λ> data S = X | Y deriving (Eq, Show)
λ> data T = W | Z deriving (Eq, Show)
λ> s2t s = case s of X -> W; Y -> Z
λ> t2s t = case t of W -> X; Z -> Y
λ> s2t' s = case s of X -> Z; Y -> W
λ> t2s' t = case t of Z -> X; W -> Y
λ> ( s2t (t2s W) == W , s2t (t2s Z) == Z , t2s (s2t X) == X , t2s (s2t Y) == Y )
(True,True,True,True)
λ> ( s2t' (t2s' W) == W , s2t' (t2s' Z) == Z , t2s' (s2t' X) == X , t2s' (s2t' Y) == Y )
(True,True,True,True)