Hi
Keith Wansbrough wrote:
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
[..]
...partiality is itself a monadic effect?
Cheers
Conor
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