Re: int to float problem
"Mike T. Machenry" <[EMAIL PROTECTED]> writes: > I am having a problem. I recently desided I wanted a bunch function to return > float instead of Int. I changed their type and wrote a new function that > returned a float. I figured it'd be okay if all the others still returned > Int since it's trivial to convert Int to Float. Perhaps the other functions could be written with a more general type? (E.g. :: Num a => ... -> a) Try to remove the type declaration, and see what Hugs or GHCi :t has to say about it! -kzm -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: fundeps for extended Monad definition
| > The reason, which is thoroughly explained in Simon Peyton-Jones' | > message, is that the given type signature is wrong: it should read | > f1 :: (exists b. (C Int b) => Int -> b) | | Right. Simon pointed out that this is a pretty useless function, but not | entirely so, since the result of it is not of type 'forall b. b', but | rather of 'forall b. C Int b => b'. Thus, if the C class has a function | which takes a 'b' as an argument, then this value does have use. I disagree. Can you give an example of its use? Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How to force UNIX text files as output instead of DOS text files??
Thanks a lot m8 but ghc says it can't the module IOExts when I try to compile, any suggestion??? Do I just use normal writeFile method to create the text file then?? Best Regards Alex - Original Message - From: "Glynn Clements" <[EMAIL PROTECTED]> To: "Alexandre Weffort Thenorio" <[EMAIL PROTECTED]> Cc: <[EMAIL PROTECTED]> Sent: Monday, March 03, 2003 2:02 AM Subject: Re: How to force UNIX text files as output instead of DOS text files?? > > Alexandre Weffort Thenorio wrote: > > > I am working on a small program which writes a text file but I need this > > file to be in UNIX format and as it is a Windows program I am compiling > > it, the output file tends to be in DSO format. Is there anyway that I > > can force it to create the file in UNIX format??? > > Use openFileEx, e.g.: > > import IOExts > ... > handle <- openFileEx filename (BinaryMode WriteMode) > > -- > Glynn Clements <[EMAIL PROTECTED]> > ___ > Haskell mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell > ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How to make Parser explicitly "fail"?
If you are using the ParseLib from the Hugs distribution, the function you are looking for is mzero: C:\Hugs98\lib\hugs\ParseLib.hs: instance MonadPlus Parser where -- mzero:: Parser a mzero= P (\inp -> []) -- mplus:: Parser a -> Parser a -> Parser a (P p) `mplus` (P q) = P (\inp -> (p inp ++ q inp)) For the explanations on how this function works and why it has such a name you can take a look at the following paper: Monadic parser combinators Graham Hutton and Erik Meijer. Technical Report NOTTCS-TR-96-4, Department of Computer Science, University of Nottingham, 1996. http://www.cs.nott.ac.uk/Department/Staff/gmh/bib.html#monparsing Regards, Leonid Bouriakovsky > Hi, there. I am glad to join Haskell community. :) > > Here I have a question about Parser: how to make a > parser explicitly "fail"? > > My question arises from the syntax below: > > ::= > ::="define" > ::=':' > ::=identifier other than > ::=()+ > ::=a..z|A..Z > > Now I am thinking of writing a Parser for variable. My > thought is: > > variable::Parser String > variable = do s<-identifier > if not (s=="define") then return s else > --let parser fail > > I will be very happy if there is some mechanism to > implement the "else" branch, for I have thought of it > for long. :) > > __ > Do you Yahoo!? > Yahoo! Tax Center - forms, calculators, tips, more > http://taxes.yahoo.com/ > ___ > Haskell mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How to make Parser explicitly "fail"?
Now I am thinking of writing a Parser for variable. My thought is: variable::Parser String variable = do s<-identifier if not (s=="define") then return s else --let parser fail You forgot to tell which parser library you use. If you use ParseLib (or Parsec) you can use the monadic zero function "mzero". Now, if you are using Parsec, you can also use "fail :: String -> Parser a" to add some error message. variable = do s<-identifier when (s=="define") (fail "define is a keyword") return s Or better: variable = do s<-identifier when (s=="define") (unexpected "keyword") return s "variable" Or even better, use the standard "identifier" parser from the "ParsecToken" module to handle this for you automatically. The user guide describes this in detail: http://www.cs.uu.nl/~daan/parsec.html All the best, Daan. I will be very happy if there is some mechanism to implement the "else" branch, for I have thought of it for long. :) __ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How to force UNIX text files as output instead of DOS text files??
OK I fixed the IOExts not found problem (-package lang) but my problem now is that I never worked with handles. How can I write the string to the file and so on?? Where can I find more info on handle data types?? Best Regards NooK - Original Message - From: "Glynn Clements" <[EMAIL PROTECTED]> To: "Alexandre Weffort Thenorio" <[EMAIL PROTECTED]> Cc: <[EMAIL PROTECTED]> Sent: Monday, March 03, 2003 2:02 AM Subject: Re: How to force UNIX text files as output instead of DOS text files?? > > Alexandre Weffort Thenorio wrote: > > > I am working on a small program which writes a text file but I need this > > file to be in UNIX format and as it is a Windows program I am compiling > > it, the output file tends to be in DSO format. Is there anyway that I > > can force it to create the file in UNIX format??? > > Use openFileEx, e.g.: > > import IOExts > ... > handle <- openFileEx filename (BinaryMode WriteMode) > > -- > Glynn Clements <[EMAIL PROTECTED]> > ___ > Haskell mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell > ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
How to force UNIX text files as output instead of DOS text files??
I am working on a small program which writes a text file but I need this file to be in UNIX format and as it is a Windows program I am compiling it, the output file tends to be in DSO format. Is there anyway that I can force it to create the file in UNIX format??? Best Regards NooK
Re: How to force UNIX text files as output instead of DOS text files??
On Mon, 3 Mar 2003 11:03:48 +0100 "Alexandre Weffort Thenorio" <[EMAIL PROTECTED]> wrote: > > OK I fixed the IOExts not found problem (-package lang) but my > problem now is that I never worked with handles. How can I write the > string to the file and so on?? Where can I find more info on handle > data types?? In the "IO" section of the prelude, http://www.zvon.org/other/haskell/Outputio/index.html Vincenzo ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
looking for Database Interface in Haskell
Dear all, we want to access a (MySQL) data base, running on a linux server, from a Haskell program. We planned to use http://www.volker-wysk.de/mysql-hs/ but it depends on earlier versions of hdirect (0.17?) and ghc(-4?). I built hdirect-0.19 (?) (from the ghc CVS) but the Foreign interfaces seem to have changed a bit since mysql-hs-0.10.1 was written. Any hints on what direction to take? Thanks, -- -- Johannes Waldmann http://www.informatik.uni-leipzig.de/~joe/ -- -- [EMAIL PROTECTED] -- phone/fax (+49) 341 9732 204/207 -- ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
The Revised Haskell 98 Report
Folks I am holding in my hands the first copy of the Haskell 98 Report to roll off the presses at Cambridge University Press. It looks great. And it has a copyright notice that says "It is intended that this Report belong to the entire Haskell community...", just as the online version does. It's in CUP's catalogue here: http://titles.cambridge.org/catalogue.asp?isbn=0521826144 Order your copy now (£35)! Not only is a physical copy useful, but CUP's agreement to the rather unique copyright terms deserves our support. Simon ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: looking for Database Interface in Haskell
Have a look here: http://haskell.cs.yale.edu/haskellDB/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: looking for Database Interface in Haskell
Johannes Waldmann <[EMAIL PROTECTED]> writes: > Dear all, we want to access a (MySQL) data base, > running on a linux server, from a Haskell program. > We planned to use http://www.volker-wysk.de/mysql-hs/ > but it depends on earlier versions of hdirect (0.17?) and ghc(-4?). > I built hdirect-0.19 (?) (from the ghc CVS) but the Foreign interfaces > seem to have changed a bit since mysql-hs-0.10.1 was written. > Any hints on what direction to take? Thanks, Also, HToolkit has working but not yet stable support for both postgresql and mysql. I haven't tried the mysql interface myself, but I have tried the postgresql code. It works, but it does explode if you do something unexpected. http://sourceforge.net/projects/htoolkit/ -- Shae Matijs Erisson - 2 days older than RFC0226 #haskell on irc.freenode.net - We Put the Funk in Funktion ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: looking for Database Interface in Haskell
On 03 Mar 2003 13:57:06 +, Steffen Mazanek <[EMAIL PROTECTED]> wrote: Dear all, we want to access a (MySQL) data base, running on a linux server, from a Haskell program. Have a look here: http://haskell.cs.yale.edu/haskellDB/ A word of warning though from the author :-), HaskellDB is somewhat outdated at the moment and only works with TREX supported systems on windows ODBC databases. It is not so hard to give it a different backend (ie. mySQL) but it is harder to give it a non-TREX type system -- I have ideas about that though and I am planning an update after my thesis is finished, but untill that moment, I don't think it would fit your particular target very well. All the best, Daan. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
GUI for Windows
What User Interface Library would you recommend for use under Windows? I tried FranTk but it crashes as soon as I run the display function (under hugs) and with ghc it won't even compile (I already tinkered with the makefiles, so finally I could make the package, but then the demos won't compile). Any ideas, suggestions? Markus ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: GUI for Windows
On Mon, 3 Mar 2003 16:21:22 +0100, <[EMAIL PROTECTED]> wrote: What User Interface Library would you recommend for use under Windows? Unfortunately, there is no official GUI library for Haskell yet (but many people are working toward this goal at the haskell gui mailing list). At the moment, the best solution on windows (in my opionion) is to use the ObjectIO library that ships with GHC 5.04.2. You need to look at some examples to get started but it is pretty stable and useable. Unfortunately, it seems though that no one actively maintains it... Another solution could be the HTk library but I have no experience with it. Fortunately though, people are working on a good portable GUI solution for Haskell so I expect that over a few months the situation will be better. All the best, Daan. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: GUI for Windows
On Mon, Mar 03, 2003 at 04:21:22PM +0100, [EMAIL PROTECTED] wrote: > What User Interface Library would you recommend for use under Windows? > I tried FranTk but it crashes as soon as I run the display function (under > hugs) > and with ghc it won't even compile (I already tinkered with the makefiles, > so > finally I could make the package, but then the demos won't compile). > > Any ideas, suggestions? There has been a lot of discussions about defining a common API for GUIs, you get an impression when you read the archives for [EMAIL PROTECTED] HToolkit+Ports+GIO tries to provide functionality that works with Gtk and Win32 as backend. In contrast, there is the more complete gtk2hs binding which uses Gtk 2.0 on Unix, Windows and Mac (more info from me). There is Object I/O wich is specific to Windows and comes with GHC. There are more options like HGL which is more for graphics AFAIK and higher-level approaches like Fudgets (not for Windows). There is no best way right now so it would be wrong to give a definite answer here. Hope this helps, Axel. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: int to float problem
Thank does sound like a pain, but it's better than putting fromIntegral all over my code. Why can't Haskell unify a an expected float with an infered int? It seems that this would make life alot easier. -mike On Sun, Mar 02, 2003 at 11:28:00AM +, Jorge Adriano wrote: > > > "Mike T. Machenry" <[EMAIL PROTECTED]> writes: > > > I recently desided I wanted a bunch function to return > > > float instead of Int. [...] I found fromInteger but it > > > didn't seem to work on the return value of the cardinality > > > function for instance. > > > > Try fromIntegral, which works for Int and Integer, too. > > > Casting an Integral value to a Fractional value to perform arithmetic > operations, is a very common need and I don't like adding fromIntegral > everywhere, so ended up writing a (very simple) module with generalized > arithmetic operators (see attachment). The » next to the operations indicate > a cast from an Integral to a Fractional value. > > J.A. > > > module CrossTypeOps where > > > -- Addition > (+«) :: (Fractional a, Integral b)=> a -> b -> a > (+«) x n = x+fromIntegral n > > (»+) :: (Integral a, Fractional b)=> a -> b -> b > (»+) n x = fromIntegral n + x > > (»+«) :: (Integral a, Fractional b)=> a -> a -> b > (»+«) m n = fromIntegral m+fromIntegral n > > > -- Difference > (-«) :: (Fractional a, Integral b)=> a -> b -> a > (-«) x n = x-fromIntegral n > > (»-) :: (Integral a, Fractional b)=> a -> b -> b > (»-) n x = fromIntegral n - x > > (»-«) :: (Integral a, Fractional b)=> a -> a -> b > (»-«) m n = fromIntegral m-fromIntegral n > > > -- Multiplication > (*«) :: (Fractional a, Integral b)=> a -> b -> a > (*«) x n = x*fromIntegral n > > (»*) :: (Integral a, Fractional b)=> a -> b -> b > (»*) n x = fromIntegral n * x > > (»*«) :: (Integral a, Fractional b)=> a -> a -> b > (»*«) m n = fromIntegral m*fromIntegral n > > > -- Division > (/«) :: (Fractional a, Integral b)=> a -> b -> a > (/«) x n = x / fromIntegral n > > (»/) :: (Integral a, Fractional b)=> a -> b -> b > (»/) n x = fromIntegral n / x > > (»/«) :: (Integral a, Fractional b)=> a -> a -> b > (»/«) m n = fromIntegral m / fromIntegral n > > > -- Priorities > infixl 6 +«, »+, »+«, -«, »-, »-« > infixl 7 *«, »*, »*«, /«, »/, »/« > > > ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Tutorial for literate Haskell
I am planning to write a small project in Haskell and stumbled upon some text that mentioned literate Haskell. Is there any good tutorial on how to write literate Haskell? I know that I could take any tutorial on latex and use that, but that's not what I am after. What I want is more like a base document with header and footer done, and perhaps a couple of example files. With that as a base I could get a hint on what the correct idioms for literate Haskell are. If there is no such tutorial a few hints on how other persons write would be very welcome. (with that I mean: how to comment (what info is relevant), how much/little one should describe a function/module, and such stuff) #Luna -- Daniel Luna | Top reasons that I have a beard: [EMAIL PROTECTED] | a) Laziness. http://www.update.uu.se/~luna/| b) I can. Don't look at my homepage (it stinks).| c) I can get away with it. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Tutorial for literate Haskell
Hello.
I do Literate Programming this way:
At first I define a Latex environment "code" as "verbatim"
e.g. so: \newenvironment{code}{\footnotesize\verbatim}{\endverbatim\normalsize}
This environment is understood by the Haskell compilers.
All my modules are own documents concluded in the main tex-file with
\input{...}.
Alternatively I sometimes use lambdaTeX which typesets the code
really nice (problem: latex2html doesn't understand it).
Hope that will help.
Ciao,
Steffen
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AW: GUI for Windows
Thanks, Axel, Daan, I will have a try with ObjectIO. (At least the examples work ;-)) Markus ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: int to float problem
>Thank does sound like a pain, but it's better than putting fromIntegral >all over my code. Why can't Haskell unify a an expected float with an >infered int? It seems that this would make life alot easier. This is my biggest gripe with Haskell, at least for what I do. The numeric class system is good, but it assumes that the sub-classes are distict, where in fact integers are a proper subset of reals, which are a proper subset of complex numbers. Personally, I would like to see module level explicit coersion, similar to the way the default numeric type is handled. Something like: > module Blah where > coerce Int to Double > in mean with fromInt > mean :: [Double] -> Double > mean x = sum x / length x So, if the typesystem infers a Double, but finds an Int, the function would be rewritten as if it had been specified as > mean x = sum x / (fromInt.length) x If you want a global declaration then you could specify > coerce Int to Double > with fromInt --Matthew Donadio ([EMAIL PROTECTED]) ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How to force UNIX text files as output instead of DOS text files??
Hi Great I got it working. Thanks a lot guys. My only problem now is when compiling. I am getting: hextodat.0(.text+0x34):fake: undefined reference to '_stginit_IOExts' Anybody can tell me what could be wrong?? I can run --make but when trying to compile with -o flag I get this error. Best Regards Alex - Original Message - From: "Nick Name" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Monday, March 03, 2003 1:34 PM Subject: Re: How to force UNIX text files as output instead of DOS text files?? > On Mon, 3 Mar 2003 11:03:48 +0100 > "Alexandre Weffort Thenorio" <[EMAIL PROTECTED]> wrote: > > > > > OK I fixed the IOExts not found problem (-package lang) but my > > problem now is that I never worked with handles. How can I write the > > string to the file and so on?? Where can I find more info on handle > > data types?? > > In the "IO" section of the prelude, > > http://www.zvon.org/other/haskell/Outputio/index.html > > Vincenzo > ___ > Haskell mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell > ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
RE: fundeps for extended Monad definition
> | entirely so, since the result of it is not of type 'forall b. b', but
> | rather of 'forall b. C Int b => b'. Thus, if the C class has a
> function
> | which takes a 'b' as an argument, then this value does have use.
>
> I disagree. Can you give an example of its use?
I believe something along the lines of the following would work:
> class C a b | a -> b where { foo :: b -> String }
> instance C Int Int where { foo x = show (x+1) }
> x :: forall b. C Int b => b
> x = 5
(Supposing that the above definition were valid; i.e., we didn't get the
type signature error, this reads that x has type "b" for all types
"b" such that C Int b -- the fact that there is only one such type (due to
the fun dep) is for us to know.)
Then, we should be able to say:
> foo x
and get "6".
>From a "translation to untyped core" perpective (*grin*), we essentially
replace class constraints with dictionaries. The definition of C
introduces a dictionary like the following (I'm not 100% familiar with
MPTC dictionaries, but I assume they're just like normal dictionaries):
> data CDict a b = CDict (b -> String)
Then the instance will give us:
> cIntIntDict :: CDict Int Int
> cIntIntDict = CDict (\x -> show (x+1))
foo will become:
> foo (CDict foo_f) x = foo_f x
and if we apply this properly, we get:
| foo cIntIntDict x
| ==> (\ (CDict foo_f) x -> foo_f x) cIntIntDict x
| ==> (\ (CDict foo_f) -> foo_f x) cIntIntDict
| ==> (\x -> show (x+1)) x
| ==> show (5+1)
| ==> "6"
or something like that?
In fact, except for the type definition on x, this is actually a valid
translation into typed core, I believe. The weird type on x is the only
stumbling block, afaics.
...it is well known that I could be wrong though...
- Hal
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RE: fundeps for extended Monad definition
| > The reason, which is thoroughly explained in Simon Peyton-Jones' | > message, is that the given type signature is wrong: it should read | > f1 :: (exists b. (C Int b) => Int -> b) > Can you give an example of its use? Yes, I can. > class (Show a, Show b) => C a b | a -> b where > doit:: a -> b -> String > instance C Int Int where >doit a b = (show a) > instance C Bool Bool where >doit a b = if a then "everything" else "nothing" > newtype M a = M (forall b.(C a b) => b) > f :: Int -> M Int > f x = M undefined > g :: Bool -> M Bool > g x = M undefined > test1 a = case (f a) of > M b -> doit a b > test2 a = case (g a) of > M b -> doit a b I wonder if the Obfuscated Haskell contest has an entry for the most useless type (with no uses). However, if a type can be used for the contest, it is no longer the most useless. This makes one wonder if the rules of the contest implicitly contain the Russel paradox. ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: How to force UNIX text files as output instead of DOS text files??
Thanks a lot m8. I got it. I was supplying it with --make but not with -o. Thanks again. I guess everything is fine now thanks to you guys.I wanna learn more about catching errors but that comes later when I finish this program. Best Regards Alex - Original Message - From: "Nils Decker" <[EMAIL PROTECTED]> To: "Alexandre Weffort Thenorio" <[EMAIL PROTECTED]> Sent: Monday, March 03, 2003 7:19 PM Subject: Re: How to force UNIX text files as output instead of DOS text files?? > "Alexandre Weffort Thenorio" <[EMAIL PROTECTED]> wrote: > > Hi Great I got it working. Thanks a lot guys. My only problem now is > > when compiling. I am getting: > > > > hextodat.0(.text+0x34):fake: undefined reference to '_stginit_IOExts' > > > > Anybody can tell me what could be wrong?? I can run --make but when > > trying to compile with -o flag I get this error. > > AFAIK, you have to supply -package lang to the linker too when using seperate > compilation. > > > e.g: > ghc -package lang -o foo.o foo.hs > ghc -package lang -o foo foo.o ... > > Regards > Nils > > -- > Unix is user friendly - it's just picky about it's friends. > > Nils Decker <[EMAIL PROTECTED]> > ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Tutorial for literate Haskell
Yo,
Steffen Mazanek wrote:
> I do Literate Programming this way:
> At first I define a Latex environment "code" as "verbatim"
> e.g. so: \newenvironment{code}{\footnotesize\verbatim}{\endverbatim\normalsize}
When I ran into the same question some time ago I tried that,
but found that the \verbatim was interpreted to0 literally, so
that the \end{code} does not terminate it. Could you give a
complete short example that works for you?
My own solution was to copy the definition of verbatim from the
base files, and define code the same way in a separate style file.
Regards,
Bruce.
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Re: looking for Database Interface in Haskell
Hi, Shae > Also, HToolkit has working but not yet stable > support for both postgresql and > mysql. I haven't tried the mysql interface myself, > but I have tried the > postgresql code. It works, but it does explode if > you do something unexpected. > http://sourceforge.net/projects/htoolkit/ Can you send me examples where the MySQL binding explode? I am interested of improving of the library. Best wishes, Krasimir __ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/ ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
First-class types
The following is a more flexible alternative to overloading. We essentially define a function on types and invoke it, seemingly at run time. No Dynamics or unsafe computations are employed. We only need existential types, multi-parameter classes and functional dependencies. The code also shows how to manipulate values which cannot be manipulated. As an example, we define a _function_ (not a method!) 'add' such that: Main> add True False 1 Main> add () (5::Int) 5 Main> add () (5::Float) 5.0 Main> add (4::Int) True 5 Main> add (10::Int) (5::Float) 15.0 The example works both in GHC and Hugs. The signature of add is quite revealing: forall b a2 a1. (Num b, Coerce a2 b, Coerce a1 b, D a1 a2 b) => a1 -> a2 -> b That is, 'add' is capable of adding any two _things_, provided they both can be "generalized" and "coerced" into a number. The "type function" D computes the generalization of two types -- in a manner we specify. The function add is actually quite simple: > add x y = let general_type = typeof x y > x' = coerce x general_type > y' = coerce y general_type > in x' + y' The function 'typeof' is also interesting. It has a type forall b a2 a1. (D a1 a2 b) => a1 -> a2 -> b Note that it returns 'b' forall b! In a sense, the function performs a type computation at run time. The code follows. We should note that we could have achieved the same effect by defining an appropriate class with the method 'add'. In our solution however, the acts of generalizing, coercion, and addition are all separated. If we later decide to subtract things rather than add them, we do not need to alter the class and all the instances. We merely need to introduce the subtraction function. This makes the maintenance of the code easier. The distinct characteristic of the following code is an indirect manipulation of an untouchable, existential value. We were able to force a value to be of a specific type without any means of accessing the value directly. > class Type a where > name :: a -> String > instance Type Bool where > name a = "Bool" > instance Type Int where > name a = "Int" > --instance Type Char where > --name a = "Char" > instance Type () where > name a = "()" > instance Type Float where > name a = "Float" > -- Type generalization function: type -> type -> type > class (Type b) => D a1 a2 b | a1 a2-> b > instance D Bool Bool Int > instance D Int Bool Int > instance D Bool Int Int > instance D Int Int Int > instance D () Int Int > instance D Int () Int > instance D () () Int > instance D Int Float Float > instance D () Float Float > instance D Float Int Float > instance D Float Float Float > -- The coercion function > class Coerce a b where > coerce :: a -> b -> b > instance Coerce () Int where > coerce _ _ = 0 > instance Coerce () Float where > coerce _ _ = 0 > instance Coerce Int Int where > coerce = const > instance Coerce Float Float where > coerce = const > instance Coerce Int Float where > coerce x _ = fromInteger $ toInteger x > instance Coerce Bool Int where > coerce True _ = 1 > coerce False _ = 0 > newtype M a1 a2 = M (forall b.(D a1 a2 b) => b) > data M1 a1 a2 = M1 a1 a2 (M a1 a2) > typeof v1 v2 = case (M1 v1 v2 (M undefined)) of M1 _ _ (M y) -> y > typeof1 v1 v2 = case (M1 v1 v2 (M undefined)) of M1 _ _ y -> y > add x y = let general_type = typeof x y > x' = coerce x general_type > y' = coerce y general_type > in x' + y' ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: Tutorial for literate Haskell
Hi,
Since I sent this to the haskell list in the first place,
I'd better let everyone know that it all worked out.
> Hmm, there were no problems in simply doing so.
Ok, I've cut your example down a bit (just from a
minimalist tendency). The complete modified code is ...
\documentclass{report}
\usepackage{verbatim}% vital
\begin{document}
\newenvironment{code}{\footnotesize\verbatim}{\endverbatim\normalsize}
HaskellModule.lhs:
Maybe some text...
We call our module Foo,
because this name is very meaningful.
\begin{code}
module Foo where
\end{code}
\end{document}
Works just fine and seems robust.
My original problem ... an over-developed tendency to go it alone?
I was not using package verbatim, which does help, doesn't it.
> I hope this will help.
Yup, thanks.
Bruce.
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Persistent data
Is there some way to reduce the cost of garbage collection over large persistent datastructures without resorting to escaping to C to malloc memory outside the heap? The program I'm working is part database, which cannot discard information. The net result is that I see figures like 82.9% of the time taken by garbage collection. The heap profile looks like a charging capacitor: a linear increase (as expected) which is slowly dilated as time increases by the garbage collector thrashing memory. When I worked on the LOLITA natural language processor we solved the problem by moving a lot of the data out to C++, so that the heap only contains things soon to be freed. I know generational garbage collection is supposed to help, but it doesn't seem to. Is there a pure Haskell solution to this problem? Sengan ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: int to float problem
"Matthew Donadio" <[EMAIL PROTECTED]> writes: >> Thank does sound like a pain, but it's better than putting fromIntegral >> all over my code. Why can't Haskell unify a an expected float with an >> infered int? It seems that this would make life alot easier. Personally, I think that one of the things that made my life easier with Haskell compared to C++, is the lack of implicit type cast/conversions/coercions. Now, obviously C++ does this in a quite byzantine way, it's possible that a simpler and better system exists. -kzm -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
