The code generated from "let open Functor(Module) in ..." is buggy.

[kxc-wraikny]

Summary

OCaml source code:

let test3 () =
  let open Samplelib.MonadOps(Promise) in
  return 2
  >>= fun x -> return (x + 40)
  >>= fun x -> return (to_be (expect x) 42)

The generated JavaScript:

function test3(param) {
  var open = Samplelib.MonadOps($$Promise$1);
  var $great$great$eq = open[1];
  var $$return = open[0];
  return Curry._2($great$great$eq, Curry._1($$return, 2), (function (x) {
                return Curry._2($great$great$eq, Curry._1($$return, x + 40 | 0), (function (x) {
                              return Curry._1($$return, (expect(x).toBe(42), undefined));
                            }));
              }));
}

Test result:

FAIL test/samplelib.test.js
  ✓ it loads (2 ms)
  ✓ test1 (1 ms)
  ✓ test2
  ✕ test3 (1 ms)

  ● test3

    TypeError: Cannot read properties of undefined (reading 'length')

      43 |   var $great$great$eq = open[1];
      44 |   var $$return = open[0];
    > 45 |   return Curry._2($great$great$eq, Curry._1($$return, 2), (function (x) {
         |                                          ^
      46 |                 return Curry._2($great$great$eq, Curry._1($$return, x + 40 | 0), (function (x) {
      47 |                               return Curry._1($$return, (expect(x).toBe(42), undefined));
      48 |                             }));

      at Object._1 (_output/node_modules/melange.runtime/curry.js:31:17)
      at Object._1 [as test3] (_output/samplelib_test.js:45:42)
      at Object.test3 (test/samplelib.test.js:17:29)

This is a reproduction repository:
https://github.com/kxc-wraikny/Melange_bug

Details

The functor is defined as

module MonadOps(M : sig
  type _ t
  val return : 'x -> 'x t
  val bind : 'x t -> ('x -> 'y t) -> 'y t
end) = struct
  let return = M.return
  let (>>=) = M.bind
end

https://github.com/kxc-wraikny/Melange_bug/blob/7f3aa164d5b0553424fe54638878717249855eb9/samplelib.ml#L1-L8

and this is the test code and the problem occurs at test3

module Test = struct
  let test1 () =
    let open Promise in
    return 2
    |> flip bind (fun x -> return (x + 40))
    |> flip bind @@ fun x -> return (to_be (expect x) 42)
    
  let test2 () =
    let module M = Samplelib.MonadOps(Promise) in
    let open M in
    return 2
    >>= fun x -> return (x + 40)
    >>= fun x -> return (to_be (expect x) 42)

  let test3 () =
    let open Samplelib.MonadOps(Promise) in
    return 2
    >>= fun x -> return (x + 40)
    >>= fun x -> return (to_be (expect x) 42)
end

https://github.com/kxc-wraikny/Melange_bug/blob/7f3aa164d5b0553424fe54638878717249855eb9/samplelib_test.ml#L19-L38

The generated code:

function test1(param) {
  var m = Promise.resolve(2);
  var m$1 = m.then(function (x) {
        return Promise.resolve(x + 40 | 0);
      });
  return m$1.then(function (x) {
              return Promise.resolve((expect(x).toBe(42), undefined));
            });
}

function test2(param) {
  var M = Samplelib.MonadOps($$Promise$1);
  return Curry._2(M.$great$great$eq, Curry._1(M.$$return, 2), (function (x) {
                return Curry._2(M.$great$great$eq, Curry._1(M.$$return, x + 40 | 0), (function (x) {
                              return Curry._1(M.$$return, (expect(x).toBe(42), undefined));
                            }));
              }));
}

function test3(param) {
  var open = Samplelib.MonadOps($$Promise$1);
  var $great$great$eq = open[1];
  var $$return = open[0];
  return Curry._2($great$great$eq, Curry._1($$return, 2), (function (x) {
                return Curry._2($great$great$eq, Curry._1($$return, x + 40 | 0), (function (x) {
                              return Curry._1($$return, (expect(x).toBe(42), undefined));
                            }));
              }));
}

https://github.com/kxc-wraikny/Melange_bug/blob/7f3aa164d5b0553424fe54638878717249855eb9/_output/samplelib_test.js#L22-L50

[anmonteiro]

Where's Jest coming from?

[kxc-wraikny]

It is added to package.json and called in the ml file using external, but I think that is not important because the generated js code itself is problematic.

open struct
  module Jest = struct
    type t
    external expect : 'a -> t = "expect" [@@bs.val]
    external to_be : t -> 'a -> unit = "toBe" [@@bs.send]
  end
end

https://github.com/kxc-wraikny/Melange_bug/blob/7b988ec6daaea197df4a8eac9059b9385b023a20/samplelib_test.ml#L7-L13

[anmonteiro]

I couldn't repro it after removing Jest and using your code. The error is a bit weird, since the function passed to Curry._1 should always have an arity.

[anmonteiro]

OK I managed to repro this, thanks.

[anmonteiro]

BTW a workaround is to give the resulting module a name like in test2

[anmonteiro]

fixing this in #664 and melange-re/melange-compiler-libs#18