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