[Haskell-cafe] TagSoup 0.9

[Ralph Hodgson]

Hello Neil ,

 

I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this
error:

 

TQ\TagSoup\TagSoupExtensions.lhs:29:17:

`Tag' is not applied to enough type arguments

Expected kind `*', but `Tag' has kind `* -> *'

In the type synonym declaration for `Bundle'

Failed, modules loaded: TQ.Common.TextAndListHandling.

 

where line 29 is the type declaration for 'bundle' in the following code:

 

> module TQ.TagSoup.TagSoupExtensions where 

 

> import TQ.Common.TextAndListHandling

> import Text.HTML.TagSoup

> import Text.HTML.Download

> import Control.Monad

> import Data.List

> import Data.Char

 

> type Bundle = [Tag]

 

[snip]

 

> tagsOnPage :: String -> IO(String)

> tagsOnPage url = do

>  tags <- liftM parseTags $ openURL url

>  let results = unlines $ map(show) $ tags

>  return (results)

 

> extractTags :: Tag -> Tag -> [Tag] -> [Tag]

> extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/=
fromTag ) tags 

 

> extractTagsBetween ::  Tag -> [Tag] -> [Tag]

> extractTagsBetween _ [] = []

> extractTagsBetween markerTag tags = if startTags == []

>  then []

>  else [head startTags] ++ (takeWhile (~/= markerTag ) $ tail
startTags) 

>  where

>startTags = dropWhile (~/= markerTag ) tags

 

I need to repair this code quickly. I am hoping you can quickly help me
resolve this. Thanks.

 

Ralph Hodgson, 

@ralphtq  

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[Haskell-cafe] Re: What do _you_ want to see in FGL?

[Heinrich Apfelmus]

Ivan Lazar Miljenovic wrote:
> Heinrich Apfelmus writes:
>> Yes; what I mean is that you can retrofit a custom vertex type to any
>> graph implementation that uses a fixed vertex type. So, let's say that
>>
>>data Gr a b = .. -- graph with vertex type  Vertex Gr = Int
>>
>> then
>>
>>type Gr' node a b = CustomVertex node Gr a b
>>
>>data CustomVertex node gr a b = CV (gr a b) (Map node (Vertex gr))
>>
>> is a graph with custom vertex type  node .
> 
> Sounds like it's more complicated than it's worth tbh ;-)

Yup. ;)


Regards,
Heinrich Apfelmus

--
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Re: [Haskell-cafe] cabal-install

[Ivan Lazar Miljenovic]

Serguey Zefirov  writes:
>> export http_proxy="http://${username}:${passwo...@${proxy_url}";
>
> I tried it and it didn't work. I don't know reason, though, maybe it
> was because my current password not entirely alphanumeric.

Shouldn't matter as long as you put it within quotes.

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Re: [Haskell-cafe] TagSoup 0.9

[Malcolm Wallace]

Neil says that the API of TagSoup changed in 0.9.
All usages of the type Tag should now take a type argument, e.g. Tag String.


Regards,
Malcolm

 
On Wednesday, May 19, 2010, at 08:05AM, "Ralph Hodgson" 
 wrote:
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[Haskell-cafe] Bug with [Double]

[Dmitry Olshansky]

Hello all,

It seems that I saw something like this in Cafe recevtly. But I am not sure...
In GHC 6.12.1 (Platform 2010 on Windows Vista) I have

Prelude> [1,1+2/3..10]
[1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327]

-- It is a bug!

Prelude> [1,5/3..10]
[1.0,1.6667,2.3335,3.0,3.6665,4.333,5.0,5.667,6.334,7.001,7.668,8.336,9.004,9.671]

-- correct, but...

Prelude> [1,5/3..4]
[1.0,1.6667,2.3335,3.0,3.6665,4.333]

-- ... wrong again

Prelude> [1,1+2/3..10] :: [Float]
[1.0,1.667,2.335,3.002,3.67,4.34,5.01,5.68,6.35,7.02,7.69,8.36,9.03,9.7]

-- correct

Prelude> [1,1+2/3..10] :: [Double]
[1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327]

-- wrong

Any comments?
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Re: [Haskell-cafe] Bug with [Double]

[Erik de Castro Lopo]

Dmitry Olshansky wrote:

> It seems that I saw something like this in Cafe recevtly. But I am not sure...
> In GHC 6.12.1 (Platform 2010 on Windows Vista) I have




> Any comments?

The problem you point out is not a problem with Haskell, but a problem
with the whole concept of floating point arithmetic as implemented on
all modern CPUs. See:

http://docs.sun.com/source/806-3568/ncg_goldberg.html

You would have got similar problems with just about any language running
on the same hardware.

Erik
-- 
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Re: [Haskell-cafe] Bug with [Double]

[Ivan Lazar Miljenovic]

Dmitry Olshansky  writes:

> Hello all,
>
> It seems that I saw something like this in Cafe recevtly. But I am not sure...
> In GHC 6.12.1 (Platform 2010 on Windows Vista) I have
>
> Prelude> [1,1+2/3..10]
> [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327]
>
> -- It is a bug!

No it isn't, because of the dodgy Ord instance for Float and Double values.

-- 
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ivan.miljeno...@gmail.com
IvanMiljenovic.wordpress.com
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Re: [Haskell-cafe] Bug with [Double]

[Serguey Zefirov]

2010/5/19 Erik de Castro Lopo :
> Dmitry Olshansky wrote:
>
>> It seems that I saw something like this in Cafe recevtly. But I am not 
>> sure...
>> In GHC 6.12.1 (Platform 2010 on Windows Vista) I have
>
> 
>
>
>> Any comments?
>
> The problem you point out is not a problem with Haskell, but a problem
> with the whole concept of floating point arithmetic as implemented on
> all modern CPUs. See:
>
>    http://docs.sun.com/source/806-3568/ncg_goldberg.html
>
> You would have got similar problems with just about any language running
> on the same hardware.

This is what used for Double list generation (Haskell Platform 2010):
--
numericEnumFromThenTo   :: (Ord a, Fractional a) => a -> a -> a -> [a]
numericEnumFromThenTo e1 e2 e3
= takeWhile predicate (numericEnumFromThen e1 e2)
where
 mid = (e2 - e1) / 2
 predicate | e2 >= e1  = (<= e3 + mid)
   | otherwise = (>= e3 + mid)
--
So normal C loop like for {double i = 1; i <= 10; i += 1+2/3) {
insert_list(i); } won't generate the same list, as Haskell does in
[1,1+2/3..10].

PS
Rationals:
Prelude> [1,1+2/3..10] :: [Rational]
[1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 %
3,25 % 3,9 % 1,29 % 3,31 % 3]

Same result.
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Re: [Haskell-cafe] Bug with [Double]

[Ivan Lazar Miljenovic]

Ivan Lazar Miljenovic  writes:

> Dmitry Olshansky  writes:
>
>> Hello all,
>>
>> It seems that I saw something like this in Cafe recevtly. But I am not 
>> sure...
>> In GHC 6.12.1 (Platform 2010 on Windows Vista) I have
>>
>> Prelude> [1,1+2/3..10]
>> [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327]
>>
>> -- It is a bug!
>
> No it isn't, because of the dodgy Ord instance for Float and Double
> values.


And by "Ord", I of course mean "Enum"...

-- 
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IvanMiljenovic.wordpress.com
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[Haskell-cafe] Type famillies & Lifting IO

[Maciej Piechotka]

I started playing with type families. I wanted to achieve, for the
beginning, something like:

> import qualified Control.Monad.IO.Class as IOC
> import Control.Monad.Trans.Class
> import Control.Monad.Trans.Cont
> import Data.Functor.Identity

> class (Monad m, Monad (IO' m)) => MonadIO m where
> type IO' m :: * -> *
> liftIO :: IO a -> IO' m a
> liftM :: m a -> IO' m a

It allows to add IO to computation even if computation originally was
'pure'.

First step was easy:

> instance MonadIO Identity where
> type IO' Identity = IO
> liftIO = id
> liftM = return . runIdentity
> 
> instance MonadIO IO where
> type IO' IO = IO
> liftIO = id
> liftM = id
> 
> instance MonadIO (ST r) where
> type IO' (ST r) = IO
> liftIO = id
> liftM = unsafeSTToIO
> 
> --instance IOC.MonadIO m => MonadIO m where
> --type IO' m = m
> --liftIO = IOC.liftIO
> --liftM = id

However I run into problems - this code doesn't want to compile:

> instance MonadIO m => MonadIO (ContT r m) where
> type IO' (ContT r m) = ContT r (IO' m)
> liftIO f = ContT $ \cont -> liftIO f >>= cont
> liftM f = ContT $ \cont -> liftM f >>= cont

Or this:

> instance MonadIO m => MonadIO (ContT r m) where
> type IO' (ContT r m) = ContT r (IO' m)
> liftIO f = lift . liftIO
> liftM f = lift . liftIO

In fact there is strange interfering types of ghci:

ghci> :t lift . liftIO
lift . liftIO
  :: (m ~ IO' m1, MonadTrans t, Monad m, MonadIO m1) => IO a -> t m a
ghci> :t lift . liftIO :: (m ~ IO' m1, MonadTrans t, Monad m, MonadIO
m1) => IO a -> t m a

:1:7:
Couldn't match expected type `IO' m' against inferred type `m1'
  `m1' is a rigid type variable bound by
   an expression type signature at :1:18
  NB: `IO'' is a type function, and may not be injective
In the second argument of `(.)', namely `liftIO'
In the expression:
lift . liftIO ::
(m ~ (IO' m1), MonadTrans t, Monad m, MonadIO m1) => IO a ->
t m a

What's the problem? I guess I don't understand something basic about
type famillies.

Regards


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Re: [Haskell-cafe] Bug with [Double]

[Dmitry Olshansky]

But

Prelude Data.List> [1,1+2/3..4] :: [Double]
[1.0,1.6665,2.333,2.9996,3.666,4.332]

Prelude Data.List> unfoldr (\n -> let n'=n+2/3 in if n' <= 4 then Just
(n',n') else Nothing) 1 :: [Double]
[1.6665,2.333,2.9996,3.666]

Prelude Data.List> takeWhile (<=4) $ iterate (+2/3) 1 :: [Double]
[1.0,1.6665,2.333,2.9996,3.666]

How 'dodgy' it should be to produce different result?
How [a,b..c] works in this case?

>
>
> 2010/5/19 Ivan Lazar Miljenovic :
>> Dmitry Olshansky  writes:
>>
>>> Hello all,
>>>
>>> It seems that I saw something like this in Cafe recevtly. But I am not 
>>> sure...
>>> In GHC 6.12.1 (Platform 2010 on Windows Vista) I have
>>>
>>> Prelude> [1,1+2/3..10]
>>> [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327]
>>>
>>> -- It is a bug!
>>
>> No it isn't, because of the dodgy Ord instance for Float and Double values.
>>
>> --
>> Ivan Lazar Miljenovic
>> ivan.miljeno...@gmail.com
>> IvanMiljenovic.wordpress.com
>>
>
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Re: [Haskell-cafe] Bug with [Double]

[Dmitry Olshansky]

Thanks, it's clear now.

2010/5/19 Serguey Zefirov :
> 2010/5/19 Erik de Castro Lopo :
>> Dmitry Olshansky wrote:
>>
>>> It seems that I saw something like this in Cafe recevtly. But I am not 
>>> sure...
>>> In GHC 6.12.1 (Platform 2010 on Windows Vista) I have
>>
>> 
>>
>>
>>> Any comments?
>>
>> The problem you point out is not a problem with Haskell, but a problem
>> with the whole concept of floating point arithmetic as implemented on
>> all modern CPUs. See:
>>
>>    http://docs.sun.com/source/806-3568/ncg_goldberg.html
>>
>> You would have got similar problems with just about any language running
>> on the same hardware.
>
> This is what used for Double list generation (Haskell Platform 2010):
> --
> numericEnumFromThenTo   :: (Ord a, Fractional a) => a -> a -> a -> [a]
> numericEnumFromThenTo e1 e2 e3
>    = takeWhile predicate (numericEnumFromThen e1 e2)
>                                where
>                                 mid = (e2 - e1) / 2
>                                 predicate | e2 >= e1  = (<= e3 + mid)
>                                           | otherwise = (>= e3 + mid)
> --
> So normal C loop like for {double i = 1; i <= 10; i += 1+2/3) {
> insert_list(i); } won't generate the same list, as Haskell does in
> [1,1+2/3..10].
>
> PS
> Rationals:
> Prelude> [1,1+2/3..10] :: [Rational]
> [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 %
> 3,25 % 3,9 % 1,29 % 3,31 % 3]
>
> Same result.
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Re: [Haskell-cafe] Bug with [Double]

[Roman Leshchinskiy]

On 19/05/2010, at 19:24, Dmitry Olshansky wrote:

> Prelude> [1,1+2/3..10]
> [1.0,1.6665,2.333,2.9996,3.666,4.332,4.998,5.664,6.33,6.9964,7.6625,8.329,8.995,9.66,10.327]
> 
> -- It is a bug!

Unfortunately, it isn't. Section 6.3.4 of the Haskell report says:

For Float and Double, the semantics of the enumFrom family is given by the 
rules for Int above, except that the list terminates when the elements become 
greater than e3+i/2 for positive increment i, or when they become less than 
e3+i/2 for negative i.

In this case, i = 2/3 so the last value in the list is 10+1/3. The same applies 
to the other examples.

Personally, I consider the Enum class itself to be broken.

Roman


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Re: [Haskell-cafe] Bug with [Double]

[Ivan Lazar Miljenovic]

Roman Leshchinskiy  writes:
> Personally, I consider the Enum class itself to be broken.

Oh?  In what sense?

It seems to work fine for data types representing bounded enumerable
values with a proper mapping to/from Int (it's not bijective since
there's no proper mapping from Int -> Bool for example); the problem is
the fact that Double, etc. are made instances of Enum when this is not
the case for those data types.

-- 
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Re: [Haskell-cafe] Bug with [Double]

[Roman Leshchinskiy]

On 19/05/2010, at 20:36, Ivan Lazar Miljenovic wrote:

> Roman Leshchinskiy  writes:
>> Personally, I consider the Enum class itself to be broken.
> 
> Oh?  In what sense?

Firstly, the enumFrom* family of functions shouldn't be methods and the class 
itself should provide enough facilities for implementing them generically. GHC, 
for instance, specialises them for all primitive numeric types just to get 
foldr/build fusion to work. That shouldn't be necessary and doesn't help with 
overloaded code anyway. For instance, this generates an intermediate list:

foo :: Enum a => a -> a -> [Int]
foo a b = [fromEnum x | x <- [a..b]]

It's even worse when you want to implement similar functionality for other data 
structures. In vector, I basically had to duplicate all those specialisations 
to get decent performance. The generic case is horribly inefficient:

enumFromTo x y = fromList [x .. y]

There is no other sensible definition.

Secondly, it should be possible to compute the length and the nth element of 
[a..b] in constant time. At the moment, it's impossible to distribute [a..b] 
efficiently across multiple threads - you have to generate the entire list 
first and then split it into chunks. It's completely unclear to me what [:a .. 
b:] should mean in DPH, for instance.

So basically, Enum only provides enough functionality to desugar [a..b] and 
friends and even here, it doesn't interact well with fusion. Of course, these 
concerns weren't relevant back when the class was designed. But it is really 
broken now, IMO.

Roman


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Re: [Haskell-cafe] Numerical Analysis

[Alberto G. Corona]

SAGE is the kind of thing that I dreamed to have available online a few
years ago.

To recode everithing in haskell perhaps does not worth the pain, but
perhapts it would be nice to do something similar to SAGE in an advanced
environment such is Google Wave, with all the collaborative facilities for
free.

Perhaps  this  would attract some developers. The utility of the whole thing
perhaps would incentivate the integration of existing haskell math software
and to translate/integrate foreign code.

Cheers

2010/5/16 Pierre-Etienne Meunier 

> Hello Cafe,
>
> Being a complete beginner in the field of numerical analysis, but anyway
> needing it to solve "real problems", I wrote a few functions recently to
> solve systems of polynomial equations using the "projected polyhedron"
> method by Maekawa and Patrikakalis.
> This requires solving systems of linear equations precisely, thus the
> simple Gauss method was not enough, and I had to write also an algorithm for
> the "SVD decomposition".
>
> Upon discovering the algol / fortran specifications of these :-( , heavily
> published in important journals, I thought it would be nice to provide the
> world with fast reliable implementation of these numerical methods (i.e. not
> simply bindings to lapack). Moreover, writing numerical things in haskell is
> much more pleasant than I thought at first. Here are a few random thoughts
> on this :
>
> - The haskell 98 norm does not require enough about IEEE-754 compliance,
> thus C bindings are still needed to guess for instance the machine epsilons,
> or manipulating ULPs. Moreover, taking advantage of hardware rounding is not
> easy, even if the hardware is IEEE-compliant : calling a C function from
> haskell screws up the speed advantages of hardware rounding, for instance.
> Maybe the new LLVM backend will make this possible ?
>
> - The current Array library is definitely not adapted to production code.
> It makes debugging tricky, requires a heavy use of Debug.Trace to actually
> see what happens, and does not seem as fast as one could expect. It seems
> that each algebra library on hackage redefines part of it, but a unified
> version would be nice : a discussion within the haskell community seems to
> be needed...
>
> - A numerical analysis library should really take advantage of the
> parallelism in GHC, especially with the arrival of hardware such as fermi
> (anyway, I do not know how much haskell is compilable to fermi code). The
> love for loops and side-effects among this community is hard to understand,
> but that's more of a cultural problem.
>
> Finally, as stated by William Stein, the creator of SAGE, of course it
> would take thousands of man-years to rewrite these codes in python, but if a
> language like haskell, and a compiler do 90% of the work, how many man-years
> are left ?
>
> If anyone here has got the time, the team and the will to start such a
> project, I'd love to contribute !
>
> Cheers,
> PE
>
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[Haskell-cafe] Proof format

[R J]

Is this how a rigorous Haskeller would lay out the proofs of the following 
theorems?  This is Bird 1.4.6.
(i)   
Theorem:  (*) x = (* x)
Proof:
  (*) x  ={definition of partial application}  \y -> x * y  =
{commutativity of "*"}  \y -> y * x  ={definition of "(* x)"}  (* x)
(ii)
Theorem:  (+) x = (x +)
Proof:
  (+) x  ={definition of partial application}  \y -> x + y  =
{definition of "(x +)"}  (x +)
(iii)
Theorem:  (-) x /= (- x)
Proof:
  (-) x  ={definition of partial application}  \y -> x - y  /=   
{definition of prefix negation, which is not a section}  (- x)
  
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[Haskell-cafe] (no subject)

[R J]

This is another proof-layout question, this time from Bird 1.4.7.
We're asked to define the functions curry2 and uncurry2 for currying and 
uncurrying functions with two arguments.  Simple enough:
curry2 :: ((a, b) -> c) -> (a -> (b -> c))curry2 f x y   =  f 
(x, y)
uncurry2   :: (a -> (b -> c)) -> ((a, b) -> c)uncurry2 f (x, y)  =  f x 
y
The following two assertions are obviously true theorems, but how are the 
formal proofs laid out?
1.  curry2 (uncurry2 f) x y = f x y
2.  uncurry2 (curry 2 f) (x, y) = f (x, y)  
  
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Re: [Haskell-cafe] Bug with [Double]

[Ben Millwood]

On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov  wrote:
>
> PS
> Rationals:
> Prelude> [1,1+2/3..10] :: [Rational]
> [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 %
> 3,25 % 3,9 % 1,29 % 3,31 % 3]
>
> Same result.

This sounds like a bug to me. The section of the Haskell Report that
deals with the Enum class mentions Float and Double, not Rational, and
there's really no sensible reason why Rationals would exhibit this
behaviour given that they don't have rounding error.
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Re: [Haskell-cafe] Bug with [Double]

[Lutz Donnerhacke]

* Ben Millwood wrote:
>> Prelude> [1,1+2/3..10] :: [Rational]
>> [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 %
>> 3,25 % 3,9 % 1,29 % 3,31 % 3]
>>
>> Same result.
>
> This sounds like a bug to me. The section of the Haskell Report that
> deals with the Enum class mentions Float and Double, not Rational, and
> there's really no sensible reason why Rationals would exhibit this
> behaviour given that they don't have rounding error.

Double is not better:

Prelude> [9,9+2/3..10]
[9.0,9.666,10.332]
Prelude> [7,9 .. 10]
[7,9]
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Re: [Haskell-cafe] Bug with [Double]

[Roman Leshchinskiy]

On 19/05/2010, at 23:44, Ben Millwood wrote:

> On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov  wrote:
>> 
>> PS
>> Rationals:
>> Prelude> [1,1+2/3..10] :: [Rational]
>> [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 %
>> 3,25 % 3,9 % 1,29 % 3,31 % 3]
>> 
>> Same result.
> 
> This sounds like a bug to me. The section of the Haskell Report that
> deals with the Enum class mentions Float and Double, not Rational, and
> there's really no sensible reason why Rationals would exhibit this
> behaviour given that they don't have rounding error.

From Section 12.1 of the Library Report:

instance  (Integral a)  => Enum (Ratio a)  where
succ x   =  x+1
pred x   =  x-1
toEnum   =  fromIntegral
fromEnum =  fromInteger . truncate  -- May overflow
enumFrom =  numericEnumFrom  -- These numericEnumXXX functions
enumFromThen =  numericEnumFromThen  -- are as defined in Prelude.hs
enumFromTo   =  numericEnumFromTo   -- but not exported from it!
enumFromThenTo   =  numericEnumFromThenTo

The numericEnum functions are defined in Section 8 of the Language Report and 
have semantics required for Float and Double.

Roman

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Re: [Haskell-cafe] Intuitive function given type signature

[Brent Yorgey]

On Wed, May 19, 2010 at 04:27:14AM +, R J wrote:
> 
> What are some simple functions that would naturally have the following type 
> signatures:
> f :: (Integer -> Integer) -> Integer

Well, this means f is given a function from Integer to Integer, and it
has to somehow return an Integer, (possibly) using the function it is
given.  For example, f could just ignore its argument:

  f _ = 6

Or it could apply it to a particular input value:

  f g = g 0

I'll let you think of some other possibilities.

> g :: (Integer -> Integer) -> (Integer -> Integer)

g is given an Integer->Integer function and has to produce a different
one.  But as someone else pointed out, you can also think of this as

  g :: (Integer -> Integer) -> Integer -> Integer

That is, g is given an Integer->Integer function as well as an
Integer, and must somehow use these things to produce another Integer.
There are lots of ways to do this--I'll let you figure this one out.

-Brent
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Re: [Haskell-cafe] Proof format

[Brent Yorgey]

On Wed, May 19, 2010 at 01:12:16PM +, R J wrote:
> 
> Is this how a rigorous Haskeller would lay out the proofs of the following 
> theorems?  This is Bird 1.4.6.
> (i)   
> Theorem:  (*) x = (* x)
> Proof:
>   (*) x  ={definition of partial application}  \y -> x * y  =
> {commutativity of "*"}  \y -> y * x  ={definition of "(* x)"}  (* 
> x)
> (ii)
> Theorem:  (+) x = (x +)
> Proof:
>   (+) x  ={definition of partial application}  \y -> x + y  =
> {definition of "(x +)"}  (x +)

I would put each step and each {reason} on a separate line (or perhaps
there is something wrong with the way your mail client handles
newlines?) but other than that these look good.

> (iii)
> Theorem:  (-) x /= (- x)
> Proof:
>   (-) x  ={definition of partial application}  \y -> x - y  /=   
> {definition of prefix negation, which is not a section}  (- x)

This is not a proof.  To prove an inequality like this you should
simply give a counterexample.

-Brent
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Re: [Haskell-cafe] (no subject)

[Brent Yorgey]

On Wed, May 19, 2010 at 01:37:49PM +, R J wrote:
> 
> This is another proof-layout question, this time from Bird 1.4.7.
> We're asked to define the functions curry2 and uncurry2 for currying and 
> uncurrying functions with two arguments.  Simple enough:
> curry2 :: ((a, b) -> c) -> (a -> (b -> c))curry2 f x y   =  f 
> (x, y)
> uncurry2   :: (a -> (b -> c)) -> ((a, b) -> c)uncurry2 f (x, y)  =  f 
> x y
> The following two assertions are obviously true theorems, but how are the 
> formal proofs laid out?

There are lots of variations, I wouldn't say there's one "right" way
to organize/lay out the proofs.  But here's how I might do it:

  curry2 (uncurry2 f) x y 
=  { def. of curry2 }
  uncurry2 f (x,y)
=  { def. of uncurry2 }
  f x y

I'll let you do the other one.

By the way, are you working through these problems just for
self-study, or is it homework for a class?

-Brent
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Re: [Haskell-cafe] cabal: problem building ffi shared library and significance of __stginit

[Duncan Coutts]

On Tue, 2010-05-18 at 17:31 -0400, Anthony LODI wrote:
> Hello,
> 
> I'm trying to build some haskell code as a .so/.dll so that it can
> ultimately be used by msvc.  I have it working when I compile by hand
> (listed below) but I can't get the exact same thing built/linked with
> cabal.  On linux everything builds fine, but when I try to link the
> resulting .so file, I get an error about a missing
> '__stginit_CInterface' reference.  Indeed I couldn't find that name in
> any of the cabal-generated .dyn_o files.  I checked the output of
> 'cabal build -v' and it seems to be executing about the same thing
> that I'm executing manually so I'm not sure what could be going wrong.
>  On windows cabal won't even configure since '--enable-shared' seems
> to imply '-dynamic' (right?), and that's not currently supported.
> 
> Also, when I remove the line 'hs_add_root(__stginit_CInterface);', and
> the corresponding forward declaration, the program runs fine!  Does
> ghc no longer need this call or are my toy programs just being lucky
> sofar?

For reference for other people, Anthony and I worked this out today.

full example:
http://pastebin.com/aLdyFMPg

The difference between doing it manually and building a library via
Cabal is the package name.

When building directly with ghc, the default package name is "" aka the
main package. When building a ghc/Haskell package, the package name gets
set (ghc -package-name test-0.0). This package name gets encoded into
the symbol names. So we get:

   __stginit_testzm0zi0_CInterface
vs __stginit_CInterface

(testzm0zi0 is the Z-encoding of test-0.0)

What is bad here is that the __stginit stuff is even necessary. Anthony
is going to file a ghc ticket and/or complain on the ghc users list,
citing this example.

Duncan

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RE: [Haskell-cafe] TagSoup 0.9

[Ralph Hodgson]

Thanks Malcolm,

 

Providing a 'String' type argument worked:

 

> type Bundle = [Tag String]

 

> extractTags :: Tag String -> Tag String -> Bundle -> Bundle

> extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= 
> fromTag ) tags

 

 

 

From: Malcolm Wallace [mailto:malcolm.wall...@me.com] 
Sent: Wednesday, May 19, 2010 1:48 AM
To: rhodg...@topquadrant.com
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] TagSoup 0.9

 

Neil says that the API of TagSoup changed in 0.9.
All usages of the type Tag should now take a type argument, e.g. Tag String.
 
 
Regards,
Malcolm
 
 
On Wednesday, May 19, 2010, at 08:05AM, "Ralph Hodgson" 
 wrote:
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> 
> 

Hello Neil ,

 

I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this 
error:

 

TQ\TagSoup\TagSoupExtensions.lhs:29:17:

`Tag' is not applied to enough type arguments

Expected kind `*', but `Tag' has kind `* -> *'

In the type synonym declaration for `Bundle'

Failed, modules loaded: TQ.Common.TextAndListHandling.

 

where line 29 is the type declaration for 'bundle' in the following code:

 

> module TQ.TagSoup.TagSoupExtensions where 

 

> import TQ.Common.TextAndListHandling

> import Text.HTML.TagSoup

> import Text.HTML.Download

> import Control.Monad

> import Data.List

> import Data.Char

 

> type Bundle = [Tag]

 

[snip]

 

> tagsOnPage :: String -> IO(String)

> tagsOnPage url = do

>  tags <- liftM parseTags $ openURL url

>  let results = unlines $ map(show) $ tags

>  return (results)

 

> extractTags :: Tag -> Tag -> [Tag] -> [Tag]

> extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= 
> fromTag ) tags 

 

> extractTagsBetween ::  Tag -> [Tag] -> [Tag]

> extractTagsBetween _ [] = []

> extractTagsBetween markerTag tags = if startTags == []

>  then []

>  else [head startTags] ++ (takeWhile (~/= markerTag ) $ tail 
> startTags) 

>  where

>startTags = dropWhile (~/= markerTag ) tags

 

I need to repair this code quickly. I am hoping you can quickly help me resolve 
this. Thanks.

 

Ralph Hodgson, 

@ralphtq  

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RE: [Haskell-cafe] TagSoup 0.9

[Ralph Hodgson]

Forgot to add: I now need to understand the following warnings on this line "> 
import Text.HTML.Download":

 

TagSoupExtensions.lhs:24:2:

Warning: In the use of `openItem'

 (imported from Text.HTML.Download):

 Deprecated: "Use package HTTP, module Network.HTTP, getResponseBody

 =<< simpleHTTP (getRequest url)"

 

TagSoupExtensions.lhs:24:2:

Warning: In the use of `openURL'

 (imported from Text.HTML.Download):

 Deprecated: "Use package HTTP, module Network.HTTP, getResponseBody

 =<< simpleHTTP (getRequest url)"

Ok, modules loaded: TQ.TagSoup.TagSoupExtensions.

*TQ.TagSoup.TagSoupExtensions>

 

 

From: haskell-cafe-boun...@haskell.org 
[mailto:haskell-cafe-boun...@haskell.org] On Behalf Of Ralph Hodgson
Sent: Wednesday, May 19, 2010 10:30 AM
To: 'Malcolm Wallace'
Cc: haskell-cafe@haskell.org
Subject: RE: [Haskell-cafe] TagSoup 0.9

 

Thanks Malcolm,

 

Providing a 'String' type argument worked:

 

> type Bundle = [Tag String]

 

> extractTags :: Tag String -> Tag String -> Bundle -> Bundle

> extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= 
> fromTag ) tags

 

 

 

From: Malcolm Wallace [mailto:malcolm.wall...@me.com] 
Sent: Wednesday, May 19, 2010 1:48 AM
To: rhodg...@topquadrant.com
Cc: haskell-cafe@haskell.org
Subject: Re: [Haskell-cafe] TagSoup 0.9

 

Neil says that the API of TagSoup changed in 0.9.
All usages of the type Tag should now take a type argument, e.g. Tag String.
 
 
Regards,
Malcolm
 
 
On Wednesday, May 19, 2010, at 08:05AM, "Ralph Hodgson" 
 wrote:
>___
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>http://www.haskell.org/mailman/listinfo/haskell-cafe
> 
> 

Hello Neil ,

 

I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this 
error:

 

TQ\TagSoup\TagSoupExtensions.lhs:29:17:

`Tag' is not applied to enough type arguments

Expected kind `*', but `Tag' has kind `* -> *'

In the type synonym declaration for `Bundle'

Failed, modules loaded: TQ.Common.TextAndListHandling.

 

where line 29 is the type declaration for 'bundle' in the following code:

 

> module TQ.TagSoup.TagSoupExtensions where 

 

> import TQ.Common.TextAndListHandling

> import Text.HTML.TagSoup

> import Text.HTML.Download

> import Control.Monad

> import Data.List

> import Data.Char

 

> type Bundle = [Tag]

 

[snip]

 

> tagsOnPage :: String -> IO(String)

> tagsOnPage url = do

>  tags <- liftM parseTags $ openURL url

>  let results = unlines $ map(show) $ tags

>  return (results)

 

> extractTags :: Tag -> Tag -> [Tag] -> [Tag]

> extractTags fromTag toTag tags = takeWhile (~/= toTag ) $ dropWhile (~/= 
> fromTag ) tags 

 

> extractTagsBetween ::  Tag -> [Tag] -> [Tag]

> extractTagsBetween _ [] = []

> extractTagsBetween markerTag tags = if startTags == []

>  then []

>  else [head startTags] ++ (takeWhile (~/= markerTag ) $ tail 
> startTags) 

>  where

>startTags = dropWhile (~/= markerTag ) tags

 

I need to repair this code quickly. I am hoping you can quickly help me resolve 
this. Thanks.

 

Ralph Hodgson, 

@ralphtq  

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Re: [Haskell-cafe] cabal-install

[Brandon S. Allbery KF8NH]

On May 19, 2010, at 04:49 , Ivan Lazar Miljenovic wrote:

Serguey Zefirov  writes:

export http_proxy="http://${username}:${passwo...@${proxy_url}";


I tried it and it didn't work. I don't know reason, though, maybe it
was because my current password not entirely alphanumeric.


Shouldn't matter as long as you put it within quotes.


I imagine things will go wrong if it includes an @... urlencoding is  
probably a smart idea.


--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon universityKF8NH




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Description: This is a digitally signed message part
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Re: [Haskell-cafe] TagSoup 0.9

[Daniel Fischer]

On Wednesday 19 May 2010 19:46:57, Ralph Hodgson wrote:
> Forgot to add: I now need to understand the following warnings on this
> line "> import Text.HTML.Download":
>
>

In Text.HTML.Download, there's the following:

{-|
/DEPRECATED/: Use the HTTP package instead:

> import Network.HTTP
> openURL x = getResponseBody =<< simpleHTTP (getRequest x)

This module simply downloads a page off the internet. It is very 
restricted,
and it not intended for proper use.

The original version was by Alistair Bayley, with additional help from
Daniel McAllansmith. It is taken from the Haskell-Cafe mailing list
\"Simple HTTP lib for Windows?\", 18 Jan 2007.

-}

and

{-# DEPRECATED openItem, openURL "Use package HTTP, module Network.HTTP, 
getResponseBody =<< simpleHTTP (getRequest url)" #-}


So, don't use Text.HTML.Download anymore, instead use the functions from 
the HTTP package.

Deprecated stuff will probably be removed in one of the next releases.
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Re: [Haskell-cafe] TagSoup 0.9

[Don Stewart]

Or use things from the download-curl package, which provides a nice
openURL function.

daniel.is.fischer:
> On Wednesday 19 May 2010 19:46:57, Ralph Hodgson wrote:
> > Forgot to add: I now need to understand the following warnings on this
> > line "> import Text.HTML.Download":
> >
> >
> 
> In Text.HTML.Download, there's the following:
> 
> {-|
> /DEPRECATED/: Use the HTTP package instead:
> 
> > import Network.HTTP
> > openURL x = getResponseBody =<< simpleHTTP (getRequest x)
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[Haskell-cafe] Re: cabal: problem building ffi shared library and significance of __stginit

[Anthony LODI]

> I'm trying to build some haskell code as a .so/.dll so that it can
> ultimately be used by msvc.  I have it working when I compile by hand
> (listed below) but I can't get the exact same thing built/linked with
> cabal.  On linux everything builds fine, but when I try to link the
> resulting .so file, I get an error about a missing
> '__stginit_CInterface' reference.  Indeed I couldn't find that name in
> any of the cabal-generated .dyn_o files.  I checked the output of
> 'cabal build -v' and it seems to be executing about the same thing
> that I'm executing manually so I'm not sure what could be going wrong.
>  On windows cabal won't even configure since '--enable-shared' seems
> to imply '-dynamic' (right?), and that's not currently supported.

Okay after some help from Duncan on #haskell, the problem seems pretty
straightforward.  I've uploaded the details here:

  http://hackage.haskell.org/trac/ghc/ticket/3252#comment:4

Cheers.
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Re: [Haskell-cafe] TagSoup 0.9

[Neil Mitchell]

Hi Ralph,

> I was using TagSoup 0.8 with great success. On upgrading to 0.9 I have this 
> error:
>
> TQ\TagSoup\TagSoupExtensions.lhs:29:17:
>`Tag' is not applied to enough type arguments
>Expected kind `*', but `Tag' has kind `* -> *'
>In the type synonym declaration for `Bundle'
> Failed, modules loaded: TQ.Common.TextAndListHandling.

My change notes have this being a change between 0.6 and 0.8. As
Malcolm says, any old uses of "Tag" should become "Tag String". The
reason is that Tag is now parameterised, and you can use Tag
ByteString etc. However, I should point out that Tag ByteString won't
be any faster than Tag String in this version (it's in the future work
pile).

>> > Forgot to add: I now need to understand the following warnings on this
>> > line "> import Text.HTML.Download":

Everyone's comments have been right. I previously included
Text.HTML.Download so that it was easy to test tagsoup against the
web. Since I first wrote that snippet the HTTP downloading libraries
have improved substantially, so people should use those in favour of
the version in tagsoup - you'll be able to connect to more websites in
more reliable ways, go through proxies etc. I don't intend to remove
the Download module any time soon, but I will do eventually.

Thanks, Neil
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Re: [Haskell-cafe] cabal-install

[Serguey Zefirov]

>>> I tried it and it didn't work. I don't know reason, though, maybe it
>>> was because my current password not entirely alphanumeric.
>> Shouldn't matter as long as you put it within quotes.
> I imagine things will go wrong if it includes an @... urlencoding is
> probably a smart idea.

Thank you very much!

I'll definitely try that.

And, as I'm on it again, switch to just ignore proxy would be better. ;)
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[Haskell-cafe] Haskell for control system [was: [reactive] A pong and integrate]

[Ben Franksen]

David Leimbach wrote:
> I find it's often the most practical chapter that I hit a lot during
> writes and changes to my server process I have in Haskell in our control
> system code :-)

Are you actually saying that you use Haskell for a control system server?
Thta would be very interesting to me.

Could you tell what kind of control system?

Cheers
Ben

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[Haskell-cafe] Extensible Records and Functional References

[Günther Schmidt]

Hi all,

I just read "Functional References  are a cheap and cheerful technique 
for working with the existing (non-extensible) record system, and may be 
of interest to extensible record implementers. A good implementation can 
be found on ..." on


http://hackage.haskell.org/trac/ghc/wiki/ExtensibleRecords


Does this mean that there is a way to implement extensible records via 
functional references?


Günther


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Re: [Haskell-cafe] ANN: has-0.4 Entity based records

[adam vogt]

On Thu, May 13, 2010 at 7:16 PM, HASHIMOTO, Yusaku  wrote:
> Sorry for spamming, what I wanted to write is I think `has' has better
> interface than other record packages in types.
>
> There are many libraries to write function "takes an record has Foo
> and Bar and returns something." But writing type of the function is
> still difficult. I can't write such types using HList or records
> without reading documents. I think, using has, There's few effort to
> write such types.

In which manner do you need to read less documentation to write:

] f :: Has Foo r => r -> ...

Instead when using HList:

] f :: HasField Foo record fieldType => ...


> I think `has' fits the needs of Haskellers who have the good habit of
> writing a type of a function before its definition.

What does this mean exactly in terms of the type inference possible?

--
Adam
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Re: [Haskell-cafe] TagSoup 0.9

[Henning Thielemann]

Don Stewart schrieb:
> Or use things from the download-curl package, which provides a nice
> openURL function.

The openURL function from TagSoup is lazy, which the proposed
replacement 'getResponseBody =<< simpleHTTP (getRequest x)' is not. Is
the openURL function from download-curl lazy?

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Re: [Haskell-cafe] TagSoup 0.9

[Don Stewart]

schlepptop:
> Don Stewart schrieb:
> > Or use things from the download-curl package, which provides a nice
> > openURL function.
> 
> The openURL function from TagSoup is lazy, which the proposed
> replacement 'getResponseBody =<< simpleHTTP (getRequest x)' is not. Is
> the openURL function from download-curl lazy?
> 

Yes, see:

Network.Curl.Download.Lazy.openLazyURI

though I think it is possible that I strictified the code. Have a play
around with it if it doesn't meet your needs -- should be /trivial/ to
ensure it is chunk-wise lazy.
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[Haskell-cafe] Devices and Webcams, The Basics

[Eitan Goldshtrom]

Hi everyone,

I would like to start working on a program that requires access to a 
camera attached to the computer probably via USB or otherwise 
internally. Unfortunately I don't know anything about using devices in 
haskell. I tried looking up how to access the microphone one too and had 
little success. Could someone just point me in the direction of 
tutorials for learning the basics of both devices in general as well as 
the webcam more specifically? The webcam is a bit more of a priority.


-Eitan
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Re: [Haskell-cafe] Devices and Webcams, The Basics

[aditya siram]

Haskell has bindings to USB [1]. I don't know of any USB tutorials or
any webcam specific libraries.

-deech
[1] http://hackage.haskell.org/package/usb

On 5/19/10, Eitan Goldshtrom  wrote:
> Hi everyone,
>
> I would like to start working on a program that requires access to a
> camera attached to the computer probably via USB or otherwise
> internally. Unfortunately I don't know anything about using devices in
> haskell. I tried looking up how to access the microphone one too and had
> little success. Could someone just point me in the direction of
> tutorials for learning the basics of both devices in general as well as
> the webcam more specifically? The webcam is a bit more of a priority.
>
> -Eitan
>
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Re: [Haskell-cafe] Devices and Webcams, The Basics

[Ivan Lazar Miljenovic]

aditya siram  writes:

> Haskell has bindings to USB [1]. I don't know of any USB tutorials or
> any webcam specific libraries.

I don't know of any, but if using Linux then maybe writing a binding to
v4l (video for linux) might be the best/easiest approach.

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[Haskell-cafe] Bird problem 1.6.2 -- is there an easier method?

[R J]

Bird problem 1.6.2 is:
If f :: (a, b) -> c, then define a function "swap" such that:
flip (curry f) = curry (f . swap).
I'd very much appreciate if someone could tell me whether there's a rigorous 
solution simpler than mine, which is:
Since (.) :: (q -> r) -> (p -> q) -> (p -> r), we have f :: q -> r and swap :: 
p -> q.  Type unification of f requires q = (a, b) and r = c.
Since f :: (a, b) -> c and curry :: ((l, m) -> n) -> (l -> m -> n), 
typeunification requires l = a, b = m, and n = c.  Therefore,curry :: ((a, b) 
-> c) -> (a -> b -> c), and (curry f) :: a -> b -> c.
Since flip :: (s -> t -> u) -> t -> s -> u, type unification requiress = a, t = 
b, and u = c.  Therefore, flip :: (a -> b -> c) -> b -> a -> c,and flip (curry 
f) :: b -> a -> c.
Therefore, curry (f . swap) ::  b -> a -> c, and p :: b -> a.  Therefore,swap 
:: b -> a -> (a, b), and:

swap   :: b -> a -> (a, b)swap x y   =  (y, 
x)

  
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[Haskell-cafe] Deducing a type signature

[R J]

Bird 1.6.3 requires deducing type signatures for the functions "strange" and 
"stranger."
Are my solutions below correct?
(i)  strange f g = g (f g)
Assume g :: a -> b.  Then f :: (a -> b) -> c.  But since g :: a -> b,f g :: a, 
so c = a.  Therefore, f :: (a -> b) -> a, and g (f g) :: a.Therefore, strange 
:: ((a -> b) -> a) -> (a -> b) -> a.
(ii)  stranger f = f f
Assume f :: a -> b.  Since "f f" is well-typed, type unification requiresa = b. 
 Therefore, f :: a -> a, and stranger :: (a -> a) -> a. 
  
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Re: [Haskell-cafe] Deducing a type signature

[Richard O'Keefe]

On May 20, 2010, at 11:03 AM, R J wrote:


stranger f = f f



This doesn't have a type in Haskell.
Suppose f :: a -> b
Then if f f made sense, a = (a -> b) would be true,
and we'd have an infinite type.

Type the definition into a file, and try loading it
into ghci:

Occurs check: cannot construct the infinite type: t = t -> t1
Probable cause: `f' is applied to too many arguments
In the expression: f f
In the definition of `stranger': stranger f = f f

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Re: [Haskell-cafe] Deducing a type signature

[Dan Weston]

> (i)  strange f g = g (f g)
>
> Assume g :: a -> b.  Then f :: (a -> b) -> c.  But since g :: a -> b,
> f g :: a, so c = a.  Therefore, f :: (a -> b) -> a, and g (f g) :: a.
> Therefore, strange :: ((a -> b) -> a) -> (a -> b) -> a.

Almost. The return type of strange is the same as the return type of g 
(the outermost function), namely b.


So strange :: ((a -> b) -> a) -> (a -> b) -> b.

Dan

R J wrote:
Bird 1.6.3 requires deducing type signatures for the functions "strange" 
and "stranger."


Are my solutions below correct?

(i)  strange f g = g (f g)

Assume g :: a -> b.  Then f :: (a -> b) -> c.  But since g :: a -> b,
f g :: a, so c = a.  Therefore, f :: (a -> b) -> a, and g (f g) :: a.
Therefore, strange :: ((a -> b) -> a) -> (a -> b) -> a.

(ii)  stranger f = f f

Assume f :: a -> b.  Since "f f" is well-typed, type unification requires
a = b.  Therefore, f :: a -> a, and stranger :: (a -> a) -> a.


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Re: [Haskell-cafe] Intuitive function given type signature

[Richard O'Keefe]

On May 20, 2010, at 3:18 AM, Brent Yorgey wrote:


On Wed, May 19, 2010 at 04:27:14AM +, R J wrote:


What are some simple functions that would naturally have the  
following type signatures:

f :: (Integer -> Integer) -> Integer


Well, this means f is given a function from Integer to Integer, and it
has to somehow return an Integer, (possibly) using the function it is
given.  For example, f could just ignore its argument:

 f _ = 6


That would NOT give f the right type.
The type would be
f :: Num r => a -> r



Or it could apply it to a particular input value:

 f g = g 0


That would NOT give f the right type.
The type would be
f :: Num a => (a -> b) -> b



I'll let you think of some other possibilities.


The key point is the 'that would NATURALLY have', which I take
to mean "as a result of type inference without any forcibly
imposed type signatures".

What kind of definition of f, *not* including "::" anywhere,
would result in Haskell inferring the desired signature?

The key thing is that the types *must* be fully constrained
to Integer.  The simplest way to get something that is
guaranteed to be the specific type Integer, as far as I can
see, is to use something like "toInteger 0".

If we have identityForA :: A -> A
   identityForB :: B -> B
then we can constrain a variable x to A using identityForA $ x
and we can constrain a variable g to A->B using
identityForB . g . identityForA.

This is, in a sense, equivalent to providing a type signature,
but it meets the formal requirements of these questions.

identityForInteger = (+ toInteger 0)

is an example.

f g = g z + z where z = toInteger 0

does the trick here.
m% ghci
Prelude> let f g = g z + z where z = toInteger 0
Prelude> :type f
f :: (Integer -> Integer) -> Integer


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Re: [Haskell-cafe] Bird problem 1.6.2 -- is there an easier method?

[John Millikin]

You've been asking a lot of very tutorial-ish questions on this list.
Although this isn't necessarily a *bad* thing, you may receive
responses more appropriate to your skill level on the
haskell-beginners list <
http://www.haskell.org/mailman/listinfo/beginners >.

I don't own the Bird book, but while reading your problem the type "f
:: (a, b) -> c" is throwing off huge warning signs. What sensible
implementation could such a function have? The only way I can think of
to implement it is "f (_, _) = undefined".

Assuming this signature is somehow valid, your reasoning for the left
side of the equation "flip (curry f)" is correct. It's a bit verbose,
but you'll learn to see the types better as you become more
experienced.

However, your reasoning for the right side is incorrect. First, lets
look at the equalities again:

flip (curry f) :: b -> a -> c
flip (curry f) = curry (f . swap)
curry (f . swap) :: b -> a -> c

The first step is to remove the "curry". Since (curry :: ((a, b) -> c)
-> a -> b -> c), there's only one possible type signature for (f .
swap):

f . swap :: (b, a) -> c

The types for (.) and f are known already. There's only one reasonable
definition for (.), so we can reason that:

(.) f g x = f (g x)

f . swap :: (b, a) -> c
f . swap = \x -> f (swap x)

>From this, it should be possible to derive the type of "swap" easily. Good 
>luck.

2010/5/19 R J :
> Bird problem 1.6.2 is:
> If f :: (a, b) -> c, then define a function "swap" such that:
> flip (curry f) = curry (f . swap).
> I'd very much appreciate if someone could tell me whether there's a rigorous
> solution simpler than mine, which is:
> Since (.) :: (q -> r) -> (p -> q) -> (p -> r), we have f :: q -> r and swap
> :: p -> q.  Type unification of f requires q = (a, b) and r = c.
> Since f :: (a, b) -> c and curry :: ((l, m) -> n) -> (l -> m -> n), type
> unification requires l = a, b = m, and n = c.  Therefore,
> curry :: ((a, b) -> c) -> (a -> b -> c), and (curry f) :: a -> b -> c.
> Since flip :: (s -> t -> u) -> t -> s -> u, type unification requires
> s = a, t = b, and u = c.  Therefore, flip :: (a -> b -> c) -> b -> a -> c,
> and flip (curry f) :: b -> a -> c.
> Therefore, curry (f . swap) ::  b -> a -> c, and p :: b -> a.  Therefore,
> swap :: b -> a -> (a, b), and:
>
> swap                       :: b -> a -> (a, b)
> swap x y                   =  (y, x)
>
>
> 
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[Haskell-cafe] Retrospective type-class extension

[Tony Morris]

We all know that "class (Functor f) => Monad f" is preferable but its
absence is a historical mistake. We've all probably tried once:

instance (Functor f) => Monad f where
...

However, is there a type system extension (even proposed but not
implemented) that allows me to retrospectively apply such a notion?

Ideally something like this would be handy if it could somehow be
retrospectively applied:
Monad <- Applicative <- Pointed <- Functor


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Re: [Haskell-cafe] Retrospective type-class extension

[Ivan Miljenovic]

On 20 May 2010 14:42, Tony Morris  wrote:
> We all know that "class (Functor f) => Monad f" is preferable but its
> absence is a historical mistake. We've all probably tried once:
>
> instance (Functor f) => Monad f where

Do you mean the reverse of this (instance (Monad m) => Functor m where) ?

>    ...
>
> However, is there a type system extension (even proposed but not
> implemented) that allows me to retrospectively apply such a notion?
>
> Ideally something like this would be handy if it could somehow be
> retrospectively applied:
> Monad <- Applicative <- Pointed <- Functor
>
>
> --
> Tony Morris
> http://tmorris.net/
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Re: [Haskell-cafe] Retrospective type-class extension

[Tony Morris]

Ivan Miljenovic wrote:
> On 20 May 2010 14:42, Tony Morris  wrote:
>   
>> We all know that "class (Functor f) => Monad f" is preferable but its
>> absence is a historical mistake. We've all probably tried once:
>>
>> instance (Functor f) => Monad f where
>> 
>
> Do you mean the reverse of this (instance (Monad m) => Functor m where) ?
>   
Yes.

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