[Haskell-cafe] Possible ghc bug
Sylvain Henry
sylvain at haskus.fr
Wed Apr 10 14:29:53 UTC 2019
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
> <mailto: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
>>
>> _______________________________________________
>> Haskell-Cafe mailing list
>> To (un)subscribe, modify options or view archives go to:
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>> Only members subscribed via the mailman list are allowed to post.
> _______________________________________________
> Haskell-Cafe mailing list
> To (un)subscribe, modify options or view archives go to:
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
> Only members subscribed via the mailman list are allowed to post.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20190410/2c0721ef/attachment.html>
More information about the Haskell-Cafe
mailing list