George Russel wrote: [snip]

I presume it would not in fact be difficult to synthesise a left identity at the cost of making things slower, thus (forgive any syntax errors, I'm not going to test this).

data MonadIO a = Action (IO a) | Return a instance Monad (MonadIO a) where return a = Return a (>>=) (Return a) k = k a (>>=) (Action act) f = Action (act >>= (\ a -> case f a of {Return a -> return a;Action act -> act}))

Or, more general: data MonadWrap m a = M (m a) | R a instance Monad m => Monad (MonadWrap m) where return = R R x >>= f = f x M x >>= f = M (x >>= \a->case f a of R b -> return b M c -> c) I don't think this really solves the problem with the left unit (not in general, and not for IO either), it merely pushes it to a different place. I think you need the left-unit law for monad m to prove the 'associativity' law for monad (MonadWrap m) The 'associativity' law: m >>= (\x -> k x >>= h) = (m >>= k) >>= h The case in which the situation occurs is m=(M x) with (k x)=(R b). Stefan Kahrs