Naming bikeshedding for opcompose (opposite composition function)

[tkf]

I can't really think about a better name but I'd like to rename it if we find a better one.

cc @jw3126

[tkf]

Another possible ASCII-based API is to come up with something based on |>. The observation is that, if |> has a vararg form

|>(x, f, g, h) == x |> f |> g |> h == h(g(f(x)))

then opcompose is (args...) -> x -> |>(x, args...). So, if we have a "hole" or argument placeholder object, we can use |>(__placeholder_object__, functions...) instead of compose(functions...).

This can be done with <|(functions..., __placeholder_object__) too.

Implementation:

struct Hole end
const _ARG_ = Hole()
Base.:|>(::Hole, functions...) = ∘(reverse(functions)...)
<|(args...) = |>(reverse(args)...)

Usage:

julia> |>(_ARG_, sin, cos, tan)
tan ∘ cos ∘ sin

julia> <|(sin, cos, tan, _ARG_)
sin ∘ cos ∘ tan

This might be a bit too "fancy" as I want CompositionsBase.jl to be super conservative. Maybe this can be put in CompositionsX.jl or something.

[jw3126]

I like compose and opcompose. I think Flux.jl uses Chain, which I also like as a name.

The Hole() trick is really clever and your tofu symbol ◻ is a good unicode name for it. I agree that this would be a better fit for a fancy CompositionsX.jl package.

Just to remind myself, ideally this package would be part of Base, it is currently not due to bad font support.

[tkf]

OK, good to have one more "vote" to opcompose :) I guess it is likely the current best approach. I'll experiment the hole thing in CompositionsX.jl.

BTW, just for the record, some more related links:

• Add ASCII alias compose of ∘ JuliaLang/julia#33573 where I failed to convince that defining const compose = ∘ in Base is a good idea.
• RFC: Add new pipe-like operator syntaxes </, |>>, etc. JuliaLang/julia#34340 where I'm asking for more pipe-like operators (e.g., |>>).