Re: Add an extra class to Control.Applicative.Alternative

At that point it is very close to but stronger than MonadLogic's msplit: class MonadPlushttp://hackage.haskell.org/package/base-4.6.0.1/docs/Control-Monad.html#t:Mo... m => MonadLogic m whereSourcehttp://hackage.haskell.org/package/logict-0.6.0.2/docs/src/Control-Monad-Log... msplit :: m a -> m (Maybehttp://hackage.haskell.org/package/base-4.6.0.1/docs/Data-Maybe.html#t:Maybe (a, m a)) -Edward On Thu, May 1, 2014 at 2:01 PM, Daniel Gorín wrote:

On 01 May 2014, at 13:19, S D Swierstra wrote:

...

I have probably not been clear enough. The relationship that should hold between getPure and getNonPure is as follows:

case (getPure p, genNonPure p) of (Nothing, Nothing) -> "should not happen since p should have at least a pure or a nonpure part" (Just pp, Just npp) -> pure pp <|> npp (Just np, Nothing) -> pure np (Nothing, Just npp) -> npp

is equivalent to p.

I do not see how this would correspond to your use of Either?

Doaitse

Would this then be equivalent to the following?

class Alternative f => Splittable f where split :: f a -> (Maybe a, f a)

isEmpty :: f a -> Bool isEmpty = isNothing . fst . split

One then would have something like:

getPure = fst . split

getNonPure x = guard (not (isEmpty fa)) >> Just fa where fa = snd (split x)

and the expected laws might be easier to state

Daniel

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