Jules Bean wrote:
However, your problem *does* have a natural underlying monad, if you care to use it.
I may be confused, but I don't think it does. It seems like the OP wants a type like data Perhaps a = Success a | Failure [Error] and semantics like liftM2 (+) (Failure [error1]) (Failure [error2]) === Failure [error1,error2] where liftM2 f a1 a2 = a1 >>= p where p v1 = a2 >>= q where q v2 = return (f v1 v2) I don't see how to define (>>=) such that this will return the appropriate value. If a1 fails you must call p in order to collect additional errors, but there's no appropriate value of v1 that you can pass. But it's easy with a custom lifting function: liftPerhaps2 f (Success x) (Success y) = Success (f x y) liftPerhaps2 f p q = Failure (errors p ++ errors q) where errors (Success _) = [] errors (Failure es) = es -- Ben