Different behavior of GHC 6.10.1 and Hugs (Sep 2006)

Hi list, GHC 6.10.1: Prelude :t let f x y = return x == return y in f let f x y = return x == return y in f :: (Eq (m a), Monad m) = a - a - Bool Hugs (Sep 2006): Hugs :t let f x y = return x == return y in f ERROR - Ambiguous type signature in inferred type *** ambiguous type : (Eq (a b),

[Haskell-cafe] Different behavior of GHC 6.10.1 and Hugs (Sep 2006)

Hi list, GHC 6.10.1: Prelude :t let f x y = return x == return y in f let f x y = return x == return y in f :: (Eq (m a), Monad m) = a - a - Bool Hugs (Sep 2006): Hugs :t let f x y = return x == return y in f ERROR - Ambiguous type signature in inferred type *** ambiguous type : (Eq (a b),

[Haskell-cafe] Why there is not standard Monoid instance for ZipList a?

I find the following instance very convenient: import Data.Monoid import Control.Applicative instance Monoid a = Monoid (ZipList a) where mempty = pure mempty mappend = liftA2 mappend

Re: [Haskell-cafe] Monotype error

See our previous discussion on this topic here: http://www.nabble.com/Fwd:-Unification-for-rank-N-types-td23942179.html Thanks, Vladimir On Wed, Oct 14, 2009 at 10:35 PM, Martijn van Steenbergen mart...@van.steenbergen.nl wrote: Dear café, {-# LANGUAGE Rank2Types #-} {-# LANGUAGE

List comprehensions and impredicative rank-N types

Hi, Consider the following definitions: --- {-# LANGUAGE RankNTypes, ImpredicativeTypes #-} foo :: [forall a. [a] - [a]] foo = [reverse] bar :: [a - b] - a - b bar fs = head fs ---

[Haskell-cafe] List comprehensions and impredicative rank-N types

Hi, Consider the following definitions: --- {-# LANGUAGE RankNTypes, ImpredicativeTypes #-} foo :: [forall a. [a] - [a]] foo = [reverse] bar :: [a - b] - a - b bar fs = head fs ---

Fwd: Unification for rank-N types

Forwarding to GH list. -- Forwarded message -- From: Vladimir Reshetnikov v.reshetni...@gmail.com Date: Tue, 9 Jun 2009 16:51:59 +0500 Subject: Re: Unification for rank-N types To: haskell-cafe haskell-c...@haskell.org One more example: This does not type-check

[Haskell-cafe] Unification for rank-N types

Hi, I have the following code: --- {-# LANGUAGE RankNTypes #-} f :: ((forall a. a - a) - b) - b f x = x id g :: (forall c. Eq c = [c] - [c]) - ([Bool],[Int]) g y = (y [True], y [1]) h :: ([Bool],[Int]) h = f g

[Haskell-cafe] Re: Unification for rank-N types

= foldr (\g - (.) g) id --- What is the reason for this? Thanks, Vladimir On 6/9/09, Vladimir Reshetnikov v.reshetni...@gmail.com wrote: Hi, I have the following code: --- {-# LANGUAGE

Re: [Haskell-cafe] Question on rank-N polymorphism

Hi Zsolt, fs :: (((a, a) - a) - t) - (t, t) fs g = (g fst, g snd) examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,2)), fs ((,)id), fs (:[]), fs repeat) No instance for (Num [Char]) arising from the literal `1' at M.hs:6:54 Possible fix: add an instance declaration for (Num

[Haskell-cafe] Question on rank-N polymorphism

Hi, I have the following code: fs g = (g fst, g snd) examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(1,2)), fs ((,)id), fs (:[]), fs repeat)

[Haskell-cafe] Trouble with types

Hi, I tried this code: --- f, g :: a - a (f, g) = (id, id) --- Hugs: OK GHC: Couldn't match expected type `forall a. a - a' against inferred type `a - a' In the expression: id In the expression: (id, id) In a pattern binding:

Re: [Haskell-cafe] Trouble with types

: Am Montag 01 Juni 2009 14:44:37 schrieb Vladimir Reshetnikov: Hi, I tried this code: --- f, g :: a - a (f, g) = (id, id) --- Hugs: OK GHC: Couldn't match expected type `forall a. a - a' against inferred type

[Haskell-cafe] Question on kind inference

Hi, Consider this Haskell code: --- class A a where foo :: a b class B a class (A a, B a) = C a --- GHC compiles it without errors, but Hugs rejects it: Illegal type in class constraint. What is the

[Haskell-cafe] Which type variables are allowed in a context?

Hi, Consider this (a bit pathological) Haskell code: -- class A a where foo :: A (b d) = a (c b) -- GHC compiles it successfully, but Hugs rejects it: Ambiguous type signature in class declaration *** ambiguous type : (A

[Haskell-cafe] (no subject)

Hi, Seems that Haskell allows to specify dummy type variables in a declaration of a type synonym, which do not appear in its right-hand side. This can lead to interesting effects, which appears differently in GHC and Hugs. I would like to know, what behavior is correct according to the haskell 98

[Haskell-cafe] GHCi vs. Hugs (record syntax)

Hi, I tried to evaluate this expression: head[[]{}] GHCi: [] Hugs: ERROR - Empty field list in update What is the correct behavior? Thanks, Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org

[Haskell-cafe] Hugs vs. GHCi

Hi, The following expression evaluates to 1 in GHCi, but results in an error in Hugs: let f x = let g y = [x,y] in (g 1, g []) in 1 What is the correct behavior? Thanks Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org

[Haskell-cafe] Strange behavior of float literals

, Vladimir Reshetnikov (aka nikov), Microsoft MVP ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe