Recursive function with no parameters generate while loop which contains assignment to undeclared _param variable

[hackwaly]

Thank you for filing! Check list:

• Is it a bug? Usage questions should often be asked in the forum instead.
• Concise, focused, friendly issue title & description.

Recursive function with no parameters generate while loop which contains assignment to undeclared _param variable

let test = () => {
  let rec loop = () => {
    switch () {
    | _ => loop()
    }
  }
  loop()
}

// Generated by ReScript, PLEASE EDIT WITH CARE


function test() {
  var loop = function () {
    while(true) {
      _param = undefined;
      continue ;
    };
  };
  return loop();
}

export {
  test ,
}
/* No side effect */

• A minimal, reproducible example.
• OS and browser versions, if relevant.
• Is it already fixed in master? Instructions

[hackwaly]

Any plan to fix this bug?

[tx46]

The biggest problem I see with this is that it will silently freeze instead of overflow the stack...

[TheSpyder]

I came across this while upgrading my project to ReScript 11 and considering a swap to uncurried mode. The specific issue is with code that takes a JS generator and turns it into a "lazy sequence".

In my case, the project still has uncurried: false with the code in question manually uncurried, so it is blocking my upgrade to ReScript 11. I could switch it to curried code, but then it would be slow (and block my attempts to use uncurried mode).
https://rescript-lang.org/try?version=v11.1.2&code=DYUwLgBAdiAekF4IGcQEcIIHwvQCgEoAoI0SAMwEtgwQAnTCPVNAGggD9yDMcBvIhAhkIdEAGMIACxDAADvUaFegoSgDulMOKm40AWiwx4EAWogAfCAGUA9gFsQeWD2wRyzngH4bDpy4gALmlZBTpCVSErADlbGF4IWJhIiABfEiEZeXoidKA

[cristianoc]

Likely place to start from: #6131

[cristianoc]

@hackwaly @TheSpyder how does this look? #6907

[TheSpyder]

The JS certainly looks good! I didn't realise there was broken code in the expected test output 😂

[cristianoc]

Improved the PR, now it simply omits the problematic assignment without reverting to using a spurious parameter in the case of recursive functions.

[cristianoc]

The JS certainly looks good! I didn't realise there was broken code in the expected test output 😂

That happens often. There are so many old and forgotten tests, that chances are the repro is already right there.

[TheSpyder]

Yes, I have worked on many old codebases where that's true 🙃