[Haskell-cafe] Possible ghc bug
Artem Pelenitsyn
a.pelenitsyn at gmail.com
Wed Apr 10 15:02:16 UTC 2019
Hello Michel,
I think this might be worth filing as a bug:
https://gitlab.haskell.org/ghc/ghc/issues/new
--
Best, Artem
On Wed, 10 Apr 2019 at 10:54 Michel Haber <michelhaber1994 at gmail.com> wrote:
> Hello Sylvain, and thanks again :)
>
> When adding the forall I can compile the code without a problem.
>
> For the types b1 b2 b3, they are of course not what I want (b must be
> unique in the context of the question).
> But the type for the function with b1, b2 and b3 is what I got when I did
> not give a type signature to the compiler/interpreter (which is
> understandable since it has no reason to assume they would be one and the
> same).
>
> I will try the Functional dependency (though I haven't learned about this
> yet).
>
> But the main question remains:
>
> "add x y = wrap $ unwrap x + unwrap y" can be compiled/interpreted, and
> when I ask about its type, the answer is:
> ":t add"
> "add :: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 ->
> b1"
>
> Then when I add that same type signature, like so:
> "add :: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1
> add x y = wrap $ unwrap x + unwrap y"
> the compiler refuses it for ambiguity reasons.
>
> So the question is: How come the compiler does not accept its own type
> signature? Why had it not refused to compile even without one, and only
> upon specification of said signature did the compilation break?
>
> Regards,
> Michel :)
>
> On Wed, Apr 10, 2019 at 4:29 PM Sylvain Henry <sylvain at haskus.fr> wrote:
>
>>
>> On 10/04/2019 15:29, Michel Haber wrote:
>>
>> Hello,
>> Thanks for the answer.
>> I tried the code you sent, but now I'm getting a "type variable not in
>> score" error. (I added both extensions)
>> This is the whole code pertaining to this problem (with the extensions
>> mentioned before):
>>
>> class Newtype a b where
>> wrap :: a -> b
>> unwrap :: b -> a
>>
>> newtype MyInt = MyInt Int
>> newtype YourInt = YourInt Int
>>
>> instance Newtype Int MyInt where
>> wrap = MyInt
>> unwrap (MyInt i) = i
>>
>> instance Newtype Int YourInt where
>> wrap = YourInt
>> unwrap (YourInt i) = i
>>
>> add :: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 -> b1
>> add x y = wrap @a @b1 $ unwrap @a x + unwrap @a y
>>
>> You need to add the "forall a b1 b2 b3" to be allowed to use "@a" (with
>> ScopedTypeVariables extension).
>>
>>
>> For further reference, the exercice to which this code should be a
>> solution can be found at:
>>
>> https://github.com/i-am-tom/haskell-exercises/blob/answers/09-MultiParamTypeClasses/src/Exercises.hs
>>
>> > c. Write a function that adds together two values of the same type, providing
>> that the type is a newtype around some type with a 'Num' instance.
>>
>> You only need a single "b" type instead of b1, b2 b3. Also I think you
>> could use a Function Dependency in the "Newtype" definition (because when
>> we know "b" we know "a"). It will make the code of "add' much simpler.
>>
>>
>> Finally, the question remains: Is it "normal" that ghci behave
>> differently depending on whether
>> the type signature is declared or not? (Remember that the signature is
>> given by ghci itself)
>>
>> Finally it is not related to GHCi (we get the same errors/warnings when
>> we compile) but to the AllowAmbiguousTypes extension:
>> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html?#extension-AllowAmbiguousTypes
>>
>> The type of "add" is ambiguous according to the definition in the manual.
>>
>>
>> My advice would be to use a Functional dependency in your definition of
>> NewType and then you can forget about ambiguous types, type applications,
>> scoped type variables, etc. for now. The reported type of "add" by ghci
>> becomes non-ambiguous and everything is well :) (and I guess that it was
>> the point of the exercise)
>>
>> Regards,
>> Sylvain
>>
>>
>>
>> Thanks again,
>> Michel :)
>>
>> On Wed, Apr 10, 2019 at 1:28 PM Sylvain Henry <sylvain at haskus.fr> wrote:
>>
>>> Hi,
>>>
>>> It looks like an effect of ExtendedDefaultRules:
>>> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html#extension-ExtendedDefaultRules
>>>
>>> It's hard to tell without the code but maybe something like that will do:
>>>
>>> {-# LANGUAGE ScopedTypeVariables #-}
>>> {-# LANGUAGE TypeApplications #-}
>>>
>>> add :: forall a b1 b2 b3. (Num a, Newtype a b1, Newtype a b2, Newtype a
>>> b3) => b2 -> b3 -> b1
>>> add x y = wrap @a @b1 $ unwrap @a x + unwrap @a y
>>>
>>> -Sylvain
>>> On 10/04/2019 12:32, Michel Haber wrote:
>>>
>>> Hello Cafe,
>>>
>>> I was trying to load a module containing this function in ghci:
>>> "add x y = wrap $ unwrap x + unwrap y"
>>> with the following extensions activated:
>>>
>>> ConstraintKinds
>>> DataKinds
>>> DeriveFunctor
>>> DuplicateRecordFields
>>> FlexibleContexts
>>> FlexibleInstances
>>> GADTs
>>> KindSignatures
>>> MultiParamTypeClasses
>>> PolyKinds
>>> TypeFamilies
>>> TypeOperators
>>> AllowAmbiguousTypes
>>>
>>> And it loaded without problem.
>>>
>>> So then I tested its type with ":t add", which gave:
>>> add :: (Num a, Newtype a b1, Newtype a b2, Newtype a b3) => b2 -> b3 ->
>>> b1
>>>
>>> Then I added this signature to the function in the module. This caused
>>> ghci
>>> to refuse to load it and give the following error:
>>>
>>> src/Exercises.hs:55:11: error:
>>> • Could not deduce (Newtype Integer b1)
>>> arising from a use of ‘wrap’
>>> from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3)
>>> bound by the type signature for:
>>> add :: forall a b1 b2 b3.
>>> (Num a, Newtype a b1, Newtype a b2, Newtype a
>>> b3) =>
>>> b2 -> b3 -> b1
>>> at src/Exercises.hs:54:1-74
>>> • In the expression: wrap $ unwrap x + unwrap y
>>> In an equation for ‘add’: add x y = wrap $ unwrap x + unwrap y
>>> |
>>> 55 | add x y = wrap $ unwrap x + unwrap y
>>> | ^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>
>>> src/Exercises.hs:55:18: error:
>>> • Could not deduce (Newtype Integer b2)
>>> arising from a use of ‘unwrap’
>>> from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3)
>>> bound by the type signature for:
>>> add :: forall a b1 b2 b3.
>>> (Num a, Newtype a b1, Newtype a b2, Newtype a
>>> b3) =>
>>> b2 -> b3 -> b1
>>> at src/Exercises.hs:54:1-74
>>> • In the first argument of ‘(+)’, namely ‘unwrap x’
>>> In the second argument of ‘($)’, namely ‘unwrap x + unwrap y’
>>> In the expression: wrap $ unwrap x + unwrap y
>>> |
>>> 55 | add x y = wrap $ unwrap x + unwrap y
>>> | ^^^^^^^^
>>>
>>> src/Exercises.hs:55:29: error:
>>> • Could not deduce (Newtype Integer b3)
>>> arising from a use of ‘unwrap’
>>> from the context: (Num a, Newtype a b1, Newtype a b2, Newtype a b3)
>>> bound by the type signature for:
>>> add :: forall a b1 b2 b3.
>>> (Num a, Newtype a b1, Newtype a b2, Newtype a
>>> b3) =>
>>> b2 -> b3 -> b1
>>> at src/Exercises.hs:54:1-74
>>> • In the second argument of ‘(+)’, namely ‘unwrap y’
>>> In the second argument of ‘($)’, namely ‘unwrap x + unwrap y’
>>> In the expression: wrap $ unwrap x + unwrap y
>>> |
>>> 55 | add x y = wrap $ unwrap x + unwrap y
>>> | ^^^^^^^^
>>> Failed, no modules loaded.
>>>
>>> This does not make sense to me, since I only used the signature that
>>> ghci itself gave me.
>>>
>>> Is this a bug? if not, could someone please explain this behaviour to me?
>>>
>>> Thanks,
>>> Michel
>>>
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