[env stanza] [feature request] More flexible behavior for wildcard pattern

[ejgallego]

In Coq's root Dune file we have:

(env
 (dev     (flags :standard -rectypes -w -9-27-50+40+60))
 (release (flags :standard -rectypes))

I'd be great if we could instead write:

(env
 (dev     (flags :standard -w -9-27-50+40+60))
 (_ (flags :standard -rectypes))

so the wildcard pattern is also applied to the profiles there.

c.f. #1155 (comment)

[ghost]

Changing the meaning of _ would be a breaking change, however to keep the syntax light, we could simply introduce a special :common key:

(env
 (:common (flags :standard -rectypes))
 (dev     (flags :standard -w -9...)))

[rgrinberg]

What about just introducing general bindings? We're pretty much going to have the mechanism for it with setenv, might as well reuse it. E.g:

(let
 (:common foo bar)
 (env
  (_ (flags %{common}))
  (dev (flags %{common} -w -9))))

[ghost]

Why not. Its just quite a bit more work. For instance, if we want to correctly interpret things like this: (let (:x :default x y) ...), then we can't evaluate :default x y immediately as the expansion of :default depend on the context. On the other hand, %-forms should be expanded in the context of where the variable was bound. So we have to create a closure.

[ejgallego]

IMVHO this may be such a common issue / use case that it would make sense to get explicit support for :common, but of course YMMV.

let scares me a bit as it reminds me of make :)