- Haskell-Cafe - Haskell.org

Java - Haskell adjustment
by Ryan Bloor 15 Oct '07

15 Oct '07

Hi, its Ryan here... I've just come from an intensive course in java and have been thrown into the imperative world of haskell. The problem that I have is extremely simple in java but I am having trouble adjusting my old mindset. A multiset is a collection of items. Each item may occur one or more times in the multiset. All items are of the same type. The scenario we could use is a students' grades for their A levels: 3A, 4B and 2C grades for our pupil 'x'. A multiset may be implemented by a list... so ['A', 'A', 'A', 'B', 'B', 'B', 'B, 'C', 'C'] but this very ineffiecient and tedious. How might one set out into putting this into action.... thanks... Any help that complements my ideas is welcome _________________________________________________________________ Feel like a local wherever you go. http://www.backofmyhand.com

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Re: [Haskell-cafe] On the verge of ... giving up! [OT]
by Brandon S. Allbery KF8NH 15 Oct '07

15 Oct '07

On Oct 15, 2007, at 7:01 , Yitzchak Gale wrote: > But I think we are still at the stage where a programmer > who wants practical results is better off starting out > by learning how to use monads in practice, not by > delving into category theory. No argument from a Haskell standpoint. Still, when people here or on #haskell start tossing CT around, it'd be nice to have more than a very vague and probably incorrect notion of what they're saying. :) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.com system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

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Let's do ListT right, finally
by Yitzchak Gale 15 Oct '07

15 Oct '07

Hi Ian, thanks for responding to my plea! I am renaming this thread and moving it to libraries. Please respond there. I wrote: >> http://haskell.org/haskellwiki/ListT_done_right >> and not the broken implementation that comes with mtl. >> (Finally fixed in 6.8? Please?) Ian Lynagh wrote: > If you want to propose that mtl switches to a different definition of > ListT then please see > http://www.haskell.org/haskellwiki/Library_submissions OK, then. This is a major rewrite of this module. So let me open up the topic for discussion even before posting a submission. Do we need to worry about breaking code? Not very much, I suspect. That "monad" really is broken - it's not a monad at all. The problem and its solution have been well known for years. I personally tried using the "standard" ListT for a while, just because it's the standard. Until I ran into enough problems that I finally slapped my forehead and said "forget it". But just in case: Is there anyone reading this who would be hurt by this change and opposes it? Should we provide a more gradual deprecation path (what is it)? Thanks, Yitz

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Re: [Haskell-cafe] haskell-curry, classical logic, excluded middle
by Tim Newsham 15 Oct '07

15 Oct '07

On Sun, 14 Oct 2007, Roberto Zunino wrote: > (Warning: wild guess follows, I can not completely follow CPS ;-)) > Adding a couple of forall's makes it compile: > propCC :: ((forall q . p -> Prop r q) -> Prop r p) -> Prop r p > func1 :: (forall q . Or r p (Not r p) -> Prop r q) > -> Prop r (Or r p (Not r p)) Yup! That did it, thanks! Now that that works, one more question. Is it possible to hide the "r" that is attached to every single type? For example to do something like this (which doesn't compile): data Prop p = Prop (forall r. (p -> r) -> r) run :: Prop p -> (p -> r) -> r run (Prop f) k = f k propCC :: ((forall q. p -> Prop q) -> Prop p) -> Prop p propCC f = Prop (\k -> run (f (\a -> Prop (\k' -> k a))) k) > Zun. Tim Newsham http://www.thenewsh.com/~newsham/

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haskell-curry, classical logic, excluded middle
by Tim Newsham 15 Oct '07

15 Oct '07

I've been struggling with this for the last day and a half. I'm trying to get some exercise with the type system and with logic by playing with the curry-howard correspondence. I got stuck on the excluded-middle, and I think now I got it almost all the way there, but its still not quite right. Is this error I'm getting (inline at the end) easily fixed, and what exactly is going on? ---- code --- {-# OPTIONS_GHC -fglasgow-exts #-} module Classic where {- Classic logic. (See Intuit.hs first). In this system propositions all take a continuation. This allows us to define the law of the excluded middle. -} import Control.Monad -- propositions are functions taking a continuation. data Prop r p = Prop ((p -> r) -> r) run :: Prop r p -> (p -> r) -> r run (Prop f) k = f k propCC :: ((p -> Prop r q) -> Prop r p) -> Prop r p propCC f = Prop (\k -> run (f (\a -> Prop (\k' -> k a))) k) instance Monad (Prop r) where return p = Prop (\c -> c p) p >>= mkq = Prop (\c -> run p (\r -> run (mkq r) c)) data TRUTH = TRUTH type FALSE r = Not r TRUTH data And r p q = And (Prop r p) (Prop r q) data Or r p q = OrL (Prop r p) | OrR (Prop r q) data Imp r p q = Imp (Prop r p -> Prop r q) data Equiv r p q = Equiv (Prop r p -> Prop r q) (Prop r q -> Prop r p) data Not r p = Not (forall q. (Prop r p -> Prop r q)) -- Truth truth :: Prop r TRUTH truth = return TRUTH -- And-Injection -- P, Q |- P /\ Q andInj :: Prop r p -> Prop r q -> Prop r (And r p q) andInj p q = return (And p q) -- And-Elimination, left and Right -- P /\ Q |- P andElimL :: Prop r (And r p q) -> Prop r p andElimL pq = pq >>= \(And p q) -> p -- P /\ Q |- Q andElimR :: Prop r (And r p q) -> Prop r q andElimR pq = pq >>= \(And p q) -> q -- Or-Injection, left and right -- P |- P \/ Q orInjL :: Prop r p -> Prop r (Or r p q) orInjL p = return (OrL p) -- Q |- P \/ Q orInjR :: Prop r q -> Prop r (Or r p q) orInjR q = return (OrR q) -- Or-Elimination. -- P \/ Q, P -> R, Q -> R |- R orElim :: Prop r (Or r p q) -> (Prop r p -> Prop r s) -> (Prop r q -> Prop r s) -> Prop r s orElim pORq p2r q2r = pORq >>= byCases where byCases (OrL p) = p2r p byCases (OrR q) = q2r q -- Implication-Injection -- (P |- Q) |- P -> Q impInj :: (Prop r p -> Prop r q) -> Prop r (Imp r p q) impInj p2q = return (Imp p2q) -- Implication-elimination (modus ponen) -- P, P -> Q |- Q impElim :: Prop r p -> Prop r (Imp r p q) -> Prop r q impElim p pIMPq = pIMPq >>= \(Imp p2q) -> p2q p -- Equivalence-Injection -- P -> Q, Q -> P |- P = Q equivInj :: Prop r (Imp r p q) -> Prop r (Imp r q p) -> Prop r (Equiv r p q) equivInj pIMPq qIMPp = do (Imp p2q) <- pIMPq (Imp q2p) <- qIMPp return (Equiv p2q q2p) -- Equivalence-Elimination, Left and Right -- P = Q |- P -> Q equivElimL :: Prop r (Equiv r p q) -> Prop r (Imp r p q) equivElimL pEQq = pEQq >>= \(Equiv p2q q2p) -> return (Imp p2q) equivElimR :: Prop r (Equiv r p q) -> Prop r (Imp r q p) equivElimR pEQq = pEQq >>= \(Equiv p2q q2p) -> return (Imp q2p) -- Absurdity -- False |- P absurd :: Prop r (FALSE r) -> Prop r p absurd false = false >>= \(Not true2p) -> true2p truth -- Not-Inj -- (P |- False) |- ~P notInj :: forall r p. (Prop r p -> Prop r (FALSE r)) -> Prop r (Not r p) notInj p2false = return (Not p2any) where p2any :: forall q. Prop r p -> Prop r q p2any assumep = absurd (p2false assumep) -- Not-Elimination -- P, ~P |- Q notElim :: Prop r p -> Prop r (Not r p) -> Prop r q notElim p np = np >>= \(Not p2any) -> p2any p -- Excluded-Middle -- P \/ ~P exclMiddle :: forall r p. Prop r (Or r p (Not r p)) exclMiddle = propCC func1 where func1 :: (Or r p (Not r p) -> Prop r q) -> Prop r (Or r p (Not r p)) -- k :: Or r p (Not r p) -> Prop r q func1 k = return (OrR (return (Not func2))) where func2 :: Prop r p -> Prop r q func2 k' = k (OrL k') {- http://www.cse.ogi.edu/~magnus/mdo-callcc-slides.pdf -- A \/ ~A excmid :: Either a (a -> b) excmid = callcc (\k. Right (\a.k (Left a))) -} {- Classic2.hs:114:27: Couldn't match expected type `q' (a rigid variable) against inferred type `q1' (a rigid variable) `q' is bound by the type signature for `func2' at Classic2.hs:113:44 `q1' is bound by the type signature for `func1' at Classic2.hs:110:45 Expected type: Prop r q Inferred type: Prop r q1 In the expression: k (OrL k') In the definition of `func2': func2 k' = k (OrL k') -} -- False-Elimination -- (~P |- False) |- P Tim Newsham http://www.thenewsh.com/~newsham/

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Categories in base
by Jean-Philippe Bernardy 15 Oct '07

15 Oct '07

Hello folks, Prompted by Ashley's proposal for Category class, I came up with the following module, which gives a summary of the overloading situation in the base package. Maybe it can help people find their way through all the available names... Failing that it surely was fun to come up with. :) -- JP {-# OPTIONS -fallow-undecidable-instances #-} import Prelude () class Functor f where (<$>) :: (a -> b) -> f a -> f b -- a.k.a: fmap, map, liftM class Functor f => Applicative f where return :: a -> f a -- a.k.a: Control.Applicative.pure (<*>) :: f (a -> b) -> f a -> f b -- a.k.a: ap -- f <$> g = return f <*> g class Applicative m => Monad m where (=<<) :: (a -> m b) -> m a -> m b -- a.k.a: flip (>>=) -- f <*> g = (\h -> (return . h) =<< g) =<< f class Category c where id :: c a a (.) :: c y z -> c x y -> c x z -- a.k.a: (<<<), flip (>>>) class Category a => Arrow a where pure :: (b -> c) -> a b c first :: a b c -> a (b, d) (c, d) instance Arrow a => Functor (a r) where -- (not defined as such in base, but ad-hoc) f <$> g = pure f . g -- instances for (->) instance Category (->) where id = \x -> x f . g = \x -> f (g x) instance Arrow (->) where pure = id first f = \(b, d) -> (f b, d)

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Re: [Haskell-cafe] Let's do ListT right, finally
by Dan Piponi 15 Oct '07

15 Oct '07

On 10/14/07, Yitzchak Gale <gale(a)sefer.org> wrote: > Interesting. What do you mean by a "commutative monad"? > It can't be a monad with some sort of additional commutative > law, because the old ListT doesn't even satisfy the monad > laws. Or does it in some sense? If m is a commutative monad, then ListT m is a fully paid up monad, not just "in some sense". On a brief definition of commutative monads, and for some examples indicating how common they are, see SPJ's http://research.microsoft.com/~simonpj/papers/haskell-retrospective/Haskell… > OK, do you propose that we keep it and change its name? +1, dump the old one completely because you can't express what kind of thing the old ListT is in Haskell. I just want to make sure it gets a good epitaph. If someone answers SPJ's challenge #2 maybe it'll come back. -- Dan

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Is this a known problem?
by ok 14 Oct '07

14 Oct '07

UltraSPARC II, Solaris 2.10, gcc 4.0.4 (gccfss), Haskell GHC 6.6.1 binary release. Trying to compile a simple file gives me oodles of errors because ghc is generating something that makes gcc generate lots of these: sethi %hi(<<some register>>),<<another register>> For people unfamiliar with SPARC assembly code, the %hi(...) operation returns the top bits of a *literal* address, it makes absolutely no sense to give it a register. I have tried this version of gccfss with lots of C code and have been unable to make it generate this construction; it's only ghc's .hc files that do it. Running gcc by hand on the .hc file produces 8 warnings ...lib/ghc-6.6.1/include/Regs.h: ###: warning: call-clobbered register used for global register variable -DNO_GLOBAL_REG_DECLS eliminated that. Comparing the .s file produced using 'ghc -S' with the .s file produced by gcc'ing the .hc produced some illumination: .section ".text",#slloc,#execinstr,#progbits .align 4 #if GHC .text .align 4 #endif s4AK_ret: save %sp,-96,%sp #if GHC sethi %hi(%l1),%i2 add %i2,%lo(%l1),%i5 ld [%i2+%lo(%l1)],%i2 ld [%i2],%l7 lduh [%l7-2],%l6 ... #else sethi %hi(MainCapability),%i5 add %i5,%lo(MainCapability),%i2 ld [%i2+8],%i5 ld [%i5],%i4 lduh [%i4-2],%i3 ... #endif What with one thing and another, I'm not really sure that it's gcc's fault.

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do
by PR Stanley 14 Oct '07

14 Oct '07

Hi "do", what's its role? I know a few uses for it but can't quite understand the semantics - e.g. do putStrLn "bla bla" So, what does do, do? Thanks, Paul

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more functions to evaluate
by PR Stanley 14 Oct '07

14 Oct '07

Hi folks Any comments and/or criticisms no matter how trivial on the following please: wordSize :: [Int] -> Int wordSize xs = head (dropWhile (<(length xs)) $ iterate (*2) 8) intToBinWord :: Int -> [Int] intToBinWord n = reverse (take elements (xs ++ repeat 0)) where xs = reverse (intToBin n) elements = wordSize xs Thanks, Paul

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Can every monad can be implemented with Cont? (was: New slogan for haskell.org)
by apfelmus 14 Oct '07

14 Oct '07

Don Stewart wrote: > allbery: > >> Didn't someone already prove all monads can be implemented in terms >> of Cont? >> > > Cont and StateT, wasn't it? > And the schemers have no choice about running in StateT :) You sure? I want to see the proof :) Last time I stumbled upon something like this, the "proof" was to embed every monad m in the type type Cont m a = forall b . (a -> m b) -> m b with an instance Monad (Cont m) where ... _independent_ of whether m is a monad or not. The problem I see with it is that we didn't really encode m with it since we're still dependent on return and (>>=) via project :: Monad m => Cont m a -> m a project f = f return and inject :: Monad m => m a -> Cont m a inject x = (x >>=) I mean, the starting point for a concrete monad M are some primitive operations like get :: M s put :: s -> M () and a function observe :: M a -> (S -> (a,S)) together with laws for the primitive operations (= operational semantics) observe (put s >>= x) = \_ -> observe (x ()) s observe (get >>= x) = \s -> observe (x s ) s and for return observe (return a) = \s -> (a,s) Now, implementing a monad means to come up with a type M and functions (>>=) and return that fulfill the monad laws. (In general, the result type of observe is _not_ M a !) This can be done with the standard trick of implementing stuff as constructors (see Unimo for details http://web.cecs.pdx.edu/~cklin/papers/unimo-143.pdf) But - and that's the problem - I don't see how it can be done with Cont in all cases. It works for the above state monad (*) but what about primitives like mplus :: m a -> m a -> m a callcc :: ((a -> m r) -> m r) -> m a that have monadic arguments in a contravariant position (possibly even universally quantified)? Regards, apfelmus *: Here you go: type Observe s a = s -> (a,s) type State s a = Cont (Observe s) a get = \x -> (\s -> observe (x s ) s) -- law for get put s = \x -> (\_ -> observe (x ()) s) -- law for put observe f = f $ \a -> (\s -> (a,s)) -- law for observe (return a)

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Dual Parser Failure???
by PR Stanley 13 Oct '07

13 Oct '07

Hi failure :: (Parser a) failure = \inp -> [] The code might contain some syntax errors and I'd be grateful for any corrections. What is a "dual parser failure"? Thanks, Paul

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WinAmp plugin?
by Andrew Coppin 13 Oct '07

13 Oct '07

Does anybody here know WinAmp? [I feel sure the answer must be yes!] How hard would it be to write a visualisation plugin in Haskell? I think this would be a neat way of demonstrating that "Haskell isn't slow". Also, WinAmp plugins (and, actually, WinAmp) are notoriously buggy and unstable. Would be a nice place to show off how reliable Haskell programs are. OTOH, I have no idea about this kind of thing, so...

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question about throwDyn
by Michael Vanier 13 Oct '07

13 Oct '07

In ghci, why does throw $ ArithException DivideByZero print *** Exception: divide by zero while throwDyn $ ArithException DivideByZero print *** Exception: (unknown) ? Mike

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Defining Tree
by PR Stanley 13 Oct '07

13 Oct '07

Hi I'm reading the chapter on parsers in the Hutton book. The text refers to the data type tree which doesn't seem to be in prelude. So, I was wondering, what would be asuitable tree for a parser? A binary tree perhaps? Are there different types of tree for different parsers? Thanks, Paul

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Re: [Haskell-cafe] WinAmp plugin?
by Andrew Coppin 13 Oct '07

13 Oct '07

pierre wrote: > > > Wouldn't it be better to write not just a visualization plugin, but a whole player from scratсh? :-) > ...this idea also occurred to me. ;-) Why, do *you* know how to decode MP3 data?

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Re: Type-level arithmetic
by Andrew Coppin 13 Oct '07

13 Oct '07

I was actually thinking more along the lines of a programming language where you can just write head :: (n > 1) => List n x -> x tail :: List n x -> List (n-1) x (++) :: List n x -> List m x -> List (n+m) x and so forth. You know, instead of the elaborate simulations crafted out of systems that weren't meant to do this stuff. Of course, the integer case is probably fairly easy. I suspect the issue is that as soon as you try to do this kind of thing, you develop a terminal case of wanting to hyper-generalise everything which then directly results in an incomprehensible mess... ;-)

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[libraries-owner@haskell.org: darcs patch: fix typo in docs.]
by David Roundy 12 Oct '07

12 Oct '07

It seems a little unfriendly to reject contributions from anyone who isn't subscribed to the libraries mailing list... I suppose one solution is to have maintainers for packages. But in this case it should be deprecated having the email of packages set to libraries(a)haskell.org. (Yes, I know I could register and then stop delivery of messages, but that's a bit much to ask random contributors to do.) David ----- Forwarded message from libraries-owner(a)haskell.org ----- Envelope-to: droundy(a)darcs.net Delivery-date: Thu, 11 Oct 2007 14:28:43 -0700 Subject: darcs patch: fix typo in docs. From: libraries-owner(a)haskell.org To: droundy(a)darcs.net You are not allowed to post to this mailing list, and your message has been automatically rejected. If you think that your messages are being rejected in error, contact the mailing list owner at libraries-owner(a)haskell.org. To: libraries(a)haskell.org From: David Roundy <droundy(a)darcs.net> Subject: darcs patch: fix typo in docs. DarcsURL: http://darcs.haskell.org/QuickCheck This fixes a typo in the docs of QuickCheck 2. David Thu Oct 11 17:26:19 EDT 2007 David Roundy <droundy(a)darcs.net> * fix typo in docs. Content-Description: A darcs patch for your repository! New patches: [fix typo in docs. David Roundy <droundy(a)darcs.net>**20071011212619] < > { hunk ./Test/QuickCheck/Arbitrary.hs 92 arbitrary :: Gen a arbitrary = error "no default generator" - -- | Produces a (possibly) empty list of all the possible + -- | Produces a (possibly empty) list of all the possible -- immediate shrinks of the given value. shrink :: a -> [a] shrink _ = [] } Context: [Haddock tweak. bringert@cs.chalmers.se**20061126234053] [Added a version of sampling that returns the test cases dpt@math.columbia.edu**20061126220704] [Added LICENSE file. bringert@cs.chalmers.se**20061126232241] [Fixed silly missed use of Positive constructor. Reported by Audrey Tang. bringert@cs.chalmers.se**20061124212824] [Added README. bringert@cs.chalmers.se**20061123144917] [Some haddock mark-up fixes to support haddock 0.7. bringert@cs.chalmers.se**20061123144831] [Removed BUGS. bringert@cs.chalmers.se**20061123144204] [Changed distribution of NonNegative to 1:5 for 0:everything else (was 9:1). bringert@cs.chalmers.se**20061120210126] [Added NonZero and reimplemented Positive in terms of NonZero and NonNegative. bringert@cs.chalmers.se**20061120205417] [Added BUGS file. bringert@cs.chalmers.se**20061120205405] [Added Integral and Real instances for Positive and NonNegative. bringert@cs.chalmers.se**20061120204544] [Some haddock tweaks. bringert@cs.chalmers.se**20061120112922] [More haddock comments. bringert@cs.chalmers.se**20061120112532] [More haddock and export list fixes. bringert@cs.chalmers.se**20061117201830] [Added lots of Haddock comments and cleaned up export lists. bringert@cs.chalmers.se**20061117200346] [Commented out generics import that Haddock choked on. bringert@cs.chalmers.se**20061117152818] [Removed shrinkSub workarounds for old GHC bugs. bringert@cs.chalmers.se**20061117152630] [Get rid of the last uses of modules from the haskell98 package, replace with base modules. bringert@cs.chalmers.se**20061117152400] [Added Cabal build system. bringert@cs.chalmers.se**20061117152230] [Removed Koen's CVS update script. bringert@cs.chalmers.se**20061117151819] [Renamed modules to Test.QuickCheck.*. bringert@cs.chalmers.se**20061117151417] [More fixes to get examples to compile. bringert@cs.chalmers.se**20061114164526] [Fixed some imports in examples. bringert@cs.chalmers.se**20061114163012] [Fixed Chalmers/QuickCheck/Monadic.hs to compile with GHC 6.6. bringert@cs.chalmers.se**20061114162953] [Moved examples to a top-level directory. bringert@cs.chalmers.se**20061114160643] [Fixed ExSet2 and ExLambda to compile with GHC 6.6. bringert@cs.chalmers.se**20061114160531] [Comment out generics stuff to compile with GHC 6.6. bringert@cs.chalmers.se**20061114160414] [Renamed QuickCheck to Chalmers to get module names right. bringert@cs.chalmers.se**20061114160326] [Tailorization of QuickCheck tailor@momo.medic.chalmers.se**20061114165514 Import of the upstream sources from the repository /users/cs/koen/CvsRoot as of revision 2006-05-03 18:37:52 by koen ] Patch bundle hash: a1cf7ea46740918443b7069e6caff5a51cff90aa ----- End forwarded message ----- -- David Roundy Department of Physics Oregon State University

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Fast Paced Haskell Tutorial
by Paulo J. Matos 12 Oct '07

12 Oct '07

Hello all, I'm interested in a freely available fast paced haskell tutorial. By fast paced, I means I want something that goes through basic in a very fast pace, presents a couple of examples and then talks about more advanced features. A set of tutorials would be also good. References to these kind of tutorials would be great. Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK

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Type Synonyms
by Tom Davies 12 Oct '07

12 Oct '07

Newbie question: I was wondering the other day if type synonyms might be more useful if they were more restricted, that is, with the definitions: type Foo = String type Bar = String foo :: Foo foo = "a foo" bar :: Bar bar = "a bar" x :: Foo -> ... x f b = ...only valid for Foo Strings... both 'x foo' and 'x bar' type check correctly. Wouldn't it be useful if Foo and Bar were both equivalent to String, but Foo and Bar were not equivalent themselves? For instance, if you are using Strings as properties of something and want to associate the type of the property with its value, without wrapping the String. Would this break a transitivity property of the type system? Am I just suffering from laziness?

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Hugs, dotnet, C#...
by Peter Verswyvelen 12 Oct '07

12 Oct '07

In the (Win)Hugs documentation, I found "Only the ccall, stdcall and *dotnet *calling conventions are supported. All others are flagged as errors." However, I fail to find any more information on how to invoke dotnet methods. This might be really handy for me, as I'm very familiar with the dotnet framework. For example, yesterday I rewrote and extended a program that I wanted to develop in Haskell in just 3 hours using dotnet, while I spend weeks trying do this in Haskell. Of course, I'm a Haskell newbie and a dotnet expert, so this is not a fair comparison. However, I got a strange feeling, which I want to share with you :) First of all, it was a *horrible* experience to program C# again; I needed to type at least 3 times the amount of code, much of which was boilerplate code, and the code is not elegant. Haskell really changed my point of view on this; before I knew Haskell, I found C# (I'm talking C# 3.0 here) a really neat and nice language. On the other hand, the great Visual Studio IDE and Resharper addin made it at least 3 times faster to type, navigate, refactor, and debug the code... Somehow, I get things done really really really fast in C#, albeit in an "ugly" way. Once again, I just wish Haskell had such an IDE... And yes, I know of the existance of Visual Haskell, EclipseFP, Haskell Mode for Emacs (which I'm using), VIM, YI, but still, these do not compare with the experience I have when using Visual Studio/Resharper (or Eclipse or IntelliJ/IDEA for Java). But that might just be me of course... A slightly frustrated Peter ;-) BTW: I don't want to bring up the IDE discussion again, no really ;-)

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symbol type?
by Michael Vanier 12 Oct '07

12 Oct '07

Is there an implementation of a symbol type in Haskell i.e. a string which has a constant-time comparison operation? Mike

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Re: [Haskell] Abstract syntax representation
by Neil Mitchell 12 Oct '07

12 Oct '07

Hi In future questions like this are usually best directed to haskell-cafe, with haskell being left for annoucements. The standard "full haskell" representation is in Template Haskell (http://haskell.org/ghc/docs/latest/html/libraries/template-haskell/Language…) If you want to work on a reduced language then I recommend Yhc.Core (http://www-users.cs.york.ac.uk/~ndm/yhc/ - there is a link to the Yhc.Core paper on that page). Thanks Neil

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ANNOUNCE: metaplug
by mad.one＠gmail.com 11 Oct '07

11 Oct '07

Hi, Recently I just decided to finish up and publish my library, metaplug: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/metaplug What is it? It's basically a very tiny GHC API wrapper for programs that want eval and plugin style functionality packaged in a basic library. The need for it spawned out of my GHC API investigations, and looking at it as an easy way to get pluggable apps across GHC releases (hs-plugins is somewhat fragile as it can break basically inbetween any release,) and with the oh-so-close ghc 6.8 release, I needed something for things like my IRC bot (which does hot-code swapping) that wanted plugin style facilities without the hassle of using the API explicitly or having dependencies on soon to break packages like hs-plugins. Right now, there is no haddock documentation (I'll add it and upload a new version when I have time) but it's simple enough to figure out how to use from the source code. Here's a quickie review: Metaplug: top level module, you only need import this Metaplug.Eval: exports one function, eval, which, well, does what it sounds like Metaplug.Initialize: exports two functions, initSession and initSession' which initialize a metaplug session. initSession' is for the case where you want to provide your own very customized DynFlags to GHC.newSession Metaplug.Loader: provides two functions, loadFiles and compileCall which load files into a session and compiles a dynamic to a function you can call. Metaplug.Types: simply provides the SessionMode and MetaSession types Metaplug.Packages: provides a very simple interface to query the package database for info, please note this has NOT been tested with Cabal 1.2.0 There're some example applications in the tests/ directory which show off how to use most of this stuff in a very simple manner. Check the README. There is one limitation to this, however. compileCall expects to compile a dynamic via GHC.dynCompileExpr; what this means is your resource must be monomorphic (for Typeable to work.) As of right now, the easiest way I can see to get around this is to simply define a datatype like such: data Plugin { rsrc :: ... -- your type here } deriving Typeable And then have the symbol you want to compile could be for example, plugin = Plugin { rsrc = dynmain } The application in tests/dynst/ provides a good example of this. If anybody knows a way around this or an easier way to approach it, I appreciate all feedback (unsafeCoerce# is an option but there's not a version of compileCall to support this as of right now. I might add it if it seems needed.) Any feedback appreciated. Thanks, Austin

1 0

help!!!! install HaskellDirect
by Sulema Rocha Betancur 10 Oct '07

10 Oct '07

Hello :) Someone knows like installing HDirect 1,2 in Windows XP with cygwin? , and that version of GHC is the one that uses???.Thank you for its collaboration!!!!!!!! Sule ;) _________________________________________________________________ News, entertainment and everything you care about at Live.com. Get it now! http://www.live.com/getstarted.aspx

1 0

help!!!! install HakellDirect
by Sulema Rocha Betancur 10 Oct '07

10 Oct '07

Hello :) Someone knows like installing HDirect 1,2 in Windows XP with cygwin? , and that version of GHC is the one that uses???.Thank you for its collaboration!!!!!!!! Sule ;) _________________________________________________________________ Invite your mail contacts to join your friends list with Windows Live Spaces. It's easy! http://spaces.live.com/spacesapi.aspx?wx_action=create&wx_url=/friends.aspx…

1 0

Manual constructor specialization
by Johan Tibell 10 Oct '07

10 Oct '07

I have a rope data type with the invariant that one of its data constructors can never appear as a leaf. module Data.Rope where import Data.Word (Word8) data Rope = Empty | Leaf | Node !Rope !Rope index :: Rope -> Int -> Word8 index Empty _ = error "empty" index Leaf _ = error "leaf" index (Node l r) n = index' l n where index' Leaf _ = error "leaf" index' (Node l r) n = index' l n I removed some of the actual details (Leafs have a ByteString member and Nodes have a length and a depth field). The point is that Empty can only appear at the top by construction (i.e. it's not possible to construct a rope that breaks this invariant using the exported API). If I understand compilation of case statements correctly they will compile into some if like construct that checks the constructor tag. This means that I would like that once we have established that the constructor is not Empty we could use a specialized function to traverse the tree without checking for Empty on each iteration. The above doesn't achieve this as GHC inserts an automatic error branch for Empty in index'. Perhaps one solution is to reorder the cases and remove index'? -- Johan

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Re: [Haskell-cafe] ANNOUNCE: binary 0.4: high performance, pure binary parsing and serialisation
by Don Stewart 10 Oct '07

10 Oct '07

ketil: > Don Stewart <dons(a)galois.com> writes: > > > The main thing is porting to ghc 6.8 -- which means the new (*faster*) > > lazy bytestring representation, and the smp parallel quickcheck driver > > for the testsuite (it'll use N cores, watch the jobs migrate around). > > Binary 0.4 seems to require bytestring 0.9, the library formerly known > as fps. I just want to mention it, as most network resources (as > found by Google) only have discoverd fps 0.7, which is not going to > work. > > (Oh, and yes, it's at darcs.haskell.org/bytestring/ ) > > It'd be nice - since I know you're at it anyway :-) - if hackage/cabal > would have some support for backwards compatibility. I know GHC 6.8 > is the thing to get, but some of us still install what comes with our > distribution, so I hope 6.6.1 still will be supported. > Normally this is farily easy to do, with cabal configurations. (See bytestring's .cabal file for example). However, the binary internals changed in such a way that it wasn't worth preserving backwards buildability. Instead, you should use binary 0.3 with ghc 6.6 libs A released version of bytestring 0.9 will be on hackage shortly. -- Don

2 2

Haskell FFI and finalizers
by Maxime Henrion 09 Oct '07

09 Oct '07

Hello all, I have recently developed a small set of bindings for a C library, and encountered a problem that I think could be interesting to others. My problem was that the C function I was writing bindings to expects to be passed a FILE *. So, I had basically two possibles routes to take: 1) Mimic the C API and have the haskell function take a Handle. Unfortunately, I can see no way to go from a Handle to a Ptr CFile, at least no portable way, so I discarded this option. 2) Deviate from the C API slightly and have the haskell function take a FilePath instead of a Handle. This is the option I chose, and this is where things get interesting. In order to pass a Ptr CFile (FILE *) to the C function, I had to call fopen() myself, using a usual FFI binding: foreign import ccall unsafe "fopen" fopen :: CString -> CString -> IO (Ptr CFile) That's the easy part. Now my problem was that I had to find a way to automatically close this FILE * when it isn't used anymore, in order not to leak FILE structures (and thus fds, etc). A finalizer is typically what I need, but unfortunately, a finalizer has a very strict shape: type FinalizerPtr a = FunPtr (Ptr a -> IO ()) That is, a finalizer can only be a pointer to a foreign function, and the foreign function itself needs a quite specific shape. And then I discovered Foreign.Concurrent, which allows one to associate a plain Haskell IO action to a pointer. The 'Foreign.Concurrent' name is a bit misleading to me; it seems this module is named so because it needs concurrency itself, rather than providing stuff for concurrency. So, in the end, I've got this code: import Foreign import Foreign.C import qualified Foreign.Concurrent as FC ... data PlayerStruct type Player = ForeignPtr PlayerStruct ... foreign import ccall unsafe "dd_newPlayer_file" dd_newPlayer_file :: Ptr CFile -> Ptr ImageStruct -> IO (Ptr PlayerStruct) foreign import ccall unsafe "&dd_destroyPlayer" destroyPlayerFinal :: FunPtr (Ptr PlayerStruct -> IO ()) foreign import ccall unsafe "fopen" fopen :: CString -> CString -> IO (Ptr CFile) foreign import ccall unsafe "fclose" fclose :: Ptr CFile -> IO CInt ... mkFinalizedPlayer :: Ptr PlayerStruct -> IO Player mkFinalizedPlayer = newForeignPtr destroyPlayerFinal newPlayerFile :: FilePath -> Image -> IO Player newPlayerFile path image = do withCString path $ \cpath -> do withCString "rb" $ \cmode -> do file <- throwErrnoIfNull "fopen: " (fopen cpath cmode) withForeignPtr image $ \ptr -> do player <- dd_newPlayer_file file ptr >>= mkFinalizedPlayer FC.addForeignPtrFinalizer player (fclose file >> return ()) return player So I'm adding the "usual" finalizer, and with the help of Foreign.Concurrent, I can add a second free-form one (fclose file >> return ()), in order to close the file I opened at an appropriate time. I'm looking forward hearing about other people's opinions, and wether this is a correct solution to the initial problem or not. I think there is another way to solve this, which is to provide the finalizer still in haskell code, but export the haskell code using FFI, so that I can use it as a plain, normal finalizer. I'm still unsure about this. Cheers, Maxime

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Some lambda identities
by Yitzchak Gale 09 Oct '07

09 Oct '07

While playing with @pl on #haskell, I noticed some weird and surprising lambda identities. For example: let {c = (.); c4 = c c c c} Then we have: c c4 == c c4 c c c c Proof: Repeatedly apply the identity: (*) x (y z) == c x y z More stuff: c c2 == c4, c c3 == c7, but c cn does not appear to be reducible for n>3. c7 c3 == c3 c7 c c c You get a lot more interesting stuff when you throw flip into the mix. The identity (*) is actually a semi-associativity condition that makes the entire lambda calculus into a semi-monoid. Apparently with very interesting structure. Anyone know more about these things? Thanks, Yitz

1 2

EnableGUI hack changes working directory
by apfelmus 09 Oct '07

09 Oct '07

Hello, the EnableGUI hack for getting a window in GHCi on Mac OS X (10.3.9 for me) unexpectedly changes the working directory for the running ghci [...] apfelmus$ ghci -i../../Hackage/SOE/src [...] GHC Interactive, version 6.6.1, for Haskell 98. *Main> :! pwd /Users/apfelmus/Documents/Programmierung/Haskell/Graphics/Kunst *Main> enableGUI >> run lambda Loading package OpenGL-2.2.1 ... linking ... done. Loading package GLFW-0.1 ... linking ... done. *Main> :! pwd /opt/local/lib/ghc-6.6.1 The effect is that reloading the Main module now fails *Main> :r <no location info>: can't find file: Main.hs Any solution? Btw, using enableGUI exactly once at the beginning of the ghci session doesn't work *Main> enableGUI <interactive>: ../../Hackage/SOE/src/EnableGUI.o: unknown symbol `_CPSEnableForegroundOperation' unless ghci is started with the -framework Carbon option. But then, the working directory remains fine, so I guess the change of working directory bug happens while dynamically loading frameworks/shared libraries. Is there a way to automate ghci ... EnableGUI -framework Carbon and typing enableGUI at the first prompt? Something like a custom shell script ghci+gui or even ghci --with-gui ? Regards, apfelmus

2 1

unsafePerformIO: are we safe?
by Jorge Marques Pelizzoni 09 Oct '07

09 Oct '07

Hi, all! This is a newbie question: I sort of understand what unsafePerformIO does but I don't quite get its consequences. In short: how safe can one be in face of it? I mean, conceptually, it allows any Haskell function to have side effects just as in any imperative language, doesn't it? Doesn't it blow up referential transparency for good? Is there anything intrinsic to it that still keeps Haskell "sound" no matter what unsafePerformIO users do (unlikely) or else what are the guidelines we should follow when using it? Thanks in advance. Cheers, Jorge.

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Type-level arithmetic
by Andrew Coppin 09 Oct '07

09 Oct '07

I've seen quite a few people do crazy things to abuse the Haskell type system in order to perform arithmetic in types. Stuff the type system was never ever intended to do. Well I was just wondering... did anybody ever sit down and come up with a type system that *is* designed for this kind of thing? What would that look like? (I'm guessing rather complex!)

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ANNOUNCE: binary 0.4: high performance, pure binary parsing and serialisation
by Don Stewart 09 Oct '07

09 Oct '07

Binary: high performance, pure binary encoding, decoding and serialisation for Haskell ---------------------------------------------------------------------- The Binary Strike Team is pleased to announce release 0.4 of Data.Binary, the pure, efficient binary serialisation library for Haskell, now available from Hackage: tarball: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary/0.4 darcs: darcs get http://darcs.haskell.org/binary The 'binary' package provides efficient serialisation of Haskell values to and from lazy ByteStrings, and a low level layer for high performance decoding and decoding binary data. ByteStrings constructed this way may then be written to disk, written to the network, or further processed (e.g. stored in memory directly, or compressed in memory with zlib or bzlib). *Very* high performance can be expected, with throughput over 1G/sec observed in practice (good enough for most networking scenarios, we suspect). Encoding and decoding are achieved by the functions: encode :: Binary a => a -> ByteString decode :: Binary a => ByteString -> a which mirror the read/show functions. Convenience functions for serialising to disk are also provided: encodeFile :: Binary a => FilePath -> a -> IO () decodeFile :: Binary a => FilePath -> IO a To serialise your Haskell data, all you need do is write an instance of Binary for your type. For example, suppose in an interpreter we had the data type: import Data.Binary import Control.Monad data Exp = IntE Int | OpE String Exp Exp We can serialise this to bytestring form with the following instance: instance Binary Exp where put (IntE i) = putWord8 0 >> put i put (OpE s e1 e2) = putWord8 1 >> put s >> put e1 >> put e2 get = do tag <- getWord8 case tag of 0 -> liftM IntE get 1 -> liftM3 OpE get get get The binary library has been heavily tuned for performance, particularly for writing speed. On average, Data.Binary is 10x faster than NewBinary, and has the advantage of a pure interface, and bytestring return values. Binary was developed by a team of 8 during the Haskell Hackathon, Hac 07, in January 2007, and this maintainence release has taken place during the second hackathon. Binary is portable, using the foreign function interface and cpp, and has been tested with Hugs and GHC. Happy hacking! -- Don

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Confusing type specialisation in ghci
by David Carter 09 Oct '07

09 Oct '07

Hello, Can someone elucidate to me this behaviour from ghci 6.6.1? Why is the type of sqlist specialised to Integer? Prelude> let sq x = x * x Prelude> :t sq sq :: (Num a) => a -> a Prelude> sq 2.5 6.25 Prelude> :t map sq map sq :: (Num a) => [a] -> [a] Prelude> map sq [2.5] [6.25] This is all as I would expect so far, but: Prelude> let sqlist = map sq Prelude> :t sqlist sqlist :: [Integer] -> [Integer] And indeed, I get Prelude> sqlist [2.5] <interactive>:1:8: No instance for (Fractional Integer) ... etc Thanks! David -- The Wellcome Trust Sanger Institute is operated by Genome Research Limited, a charity registered in England with number 1021457 and a company registered in England with number 2742969, whose registered office is 215 Euston Road, London, NW1 2BE.

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Re: [Haskell] Converting Double from String
by Brent Yorgey 09 Oct '07

09 Oct '07

(moving to haskell-cafe...) On 10/8/07, Johan Tibell <johan.tibell(a)gmail.com> wrote: > > On 10/8/07, bjornie <bjorn.wikstrom(a)gmail.com> wrote: > > > > Hi! I'm kind of new to functional programming with Haskell, I'm reading > a > > university course you see. Now I'm having some problems with one of our > lab > > assignments, we're supposed to write a program handling numeric > operations > > (addition, multiplication, and some functionality like sin and cos). > > > > I've made a data type; > > data Expr = Num Double | Add Expr Expr | Mul Expr Expr... > > > > I.e for representing the expression: > > 2.4 * (4.0 + 1.5) > > The Haskell representation is: > > Mul (Num 2.4) (Add (Num 4.0) (Num 1.5)) > > > > My problem is to convert a String to an expression like the one above. > > I.e the String: "2.4*(4.0+1.5)" > > > > If we would use plain Integers there would not be a problem, since the > > Prelude function 'digitToInt' :: Char -> Int does nearly what I would > like > > to do. But I'm stuck on the Doubles! How can I solve this in a smooth > way? > > You can use read. Here's a session from GHC's interpreter: > > $ ghci > Prelude> read "1.0" :: Double > 1.0 > > Since read is polymorphic (i.e. works for different types) you need to > specify the type of the result you want using ":: Double". ...although if you have something like Num (read x), you don't need the :: Double since the typechecker can figure it out (the Num constructor has to take a Double). Note that you would use the 'read' function to read integer values, too; digitToInt is just for dealing with single Chars, not entire integral values. The read function takes a String representation of anything that is "readable" (any type that is an instance of the Read type class) and converts it into a value of the requested type, so it works for Doubles, Ints, Integers, lists, and so on... Also, if I were you I'd use something other than "Num" as the name of the data constructor for numeric values in your Expr type. Num is already used for something else (it's the name of the general numeric type class), and although it's legal to have the same name for both, it might lead to confusion, especially when dealing with error messages. Good luck with learning Haskell and with your course! If you need more help there's also the #haskell IRC channel on freenode.net<http://haskell.org/haskellwiki/IRC_channel>, where you'll find lots of friendly people who would be glad to answer your questions. -Brent

1 0

Data types, opengl display loop and readIORef/writeIORef
by bbrown 09 Oct '07

09 Oct '07

This is more an aesthetics question, I have a simple opengl application that has a singleton like object I need to pass around to the display loop and possibly to the keyboard action handler. (pseudo code) data SimpleMech = SimpleMech { mechPos :: !MVector, mechRot :: !MVector } deriving (Read) mainMech = initMech (0, 0, 0) (0, 0, 0) mainMechRef <- newIORef(mainMech) displayCallback $= displayGameLoop mainMechRef rotateMech :: (Double, SimpleMech) -> SimpleMech rotateMech (angle, mech) = -- Calculate a new rotation let (xrot,yrot,zrot) = (mechRot mech) newyrot = newyrot + angle in SimpleMech { mechRot = (xrot, newyrot, zrot), mechPos = (mechPos mech) } displayGameLoop mechRef = do mech <- get mechRef mechRef $= (rotateMech (0.1, mech)) ... ... For some reason in that code above, the "mechRef" doesnt update the value I want to change. It is like I am getting an instance of the initial valyue back. Also, when I use writeIORef, eg writeIORef mechRef (rotateMech (0.1, mech) I get stack overflow exceptions. The displayGameLoop will get called an infinite amount of times, so I think I have to be careful there. But I wonder why the reference isn't getting updated? Is this an approach you would take, eg passing "mechRef" around, a singleton game object. -- Berlin Brown [berlin dot brown at gmail dot com]

2 2

cabal library dependencies
by Nehir Sonmez 08 Oct '07

08 Oct '07

Hi All, To test some programs written in CABAL, I need to use a server (where I do not have root access) with GHC 6.6.1 (in-place) installed. However, many programs need more libraries that have to be installed, (such as binary, or derive) for which, even when I use ./Setup configure --prefix=something, 1) After the ./Setup build, it can not do ./Setup install correctly because I am not root, ie. Installing: /usr/local/lib/derive-0.1/ghc-6.6.1 & /usr/local/bin derive-0.1... Setup: /usr/local/lib/derive-0.1: createDirectory: permission denied (Permission denied) 2) But the .o and the .a files are created. However possibly due to (1), I can not make the program see the library, so in the end when I try configuring the program, I get (for example): Setup: cannot satisfy dependency derive-any possible solutions I could think of were: 1) LD_LIBRARY_PATH 2) use configure with a --with flag 3) copying the library files to the prefix directory of the program I ultimately want to run 4) The -L flag and still no luck, so here I am asking for your help... Thanks a lot in advance, Sincerely, -Nehir Sonmez

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Re: [Haskell] reading from stdin
by Reinier Lamers 08 Oct '07

08 Oct '07

Axel Simon wrote: >a) is the behaviour I want, but unfortunately for platform d) >b) must be due to ghci and Hugs having different opinions on whether >stdin should be line buffered or unbuffered >c) this is weird >d) this is broken > You can probably unify the behaviors of platforms a, b and c by calling hSetBuffering on stdin to turn off buffering. Reinier

2 1

Typechecker to GADT: the full implementation of a typed DSL
by oleg＠pobox.com 08 Oct '07

08 Oct '07

Pasqualino 'Titto' Assini wrote: > I am trying to write an interpreter for a little functional language but I am > finding very problematic to dynamically create a typed representations of the > language terms. > > > The problem is to write a function that converts between Exp and Term > > t as in: > > > > > test :: Term Double > > > test = typecheck $ EApp (EPrim "inc") (EDouble 10.0) The specification leaves out a few important details. The typechecker can't be a total function: what happens when typechecking this code? EApp (EDouble 10.0) (EPrim "inc") The error has to be reported somehow, and it is important to know how. There are basically two choices. One is to keep the above interface > test :: Term Double > test = typecheck $ EApp (EPrim "inc") (EDouble 10.0) which means using Template Haskell and means letting TH reporting type errors (with no hope of catching them). The second choice is `typing dynamic typing'. Which means we can't write a typechecker with the signature "Exp -> Term t". Rather, we have to write typecheck :: Exp -> (forall t. Typ t -> Term t -> w) -> Maybe w or, equivalently, using an existential data MostlyStatic = forall t. MostlyStatic (Typ t, Term t) typecheck :: Exp -> Maybe MostlyStatic Although both MostlyStatic and Exp are `untyped', the latter is `deeply' untyped, whereas the former is only untyped at the surface. In the case of MostlyStatic, the term is built out of typed components, and the type is erased only at the end. These choices aren't contradictory: using TH, we can convert MostlyStatic to the proper Haskell term. Because the term has already been typechecked, we are guaranteed the absence of errors during such a conversion. For today, perhaps the implementation of the second choice will be sufficient. This is the complete representation of a typed DSL given in the untyped AST form. We typecheck a term. We either report a type error, or evaluate the typed term and then report result, if can be shown. We also show the inferred type of the result. Examples. Let's assume the following terms (not all of them well-typed) te1 = EApp (EPrim "inc") (EDouble 10.0) te2 = EApp (EDouble 10.0) (EPrim "inc") te3 = EApp (EApp (EPrim "+") (EApp (EPrim "inc") (EDouble 10.0))) (EApp (EPrim "inc") (EDouble 20.0)) te4 = EApp (EPrim "rev") te3 te5 = EApp (EPrim "rev") (EApp (EPrim "show") te3) *Eval> teval te1 "type Double, value 11.0" *Eval> teval te2 "Type error: Trying to apply not-a-function: Double" *Eval> teval te3 "type Double, value 32.0" *Eval> teval te4 "Type error: incompatible type of the application: (String->String) and Double" *Eval> teval te5 "type String, value 0.23" The complete code follows {-# OPTIONS -fglasgow-exts #-} -- Typechecker to GADT -- Implementing a typed DSL with the typed evaluator and the -- the typechecker from untyped terms to typed ones module Eval where -- Untyped terms (what I get from my parser): data Exp = EDouble Double | EString String | EPrim String | EApp Exp Exp deriving (Show) -- Typed terms: data Term a where Num :: Double -> Term Double Str :: String -> Term String App :: Term (a->b) -> Term a -> Term b Fun :: (a->b) -> Term (a->b) -- Typed evaluator eval :: Term a -> a eval (Num x) = x eval (Str x) = x eval (Fun x ) = x eval (App e1 e2) = (eval e1) (eval e2) -- Types and type comparison data Typ a where TDouble :: Typ Double TString :: Typ String TFun :: Typ a -> Typ b -> Typ (a->b) data EQ a b where Refl :: EQ a a -- checking that two types are the same. If so, give the witness eqt :: Typ a -> Typ b -> Maybe (EQ a b) eqt TDouble TDouble = Just $ Refl eqt TString TString = Just $ Refl eqt (TFun ta1 tb1) (TFun ta2 tb2) = eqt' (eqt ta1 ta2) (eqt tb1 tb2) where eqt' :: (Maybe (EQ ta1 ta2)) -> Maybe (EQ tb1 tb2) -> Maybe (EQ (ta1 -> tb1) (ta2 -> tb2)) eqt' (Just Refl) (Just Refl) = Just Refl eqt' _ _ = Nothing eqt _ _ = Nothing instance Show (Typ a) where show TDouble = "Double" show TString = "String" show (TFun ta tb) = "(" ++ show ta ++ "->" ++ show tb ++ ")" -- Type checking data MostlyStatic = forall t. MostlyStatic (Typ t, Term t) -- Typing environment type Gamma = [(String,MostlyStatic)] -- Initial environment (the types of primitives) env0 = [("rev", MostlyStatic (TFun TString TString, Fun (reverse::String->String))), -- sorry, no polymorphism! ("show", MostlyStatic (TFun TDouble TString, Fun (show::Double->String))), ("inc", MostlyStatic (TFun TDouble TDouble, Fun ((+ (1.0::Double))))), ("+", MostlyStatic (TFun TDouble (TFun TDouble TDouble), Fun ((+)::Double->Double->Double))) ] typecheck :: Gamma -> Exp -> Either String MostlyStatic -- literals typecheck _ (EDouble x) = Right $ MostlyStatic (TDouble, Num x) typecheck _ (EString x) = Right $ MostlyStatic (TString, Str x) typecheck env (EPrim x) = maybe err Right $ lookup x env where err = Left $ "unknown primitive " ++ x typecheck env (EApp e1 e2) = case (typecheck env e1, typecheck env e2) of (Right e1, Right e2) -> typechecka e1 e2 (Left err, Right _) -> Left err (Right _, Left err) -> Left err (Left e1, Left e2) -> Left (e1 ++ " and " ++ e2) -- typecheck the application typechecka (MostlyStatic ((TFun ta tb),e1)) (MostlyStatic (t2,e2)) = typechecka' (eqt ta t2) tb e1 e2 where typechecka' :: Maybe (EQ ta t2) -> Typ tb -> Term (ta->tb) -> Term t2 -> Either String MostlyStatic typechecka' (Just Refl) tb e1 e2 = Right $ MostlyStatic (tb,App e1 e2) typechecka' _ tb e1 e2 = Left $ unwords ["incompatible type of the application:", show (TFun ta tb), "and", show t2] typechecka (MostlyStatic (t1,e1)) _ = Left $ "Trying to apply not-a-function: " ++ show t1 -- tests te1 = EApp (EPrim "inc") (EDouble 10.0) te2 = EApp (EDouble 10.0) (EPrim "inc") te3 = EApp (EApp (EPrim "+") (EApp (EPrim "inc") (EDouble 10.0))) (EApp (EPrim "inc") (EDouble 20.0)) te4 = EApp (EPrim "rev") te3 te5 = EApp (EPrim "rev") (EApp (EPrim "show") te3) -- typecheck-and-eval teval :: Exp -> String teval exp = either (terror) (ev) (typecheck env0 exp) where terror err = "Type error: " ++ err ev (MostlyStatic (t,e)) = "type " ++ show t ++ ", value " ++ (tryshow (eqt t TString) (eqt t TDouble) (eval e)) tryshow :: Maybe (EQ t String) -> Maybe (EQ t Double) -> t -> String tryshow (Just Refl) _ t = t tryshow _ (Just Refl) t = show t tryshow _ _ _ = "<fun>"

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SOE/GLFW compilation problems
by Joel Stanley 08 Oct '07

08 Oct '07

Hi folks, I've recently been able to get the SOE/GLFW code up and running from http://haskell.org/soe/software1.htm and things are working well under GHCi under Mac OS X. However, when attempting to compile a simple graphics script using ghc, I'm getting some unexpected link errors: $ ghc -o mygfx mygfx.hs -iSOE/src compilation IS NOT required /usr/bin/ld: Undefined symbols: _SOE_Blue_closure _SOE_Red_closure _SOE_closeWindow_closure _SOE_drawInWindow_closure _SOE_ellipse_closure _SOE_getKey_closure _SOE_openWindow_closure _SOE_polyline_closure _SOE_runGraphics_closure _SOE_withColor_closure collect2: ld returned 1 exit status I'm quite new to Haskell, so I'm not sure if there was something special I had to do in order to get the code for the closures generated by ghc. Any ideas? Thanks in advance. Joel

2 1

Extract source code from literate Haskell (LHS) files
by Peter Verswyvelen 07 Oct '07

07 Oct '07

This is of course very easy to do manually, but does a command line tool exist for extracting source code from literate Haskell files? Thanks, Peter

7 6

GLFW for WinHugs
by Peter Verswyvelen 07 Oct '07

07 Oct '07

The latest version of SOE comes with a wrapper for a nice GLFW library. This library comes with a demo of a 3D bouncing "Amiga" ball so it must be the best library in the world ;-) ;-) Since I'm letting my students play with WinHugs, I would prefer to have a WinHugs compatible version of that library. I tried to convert it, but I got stuck when ffihugs complained about not finding RTS.h, which seems to be a GHC-only include file. Would it be possible to convert this library to WinHugs? I guess similar work has been done for other libraries, so any hints are welcome. Thanks, Peter

2 1

Puzzled
by Peter Verswyvelen 06 Oct '07

06 Oct '07

The following code, when compiled with GHC 6.6.1 --make -O gives a stack overflow when I enter 1000000 as a command line argument: (please don't look at the efficiency of the code, it can of course be improved a lot both in time performance and numeric precision...) import System leibnizPI :: Integer -> Double leibnizPI n = sum (map leibnizTerm [0..n]) where leibnizTerm n = let i = fromIntegral n in 4 * (((-1) ** i) / (2*i+1)) main = do args <- getArgs let n = read (head args) print (leibnizPI n) However, if I replace main = print (leibnizPI 1000000) is does not stack overflow. Now, if I leave the original main, but replace sum in leibnizPI by mysum xs = aux 0 xs where aux s (x:xs) = aux (s+x) xs aux s [] = s then I don't get a stack overflow. However, I do get a stack overflow when I compile it without -O, in all cases. This puzzles me. I don't see any non-tail calls in my code... I guess it has to do with strictness? http://www.haskell.org/haskellwiki/Performance/Strictness Why isn't it possible to annotate strictness on the type signature in Haskell as in Clean? Is this on the TODO list? Many thanks, Peter

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more basic articles on functional programming using haskell
by Vikrant 06 Oct '07

06 Oct '07

part I -> http://www.ias.ac.in/resonance/August2007/p27-48.pdf part II -> http://www.ias.ac.in/resonance/September2007/p40-63.pdf == Vikrant

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Hackathon underway
by Don Stewart 06 Oct '07

06 Oct '07

Just to let everyone know , the Haskell Hackathon is underway (the 2nd this year), in Freiburg, Germany, and library and infrastructure code is being worked on furiously -- Cabal and related tools in particular. You can follow all the action from planet.haskell.org, and from the hackathon site, including photos. http://www.haskell.org/haskellwiki/Hac_2007_II#Photos_.2F_blogs -- Don

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Re: [Haskell-cafe] Curry and uncurry
by PR Stanley 05 Oct '07

05 Oct '07

No, still no idea! I tried curry f where f :: (Num a) => (a, a) -> a and it didn't like it. For some reason I'm finding this a little chalenging. Thanks, Paul At 17:03 03/10/2007, you wrote: >On 10/3/07, PR Stanley ><<mailto:prstanley@ntlworld.com>prstanley(a)ntlworld.com> wrote: >I didn't even know about the curry and uncurry functions. I'm not >looking for the answer but some guidance would be much appreciated. >Thanks, Paul > > >You can look at the types without seeing the implementation, too. >Just start up GHCI and type: > > :t curry > >or > > :t uncurry > >Justin

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Space and time leaks
by Ronald Guida 05 Oct '07

05 Oct '07

I need some help with space and time leaks. I know of two types of space leak. The first type of leak occurs when a function uses unnecessary stack or heap space. GHCi> sum [1..10^6] *** Exception: stack overflow Apparently, the default definition for "sum" has a space leak. I can define my own sum in terms of strict foldl ... > sum' xs = foldl' (+) 0 xs ... and it doesn't overflow the stack. GHCi> sum' [1..10^6] 500000500000 (0.27 secs, 112403416 bytes) GHCi> sum' [1..10^7] 50000005000000 (2.73 secs, 1161223384 bytes) GHCi> sum' [1..10^8] 5000000050000000 (27.83 secs, 11645261144 bytes) I think there's still a space leak; I don't understand why GHCi using 10^8, 10^9, 10^10 bytes of memory for these calculations. The other type of space leak I know of is when I have a chunk of data in memory that I no longer need, such that the data is still being referenced somewhere. Since, in theory, I might still access the data, the garbage collector can't reclaim the memory. I'm not sure how to construct an example though. Regarding time leaks, I only know of one kind of leak. If I have a calculation that accumulates data over time, and I don't ask for any results until the end, then, due to laziness, that calculation might accumulate a chain of unevaluated thunks. When I get the the end and "demand" the final result, I have to wait for it while the RTS evaluates a long chain of thunks. The chain of unevaluated thunks is a space leak. The time leak occurs because the capture process and the accumulate process are supposed to be interleaved, such that I perform some computations after I capture each piece of data. If I have to wait around at the end, then it means the capture process and the accumulate process happened in sequence. As far as I know, every time leak has a companion space leak; it's not possible to create a time leak without a space leak to go with it. Is this really true? Now for the hard questions. 1. How do I go about detecting space and time leaks? 2. Once I find a leak, how do I fix it? 3. Are there any programming techniques I can use to avoid leaks? -- Ron

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The "Exp -> Term a" problem (again), how to dynamically create (polymorphic) typed terms in Haskell ??
by Pasqualino 'Titto' Assini 05 Oct '07

05 Oct '07

Hi, I am trying to write an interpreter for a little functional language but I am finding very problematic to dynamically create a typed representations of the language terms. I have googled around and found a few solutions but none seem to solve the problem. This is the example code: > {-# OPTIONS -fglasgow-exts #-} > module Eval where These are my untyped terms (what I get from my parser): > data Exp = EDouble Double | EString String | EPrim String | EApp Exp Exp deriving (Show) And these are the typed terms: > data Term a where > Num :: Double -> Term Double > Str :: String -> Term String > App :: Term (a->b) -> Term a -> Term b > Fun :: (a->b) -> Term (a->b) The problem is to write a function that converts between Exp and Term t as in: > test :: Term Double > test = typecheck $ EApp (EPrim "inc") (EDouble 10.0) So this is the conversion function: > class TypeCheck t where > typecheck :: Exp -> Term t A few primitives: > instance TypeCheck (String->String) where > typecheck (EPrim "rev") = Fun reverse > typecheck (EPrim "show") = Fun show > instance TypeCheck (Double->Double) where > typecheck (EPrim "inc") = Fun ((+1) :: Double -> Double) > instance TypeCheck (Double->String) where > typecheck (EPrim "show") = Fun show > instance TypeCheck Double where > typecheck (EDouble x) = Num x > instance TypeCheck String where > typecheck (EString x) = Str x The problem arises in the conversion of the function application (EApp). It does not seem to be possible to define "typecheck" on EApp in a generic way and is also not possible to distinguish between the different cases: > typecheck (EApp f a) = App (typecheck f :: Term (String->String)) (typecheck a:: Term String) The following pattern overlaps the previous one: > typecheck (EApp f a) = App (typecheck f :: Term (Double->String)) (typecheck a:: Term Double) To avoid this problem I could split my untyped terms in different data types as in: data EDouble = EDouble Double data App a b c = App a b c ... and define TypeCheck separetely on every data type. However, in that case what would be the type of my parser?? parser :: String -> ?? Any suggestion woud be very welcome indeed, titto

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Re: [Haskell] reading from stdin
by Stefan O'Rear 05 Oct '07

05 Oct '07

On Thu, Oct 04, 2007 at 05:46:09PM +0100, Axel Simon wrote: > Hi, > > I'm trying to continuously output data to a file handle while reading > single characters from the user to adjust the speed at which things are > output. I'm interested to get this to work in Hugs on Windows. > I successfully used the following function ghci under Mac OS: > > {{{ > getUserInput :: IO (Maybe Char) > getUserInput = do > hasInput <- hReady stdin > if hasInput then liftM Just (hGetChar stdin) else return Nothing > }}} > > This function returns a character to me if there's one available. > If anybody could give me a hint how to get this working in Hugs under > Windows, please tell me. Something in the 'win32' package, or maybe the FFI. MSDN will be very handy. > P.S.: Sorry to post to haskell@..., but nothing else seemed to match. This is a thread for haskell-cafe@. Stefan

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