Experiment: Playing with currying, chaining and underscores

[c42f]

A super hacky, quick implementation of "pipefirst / pipelast" ideas inspired by

https://discourse.julialang.org/t/fixing-the-piping-chaining-issue

Tiny demo:

julia> 1:10 />> filter(isodd) />> map(z->z^2)
5-element Vector{Int64}:
  1
  9
 25
 49
 81

The meaning of />> is roughly "pipelast", so the above expression is like x |> (y->filter(isodd, y)) |> (y->map(z->z^2, y)) though with different lowering. Similarly /> is "pipefirst".

[Note: in the original version of this PR, />> was spelled \> but this is visually confusing.]

Implementation

Parse chains of /> and />> at the rough same precedence as |>, treating them as a currying operator for function calls such that the succeeding function call becomes curried with the only free argument as either the first or last, respectively. Thus, the following

x  />  f(y)  />>  g(z)

is parsed as

(chain x (/> (call f y)) (/>> (call g z)))

and lowered to

chain(x, fixbutfirst(f, y), fixbutlast(g, z))

Also, this can be used without data on the left hand side to create a first class data pipeline:

h = />  f(y)  />>  g(z)

Lowers to

compose_chain(fixbutfirst(f, y), fixbutlast(g, z))

Additionally, add lowering of underscore as strictly tight-binding placeholder syntax. (Super hacky - more forms should be allowed! This is just for experimentation).

---

Note, I don't really expect to ever merge this, given how difficult https://github.com/JuliaLang/julia/pull/24990 has proven :grimacing:

It's just an experiment for people to play with.

You can try this by checking out this branch and using

JuliaSyntax.enable_in_core!(freeze_world_age=false)

(With this setup, you can also edit JuliaSyntax.jl to play with different lowering scenarios and see them almost immediately in the REPL using Revise. (May need to run a command twice for Revise to pick things up.))