append/prepend to PATH-like variables in dune-workspace

[avsm]

It would be useful to be able to prepend/append to PATH in dune-workspace files, which currently support:

(paths (<var1> <val1>) .. (<varN> <valN>))

This can be used to introduce a local OCaml compiler and associated tools to a dune build seamlessly, without interfering with the main dune build graph.

Discussed with @rgrinberg offline.

[ghost]

Doesn't (paths (PATH <path> :standard)) already works?

/cc @nojb who added this feature IIRC

[nojb]

Indeed it should work I think. It was added in #2426. Did you try it @avsm?

[avsm]

It works, and allows a dune build to proceed without an OCaml or opam installation on the system. This is how the duniverse bootstrap will work shortly. Thanks!

(lang dune 2.5)
(context (default (paths (PATH _ocaml/bin :standard))))