[Haskell-cafe] Possible ghc bug

[Michel Haber]

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

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

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)

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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