Change compiler flags based on OCaml version

[talex5]

I'd like to compile a library with -O3. Since OCaml 4.02 doesn't support that flag, I'd like to skip it for that version of the compiler.

I tried using :include, but it looks like ocamlopt_flags doesn't support that (undefined symbol :include). Is there another way (short of switching to the OCaml jbuild syntax)?

I guess #56 would also solve my problem, if implemented.

[ghost]

:include is only supported for C compilation/link flags for historical reasons. I guess we could support it everywhere for uniformity, although you rarely need complex computation to determine the ocaml flags.

I don't think #56 will ever be implemented. It won't be enough in the long term; jbuilder will have to support OCaml plugins properly.

For simple things, I have been working on a small meta-programming language for jbuild files, see this jbuild for instance. Eventually there'll also be a way to change the default flags for a given scope, so you'll be able to write this in a toplevel jbuild file:

(set_defaults
 ((ocamlopt_flags
   (:standard
    (:if (:ver> %{ocaml_version} 4.02) -O3)
    (:if %{dev} -w @a)))))

In the meantine, you indeed need to resort to the OCaml syntax.

[dra27]

#235 makes the suggestion that we could automate the removal of flags (which I think is a good idea)

[kit-ty-kate]

Here is an example where this can be useful: ocurrent/ocaml-ci#265
Also I just noticed that the documentation says this feature is definitely coming in the future: https://dune.readthedocs.io/en/stable/concepts.html?highlight=flags#ordered-set-language.

Note that inside an ordered set, the first element of a list cannot be an atom except if it starts with -
or :. The reason for this is that we are planning to add simple programmatic features in the futures
so that one may write:

(flags (if (>= %{ocaml_version} 4.06) ...))

Any news on that?

[emillon]

:include in all flags has been added in #153, so I'm closing this. Thanks!