Off the top of my head, I can't think of anything about sequence or sequenceA to justify their existence as class methods from the standpoint of practical programming. I could imagine they might be helpful to beginners who may find it easier to think about a container full of actions than about a container full of elements and a function that can be applied to them to get actions. But it's also not clear to me that removing methods is very valuable unless either: 1. We get down to just one method, which can be good for performance with non-regular types, or 2. We end up with a class that works with GND/DerivingVia, such as class (Foldable t, Functor t) => Traversable t where mapTraverse :: Applicative f => (t b -> r) -> (a -> f b) -> t a -> f r The one-method bonus can generally be worked around with auxiliary classes in the rare cases where it comes up, so I wouldn't worry about that too much here. On Sat, Nov 7, 2020, 9:47 PM Carter Schonwald <carter.schonwald@gmail.com> wrote:
Yeah, mapM is semantically different from traverse. So dropping mapm seems ill advised.
@david: is there an analogue for sequence?
On Sat, Nov 7, 2020 at 5:15 PM David Feuer <david.feuer@gmail.com> wrote:
In light of this example, I oppose the proposal.
On Sat, Nov 7, 2020, 5:04 PM David Feuer <david.feuer@gmail.com> wrote:
I don't know about measurements or anything. There are certainly *implementation strategies* for mapM that don't translate well to traverse. Imagine a queue, for example. One way to write mapM is this:
mapM f = go empty where go !acc xs = case uncons xs of Just (x, xs') -> do y <- f x go (acc `snoc` y) Nothing -> pure acc
There's no way to do anything operationally equivalent with just Applicative.
Is this a good way to write it? Well, that presumably depends on the queue and on the Monad, but I'd give it a good "possibly".
On Sat, Nov 7, 2020, 3:42 PM A S <masaeedu@gmail.com> wrote:
Personally I would love to know of some kind of reasoning regarding these things, as I'm not aware of any! (efficiency of Applicative vs Monad based functions)
I agree. I'd be very interested in seeing an example (contrived or otherwise) of a specific Monad which is necessarily more efficient to `mapM` over some arbitrarily selected Traversable container than to `traverse`. That would be a good first step I think.
On Sat, Nov 7, 2020 at 3:29 PM Georgi Lyubenov <godzbanebane@gmail.com> wrote:
Hi!
Regarding the "there can be no instance for which mapM is more efficient than traverse": There have been issues with Applicative functions leaking memory where Monad ones aren't in Polysemy - some of these have been fixed, but it's not clear that there are none left. There is also this claim in parser-combinators <https://hackage.haskell.org/package/parser-combinators-1.2.1/docs/Control-Applicative-Combinators.html> :
Due to the nature of the Applicative <https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#t:Applicative> and Alternative <https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#t:Alternative> abstractions, they are prone to memory leaks and not as efficient as their monadic counterparts. Although all the combinators we provide in this module are perfectly expressible in terms of Applicative <https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#t:Applicative> and Alternative <https://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.html#t:Alternative>, please prefer Control.Monad.Combinators <https://hackage.haskell.org/package/parser-combinators-1.2.1/docs/Control-Monad-Combinators.html> instead when possible.
I have not verified it, but it is a bit worrying.
Personally I would love to know of some kind of reasoning regarding these things, as I'm not aware of any! (efficiency of Applicative vs Monad based functions)
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