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research topics
by Henrique Ferreiro 31 Dec '07

31 Dec '07

Hi haskellers! I am a postgraduate student interested in functional programming and Haskell. I would appreciate if someone could elaborate on which are the current hot topics of research in this area, and related to Haskell, of course. In particular, I am interested in compiler implementation. Thanks in advance. -- Henrique Ferreiro <henrique.ferreiro(a)gmail.com>

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Re: [Haskell-cafe] How to convert number types.
by L.Guo 31 Dec '07

31 Dec '07

Ah, thanks, it is my fault, for mis-understanding the hint from GHC. And what is the difference between fromIntegral and fromInteger ? ------------------ L.Guo 2008-01-01 ------------------------------------------------------------- From: Mark T.B. Carroll At: 2008-01-01 01:42:37 Subject: Re: [Haskell-cafe] How to convert number types. Perhaps fromIntegral does what you want? Mark

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How to convert number types.
by L.Guo 31 Dec '07

31 Dec '07

Hi MailList Haskell-Cafe: I am a new haskeller. And was farmilar with C. When tring to do some calculate, like this: input = 5 :: Int factor = 1.20 :: Float output = factor ** (toFloat input) I found that I do not know any function could do just what 'toFloat' should do. What should I do then ? Regards -------------- L.Guo 2008-01-01

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A "commutative diagram" conjecture about applicative functors
by Robin Green 31 Dec '07

31 Dec '07

I am proving various statements relating to applicative functors, using the Coq proof assistant (I am considering only Coq terms, which always terminate so you don't have to worry about _|_). However, I'm not sure how to go about proving a certain conjecture, which, translated back into Haskell and made more specific to make it easier to think about, looks like this (assuming Control.Applicative and Control.Arrow are imported): "For all applicative functors: \f x -> fmap second f <*> fmap ((,) (0::Int)) x is equivalent to \f x -> fmap ((,) (0::Int)) (f <*> x)" McBride and Patterson give four laws for applicative functors in their paper "Functional Pearl: Applicative programming with effects". Can these laws be used to prove this conjecture for all applicative functors satisfying those laws? -- Robin P.S. I realise this might not look like a "commutative diagram conjecture", but as I said, I've made it more specific - my actual conjecture is more general, but hopefully if I can understand how to prove the specific one, I'll be able to prove my real one in short order.

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Getting links to Haddock documentation in Hackage
by Adam Langley 30 Dec '07

30 Dec '07

Some packages[1] have links on their Hackage pages to the haddock generated documentation for each exported module[2]. However, many[3] don't. What's the secret to getting this generated documentation to work with Hackage? Even packages for which `runhaskell Setup.lhs haddock` runs don't (always) get the magic links. Cheers, AGL [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bzlib-0.4.0.1 [2] http://hackage.haskell.org/packages/archive/bzlib/0.4.0.1/doc/html/Codec-Co… [3] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tar-0.1.1.1 -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org 650-283-9641

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The semantics of constructor patterns
by Cristian Baboi 30 Dec '07

30 Dec '07

In section 4.3.3., chapter 4: Structured types and the semantics of pattern-matching, by S.Peyton Jones and Philip Wadler, there is this equation: Eval[[\(s p1 p2 ... pt).E]] (s a1 a2 ...at) = Eval[[\p1 ... \pt.E]] a1 ... at The text say: "To apply \(s p1 ... pt).E to an argument A we first evaluate A to find out what sort of object it is." "Finally, if A was built with the same constructor as the patern, then the first rule applies." What I don't get it : (s a1 a2 ... at) must be the value of A in the semantic domain. Let call that value a. Then how can one know if a was built with (s a1 a2 ... at) and not with (egg b1 b2) ?

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easy cabal tasks
by Duncan Coutts 30 Dec '07

30 Dec '07

People are often unsure about where they can help out with Cabal. One thing we can do to make that easier is to point out smaller simpler tasks that people might like to have a go at. We have a list of tasks that are marked as easy or very easy: http://hackage.haskell.org/trac/hackage/report/13 This list is also linked from the hackage trac web page ("Easy tickets") http://hackage.haskell.org/trac/hackage/ For people reporting bugs and working with the hackage/cabal trac, look out for tasks that you think could be done by a new or relatively new contributor and mark the bug's difficulty appropriately. Please also give plenty of detail on what the solution should involve and pointers to other documentation as appropriate. Duncan

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Type-system fun: a type-safe embedding of System F Lambda Calculus into Haskell
by Ryan Ingram 30 Dec '07

30 Dec '07

(The full code used for this message is available at http://ryani.freeshell.org/haskell/systemf.hs) System F is the polymorphically typed lambda calculus. It is strongly typed but allows polymorphic functions like "id". id is represented as follows: > eId = EGamma "A" (ELam "x" TVar EVar) which represents > \A -> \x:A -> x that is, a function that takes a type and a single argument of that type and returns its argument. This embedding only allows well-typed terms to be constructed; the Haskell typechecker verifies that a term is well-typed. I include a full compiler which converts terms into Haskell functions (albeit not-very-efficient ones). A sample session with GHCi follows: GHCi, version 6.8.1: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Ok, modules loaded: SystemF. Prelude SystemF> eId \A a:A -> a Prelude SystemF> :t eId eId :: NExpr (a :-> (a -> a)) Prelude SystemF> :t eBottom eBottom :: NExpr (a :-> a) Prelude SystemF> eBottom \A -> fix (\a:A -> a) Prelude SystemF> putStrLn $ showExprExplicit eBottom (EGamma "A" (EFix (ELam "a" TVar EVar))) Prelude SystemF> tInt I# Prelude SystemF> :t (TPrim "I#" (undefined :: Integer)) (TPrim "I#" (undefined :: Integer)) :: TypRep Integer ts Prelude SystemF> :t tInt tInt :: TypRep Integer ts Prelude SystemF> eK \A B a:A b:B -> a Prelude SystemF> eFact fix (\rec:(I#->I#) z:I# -> (\A -> if#) I# (eq# z 0) 1 (times# z (rec (minus# z 1)))) Prelude SystemF> :t eFact eFact :: NExpr (Integer -> Integer) Prelude SystemF> compile eFact 5 120 Prelude SystemF> putStrLn $ showExprExplicit eFat <interactive>:1:28: Not in scope: `eFat' Prelude SystemF> putStrLn $ showExprExplicit eFact (EFix (ELam "rec" (TArrow (TPrim I#) (TPrim I#)) (ELam "z" (TPrim I#) (EAp (EAp (EAp (ETypAp (EGamma "A" (EPrim if#)) (TPrim I#)) (EAp (EAp (EPrim eq#) EVar) (EPrim 0))) (EPrim 1)) (EAp (EAp (EPrim times#) EVar) (EAp (EPush EVar) (EAp (EAp (EPrim minus#) EVar) (EPrim 1)))))))) Prelude SystemF> :t (whnf (EAp eFact (EPrim "6" 6))) (whnf (EAp eFact (EPrim "6" 6))) :: Expr Integer NDecl NDecl Prelude SystemF> (whnf (EAp eFact (EPrim "6" 6))) if#*** Prelude SystemF> extractPrim (whnf (EAp eFact (EPrim "6" 6))) Just 720

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Specializing classes with classes
by alex 30 Dec '07

30 Dec '07

Hi, I am new to everything Haskell and am stumped on an aspect of Type Classes. I would like to do this: class Foo t where hi :: t -> Bool class Foo t => Bar t where hi x = True But using GHC 6.8.1 on PPC I get this error: `hi' is not a (visible) method of class `Bar' The actual problem is more like this: class Num a => Alg a where (<>) :: Mat m => m -> a -> a -- matrix multiply class Alg v => Vec v where toRow, toColumn :: Mat m => v -> m fromRow, fromColumn :: Mat m => m -> v fromRow = fromColumn . transpose toRow = transpose . toColumn (<>) m v = fromColumn (m <> (toColumn v)) class Alg m => Mat m where transpose :: m -> m (I hope that's readable...) I'm having trouble finding what I need with Google, and SOE doesn't really go into specifics. Thanks for any help... Alex Make the switch to the world's best email. Get the new Yahoo!7 Mail now. www.yahoo7.com.au/worldsbestemail

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encountering another STM problem .. "teething problems" ; ^)
by Galchin Vasili 30 Dec '07

30 Dec '07

Hello, Here is a code snippet: type Resource = TVar Int putR :: Resource -> Int -> STM () putR r i = do { v <- readTVar r; writeTVar r (v + i) } getR :: Resource -> Int -> STM () getR r i = do { v <- readTVar r; if (v < i) then retry else writeTVar r (v-i)} createR :: Int -> STM (Resource) createR i = newTVar i main = do {atomic (r <- (createR 1); putR r 1)} Strangely I am not getting a type error rather a syntax on the "<-" in main. ?? Kind regards, Vasya

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