 0 --should return (False,0) 4.matchReverse str1 str1 0 --should return (True,0) where str1 and str2 are defined as below: str1 = replicate 1000 'a' str2 = 'b':(replicate 999 'a') what confounds me is that it is able to identify the first element of the tuple in ALL the cases. Invoking fst on the each of the four calls instantly returns the expected value.(even for the cases 3 and 4 where, there are thousand elements) But it seems to go into an infinite loop while calculating the 'snd' of the tuple. Even for strings containing just one element each. can someone throw some light on this please? Why does it go into an infinite loop? Many thanks Kapilash -- I drink I am thunk. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question related to list evaluation
On Jan 6, 2008 4:40 PM, Jonathan Cast [EMAIL PROTECTED] wrote: let is always recursive in Haskell, so this is a recursive definition of pos. To break the recursion, use matchReverse (x:xs) (y:ys) pos = let (matched, pos') = matchReverse xs ys (pos + 1) in if matched then ((x==y), pos') else (False, pos') Actually, I think he meant matchReverse (x:xs) (y:ys) pos = let (matched, pos') = matchReverse xs ys (pos + 1) in if matched then ((x==y), pos) else (False, pos') As as side note, GHC's flag -Wall would have warned about creating a variable with a name already in scope. -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question related to list evaluation
AAh! Thanks a ton! Hemanth K On Jan 7, 2008 12:10 AM, Rodrigo Queiro [EMAIL PROTECTED] wrote: You have used the name 'pos' twice, for both the parameter and the returned value of the recursive call. The reason this results in an infinite loop is that in code like let x = x + 1 Haskell treats both xs to be references to the same thing, so evaluates: x = x + 1 = (x + 1) + 1 = ((x + 1) + 1) + 1 ... which results in the infinite loop. On 06/01/2008, Sai Hemanth K [EMAIL PROTECTED] wrote: Hi, I am new to functional and lazy programming languages ( that's correct, my life has been pretty pathetic so far) and am not able to understand GHC's behaviour for a particular function. Can someone help me please? I am trying to write a function which would compare two strings (from reverse) and return the position of first mismatch. This is part of the right-to-left scan of bayer-moore algorithm.. and it is essential for me to do it from reverse. Since my goal is to learn haskell, I am not using Data.ByteString. My function is as follows: matchReverse :: String - String -Int-(Bool,Int) matchReverse [] [] pos = (True,pos) matchReverse _ [] pos = (False,pos) matchReverse [] _ pos = (False,pos) matchReverse (x:xs) (y:ys) pos = let (matched,pos) = matchReverse xs ys (pos +1) in if matched then ((x==y),pos) else (False,pos) The behaviour I expected in four scenarios is as below: 1.matchReverse kapilash kapilash 0 --should return (True,0) 2.matchReverse kapilash kapilast 0 --should return (False,8) 3.matchReverse str1 str2 0 --should return (False,0) 4.matchReverse str1 str1 0 --should return (True,0) where str1 and str2 are defined as below: str1 = replicate 1000 'a' str2 = 'b':(replicate 999 'a') what confounds me is that it is able to identify the first element of the tuple in ALL the cases. Invoking fst on the each of the four calls instantly returns the expected value.(even for the cases 3 and 4 where, there are thousand elements) But it seems to go into an infinite loop while calculating the 'snd' of the tuple. Even for strings containing just one element each. can someone throw some light on this please? Why does it go into an infinite loop? Many thanks Kapilash -- I drink I am thunk. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- I drink I am thunk. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question related to list evaluation
Felipe Lessa wrote: As as side note, GHC's flag -Wall would have warned about creating a variable with a name already in scope. *makes a mental note* I've created bugs like this far too many times... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: can laziness lead to space compression?
Brian Hurt [EMAIL PROTECTED] wrote: But I was wondering if it is possible that lazy evaluation could lead to space compression, especially under heavily persistant usage patterns? Note that the benefit isn't *big*- we're talking about 40 words of memory when the main data structure is taking up 5K plus words of memory- so it's less than 1% different. But there is a (small) upside in memory usage at least occassionally, right? Actually, a lazy evaluation strategy can sometimes change the entire complexity class of space usage, not just the constant factors. For instance, lazy streaming algorithms (where the data is produced and consumed in lock-step) may use a small constant amount of memory, independent of the size of the data, whereas an eager strategy would use memory linearly proportional to the dataset size. Regards, Malcolm ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question: can laziness lead to space compression?
My apologies if this has been beat to death before, I'm still new to Haskell. But I was wondering if it is possible that lazy evaluation could lead to space compression, especially under heavily persistant usage patterns? Here's the argument I'm making. Say we have a tree-based Set with, say, 1024 values in it. For ease of math we'll assume that it's perfectly balanced. Say each node in the tree takes 5 words of memory. So in an eager language (for example, Ocaml), adding a new node to this tree requires the allocation of 10 new nodes, or 50 words of memory. In Haskell, what would happen (as I understand it) is that just a new lazy thunk would be allocated- say, 10 words of memory. Conceptually, we could think of the returned value as the original tree plus a small delta. The lazy implementation is using 40 fewer words of memory than the eager implementation. Note that the benefit isn't *big*- we're talking about 40 words of memory when the main data structure is taking up 5K plus words of memory- so it's less than 1% different. But there is a (small) upside in memory usage at least occassionally, right? Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: can laziness lead to space compression?
Am Samstag, 29. Dezember 2007 16:00 schrieb Brian Hurt: My apologies if this has been beat to death before, I'm still new to Haskell. But I was wondering if it is possible that lazy evaluation could lead to space compression, especially under heavily persistant usage patterns? Here's the argument I'm making. Say we have a tree-based Set with, say, 1024 values in it. For ease of math we'll assume that it's perfectly balanced. Say each node in the tree takes 5 words of memory. So in an eager language (for example, Ocaml), adding a new node to this tree requires the allocation of 10 new nodes, or 50 words of memory. In Haskell, what would happen (as I understand it) is that just a new lazy thunk would be allocated- say, 10 words of memory. Conceptually, we could think of the returned value as the original tree plus a small delta. The lazy implementation is using 40 fewer words of memory than the eager implementation. Note that the benefit isn't *big*- we're talking about 40 words of memory when the main data structure is taking up 5K plus words of memory- so it's less than 1% different. But there is a (small) upside in memory usage at least occassionally, right? Oh yes. Imagine how much memory an eager language would need for [1 .. ]. But laziness can also induce space leaks. That's not too uncommon either. Finding out when which case applies is the art to be learned. Brian Cheers, Daniel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie question about list performance
John Lato wrote: I'm working with moderate-sized files (tens to hundreds of MBs) that have some ascii header data followed by a bunch of 32-bit ints. but I don't know if [Int32] is actually the best choice. It seems to me that something like a lazy list of strict arrays (analogous to a lazy bytestring) would be better. Depends on your data access pattern. If you access the words strictly linearly, from the beginning of the file to the end, and that's all, then [Int32] is absolutely fine. A list is a data-structure equivalent of a for loop; it's the correct structure if you are dealing with things linearly or nearly-linearly. If you were using adjacent words together, that would be fine too (as in, e.g., zip xs (tail xs)). If your data access pattern is more scattered or random-access in style, then [Int32] does not scale well to 10s of MBs. If you keep the data around, the overhead for [] is inappropriate (around 600-800% memory usage overhead on [Int32]) and its performance guarantees are not good either, for random access. In this case, as a first approximation, I would be inclined to try a library which simple backended onto lazy bytestring. For example the 'index' operation to fetch a single word would fetch four bytes and bit-twiddle them into a word. If that doesn't give the high speed you're after, then perhaps something *like* LBS, i.e. foreignptr behind the scenes, but directly accessing word-at-a-time. Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question on Setting the GHC Search Path
One factor that is slightly unusual about this phenomenon is that it only occurs with GHC, but not with Hugs 98. Typing :cd D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC in Hugs 98 does not cause an error, but typing the same command in GHC does. It seems that Hugs 98 allows spaces in filenames/paths, but GHC doesn't. Is there any way to configure GHC so that it accepts the same type of filenames/paths as Hugs 98? Benjamin L. Russell --- Benjamin L. Russell [EMAIL PROTECTED] wrote: Please pardon this intrusion for an elementary question on setting the GHC search path. I have installed GHC on my work Windows XP machine, and would like to be able to search for files in the following directory: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC However, when I type the following command into the GHC interpreter: :cd D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC I get the following error message: *** Exception: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC: setCurrentDirectory: invalid argument (Invalid argument) Yet, for testing purposes, when I type the following command: :cd cygwin I do not get any error message. There seems to be a problem with the spaces in the filename. However, I would like to be able to use the D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC directory for GHC programming practice, because I keep my practice work for my other programming languages in the same super-super-directory. Does anybody know a way to specify D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC as a directory in the search path for GHC? Benjamin L. Russell ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question on Setting the GHC Search Path
On Mon, Oct 29, 2007 at 04:25:45AM -0700, Benjamin L. Russell wrote: One factor that is slightly unusual about this phenomenon is that it only occurs with GHC, but not with Hugs 98. Typing :cd D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC Are you sure it has anything to do with spaces? Exactly one of your test paths has backslashes, and it's not the working one. Stefan signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question on Setting the GHC Search Path
In GHC it works without the and don't work with them: Prelude :cd C:\Documents and Settings Prelude :! pwd C:\Documents and Settings Olivier. On 10/29/07, Benjamin L. Russell [EMAIL PROTECTED] wrote: Please pardon this intrusion for an elementary question on setting the GHC search path. I have installed GHC on my work Windows XP machine, and would like to be able to search for files in the following directory: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC However, when I type the following command into the GHC interpreter: :cd D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC I get the following error message: *** Exception: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC: setCurrentDirectory: invalid argument (Invalid argument) Yet, for testing purposes, when I type the following command: :cd cygwin I do not get any error message. There seems to be a problem with the spaces in the filename. However, I would like to be able to use the D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC directory for GHC programming practice, because I keep my practice work for my other programming languages in the same super-super-directory. Does anybody know a way to specify D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC as a directory in the search path for GHC? Benjamin L. Russell ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Question on Setting the GHC Search Path
You're right; it works without the double-quotes. Problem solved. Thanks! Benjamin L. Russell --- Olivier Boudry [EMAIL PROTECTED] wrote: In GHC it works without the and don't work with them: Prelude :cd C:\Documents and Settings Prelude :! pwd C:\Documents and Settings Olivier. On 10/29/07, Benjamin L. Russell [EMAIL PROTECTED] wrote: Please pardon this intrusion for an elementary question on setting the GHC search path. I have installed GHC on my work Windows XP machine, and would like to be able to search for files in the following directory: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC However, when I type the following command into the GHC interpreter: :cd D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC I get the following error message: *** Exception: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC: setCurrentDirectory: invalid argument (Invalid argument) Yet, for testing purposes, when I type the following command: :cd cygwin I do not get any error message. There seems to be a problem with the spaces in the filename. However, I would like to be able to use the D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC directory for GHC programming practice, because I keep my practice work for my other programming languages in the same super-super-directory. Does anybody know a way to specify D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC as a directory in the search path for GHC? Benjamin L. Russell ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] newbie question about list performance
Hello, I've been following the list optimization thread with great interest, as it pertains to something I'm working on at the moment. I'm working with moderate-sized files (tens to hundreds of MBs) that have some ascii header data followed by a bunch of 32-bit ints. I can read the files into a lazy ByteString (and parse the header), but I'd like some advice as to the best data type to convert the ints into. Ideally, I'd have some functions like this: decode :: ByteString - (FileFormat, [Int32]) encode :: FileFormat - [Int32] - ByteString but I don't know if Int32 is actually the best choice. It seems to me that something like a lazy list of strict arrays (analogous to a lazy bytestring) would be better. Is there a library like this already? Or is this a case of premature optimization, and I should just try the list and see if it's good enough? Any suggestions would be appreciated. Also, I'd like to let the maintainers and implementors know that I really appreciate the work that's been done on optimizing Haskell. I haven't used Haskell much yet, but I've fallen in love with the language and it's great to see that performance even for heavy I/O tasks can be comparable to or exceed C. Thank you, John Lato ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie Question on Setting the GHC Search Path
Please pardon this intrusion for an elementary question on setting the GHC search path. I have installed GHC on my work Windows XP machine, and would like to be able to search for files in the following directory: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC However, when I type the following command into the GHC interpreter: :cd D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC I get the following error message: *** Exception: D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC: setCurrentDirectory: invalid argument (Invalid argument) Yet, for testing purposes, when I type the following command: :cd cygwin I do not get any error message. There seems to be a problem with the spaces in the filename. However, I would like to be able to use the D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC directory for GHC programming practice, because I keep my practice work for my other programming languages in the same super-super-directory. Does anybody know a way to specify D:\From C Drive\Documents and Settings\DekuDekuplex\Programming Practice\Haskell\GHC as a directory in the search path for GHC? Benjamin L. Russell ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] newbie question about list performance
jwlato: Hello, I've been following the list optimization thread with great interest, as it pertains to something I'm working on at the moment. I'm working with moderate-sized files (tens to hundreds of MBs) that have some ascii header data followed by a bunch of 32-bit ints. I can read the files into a lazy ByteString (and parse the header), but I'd like some advice as to the best data type to convert the ints into. Ideally, I'd have some functions like this: decode :: ByteString - (FileFormat, [Int32]) encode :: FileFormat - [Int32] - ByteString but I don't know if Int32 is actually the best choice. It seems to me that something like a lazy list of strict arrays (analogous to a lazy bytestring) would be better. Is there a library like this already? Or is this a case of premature optimization, and I should just try the list and see if it's good enough? Any suggestions would be appreciated. could you use Data.Binary.encode/decode (with custom put and get instances)? They read fomr lazy bytestrings into custom structures, such as arrays. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-0.4.1 Also, I'd like to let the maintainers and implementors know that I really appreciate the work that's been done on optimizing Haskell. I haven't used Haskell much yet, but I've fallen in love with the language and it's great to see that performance even for heavy I/O tasks can be comparable to or exceed C. Yay! -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question: Why gfoldl has this strange type?
Hi! I am a novice in Haskell, and particularly I have interested in generic programming. This interest motivated me to read paper Scrap your boilerplate: A practical design pattern for generic programming, but I didn't understand the type of the function gfoldl, that was present in class Term (Data). Somebody could help me to understand the type of this function? Thanks... Rodrigo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Why gfoldl has this strange type?
Just so nobody else has to look it up: Data.Generics.Basics.gfoldl :: Data a = (c (a - b) - a - c b) - (g - c g) - a - c a -- ryan On 8/30/07, Rodrigo Geraldo [EMAIL PROTECTED] wrote: Hi! I am a novice in Haskell, and particularly I have interested in generic programming. This interest motivated me to read paper Scrap your boilerplate: A practical design pattern for generic programming, but I didn't understand the type of the function gfoldl, that was present in class Term (Data). Somebody could help me to understand the type of this function? Thanks... Rodrigo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Why gfoldl has this strange type?
Actually, it's a higher rank type and that doesn't show up on hoogle's main page. gfoldl :: (forall a b . Data a = c (a - b) - a - c b) - (forall g . g - c g) - a - c a ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
On 8/20/07, Stefan O'Rear [EMAIL PROTECTED] wrote: ... (I need to find some way to automate making these trails :) ) ... I think you can come a long way with the debugger in GHC HEAD. It provides a :trace command that, when applied to an expression with some breakpoint in it, remembers the history of evaluation steps. You can view the history with :hist. See: http://www.haskell.org/ghc/dist/current/docs/users_guide/ghci-debugger.html#tracing regards, Bas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
Stefan O'Rear wrote: sum = sum' 0 sum' k [] = k sum' k (x:xs) = (sum' $! (k+x)) xs enum x y | x = y= 0 | otherwise = x : enum (x+1) y sum (enum 1 10) = sum' 0 (enum 1 10) = sum' 0 (1 : enum (1+1) 10) = (sum' $! (0+1)) (enum (1+1) 10) = sum' 1 (enum (1+1) 10) = sum' 1 (2 : enum (2+1) 10) = (sum' $! (1+2)) (enum (2+1) 10) = sum' 3 (enum (2+1) 10) = sum' 3 (3 : enum (3+1) 10) = (sum' $! (3+3)) (enum (3+1) 10) = sum' 6 (enum (3+1) 10) = sum' 6 (4 : enum (4+1) 10) = (sum' $! (6+4)) (enum (4+1) 10) = sum' 10 (enum (4+1) 10) = ... sum' 36 (9 : enum (9+1) 10) = (sum' $! (36+9)) (enum (9+1) 10) = sum' 45 (enum (9+1) 10) = sum' 45 [] = 45 (I need to find some way to automate making these trails :) ) I did have a fairly small Tcl implementation for this... I don't have the code now, and I wrote it early in my Haskell career, so there's masses of stuff it didn't handle. (*cough* type classes) Actually, I've often longed for some tool (maybe even integrated into Lambdabot) to show the reduction sequence of an arbitrary expression. But none exists, AFAIK... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
Not really more efficient but plays to the language implementation's strengths. Imagine take 10 $ foo (10^9) and take 10 $ bar (10^9) bar wouldn't evaluate until the 10^9 was done. (And I just ground my laptop to a halt checking that. :) foo on the other hand would run out to 10^6 and then conveniently finish the rest of your program waiting for the need of the other 10^9-10 values. If you *always* needed the result of the 10^9 calculations then tail-recursion should be better since you won't be holding onto the evaluation frames. -ljr Peter Verswyvelen wrote: Now if I understand this correctly, this just means that when writing something like: foo n = if n0 then [] else n : foo (n-1) bar n = aux 0 [] where aux i xs = if in then xs else aux (i+1) (i:xs) that foo is more efficient than bar because lazy evaluation of foo just puts the delayed computation in the cdr of the list, while lazy evaluation of bar has to keep track of all aux calls (the closures) which gives much more overhead, maybe even stack overflow? Something like that? Thanks, Peter -- Lanny Ripple [EMAIL PROTECTED] ScmDB / Cisco Systems, Inc. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
Lanny Ripple wrote: Not really more efficient but plays to the language implementation's strengths. Imagine take 10 $ foo (10^9) and take 10 $ bar (10^9) bar wouldn't evaluate until the 10^9 was done. (And I just ground my laptop to a halt checking that. :) foo on the other hand would run out to 10^6 and then conveniently finish the rest of your program waiting s/10^6/10/ That's what I get for not proof-reading after making a change after the first proof-read. for the need of the other 10^9-10 values. If you *always* needed the result of the 10^9 calculations then tail-recursion should be better since you won't be holding onto the evaluation frames. -ljr Peter Verswyvelen wrote: Now if I understand this correctly, this just means that when writing something like: foo n = if n0 then [] else n : foo (n-1) bar n = aux 0 [] where aux i xs = if in then xs else aux (i+1) (i:xs) that foo is more efficient than bar because lazy evaluation of foo just puts the delayed computation in the cdr of the list, while lazy evaluation of bar has to keep track of all aux calls (the closures) which gives much more overhead, maybe even stack overflow? Something like that? Thanks, Peter -- Lanny Ripple [EMAIL PROTECTED] ScmDB / Cisco Systems, Inc. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
On Mon, Aug 20, 2007 at 11:21:01AM -0500, Lanny Ripple wrote: Not really more efficient but plays to the language implementation's strengths. Imagine take 10 $ foo (10^9) and take 10 $ bar (10^9) bar wouldn't evaluate until the 10^9 was done. (And I just ground my laptop to a halt checking that. :) foo on the other hand would run out to 10^6 and then conveniently finish the rest of your program waiting for the need of the other 10^9-10 values. If you *always* needed the result of the 10^9 calculations then tail-recursion should be better since you won't be holding onto the evaluation frames. Even if you did, in the presense of laziness it's not useful to make list producers tail recursive. Consider: sum = sum' 0 sum' k [] = k sum' k (x:xs) = (sum' $! (k+x)) xs enum x y | x = y= 0 | otherwise = x : enum (x+1) y sum (enum 1 10) = sum' 0 (enum 1 10) = sum' 0 (1 : enum (1+1) 10) = (sum' $! (0+1)) (enum (1+1) 10) = sum' 1 (enum (1+1) 10) = sum' 1 (2 : enum (2+1) 10) = (sum' $! (1+2)) (enum (2+1) 10) = sum' 3 (enum (2+1) 10) = sum' 3 (3 : enum (3+1) 10) = (sum' $! (3+3)) (enum (3+1) 10) = sum' 6 (enum (3+1) 10) = sum' 6 (4 : enum (4+1) 10) = (sum' $! (6+4)) (enum (4+1) 10) = sum' 10 (enum (4+1) 10) = ... sum' 36 (9 : enum (9+1) 10) = (sum' $! (36+9)) (enum (9+1) 10) = sum' 45 (enum (9+1) 10) = sum' 45 [] = 45 (I need to find some way to automate making these trails :) ) It runs in constant space, despite the producer's non-tail-recursion. Stefan signature.asc Description: Digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
Thanks. I got confused because the StackOverflow link on http://www.haskell.org/haskellwiki/HaWiki_migration is dead. -Original Message- From: Derek Elkins [mailto:[EMAIL PROTECTED] Sent: Saturday, August 18, 2007 8:54 PM To: Peter Verswyvelen Cc: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki? On Sat, 2007-08-18 at 20:35 +0200, Peter Verswyvelen wrote: When reading an article about tail recursion (http://themechanicalbride.blogspot.com/2007/04/haskell-for-c-3-programmers. html) I came across the follow statements: If you can write a non-recursive function that uses the colon syntax it is probably better than a tail recursive one that doesn't. This is because Haskell's lazy evaluation enabled you to use the non-tail recursive version on an infinite stream without getting a stack overflow. and Unfortunately, laziness gets in the way. While transforming non-tail-recursive code to a tail-recursive form is important and useful for functional programming in general, dealing with laziness requires a little more care, and often non-tail-recursive versions are preferrable. flatten is an example of this, the first version is better in many ways. While I don't believe it happens in this case, oftentimes naively writing code tail-recursively in Haskell will actually -make- it overflow the stack. Another (actual) benefit of the first version of flatten is that it will work on infinite lists. http://www.haskell.org/hawiki/StackOverflow gives a simple example and some explanation. That page was migrated here: http://www.haskell.org/haskellwiki/Stack_overflow ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
When reading an article about tail recursion (http://themechanicalbride.blogspot.com/2007/04/haskell-for-c-3-programmers. html) I came across the follow statements: If you can write a non-recursive function that uses the colon syntax it is probably better than a tail recursive one that doesn't. This is because Haskell's lazy evaluation enabled you to use the non-tail recursive version on an infinite stream without getting a stack overflow. and Unfortunately, laziness gets in the way. While transforming non-tail-recursive code to a tail-recursive form is important and useful for functional programming in general, dealing with laziness requires a little more care, and often non-tail-recursive versions are preferrable. flatten is an example of this, the first version is better in many ways. While I don't believe it happens in this case, oftentimes naively writing code tail-recursively in Haskell will actually -make- it overflow the stack. Another (actual) benefit of the first version of flatten is that it will work on infinite lists. http://www.haskell.org/hawiki/StackOverflow gives a simple example and some explanation. Unfortunately I can't find the StackOverflow page anymore. Now if I understand this correctly, this just means that when writing something like: foo n = if n0 then [] else n : foo (n-1) bar n = aux 0 [] where aux i xs = if in then xs else aux (i+1) (i:xs) that foo is more efficient than bar because lazy evaluation of foo just puts the delayed computation in the cdr of the list, while lazy evaluation of bar has to keep track of all aux calls (the closures) which gives much more overhead, maybe even stack overflow? Something like that? Thanks, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
On Sat, 2007-08-18 at 20:35 +0200, Peter Verswyvelen wrote: When reading an article about tail recursion (http://themechanicalbride.blogspot.com/2007/04/haskell-for-c-3-programmers. html) I came across the follow statements: If you can write a non-recursive function that uses the colon syntax it is probably better than a tail recursive one that doesn't. This is because Haskell's lazy evaluation enabled you to use the non-tail recursive version on an infinite stream without getting a stack overflow. and Unfortunately, laziness gets in the way. While transforming non-tail-recursive code to a tail-recursive form is important and useful for functional programming in general, dealing with laziness requires a little more care, and often non-tail-recursive versions are preferrable. flatten is an example of this, the first version is better in many ways. While I don't believe it happens in this case, oftentimes naively writing code tail-recursively in Haskell will actually -make- it overflow the stack. Another (actual) benefit of the first version of flatten is that it will work on infinite lists. http://www.haskell.org/hawiki/StackOverflow gives a simple example and some explanation. That page was migrated here: http://www.haskell.org/haskellwiki/Stack_overflow ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?
foo n = if n0 then [] else n : foo (n-1) bar n = aux 0 [] where aux i xs = if in then xs else aux (i+1) (i:xs) that foo is more efficient than bar because lazy evaluation of foo just puts the delayed computation in the cdr of the list, while lazy evaluation of bar has to keep track of all aux calls (the closures) which gives much more overhead, maybe even stack overflow? Something like that? There is absolutely no problem with bar, it will not stack overflow since it _is_ tail-recursive _and_ the comparison i n force the evaluation of i avoiding the risk of constructing a too big thunk for the first parameter of aux which could bring a stack overflow like in this example : nonStrictLength n [] = n nonStrictLength n (_:xs) = nonStrictLength (n+1) xs (try nonStrictLength 0 [1..1000] in GHCi to see the stack overflow, GHC strictness analysis would avoid the problem with -O) Though foo is still more interesting than bar since it will work on infinite list, bar is too strict. -- Jedaï ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question (again!) about phantom types
I’m having difficulty to understand what phantom types are good for. Is this just for improving runtime performance? I read the wiki, and it says this is useful if you want to increase the type-safety of your code, but the code below does not give a compiler error for the function test1, I get a runtime error, just like test2. Thanks, Peter -- CODE -- -- With phantom types data T1 a = TI1 Int | TS1 String deriving Show foo1 :: T1 String - T1 String - T1 String foo1 (TS1 x) (TS1 y) = TS1 (x++y) test1 = foo1 (TI1 1) (TI1 2) -- Shouldn't this give a compiler error instead of a runtime error? -- Without phantom types data T2 = TI2 Int | TS2 String deriving Show foo2 :: T2 - T2 - T2 foo2 (TS2 x) (TS2 y) = TS2 (x++y) test2 = foo2 (TI2 1) (TI2 2) No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.11.8/941 - Release Date: 07/08/2007 16:06 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question (again!) about phantom types
peterv wrote: I’m having difficulty to understand what phantom types are good for. I read the wiki, and it says this is useful if you want to increase the type-safety of your code, but the code below does not give a compiler error for the function test1, I get a runtime error, just like test2. -- With phantom types data T1 a = TI1 Int | TS1 String deriving Show foo1 :: T1 String - T1 String - T1 String foo1 (TS1 x) (TS1 y) = TS1 (x++y) test1 = foo1 (TI1 1) (TI1 2) -- Shouldn't this give a compiler error instead -- of a runtime error? You have to manually instantiate the phantom type variable. In your code, the type of TI1 1 is just T1 a for an unrestricted type variable a. This unifies fine with the expected argument type of foo1, wich is T1 String, by setting a = String. Consider this variant of your code: -- the type of things wich can hold numbers or text -- what exactly they hold is encoded dynamically by the -- constructor used and statically by the phantom type data Container a = NumberContainer Int | TextContainer String data Number = Number data Text = Text -- approbiate smart constructors. only use these for creation -- of containers, never use the real constructors. number :: Int - Container Number number x = NumberContainer x text :: String - Container Text text x = TextContainer x -- a function wich works only on containers holding text foo :: Container Text - Container Text - Container Text foo (TextContainer a) (TextContainer b) = text (a ++ b) -- testing test1 = text hello `foo` text world -- works test2 = number 13 `foo` number 19-- static error This works fine when you can decide statically how to instantiate the phantom type variable (by using the approbiate smart constructor). If you can't (because you read data from same external source, for example), you can restrict the position of dynamic failures to a well-defined point in program execution by defining asText :: Container a - Maybe (Container Text) asText (TextContainer x) = Just $ text x asText _ = Nothing asNumber :: Container a - Maybe (Container Number) asNumber (NumberContainer x) = Just $ number x asNumber _ = Nothing Using these functions, you can lift a dynamic typecheck (is it the right constructor?) to a static typecheck (has it the right phantom type?). So you can for example check user input once for having the correct form and then enter the statically typesafe part of your program, where you don't have to worry about people entering numbers instead of text, because you statically know that at this point in program execution, the dynamic typecheck already suceeded. Tillmann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question (again!) about phantom types
Im having difficulty to understand what phantom types are good for. Is this just for improving runtime performance? No. As the wiki says, you can use them to add static guarantees. I read the wiki, and it says this is useful if you want to increase the type-safety of your code, but the code below does not give a compiler error for the function test1, I get a runtime error, just like test2. It seems you're mixing up GADT's and phantom types. -- CODE -- -- With phantom types data T1 a = TI1 Int | TS1 String deriving Show Here, the 'a' is an extra type parameter, which has no manifestation on the value level. Note the type of the constructors: TI1 :: Int - T1 a TS1 :: String - T1 a In particular, the 'a' is not related to the 'Int' or 'String' arguments. foo1 :: T1 String - T1 String - T1 String foo1 (TS1 x) (TS1 y) = TS1 (x++y) test1 = foo1 (TI1 1) (TI1 2) -- Shouldn't this give a compiler error instead of a runtime error? 'TI1 1' has type 'T1 a', so this unifies with 'T1 String' (the type of the argument of 'foo1'. The type parameter 'a' can still be useful, but you have to use an explicit type signature to constrain the type 'a': ti1 :: Int - T1 Int ti1 x = TI1 x Now, 'ti1' will create values with the restricted type 'T1 Int', that you can't use as arguments for your 'foo1'. GADTs are perhaps more useful for what you seem to want. Try something like this: data T1 :: * - * where -- T1 has one type parameter TI1 :: Int - T1 Int TS1 :: String - T1 String Now, the type systems guarantees that all values of the form 'TI1 x' have type 'T1 Int'. Greetings, Arie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question (again!) about phantom types
On Aug 8, 2007, at 11:16 AM, peterv wrote: I’m having difficulty to understand what phantom types are good for. Is this just for improving runtime performance? I found phantom types to be useful in the implementation of bit-wise set operations. You can find the code Edison: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ EdisonCore-1.2.1 in the Data.Edison.Coll.EnumSet module. David F. Place mailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Newbie question (again!) about phantom types
Thanks, that is a much clearer explanation than http://en.wikibooks.org/wiki/Haskell/Phantom_types Peter -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Arie Peterson Sent: Wednesday, August 08, 2007 5:59 PM To: haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Newbie question (again!) about phantom types I'm having difficulty to understand what phantom types are good for. Is this just for improving runtime performance? No. As the wiki says, you can use them to add static guarantees. I read the wiki, and it says this is useful if you want to increase the type-safety of your code, but the code below does not give a compiler error for the function test1, I get a runtime error, just like test2. It seems you're mixing up GADT's and phantom types. -- CODE -- -- With phantom types data T1 a = TI1 Int | TS1 String deriving Show Here, the 'a' is an extra type parameter, which has no manifestation on the value level. Note the type of the constructors: TI1 :: Int - T1 a TS1 :: String - T1 a In particular, the 'a' is not related to the 'Int' or 'String' arguments. foo1 :: T1 String - T1 String - T1 String foo1 (TS1 x) (TS1 y) = TS1 (x++y) test1 = foo1 (TI1 1) (TI1 2) -- Shouldn't this give a compiler error instead of a runtime error? 'TI1 1' has type 'T1 a', so this unifies with 'T1 String' (the type of the argument of 'foo1'. The type parameter 'a' can still be useful, but you have to use an explicit type signature to constrain the type 'a': ti1 :: Int - T1 Int ti1 x = TI1 x Now, 'ti1' will create values with the restricted type 'T1 Int', that you can't use as arguments for your 'foo1'. GADTs are perhaps more useful for what you seem to want. Try something like this: data T1 :: * - * where -- T1 has one type parameter TI1 :: Int - T1 Int TS1 :: String - T1 String Now, the type systems guarantees that all values of the form 'TI1 x' have type 'T1 Int'. Greetings, Arie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question: multi-methods in Haskell
In de book Modern C++ design, Andrei Alexandrescu writes that Haskell supports “multi-methods” http://books.google.com/books?id=aJ1av7UFBPwCpg=PA3ots=YPiJ_nWi6Ydq=moder n+C%2B%2Bsig=FWO6SVfIrgtCWifj9yYHj3bnplQ#PPA263,M1 How is this actually done in Haskell? Maybe this is just a basic feature of Haskell which I don't grasp yet because of my object-oriented background? A good example is collision between pairs of objects of type (a,b). In object oriented languages this cannot be handled in a nice way, because neither a.Collide(b) or b.Collide(a) is the correct approach; one would like to write (a,b).Collide() A specific example might be better here. Assume the following class hierarchy: Solid | +-- Asteroid | +-- Planet | + -- Earth | + -- Jupiter Using multi-methods, I could write (in pseudo code) collide (Asteroid, Planet) = an asteroid hit a planet collide (Asteroid, Earth) = the end of the dinos collide (Solid,Solid) = solids collided collide (Planet, Asteroid) = collide (Asteroid, Planet) collide (Earth, Asteroid) = collide (Earth, Asteroid) So basically, the best collide function is picked, depending on the type of the arguments. How should I write Haskell code for something like this in general, in the sense that this hierarchy is typically huge and the matrix (of collide functions for each pair of types) is very sparse. Thanks, Peter No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.11.6/938 - Release Date: 05/08/2007 16:16 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: multi-methods in Haskell
peterv wrote:
In de book Modern C++ design, Andrei Alexandrescu writes that Haskell
supports “multi-methods”
Using multi-methods, I could write (in pseudo code)
collide (Asteroid, Planet) = an asteroid hit a planet
collide (Asteroid, Earth) = the end of the dinos
...
collide (Planet, Asteroid) = collide (Asteroid, Planet)
collide (Earth, Asteroid) = collide (Earth, Asteroid)
Hi, In Haskell you can use multi parameter type classes to solve this
problem:
{-# OPTIONS_GHC -fglasgow-exts
-fallow-undecidable-instances
-fallow-overlapping-instances #-}
module Collide where
class Collide a b where
collide :: (a,b) - String
data Solid = Solid
data Asteroid = Asteroid
data Planet = Planet
data Jupiter = Jupiter
data Earth = Earth
instance Collide Asteroid Planet where
collide (Asteroid, Planet) = an asteroid hit a planet
instance Collide Asteroid Earth where
collide (Asteroid, Earth) = the end of the dinos
-- Needs overlapping and undecidable instances
instance Collide a b = Collide b a where
collide (a,b) = collide (b, a)
-- ghci output
*Collide collide (Asteroid, Earth)
the end of the dinos
*Collide collide (Earth, Asteroid)
the end of the dinos
Best regards, Brian.
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Re: [Haskell-cafe] Newbie question: multi-methods in Haskell
Remember that type classes do not provide object-oriented functionality.
The dispatch is static, not dynamic. Although OOP can be simulated in
Haskell, it is not a natural idiom. If you need dynamic dispatch
(including multiple dispatch), you may want to reconsider your solution.
Dan Weston
Brian Hulley wrote:
peterv wrote:
In de book Modern C++ design, Andrei Alexandrescu writes that Haskell
supports “multi-methods”
Using multi-methods, I could write (in pseudo code)
collide (Asteroid, Planet) = an asteroid hit a planet
collide (Asteroid, Earth) = the end of the dinos
...
collide (Planet, Asteroid) = collide (Asteroid, Planet)
collide (Earth, Asteroid) = collide (Earth, Asteroid)
Hi, In Haskell you can use multi parameter type classes to solve this
problem:
{-# OPTIONS_GHC -fglasgow-exts
-fallow-undecidable-instances
-fallow-overlapping-instances #-}
module Collide where
class Collide a b where
collide :: (a,b) - String
data Solid = Solid
data Asteroid = Asteroid
data Planet = Planet
data Jupiter = Jupiter
data Earth = Earth
instance Collide Asteroid Planet where
collide (Asteroid, Planet) = an asteroid hit a planet
instance Collide Asteroid Earth where
collide (Asteroid, Earth) = the end of the dinos
-- Needs overlapping and undecidable instances
instance Collide a b = Collide b a where
collide (a,b) = collide (b, a)
-- ghci output
*Collide collide (Asteroid, Earth)
the end of the dinos
*Collide collide (Earth, Asteroid)
the end of the dinos
Best regards, Brian.
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Re: [Haskell-cafe] Newbie question: multi-methods in Haskell
peterv schrieb: In de book Modern C++ design, Andrei Alexandrescu writes that Haskell supports “multi-methods” http://books.google.com/books?id=aJ1av7UFBPwCpg=PA3ots=YPiJ_nWi6Ydq=moder n+C%2B%2Bsig=FWO6SVfIrgtCWifj9yYHj3bnplQ#PPA263,M1 Chapter 11, Page 263 of this books: The C++ virtual function mechanism allows dispatching of a call depending on the dynamic type of one object. The multimethods feature allows dispatching of a function call depending on the types of multiple objects. A universally good implementation requires language support, wich is the route that languages such as CLOS, ML, Haskell, and Dylan have taken. C++ lacks such support, so it's emulation is left to library writers. I do not see why the author of this book included Haskell in this list. (But from what I know, CLOS is more like a combinator library then like a language, so I don't understand the point of this list at all). Since Haskell has no language support for subtype polymorphism or dynamic dispatch of method calls, there are no dynamic multimethods either. But with multi-parameter typeclasses, we have statically dispatched multimethods, of course. (See Brian's answer). But the author speaks specifically about dynamic dispatch. Sometimes, class hierarchies from an OO design are naturally represented by algebraic data types. Then OO methods become ordinary haskell function, and dynamic dispatch becomes pattern matching, wich is of course possible on all argument positions: data Solid = Asteroid | Planet Planet data Planet = Earth | Jupiter collide :: Solid - Solid - String collide Asteroid (Planet Earth) = the end of the dinos collide Asteroid (Planet _) = an asteroid hit a planet collide p@(Planet _) Asteroid = collide Asteroid p collide _ _ = solids collided But you have to sort the definitons for collide yourself, because there is no selection of the most specific automatically. While this is a sometimes sensible translation of an OO design into an FP design, it is not the same thing as having objects and subtypes and dynamic dispatch. Tillmann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question: multi-methods in Haskell
Dan Weston wrote:
Remember that type classes do not provide object-oriented
functionality. The dispatch is static, not dynamic. Although OOP can
be simulated in Haskell, it is not a natural idiom. If you need
dynamic dispatch (including multiple dispatch), you may want to
reconsider your solution.
Dynamic dispatch is easily added to Haskell code by using an existential
to represent any collision:
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances
-fallow-overlapping-instances #-}
module Collide where
-- Changed to a single param to make life easier...
class Collide a where
collide :: a - String
data Solid = Solid
data Asteroid = Asteroid
data Planet = Planet
data Jupiter = Jupiter
data Earth = Earth
instance Collide (Asteroid, Planet) where
collide (Asteroid, Planet) = an asteroid hit a planet
instance Collide (Asteroid, Earth) where
collide (Asteroid, Earth) = the end of the dinos
-- Needs overlapping and undecidable instances
instance Collide (a, b) = Collide (b, a) where
collide (a,b) = collide (b, a)
-- This is how you get dynamic dispatch in Haskell
data Collision = forall a. Collide a = Collision a
instance Collide Collision where
collide (Collision a) = collide a
-- ghci output
*Collide let ae = Collision (Asteroid, Earth)
*Collide let pa = Collision (Planet, Asteroid)
*Collide collide ae
the end of the dinos
*Collide collide pa
an asteroid hit a planet
*Collide map collide [ae, pa]
[the end of the dinos,an asteroid hit a planet]
Best regards, Brian.
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RE: [Haskell-cafe] Newbie question: multi-methods in Haskell
This is very nice, but it does not really solve the original problem.
In your code, evaluating
collide (Jupiter, Asteroid)
will result in an endless loop. This is expected in your code, because no
inheritance relation is present between e.g Jupiter and Planet. With
multi-dispatch, it should pick the best matching collide function based on
inheritance, or raise an error when ambiguous types.
I could fix that be just keeping the leafs (Earth, Jupiter, Asteroid) as
datatypes, and adding type classes for the super classes (Planet, Solid),
like the code below, but I could not check Asteroid-Asteroid collision with
that, GHCi gives an error.
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances
-fallow-overlapping-instances #-}
module Collide where
class Collide a where
collide :: a - String
data Asteroid = Asteroid
data Jupiter = Jupiter
data Earth = Earth
class IsSolid a
class IsSolid a = IsPlanet a
instance IsSolid Asteroid
instance IsSolid Jupiter
instance IsSolid Earth
instance IsPlanet Earth
instance IsPlanet Jupiter
instance (IsSolid a, IsSolid b) = Collide (a, b) where
collide (x,y) = generic collision
instance (IsPlanet a) = Collide (Asteroid, a) where
collide (x,y) = an asteroid hit a planet
instance (IsPlanet a) = Collide (a, Asteroid) where
collide (x, y) = an asteroid hit a planet
instance Collide (Asteroid, Earth) where
collide (_,_) = the end of the dinos
instance Collide (Earth, Asteroid) where
collide (_,_) = the end of the dinos
-- This is how you get dynamic dispatch in Haskell
data Collision = forall a. Collide a = Collision a
instance Collide Collision where
collide (Collision a) = collide a
ae = collide (Asteroid, Earth)
ea = collide (Earth, Asteroid)
ja = collide (Jupiter, Asteroid)
aj = collide (Asteroid, Jupiter)
-- However, this one gives an error?
--aa = collide (Asteroid, Asteroid)
-Original Message-
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Brian Hulley
Sent: Monday, August 06, 2007 9:15 PM
To: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] Newbie question: multi-methods in Haskell
Dan Weston wrote:
Remember that type classes do not provide object-oriented
functionality. The dispatch is static, not dynamic. Although OOP can
be simulated in Haskell, it is not a natural idiom. If you need
dynamic dispatch (including multiple dispatch), you may want to
reconsider your solution.
Dynamic dispatch is easily added to Haskell code by using an existential
to represent any collision:
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances
-fallow-overlapping-instances #-}
module Collide where
-- Changed to a single param to make life easier...
class Collide a where
collide :: a - String
data Solid = Solid
data Asteroid = Asteroid
data Planet = Planet
data Jupiter = Jupiter
data Earth = Earth
instance Collide (Asteroid, Planet) where
collide (Asteroid, Planet) = an asteroid hit a planet
instance Collide (Asteroid, Earth) where
collide (Asteroid, Earth) = the end of the dinos
-- Needs overlapping and undecidable instances
instance Collide (a, b) = Collide (b, a) where
collide (a,b) = collide (b, a)
-- This is how you get dynamic dispatch in Haskell
data Collision = forall a. Collide a = Collision a
instance Collide Collision where
collide (Collision a) = collide a
-- ghci output
*Collide let ae = Collision (Asteroid, Earth)
*Collide let pa = Collision (Planet, Asteroid)
*Collide collide ae
the end of the dinos
*Collide collide pa
an asteroid hit a planet
*Collide map collide [ae, pa]
[the end of the dinos,an asteroid hit a planet]
Best regards, Brian.
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Re: [Haskell-cafe] Newbie question: multi-methods in Haskell
peterv wrote:
This is very nice, but it does not really solve the original problem.
To get Haskell to choose the best fit it's necessary to encode the
location of each element in the hierarchy, so that elements deeper in
the hierarchy are more instantiated than those at the top. Then instance
selection chooses the best fit by just choosing the most instantiated match.
Encoding can be done using phantom types, so a generic solid has the path
IsSolid s
a planet has
IsSolid (IsPlanet p)
and a specific planet eg Earth has path
IsSolid (IsPlanet Earth)
A newtype can be used to associate the path with the actual object:
newtype InH path body = InH body
so Earth is represented by
InH Earth :: InH (IsSolid (IsPlanet Earth)) Earth
A class with a functional dependency gives us the mapping between
concrete objects and the objects as viewed by the hierarchy:
class ToH body path | body - path where
toH :: body - InH path body
toH = InH
The functional dependency means that the path (location in the
hierarchy) is uniquely determined by the body, and instance decls then
define this relationship:
instance ToH Asteroid (IsSolid Asteroid)
instance ToH Jupiter (IsSolid (IsPlanet Jupiter))
instance ToH Earth (IsSolid (IsPlanet Earth))
The code is below but as you can see the OOP encoding in Haskell becomes
quite heavy and clunky so this style is probably not ideal for a real
program - Tillmann's suggestion to use algebraic datatypes instead is
more idiomatic - but anyway here goes:
{-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances
-fallow-overlapping-instances #-}
module Collide where
class Collide a where
collide :: a - String
data Asteroid = Asteroid
data Jupiter = Jupiter
data Earth = Earth
data IsSolid a
data IsPlanet a
newtype InH path body = InH body
class ToH body path | body - path where
toH :: body - InH path body
toH = InH
instance ToH Asteroid (IsSolid Asteroid)
instance ToH Jupiter (IsSolid (IsPlanet Jupiter))
instance ToH Earth (IsSolid (IsPlanet Earth))
data Collision = forall a. Collide a = Collision a
mkCollision
:: (ToH a pa, ToH b pb, Collide (InH pa a, InH pb b))
= a - b - Collision
mkCollision a b = Collision (toH a, toH b)
instance Collide (InH (IsSolid a) aa, InH (IsSolid b) bb) where
collide _ = generic collision
instance Collide (InH (IsSolid Asteroid) Asteroid, InH (IsSolid
(IsPlanet bb)) cc) where
collide _ = an asteroid hit a planet
instance Collide (InH (IsSolid (IsPlanet a)) aa, InH (IsSolid Asteroid)
Asteroid) where
collide _ = an asteroid hit a planet
instance Collide (InH (IsSolid Asteroid) Asteroid, InH (IsSolid
(IsPlanet Earth)) Earth) where
collide _ = the end of the dinos
instance Collide (InH (IsSolid (IsPlanet Earth)) Earth, InH (IsSolid
Asteroid) Asteroid) where
collide _ = the end of the dinos
instance Collide Collision where
collide (Collision a) = collide a
--- ghci output
*Collide mapM_ putStrLn (map collide
[ mkCollision Asteroid Earth
, mkCollision Earth Asteroid
, mkCollision Jupiter Asteroid
, mkCollision Asteroid Jupiter
, mkCollision Asteroid Asteroid
])
the end of the dinos
the end of the dinos
an asteroid hit a planet
an asteroid hit a planet
generic collision
*Collide
Best regards, Brian.
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Re: [Haskell-cafe] Newbie question about automatic memoization
Bryan Burgers wrote: On 7/30/07, peterv [EMAIL PROTECTED] wrote: Does Haskell support any form of automatic memorization? For example, does the function iterate f x which expands to [x, f(x), f(f(x)), f(f(f(x))), … gets slower and slower each iteration, or can it take advantage of the fact that f is referentially transparent and hence can be memoized / cached? Thanks, Peter For 'iterate' the answer does not really need to be memoized. Or, another way of phrasing that answer is 'yes'. The definition of iteration does memoize - although normally one would say 'share' - the intermediate results. I imagine the definition of 'iterate' looks something like this: iterate f x = x : iterate f (f x) Haskell doesn't automatically memoize. But you are entitled to assume that named values are 'shared' rather than calculated twice. For example, in the above expression x, being a named value, is shared between (a) the head of the list and (b) the parameter of the function f inside the recursive call to iterate. Of course sharing x may not seem very interesting, on the outermost call, but notice that on the next call the new x is the old f x, and on the call after that the new x is f (f x) w.r.t the original x. Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Newbie question about Haskell skills progress
Having only a couple of days of practice programming Haskell (but having read lots of books and docs), I find myself writing very explicit low level code using inner aux functions (accumulators and loops). Then I force myself to revise the code, replacing these aux functions with suitable higher-order functions from the library. However, I would like to use these higher order functions right away, without using low-level aux constructs, which is most likely caused by my very long history of imperative programming. Is this the normal way of progressing in Haskell, or should I consider a different approach? Thanks, Peter ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] Newbie question about automatic memoization
Thanks! Is this is also the case when using let and where, or is this just syntactic sugar? -Original Message- From: Jules Bean [mailto:[EMAIL PROTECTED] Sent: Tuesday, July 31, 2007 5:09 PM To: Bryan Burgers Cc: peterv; haskell-cafe@haskell.org Subject: Re: [Haskell-cafe] Newbie question about automatic memoization Bryan Burgers wrote: On 7/30/07, peterv [EMAIL PROTECTED] wrote: Does Haskell support any form of automatic memorization? For example, does the function iterate f x which expands to [x, f(x), f(f(x)), f(f(f(x))), . gets slower and slower each iteration, or can it take advantage of the fact that f is referentially transparent and hence can be memoized / cached? Thanks, Peter For 'iterate' the answer does not really need to be memoized. Or, another way of phrasing that answer is 'yes'. The definition of iteration does memoize - although normally one would say 'share' - the intermediate results. I imagine the definition of 'iterate' looks something like this: iterate f x = x : iterate f (f x) Haskell doesn't automatically memoize. But you are entitled to assume that named values are 'shared' rather than calculated twice. For example, in the above expression x, being a named value, is shared between (a) the head of the list and (b) the parameter of the function f inside the recursive call to iterate. Of course sharing x may not seem very interesting, on the outermost call, but notice that on the next call the new x is the old f x, and on the call after that the new x is f (f x) w.r.t the original x. Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie question about Haskell skills progress
On 31/07/07, peterv [EMAIL PROTECTED] wrote: Having only a couple of days of practice programming Haskell (but having read lots of books and docs), I find myself writing very explicit low level code using inner aux functions (accumulators and loops). Then I force myself to revise the code, replacing these aux functions with suitable higher-order functions from the library. However, I would like to use these higher order functions right away, without using low-level aux constructs, which is most likely caused by my very long history of imperative programming… Seems sensible to me! It'll come with time, I'm sure. I often find it useful to think about general abstractions and then choose an approach from there: - many-to-one - fold - many-to-many - map - one-to-many - unfold And so on in a similar fashion. This might mean you do something stupid (as witnessed by my most recent visit to Haskell Cafe, where I said some very silly things [1] but also got some enormously clever/silly pointers [2]). But it's all part of life's rich pattern, and I can't think of a nicer place to make a fool of one's self than in this community. [1]: http://www.haskell.org/pipermail/haskell-cafe/2007-July/029274.html [2]: http://www.haskell.org/pipermail/haskell-cafe/2007-July/029285.html Is this the normal way of progressing in Haskell, or should I consider a different approach? There are probably some people here who were imbibing type theory and lambda calculus with their mammy's milk... but for the rest of us, it's just one small step at a time. Cheers D. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
