[core libraries] Rename NonEmpty.groupWith/sortWIth to *On*

[Philip Hazelden]

On Mon, Feb 17, 2020 at 3:59 PM Carter Schonwald <carter.schonwald at gmail.com>
wrote:

> It seems like the *on family in general has a nontrivial increase in space
> usage and the only differences have to do with application specific time vs
> memory tradeoffs that need to measured at the application level.
>

To be clear, this particular tradeoff isn't relevant for every *On
function. Of the ones under discussion, it applies to sortWith,
groupAllWith and groupAllWith1, but not groupWith or groupWith1.

Some things I think are worth noting:

* There must be lots of common functions that could choose different
implementation tradeoffs, but we don't keep multiple versions of them
around.
* The existing performance differences aren't documented. (Data.List.sortOn
tells us that it only evaluates `f` once per element, but it doesn't tell
us about the costs; the others don't mention performance at all.) I predict
very few people currently choose between GHC.Exts.sortWith and
Data.List.sortOn according to their performance needs.
* The Data.List.sortOn implementation is somewhat subtle and complicated.
The NonEmpty.sortWith implementation is literally `sortBy . comparing` and
GHC.Exts.sortWith isn't much more complicated. (`sortWith f = sortBy (\x y
-> compare (f x) (f y))`; I don't know if there are reasons to prefer or
disprefer this compared to `sortBy . comparing`.) If someone wants this
behavior, it's not hard for them to get.

Based on these, I don't think there's much value in deliberately[1] making
sure both implementations have a name, which is why I proposed to replace
`sortWith` with `sortOn` instead of keeping them both around.

[1] Of course, there's value in not changing things once they're there. By
"deliberately", I mean that if we weren't already in this situation, and
someone proposed adding a separate function that was semantically
equivalent to `sortOn` but implemented as `sortBy . comparing`, I don't
think that would see much support.
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