[Haskell-cafe] One-way and two-way monads.
Keith.Wansbrough at cl.cam.ac.uk
Mon Oct 18 10:34:01 EDT 2004
> 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).
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