- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] do
by PR Stanley 15 Oct '07

15 Oct '07

Thanks for the very clear explanation. More questions: What is the role of ">>"? How is ">>" different to ">>="? I am aware that ">>=" is used for sequencing parsers but that's all I know about it. Thanks, Paul At 22:28 13/10/2007, you wrote: >On 10/13/07, PR Stanley <prstanley(a)ntlworld.com> wrote: > > 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? > >In this example, do doesn't do anything. do doesn't do anything to a >single expression (well, I think it enforces that its return value is >a monad...). It's only when you give it multiple expressions that it >"rewrites" them into more formal notation. For example: > > do putStrLn "bla" > putStrLn "blah" > >Will be rewritten into: > > putStrLn "bla" >> putStrLn "blah" > >It introduces a block of "sequential actions" (in a monad), to do each >action one after another. Both of these (since they're equivalent) >mean print "bla" *and then* print "blah". > >do also allows a more imperative-feeling variable binding: > > do line <- getLine > putStr "You said: " > putStrLn line > >Will be rewritten into: > > getLine >>= (\line -> putStr "You said: " >> putStrLn line) > >Looking at the do notation again: execute getLine and bind the return >value to the (newly introduced) variable 'line', then print "You said: >", then print the value in the variable line. > >You can think of the last line in the block as the return value of the >block. So you can do something like: > > do line <- do putStr "Say something: " > getLine > putStr "You said: " > putStrLn line > >In this example it's kind of silly, but there are cases where this is useful. > >Luke

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Hosting of Haskell project
by Magnus Therning 15 Oct '07

15 Oct '07

I recently had reason to do some encoding-related coding and noticed that Haskell was somewhat lacking (I could only find code for base64, on the other hand there are two implementations of it :-). I've almost reached a state where I wouldn't be ashamed of sharing the code so I looked into my options of free hosting. It seems I only have one option for publishing the code: - Request a project on code.haskell.org. I could only find one option for a "homepage" for the project: - Create a page on the Wiki. There seems to be no option when it comes to tracking bugs. :-( I could also not locate any option for publishing haddock pages. :-( Have I missed anything? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus The British have "the perfect temperament to be hackers--technically skilled, slightly disrespectful of authority, and just a touch of criminal behavior". -- Mary Ann Davidson, Oracle's Security Chief

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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 14 Oct '07

14 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 14 Oct '07

14 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 14 Oct '07

14 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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