[Belt] Add List.sort

[zploskey]

Implements Belt.List.sort as specified in the issue.
Adds a couple of simple tests and an example.

Fixes #2586.

[zploskey]

Apparently Array.prototype.sort() sorts everything like a string, and is what Js.Array.sortInPlace() is bound to. Although I could easily fix the test, I don't think that function is what we want to use here. It's unintuitive for a sort function give a result like [1; 3; 3; 37; 9]. Maybe better to use sortInPlaceWith (fun a b -> a - b). But the other behavior is fine if we're dealing with strings. Seems like we'll need break it down by type.

[chenglou]

Oh no, definitely don't use Js.Array.sortInPlace. That behavior is famously confusing. Maybe SortArray.stableSortInPlaceBy? cc @bobzhang

[zploskey]

I went ahead and adjusted it to use SortArray.stableSortInPlaceBy. I figure whatever solution we settle on will use a compare function.

[chenglou]

The asymmetry between SortArray and List.sort is a little weird. I'm wondering how we should arrange these

[zploskey]

What was the reasoning behind putting Sort functions on arrays in their own separate module?

[bobzhang]

why a ref needed here

[bobzhang]

sortU/sort

[zploskey]

Ok, I'll add a sortU version. I think I was confused about whether I could use let and it still be mutated by the in-place sort. It works fine without the ref.

[bobzhang]

A.stableSortInPlaceBy -- it is already aliased in the beginning

[zploskey]

Belt_Array is aliased as A in this module. It doesn't have the sort functions from Belt_SortArray. Should Belt_SortArray be opened in Belt_Array?

[bobzhang]

This is fine then, sorry my mistake

[zploskey]

I addressed the concerns and rebased on master so that all the tests run. I'm not sure why we're seeing a build failure on Travis. It builds fine for me locally. Somehow it can't find Belt_SortArray.cmj, but it just built it a few lines before. 🤷‍♂️

[zploskey]

That seems to have worked. Let me know if there are any other concerns.

[bobzhang]

In general, in belt, we should define sortU first and wrap sort like this

let sort xs cmp = sortU xs (fun[@bs] x y -> cmp x y)

The thing is in your sort, you are calling stableSortInPlaceBy instead of stableSortInPlaceByU which is slower

[zploskey]

Fixed. Thanks!