unsafeCoerce from TypeFamilies and GeneralizedNewtypeDeriving

unsafeCoerce.hs

{-# LANGUAGE TypeFamilies, GeneralizedNewtypeDeriving #-}

type family Switch b a
type instance Switch b (A a) = a
type instance Switch b (B a) = b

newtype A a = A a
newtype B a = B (A a)
    deriving (Switchable)

class Switchable a where
    switch :: a -> Switch b a

instance Switchable (A a) where
    switch (A x) = x

unsafeCoerce :: a -> b
unsafeCoerce x = switch (B (A x))