Eliminating `return` after `loop` in some cases

[bbrk24]

When writing an infinite generator, such as the below, Civet emits an unreachable return statement:

f := (a, b) ->
 yield a
 loop
   yield b
   [a, b] = [b, a + b]

const f = function* (a, b) {
  yield a;
  const results = [];
  while (true) {
    yield b;
    results.push(([a, b] = [b, a + b]));
  }
  return results;
};

While this could be removed, there are some things we have to be careful of:

• If there's a break inside the loop, the return statement becomes reachable
• Only return statements in the same scope as the loop itself may be omitted -- conditionals may not trigger, and try blocks may terminate due to an error.

[STRd6]

Adding a void type annotation to the function or a trailing ; will omit the implicit return.

f := (a, b): void ->
  yield a
  loop
    yield b
    [a, b] = [b, a + b]

f := (a, b) ->
  yield a
  loop
    yield b
    [a, b] = [b, a + b]
  ;

We may be able to omit the return in this simple case but probably not generally.

[edemaine]

Does ESLint complain about the inaccessible return in this case? We could try to mimic their algorithm for conservative unreachability testing, to avoid warnings. But I imagine you were concerned more about efficiency, for which I think Daniel's response is the right one.

[bbrk24]

Not just ESLint, even TypeScript complains about it: https://www.typescriptlang.org/play?#code/MYewdgzgLgBAZjAvPArmYUCW4BUMAUAhgDQwBGAlDAN4CwAUDDAJ6YCmANgCYyEDcDJqEiwATmwgoOUCEhgBtALoDGMAO4ALTBzYEoolGyp1VTVpx5kVTJuMnSIAOgAOKCBvz55JcornyyUkIYAGpfCgprGABfQRhxKBRRMHiJKRkVaL4gA