CoerceTest.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE RoleAnnotations #-}
module CoerceTest where

import Data.Coerce

type role A phantom
data A a = MkA Int

-- works (CORRECT)
fooA :: Coercible o (A a) => o -> ()
fooA _ = ()

type role B representational
data B a = MkB Int

{-
-- does not work (CORRECT)
fooB :: Coercible o (B a) => o -> ()
fooB _ = ()
-}

type role C representational
newtype C a = MkC Int

-- works, but should not
fooC :: Coercible o (C a) => o -> ()
fooC _ = ()

newtype D a = MkD a

{-
-- does not work (CORRECT)
fooD :: Coercible o (D a) => o -> ()
fooD _ = ()
-}