-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 In the code below, the Applicative instance accumulates on the Left constructor using Monoid/mappend. Is it possible to write an equivalent Monad such that ap = (<*>) ? I'm finding difficulty in proving to myself either way. import Control.Monad.Instances import Control.Applicative import Data.Monoid newtype Z e a = Z { either :: Either e a } instance Functor (Z e) where fmap f (Z e) = Z (f `fmap` e) instance (Monoid e) => Applicative (Z e) where pure = Z . Right (Z (Left e1)) <*> (Z (Left e2)) = Z (Left (e1 `mappend` e2)) (Z (Left e1)) <*> (Z (Right _)) = Z (Left e1) (Z (Right _)) <*> (Z (Left e2)) = Z (Left e2) (Z (Right f)) <*> (Z (Right a)) = Z (Right (f a)) instance (Monoid e) => Monad (Z e) where return = pure (Z e) >>= f = error "todo" -- ? - -- Tony Morris http://tmorris.net/ S, K and I ought to be enough for anybody. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFJd5vImnpgrYe6r60RAoUNAJ4jn0GfC6zsP9giPGop1ILExiHrLQCfSoc2 0QXf533sWb3HyrL0pQNjMww= =R36O -----END PGP SIGNATURE-----