Support first class modules to treat type variables safely

[cannorin]

This PR allows value bindings to take a limited form of first-class modules as arguments:

val console_log:
  (module[@js] Ojs.T with type t = 'a) -> 'a -> unit
  [@@js.global "console.log"]

Users can now pass first-class modules to safely convert the values of types with variables:

console_log (module Ojs.String) "hello, world!";;
console_log (module Ojs.Int) 42;;

The first-class modules must have the form of (module[@js] Ojs.T with type t = 'a), and cannot be used outside of value bindings.

This PR also introduces several pre-defined modules for each built-in type to make life easier:

(* ojs.ml *)

module String = struct
  type t = string
  let t_to_js = string_to_js
  let t_of_js = string_of_js
end

...

module List (A: T) = struct
  type t = A.t list
  let t_to_js = list_to_js A.t_to_js
  let t_of_js = list_of_js A.t_of_js
end

...

[cannorin]

It seems CI is broken since https://github.com/LexiFi/gen_js_api/commit/347a18c1321e6bac18627760911c97169511dadc.

I also fixed it, and now it's failing because ppxlib somehow generates code in a slightly different format in *nix than in Windows (it's not a version difference: both uses 0.26.0).

Do you think this a bug of ppxlib...?

[cannorin]

It's also failing in issues.ml and types.ml, which I didn't edit: https://github.com/LexiFi/gen_js_api/runs/6227945812?check_suite_focus=true#step:7:481

It seems there is some problem on printing locally abstract types?

[cannorin]

I also upgraded ppxlib to 0.26 and disabled dune cache for Windows.

[cannorin]

I will revert the CI fixes and make a separate PR to make it easy to review.

[cannorin]

CI is broken again...?

[mlasson]

I also fixed it, and now it's failing because ppxlib somehow generates code in a slightly different format in *nix than in Windows (it's not a version difference: both uses 0.26.0). Do you think this a bug of ppxlib...?

Weird. I accepted the test on windows/cygwin/ocaml 4.14/ppxlib.0.26 and it has the same output as the CI. what is your setup to promote tests ?

[mlasson]

The PR is nice and well documented, thanks ! Out of curiosity, are you using this somewhere ?

[cannorin]

what is your setup to promote tests ?

I'm using Debian on WSL1, which should be the same as in a normal Debian.

Also by explicitly upgrading to ppxlib 0.26, the issue somehow disappeared. So I think we can regard it as solved?

Out of curiosity, are you using this somewhere ?

I'm using gen_js_api as the output of https://github.com/ocsigen/ts2ocaml 😄

ts2ocaml would not be possible without gen_js_api. Thank you!

[mlasson]

Thanks again for this PR. I made a new release containing this new feature. Hope this helps.