[Haskell-cafe] Possible ghc bug

[Sylvain Henry]

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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