> > 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-) _______________________________________________ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
