[Haskell-cafe] Re: what is inverse of mzero and return?
k.schupke at imperial.ac.uk
Sun Jan 23 04:50:13 EST 2005
Jorge Adriano Aires wrote:
>On the list monad, I think of the mplus operation as the "union" two
>non-deterministic states. Mzero is the state that works as the identity
>(which is when you have no possible state at all).
Okay... thats a definition of a monoid.
>>What would happen if this was the definition?
>>instance MonadPlus  where
>> mzero = 
>> mplus a b
>> | a ==  = b
>> | otherwise = a
Isn't the above a monoid as well?
a `mplus`  = a
 `mplus` b = b
Still looks like an identity to me....
Is there only on correct definition of a monad/monoid on lists - or does
anything that satisfies the monad laws count? I got the impression you
could define anthing you liked for mzero and mplus - providing the laws
>Then, I'd say you're not thinking of monadic sums, but of catching errors, and
>the appropriate place for that is the class MonadError.
I am thinking about how some monads are summed - like Maybe and
the Parser monad.
It seems there are two possibilities - either the definitions of MonadPlus
for Maybe and Parser monads are in Error, or there can be two different
acceptable definitions of MonadPlus on the List?
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