Proposal: Move `mapM` and `sequence` out of Traversable

[A S]

> 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
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