[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 http://www.haskell.org/mailman/listinfo/haskell-cafe
[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 correct behavior, and which part of the haskell 98 report explains this? Thanks, Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[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 a, A (b c)) => a (d b) *** assigned to: foo What is the correct behavior, and which part of the haskell 98 report explains this? Thanks, Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[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 report. 1) -- type F a = Int class A a where foo :: A b => a (F b) -- GHC - OK Hugs - Illegal type "F b" in constructor application 2) -- type F a = Int class A a where foo :: F a instance A Bool where foo = 1 instance A Char where foo = 2 xs = [foo :: F Bool, foo :: F Char] -- GHC: M.hs:14:6: Ambiguous type variable `a' in the constraint: `A a' arising from a use of `foo' at M.hs:14:6-8 Probable fix: add a type signature that fixes these type variable(s) M.hs:14:21: Ambiguous type variable `a1' in the constraint: `A a1' arising from a use of `foo' at M.hs:14:21-23 Probable fix: add a type signature that fixes these type variable(s) Hugs: [1,2] Thanks, Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[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
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[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: (f, g) = (id, id) What does mean this error message? And what of them (Hugs, GHC) is correct? Thanks Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble with types
Hi Daniel, Could you please explain what does mean 'monomorphic' in this context? I thought that all type variables in Haskell are implicitly universally quantified, so (a -> a) is the same type as (forall a. a -> a) Thank you, Vladimir On 6/1/09, Daniel Fischer wrote: > 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 `a -> a' >> In the expression: id >> In the expression: (id, id) >> In a pattern binding: (f, g) = (id, id) >> >> What does mean this error message? >> And what of them (Hugs, GHC) is correct? > > http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs-and-infelicities.html > Section 12.1.1.4, Declarations and bindings > > GHC's typechecker makes all pattern bindings monomorphic by default; this > behaviour can be > disabled with -XNoMonoPatBinds. See Section 7.1, “Language options”. > > Hugs is correct, it's a known infelicity in GHC which can be disabled. >> >> Thanks >> Vladimir > > > ___ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[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 ($(1,"2")), fs ((,)id), fs (:[]), fs repeat) The idea is that fs accepts a polymorphic function as its argument. What type signature can I specify for f in order to compile this code? If it is not possible in Haskell, is there another language with static typing which allows this? Thanks, Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Question on rank-N polymorphism
Hi Zsolt, fs :: (((a, a) -> a) -> t) -> (t, t) fs g = (g fst, g snd) examples = (fs fmap, fs liftA, fs liftM, fs id, fs ($(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 [Char]) In the expression: 1 In the second argument of `($)', namely `(1, "2")' In the first argument of `fs', namely `($ (1, "2"))' Anyways, this signature is not what intended. I want it to work for all tuples, regardless of their element types. Thanks Vladimir On 6/7/09, Zsolt Dollenstein wrote: > On Sun, Jun 7, 2009 at 9:17 AM, Vladimir > Reshetnikov wrote: >> Hi Zsolt, >> >> It does not compiles with GHC without type annotations. > > It does with mine: The Glorious Glasgow Haskell Compilation System, version > 6.10.2 > > Anyway, try this: fs :: (((a, a) -> a) -> t) -> (t, t) > >> >> Thanks, >> Vladimir >> >> On 6/7/09, Zsolt Dollenstein wrote: >>> Hi Vladimir, >>> >>> On Sun, Jun 7, 2009 at 12:06 AM, Vladimir >>> Reshetnikov wrote: >>>> 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) >>>> >>>> >>>> The idea is that fs accepts a polymorphic function as its argument. >>>> What type signature can I specify for f in order to compile this code? >>> >>> Have you tried putting the above into ghci for example, then asking for >>> ":t >>> fs"? >>> Or am I misunderstanding your point? >>> >>> Cheers, >>> Zsolt >>> >>>> If it is not possible in Haskell, is there another language with >>>> static typing which allows this? >>>> >>>> Thanks, >>>> Vladimir >>>> ___ >>>> Haskell-Cafe mailing list >>>> Haskell-Cafe@haskell.org >>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>> >>> >> > > ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[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
---
GHC rejects it:
Couldn't match expected type `forall a. a -> a'
against inferred type `forall c. (Eq c) => [c] -> [c]'
Expected type: forall a. a -> a
Inferred type: forall c. (Eq c) => [c] -> [c]
In the first argument of `f', namely `g'
In the expression: f g
But, intuitively, this code is type-safe, and actually I can convince
the typechecker in it with the following workaround:
---
h :: ([Bool],[Int])
h = let g' = (\(x :: forall a. a -> a) -> g x) in f g'
---
So, is the current behavior of GHC correct ot it is a bug?
How unification for rank-N types should proceed?
Thanks,
Vladimir
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[Haskell-cafe] Re: Unification for rank-N types
One more example:
This does not type-check:
---
{-# LANGUAGE RankNTypes, ImpredicativeTypes #-}
f :: [forall a. t a -> t a] -> t b -> t b
f = foldr (.) id
---
Couldn't match expected type `forall a. f a -> f a'
against inferred type `b -> c'
In the first argument of `foldr', namely `(.)'
But this, very similar, does type-check:
---
{-# LANGUAGE RankNTypes, ImpredicativeTypes #-}
f :: [forall a. t a -> t a] -> t b -> t b
f = foldr (\g -> (.) g) id
---
What is the reason for this?
Thanks,
Vladimir
On 6/9/09, Vladimir Reshetnikov wrote:
> 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
> ---
>
> GHC rejects it:
> Couldn't match expected type `forall a. a -> a'
>against inferred type `forall c. (Eq c) => [c] -> [c]'
> Expected type: forall a. a -> a
> Inferred type: forall c. (Eq c) => [c] -> [c]
> In the first argument of `f', namely `g'
> In the expression: f g
>
> But, intuitively, this code is type-safe, and actually I can convince
> the typechecker in it with the following workaround:
> ---
> h :: ([Bool],[Int])
> h = let g' = (\(x :: forall a. a -> a) -> g x) in f g'
> ---
>
> So, is the current behavior of GHC correct ot it is a bug?
> How unification for rank-N types should proceed?
>
> Thanks,
> Vladimir
>
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[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
---
According to the Haskell Report, [f | f <- foo] translates to (let ok
f = [f]; ok _ = [] in concatMap ok foo), right?
So, I wonder why (bar [f | f <- foo]) typechecks, but (bar (let ok f =
[f]; ok _ = [] in concatMap ok foo)) and (bar foo) do not typecheck?
Thanks,
Vladimir
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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 ImpredicativeTypes #-}
>>
>> type Void = forall a. a
>>
>> newtype Mono a = Mono { runMono :: [Void] }
>>
>> beep :: Mono a -> Mono a
>> beep (Mono vs) = Mono (map undefined vs)
>>
>
> Compiling this with GHC results in:
>
> Monotype.hs:9:28:
>>Cannot match a monotype with `Void'
>> Expected type: Void
>> Inferred type: a
>>
>
> What does this error mean and why does the code not compile?
>
> Thanks!
>
> Martijn.
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[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 Any reason why it is not in the standard library? Thanks, Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Strange behavior of float literals
The float literal 1e-1 in GHC evaluates to 1.0, and 1e-9 evaluates to 10.0. Is it a bug, or a documented overflow behavior? What it the correct place to submit bug reports concerning GHC? -- Thank you, Vladimir Reshetnikov (aka nikov), Microsoft MVP ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[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), Monad a) => b -> b -> Bool *** assigned to: f Who is right? -- Thanks Vladimir ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
