Adding some conditionals in the language

[trefis]

This is similar to #819 but with a somewhat broader scope.
The idea would be to support (if <cond> <then> [else]) at the toplevel of a jbuild.

This would allow encoding the example of #819, which was:

(alias
 ((name runtest)
  (deps (foo.exe))
  (action (run ${<}))
  (enabled_if (and (<> ${os} win32) (>= ${ocaml_version} 4.0.5)))))

as

(if (and (<> ${os} win32) (>= ${ocaml_version} 4.0.5))
    (alias
     ((name runtest)
      (deps (foo.exe))
      (action (run ${<})))))

(of course I have nothing with the sugar proposed in #819).

But would also allow doing things like:

(if (= ${ocaml_version} "4.07._")
    (copy_files# 407/*.ml{,i}))
(if (= ${ocaml_version} "4.06._")
    (copy_files# 406/*.ml{,i}))
...

which are needed for merlin and currently require us to use ocaml jbuilds.

---

I'm only proposing to support this at the toplevel, but this could be pushed down to arbitrary places (after all, (select ...) already does similar things).

[avsm]

Related to #838

[rgrinberg]

#819 I've implemented the boolean sub language that is required for this feature.

Btw, I would prefer if we supported conditionals via pattern matching or a cond form.

I'd much rather write something like this:

(case %{os}
 ("win32" (copy_files ...))
 ("linux" (copy_files ...))
 (_ (copy_files ...)))

or

(cond
 (= %{os} "win32" (copy_files ...))
 (= %{os} "linux" (copy_files ...))
 (else (copy_files ...))

than the equivalent with if/else.

[trefis]

That absolutely fine by me.