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:

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@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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