Stdlib: Add function composition combinator

[laelath]

Add Fun.compose and Stdlib.(@.) for function composition. Useful when wanting to apply multiple functions when wanting to map over a list as List.map (f @. g) l exposes the high-level logic in a clearer manner than List.map (fun x -> f (g x)) l. The choice of @. was made because Haskell uses . for function composition and there is probably a clearer choice that could be made for OCaml.

[nojb]

My opinion based on a quick look:

• In favour of adding Fun.compose,
• Not really in favour of adding the infix operator: 1) I think the general direction is not to add anything else to Stdlib itself to avoid polluting the global namespace, and 2) infix operators tend to harm code readability, and I'm not sure we want to encourage that.

[laelath]

While overuse of infix operators can certainly harm code readability, I think that more sparing/deliberate usages of them can make things clearer. For the purposes of this PR consider f @. g @. h vs compose f (compose g h). Maybe the infix operators could be added without polluting the global namespace by defining a Fun.Operators sub module that can be opened to access?

[Octachron]

There is also the issue that the choice of @. is non-standard. A [quick look on sherlocode](('[a-z] -> '[a-z]) -> ('[a-z] -> '[a-z]) -> '[a-z] -> '[a-z]) yields the following operator name for compose.

• |., -|
• <<<, <<
• $, %
• @*
• ++

In other words, the name of the operator is controversial, and it is better to split this discussion on the operator from the simple compose function.

I am personally in favor of settling a name for the composition operator, but this is better done in a large discussion rather than in a PR.

[dbuenzli]

This has already been discussed to death here.

[xvw]

In Preface we use f % g for f . g (and f %> g for g . f, wich is never really used) and we have some case where it is useful and probably more readable than (fun x -> f (g x)). And since we have a module Arrow for type ('a, 'b) t = 'a -> 'b) we can also use >>> and <<< but we take the habit to use %.

[laelath]

I removed the infix operator for now

[nojb]

LGTM

I am in favour of this addition. As it touches the standard library, a second approval from a core dev is needed.

[nojb]

Friendly ping. This is a simple PR to review; we still need a second reviewer; volunteers welcome!

[dbuenzli]

I looked at it today. It's ok with me if someone wants to ack on my behalf.

[yallop]

Approved (on behalf of @dbuenzli, but I'm happy with it, too)

[nojb]

Thanks for the prompt review! I took the liberty of amending the Changes file accordingly.