Re: Adding more instances to Kleisli: Generic, Monad, etc

The issue is that you can't prove that whicher set of MonadPlus laws you are using _follows_ from whichever set of Alternative laws you are using. This discussion is somewhat hampered by neither one having a well-written set of laws, because we use MonadPlus in two or three mutually incompatible ways and for the most part "it just works" so people don't notice. Consider the space of laws mentioned in: https://wiki.haskell.org/MonadPlus_reform_proposal These don't necessarily follow from any of the popular Alternative law proposals. Left Zero: On the Applicative side you can do more to explore the tree, can "run" effects right to left, etc. We have a _left_ zero law for a monadplus: mzero >>= f = mzero but not a right one, just because of the existence of the function space in (>>=) that we can't see through (do putStrLn "die"; mzero) /= mzero But nothing keeps us from seeing through an (*>) to count up something on the right. Control.Applicative.Backwards exists and would satisfy a "right zero" law if the original satisfied a left zero law. Left Distribution: Similarly, you can imagine a data type that is Alternative and also a Monad, but where mplus a b >>= k = mplus (a >>= k) (b >>= k) fails to hold in the left distributive style, and it also fails to be in the left catch style. Without _retiring_ MonadPlus, we should at least use it consistently to imply the requirement of this sort of extra structure, even if it is ambiguous what that structure should be. -Edward On Wed, Apr 3, 2019 at 3:16 AM Fumiaki Kinoshita wrote:

I'm not quite sure what the point of MonadPlus is; the default definitions are the Alternative methods. Would we ever want to have MonadPlus different from Alternative?

2019年4月2日(火) 13:44 David Feuer :

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I think the MonadPlus instance should probably have a MonadPlus constraint, since MonadPlus makes a sort of statement about the interaction between >>= and mplus, even if it's a bit of an ambiguous one.

On Tue, Apr 2, 2019, 12:30 AM Fumiaki Kinoshita wrote:

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This is another part of https://gitlab.haskell.org/ghc/ghc/merge_requests/644, which is presumably much less controversial:

Generic (Kleisli m a b) Functor m => Functor (Kleisli m a) Applicative m => Applicative (Kleisli m a) Alternative m => Alternative (Kleisli m a) Monad m => Monad (Kleisli m a) (Alternative m, Monad m) => MonadPlus (Kleisli m a) _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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