Add "else" clause to loop constructs.

[lrhn]

Please add an "else" clause to loop constructs.
The else clause would be written as:

  for/while(...) { 
    body 
  } else {
    else-part
  }

It will only be executed when the loop condition fails. An unlabeled break inside the body will break past the else-part. Continues inside the else part can not continue the loop, unlabeled breaks in the else part do not refer to the loop.

This feature would make some algorithms much easier to expression, and avoid the labeled breaks that is often used instead.

Example code:

    for (int i = 0; i < string.length; i++) {
      if (string.codeUnitAt(i) >= 0x80) break;
    } else {
      return string.codeUnits;
    }
    return UTF8.encode(string);

What I currently have to write:

    checkAscii: {
      for (int i = 0; i < string.length; i++) {
        if (string.codeUnitAt(i) >= 0x80) break checkAscii;
      }
      return string.codeUnits;
    }
    return UTF8.encode(string);

This also shows a valid rewrite to, e.g., JavaScript.

I'm only aware of one language that currently has this feature: Python.

[lrhn]

Refer to dart-lang/language#171 in the language-repo instead.