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

A few thoughts: * Splittable has been used as a name historically for splitting random number generators and the like, so the name conflict (especially from somewhere so prominent) would be unfortunate. That can of course be fixed by a simple bikeshedding exercise. * You should be able to implement this in one pass. e.g. f a -> Either a (f a) rather than as two combinators, but it isn't clear to me what the getNonPure branch can do other than return the original when the scan fails or some partially zonked/expanded version of it. * Personally, I'm rather hesitant, as there are a lot of points in the design space and it isn't apparent how to implement/lift it for instance over any of the monad transformers we have, so it is the kind of class that lifting it too high up the import hierarchy will lead to users being forced to write orphans, when they disagree about whether, say writer (mempty, a) should be an effect or not or if you should be have instance (Monad m, Splittable m, Eq s) => Splittable (StateT s m). * The lack of any constraints on `f` tying it to anything else in Control.Applicative gives me pause. Without any relationship to other types it is harder to specify the laws and make them feel coherent rather than bolted on. Off the cuff, I'm currently -1 on this proposal, almost entirely because of the concern I'd have that pushing it too far up the class hierarchy actually invites a worse experience due to orphans than leaving it closer to the use site where such ambiguous cases can be resolved unilaterally by the author or ignored as irrelevant by them safely. -Edward On Wed, Apr 30, 2014 at 5:58 AM, Doaitse Swierstra wrote:

In the package uu-interleaved I introduce a new class

class Splittable f where getNonPure :: f a -> Maybe (f a)Source getPure :: f a -> Maybe aSource

which I use for splitting an applicative value into its pure and its non-pure part. This is then used in the rest of this package to define (non-ambiguous) interleaved structures (as a generalisation of permuted structures).

My feeling that this class should be better located in Control.Applicative.Alternative.

If you agree what are the steps to be taken?

Doaitse

On 01 Nov 2013, at 14:08 , Twan van Laarhoven wrote:

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On 01/11/13 12:44, Nathan van Doorn wrote:

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Firstly, I don't see how IO is relevant here, it has neither a MonadPlus instance nor an Alternative instance.

You are right. I thought it was an instance with mzero=fail "foo" and mplus=catch. But I was apparently mistaken. Objection withdrawn.

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Secondly, the MonadPlus laws are documented in Control.Monad to be:

mzero >>= f = mzero v >> mzero = mzero

I missed them, because they are written in the documentation of mzero rather than the documentation of the class where I expected them.

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Thirdly, the monoid laws are already documented. (<|>) must be "An associative binary operation", and empty "The identity of <|>". These are exactly the monoid laws. Perhaps they should be made more explicit, but that is a different issue.

Missed this as well.

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Fourthly, [] fulfils neither the left-distribution law or the left-catch law, and I doubt many people would be happy to lose []'s MonadPlus instance.

List does satisfy left distribution:

λ> (,) <$> ([1,2] <|> [3]) <*> [4,5] [(1,4),(1,5),(2,4),(2,5),(3,4),(3,5)] λ> (,) <$> [1,2] <*> [4,5] <|> (,) <$> [3] <*> [4,5] [(1,4),(1,5),(2,4),(2,5),(3,4),(3,5)] λ> quickCheck (\x y z -> ((,) <$> (x <|> y :: [Int]) <*> (z :: [Int])) == (((,) <$> x <*> z) <|> ((,) <$> y <*> z))) +++ OK, passed 100 tests.

See also http://www.haskell.org/haskellwiki/MonadPlus. Then law which it doesn't is right distribution.

Consider Maybe. it does satisfies left catch but not left distribution for MonadPlus. Since mplus (Just False >>= guard) (Just True >>= guard) = Just () while mplus (Just False) (Just True) >>= guard = Nothing

But for Alternative, you can't have the failure of the second argument of (<*>) depend on the first. So Maybe *does* satisfy left distribution for Alternative. IMO that makes it a good candidate law.

Twan

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I believe I have addressed all your issues. If I've missed something, please point it out to me.

Nathan.

On 1 November 2013 12:09, Twan van Laarhoven mailto:twanvl@gmail.com> wrote:

On 01/11/13 11:42, Nathan van Doorn wrote:

Proposal: add the following laws to the documentation of Control.Applicative.__Alternative:

* empty <*> a = empty * f <*> empty = empty

> These laws correspond to the laws given in MonadPlus- if you take mzero = > empty and ap = (<*>), the ones in MonadPlus imply these- and I don't think > this proposal should be too controversial.

As far as I can see, the documentation for MonadPlus does not specify these laws anywhere [1,2].

Consider the IO monad. These laws claim that

launchMissiles *> fail "empty" = fail "empty"

This is clearly *not* true.

--

If we add laws, I think we should first consider the much more reasonable monoid laws

identity empty <|> a = a a <|> empty = a associativity: (a <|> b) <|> c = a <|> (b <|> c)

In the MonadPlus world, the controversial part is the choice between

left distribution (f <|> g) <*> a = (f <*> a) <|> (g <*> a)

or

left catch pure a <|> b = pure a

Your proposal would be

left zero

empty <*> a = empty right zero

f <*> empty = empty

And as mentioned above, right zero is problematic. The fmap version should be okay though

map zero f <$> empty = empty

Twan

[1]

http://hackage.haskell.org/__package/base-4.6.0.1/docs/__Control-Monad.html#...

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< http://hackage.haskell.org/package/base-4.6.0.1/docs/Control-Monad.html#t:Mo...

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[2] http://www.haskell.org/__haskellwiki/MonadPlus http://www.haskell.org/haskellwiki/MonadPlus _________________________________________________ Libraries mailing list Libraries@haskell.org mailto:Libraries@haskell.org http://www.haskell.org/__mailman/listinfo/libraries http://www.haskell.org/mailman/listinfo/libraries

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