[BUG] RuntimeError: illegal cast, when performing an effect with wasm_of_ocaml

[Nymphium]

Describe the bug
When running a program that uses an effect, a runtime error (RuntimeError: illegal cast) occurs instead of the expected uncaught effect exception. The minimal reproducible example is:

type _ Effect.t += Print : string -> unit Effect.t
let () = Effect.perform (Print "Hello, WASM!")

The bug can be reproducable in this repo: https://github.com/Nymphium/hellowasm/tree/effect

open Effect

type _ Effect.t += Print : string -> unit Effect.t

let () =
  try perform (Print "Hello, WASM!") with
  | effect Print s, k -> print_endline s |> Deep.continue k
;;

$ dune build ./app/main.bc.wasm.js
$ node `./_build/default/app/main.bc.wasm.js`
/Users/nymphium/works/github/ocaml-helloworld/_build/default/app/main.bc.wasm.js:455
           throw: e=>{throw e;},
                      ^

RuntimeError: illegal cast
    at capture_continuation (wasm://wasm/0003914e:wasm-function[100]:0x7e98)
    at toplevel (wasm://wasm/0003914e:wasm-function[162]:0x9d50)
    at caml_main (wasm://wasm/0003914e:wasm-function[102]:0x7ecd)
    at entry_point (wasm://wasm/0003914e:wasm-function[122]:0x8439)
    at link (/Users/nymphium/works/github/ocaml-helloworld/_build/default/app/main.bc.wasm.js:559:14)

Node.js v22.13.1

Expected behavior
The program should print Hello, WASM! (or, in the case where the effect is handled, print the string and continue execution) rather than throwing a runtime error.

Versions
Version of packages used to reproduce the bug
(ocamlc, js_of_ocaml, ..)

• OCaml Compiler 5.3.0
• dune 3.17.2
• wasm_of_ocaml 6.0.1
• wasm-opt 119
• NodeJS 22.13.1

[vouillon]

I think you need nodejs 23. There has been recent changes to the JSPI API that we are using to implement effect handlers.

[Nymphium]

Thank you, but the same error raised with node 23.8.0 🤔

[vouillon]

Oh, I forgot that the JSPI was not yet enabled by default.
Try:

node --experimental-wasm-jspi ./_build/default/app/main.bc.wasm.js

Or you can use the implementation of effects handlers based on a CPS transformation by adding this option to the executable stanza in your dune file:

 (wasm_of_ocaml (flags --effects=cps))

[Nymphium]

It works with the first opinion! Thank you!

On the other hand, the second makes an error during compiling:

$ dune build ./app/main.bc.wasm.js  
File "app/dune", line 2, characters 7-11:
2 |  (name main)
           ^^^^
/nix/store/k96z4j3s6vchdiwsbw5lj67ml2r6khlr-wasm_of_ocaml-compiler-6.0.1/bin/wasm_of_ocaml: Error: Incompatible build info detected while linking.
 - .main.eobjs/jsoo/main.bc.runtime.wasma: effects=jspi
 - .main.eobjs/jsoo/dune__exe__Main.wasmo: effects=cps

I close the issue but the latter might be another bug 👀

[OlivierNicole]

Hopefully this will be fixed once ocaml/dune#11222 is merged.

[vouillon]

On the other hand, the second makes an error during compiling:

Indeed, dune does not know yet about the --effect flag, so you can get incoherent results in case of separate compilation. In your case, you can just use --enable effects instead, which was the previous way to use the CPS-based implementation of effects.