Support user-define variables where possible

[giltho]

It'd be nice to be able to specify variables in the dune-project file such that one could write. Ideally, these variables would also be usable across the project in other dune files.

(define-var min-ocaml-version 5.4.0)

(package
 (name package1)
 (depends
  (ocaml
   (>= %{min-ocaml-version))))

(package
 (name package2)
 (depends
  (ocaml
   (>= %{min-ocaml-version))))

[rgrinberg]

This feature has been discussed more than times that I can count, but I'm still unable to find an existing issue.

In any case, it's a feature that we certainly want to implement but I'd like to very careful designing this. In particular, I would like us to think through scoping, shadowing, and accessing the same variable from multiple dune files.

[giltho]

I would like us to think through scoping, shadowing

Yeah, agreed it's non-trivial. That's why I edited my first attempt at writing this issue to specify that one could only write variables in the dune-project file (at least at first).