coercewoes.hs

import Data.Coercible

newtype KM a x = KM (Maybe a)
newtype K1 a x = K1 a

data Id x = Id x
newtype IdF f x = IdF (f x)
data IdF' f x = IdF' (f x)

-- this works: K1 (Maybe a) x is coercible with KM a x
coerce (K1 (Just True)) :: KM Bool ()
-- this also works, if we wrap it in a non Higher Kinded data type
coerce (Id (K1 (Just True))) :: Id (KM Bool ())
-- this also works if we wrap it in a higher kinded newtype
coerce (IdF (K1 (Just True))) :: IdF (KM Bool) ()
-- but this doesn't work if we wrap it in higher-kinded data decl
coerce (IdF' (K1 (Just True))) :: IdF' (KM Bool) ()
{-
    • Couldn't match representation of type ‘K1 (Maybe Bool)’
                               with that of ‘KM Bool’
-}
-- note the error is referring to the functor and not the fully saturated type