[FEATURE REQUEST] Partial evaluation of `if Sys.backend_type = Other "js_of_ocaml"`

[msprotz]

Hi,

I believe this is a super low-hanging fruit that would greatly improve interop with external libraries. Please read on for a proposed really simple fix.

Is your feature request related to a problem? Please describe.
Cross-referencing ocaml/dune#4854

I have externals that are implemented with bindings to cstubs via ctypes in the general case, and via a custom WASM implementation in the JS case.

While I wait for dune to implement conditional linking or something comparable, I now have in my code:

external js_hook_sha256: bytes -> bytes = "js_hook_sha256"
let sha256 =
  if Sys.backend_type = Other "js_of_ocaml" then
    js_hook_sha256
  else
    FastNativeCryptoLib.SHA256.hash

along with a dummy C stub for js_hook_sha256 just to guarantee successful linking in non-JS scenarios.

The issue is that right now I end up with an ENORMOUS amount of dependencies onto ctypes that are not eliminated because js_of_ocaml does not know that the else branch can be eliminated at compile-time.

Describe the solution you'd like

I would like js_of_ocaml to partially evaluate if Sys.backend_type = Other "js_of_ocaml" then e1 else e2 into e1

Quoting @hhugo :

Jsoo already does static evaluation. It's just a matter of adding a new case I guess. See

js_of_ocaml/compiler/lib/eval.ml

Line 147 in c929312

| "caml_sys_const_big_endian", [ _ ] -> Some (Int 0l)

Describe alternatives you've considered
See original bug report on the dune bug tracker for every other solution I've considered.

Thanks,

Jonathan

[hhugo]

#1166