coerce and phantom types

[claudeha]

{-# LANGUAGE RoleAnnotations #-}
module Coerce where
                                                  
import Data.Coerce (coerce)
import Data.Kind (Type)                           

type role Foo phantom                             
data Foo a = Foo
                                                 
foo :: forall (a :: Type) (b :: Type) . Foo a -> Foo b                                              
foo = coerce

accepted by ghc 9.12.1
rejected by mhs 0.15.5.0

*** Exception: error: "./Coerce.hs": line 11, col 7: 
Cannot satisfy constraint: (Coercible a b)
     fully qualified: (Data.Coerce.Coercible a b)

[augustss]

Mhs does not implement roles. It just parses the declaration and throws it away.

[augustss]

I'll see if there is an easy fix for phantom type coercion.

[augustss]

When I have type families working I'll need to change the coercion code, so I'll postpone a fix until then.