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

[Conor McBride]

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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