Mistranslation of recursively forwarded effects in JS backend

[marzipankaiser]

When running the following program in the JavaScript backend, it runs forever producing < on the console, until it stack-overflows:

effect stacktrace(): Unit

def withStacktrace{ body: => Unit / stacktrace }: Unit / { stacktrace } = {
  def st() = do stacktrace()
  try body()
  with stacktrace { () => println("<"); st(); resume(()) }
}
def allowStacktrace{ body: => Unit / stacktrace }: Unit = {
  try body()
  with stacktrace { () => resume(()) }
}

def foo(n: Int): Unit / stacktrace = withStacktrace {
  if (n > 0) {
    foo( n - 1 )
  } else { do stacktrace() }
}

def main() = allowStacktrace {
  foo(2)
}

(no playground link, since the playground just hangs...)

This is not the case in LLVM, Chez (also CPS), and also should not be the case.

[marzipankaiser]

Generated JavaScript:

function foo_0(n_0, stacktrace_0, ks_5, k_5) {
  foo_2: while (true) {
    const tmp_stacktrace_1 = stacktrace_0;
    const stacktrace_4 = {
      stacktrace_2: (ks_4, k_4) => {
        const v_r_2 = $effekt.println("<");
        return stacktrace_0.stacktrace_2(ks_4, k_4);
      }
    };
    if ((n_0 > (0))) {
      /* prepare call */
      const tmp_n_1 = n_0;
      n_0 = (tmp_n_1 - (1));
      stacktrace_0 = stacktrace_4;
      continue foo_2;
    } else {
      const v_r_3 = $effekt.println("<");
      return stacktrace_0.stacktrace_2(ks_5, k_5);
    }
  }
}

function main_0(ks_7, k_7) {
  const stacktrace_5 = {
    stacktrace_2: (ks_6, k_6) => () => k_6($effekt.unit, ks_6)
  };
  return foo_0(2, stacktrace_5, ks_7, k_7);
}

[marzipankaiser]

I think the problem is that stacktrace_0 is set to stacktrace_4, and captured by the implementation of stacktrace_4 by reference in JavaScript.

[marzipankaiser]

This is generated due to the transformation of recursive functions to loops in this case:

effekt/effekt/shared/src/main/scala/effekt/generator/js/TransformerCps.scala

Line 308 in ad6f80f

case cps.Stmt.App(Recursive(id, label, vparams, bparams, ks1, k1, used), vargs, bargs, MetaCont(ks), k) =>

This for some reason does not substitute (in line 7):

- return stacktrace_0.stacktrace_2(ks_4, k_4);
+ return tmp_stacktrace_1.stacktrace_2(ks_4, k_4);

[jiribenes]

This sounds a bit similar to #1235 (issue #1229)