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

[David Feuer]

I'm generally in favor of removing `sequence`, and probably also
`sequenceA`. Can you demonstrate that there can be no instance for which
`mapM` is much more efficient than `traverse`? Consider especially types
based on monadic streams.

On Sat, Nov 7, 2020 at 2:56 PM Emily Pillmore <emilypi at cohomolo.gy> wrote:

> Hi All,
>
> I have a proposal for the `base` library:  I would like to move `mapM` and
> `sequence` out of the class definition for `Traversable`, redefining them
> as toplevel aliases:
>
> ```haskell
> mapM :: Traversable t ⇒ Monad m ⇒ (a -> m b) -> t a -> m (t b)
> mapM = traverse
>
> sequence ::  Traversable t ⇒ Monad m=> t (m a) -> m (t a)
> sequence = sequenceA
> ```
>
> This slims `Traversable` by 50%. This would be a very small breaking
> change, which is completely tractable, but a great improvement for the
> ecosystem imo. Thoughts? What timeline should we shoot for with a change
> like this?
>
> Cheers,
> Emily
>
>
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