const_functor.hs

data Val a = Val Int deriving Show
instance Functor Val where
  fmap _ (Val x) = Val x
coerce :: Val a -> Val b
coerce = fmap undefined

data Void

x :: Val Int
x = Val 1

y :: Val Void
y = coerce x

-- Fails typechecking with:
--     Couldn't match type ‘Int’ with ‘Void’
--    Expected type: Val Void
--      Actual type: Val Int
--    In the expression: x
--    In an equation for ‘z’: z = x
-- z :: Val Void
-- z = x

main = do
  print x
  print y