data Cond a = Then a | Else a then' :: (Monad m) => m a -> Cond (m a) then' = Then else' :: (Monad m) => m a -> Cond (m a) else' = Else if' :: (Monad m) => Bool -> Cond (m a) -> Cond (m a) -> m a if' True (Then t) _ = t if' False _ (Else f) = f if' _ _ _ = error "syntax error" main = do if' (4 < 3) (then' $ print "foo") (else' $ print "bar") if' (4 > 3) (then' $ print "baz") (else' $ print "bat")