Comment by teo_zero

3 months ago

In some way it's the dual of break, in that you want to jump into the middle of the loop, while break is to jump out of it.

Let's rewrite the loop this way, with 'break' expanded to 'goto':

  while (true) {
    prepare...
    if (!cond) goto exitpoint;
    process...
  }
  exitpoint:

The dual would be:

  goto entrypoint;
  do {
    process...
  entrypoint:
    prepare...
  } while(cond);

Both constructs need two points: where the jump begins and where it lands. The 'break' is syntactic sugar that removes the need to specify the label 'exitpoint'. In fact with 'break' the starting point is explicit, it's where the 'break' is, and the landing point is implicit, after the closing '}'.

If we want to add the same kind of syntactic sugar for the jump-in case, the landing point must be explicit (no way for the compiler to guess it), so the only one we can make implicit is the starting point, that is where the 'do' is.

So we need: a new statement, let's call it 'entry', that is the dual of 'break' and a new semantic of 'do' to not start the loop at the opening '{' but at 'entry'.

  do {
    process...
    entry;
    prepare...
  } while (cond);