Proposal: Move `mapM` and `sequence` out of Traversable
David Feuer
david.feuer at gmail.com
Sat Nov 7 22:15:09 UTC 2020
In light of this example, I oppose the proposal.
On Sat, Nov 7, 2020, 5:04 PM David Feuer <david.feuer at 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 at 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 at 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)
>>>
>>>
>>> ======
>>> Georgi
>>> _______________________________________________
>>> Libraries mailing list
>>> Libraries at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>
>>
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