How to tweak compiler warnings when configuring build with Oasis?

how_to_set_granular_compiler_flags_via_myocamlbuild.md

How to tweak compiler warnings when configuring build with Oasis?

When using bare ocamlbuild, it is simple-enough - just add -cflags -w <warnings> to the ocamlbuild command line options. But when using Oasis to configure the build - it is less straight forward. Your options are:

• A) use _tags file, which is simple, but incomplete in functionality and documentation
• B) use myocamlbuild.ml, which is not documented at all

I first tried using option (A), but found that it only allows you to add one of the letter options and does not allow disabling individual, numbered, warnings:

https://github.com/ocaml/ocaml/blob/a18bc7950b385ba1668d5efe7b277591b8638802/ocamlbuild/ocaml_specific.ml#L716-L730

Looking at how the above is implemented and a bit more grepping, I found:

val flag : Tags.elt list -> Command.spec -> unit

Which can be used to set arbitrary compiler flags.

This is what I ended-up appending to the end of myocamlbuild.ml, to enable all compiler warnings but 33 (Unused open statement) and 44 (Open statement shadows an already defined identifier):

flag ["ocaml"; "compile"] (S [ A "-w"; A "A-33-44"])