Re: [Haskell-cafe] One-way and two-way monads.

Dear All,

List and IO are both monads when appropriate operations are defined.

The IO monad, which is a one-way monad, does not have a function of type IO a -> a, in general, except an unsafe function "unsafePerformIO :: IO a -> a". A two-way monad, such as List, has such a function "head :: [a] -> a".

But "head" sometimes fails. So List isn't a two-way monad either. (if you accept partial functions, then *all* monads are two-way, since otherway :: m a -> a otherway x = error "foo" has the desired type). --KW 8-)