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

Conor McBride conor at cs.rhul.ac.uk
Mon Oct 18 11:15:45 EDT 2004


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?




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