Warn when whitespace is misleading when compared to operator precedence

[rakudrama]

I have not been convinced by some of the puzzlers. They rely to some degree on sleight-of-hand. Consider:

    main() {
      print("A simple sum: 2 + 2 = " +
            2 + 2);
    }

In Java this prints "A simple sum: 2 + 2 = 22".

Why do I say 'sleight-of-hand'? Operator + groups from the left. In the expression a+b+c, the first + binds tighter, giving an evaluation equivalent to (a+b)+c. In the puzzler, there is more whitespace following the first operator, even though it binds tighter due to grouping from the left. The choice of whitespace is misleading in the same way that

print(1 + 2 *
      3 + 4);

might mislead some to think it prints 21.

The 'Animal farm' puzzler does the same:

main() {
    final String pig = "length: 10";
    final String dog = "length: " + pig.length;

    print("Animals are equal: "
          + pig == dog);
}

• binds tighter than == but is surrounded by more whitespace.

We might be able to do something about misleading whitespace. The editor could flag expressions where the whitespace suggests operator binding that is contrary to the actual binding.

Suggested implementation:
Attach to each binary operation node the whitespace to the left of the operator and to the right of the operator.
Warn when the whitespace in a subtree is looser than the whitespace on the binary operation node.
One whitespace string is looser than another if it has more newlines, or if it has more spaces when both have no newlines.

Although Dart no longer has + for strings, this is still a useful warning - see Issue #114 for a precedence issue inherited from C/C++/Java that has not been fixed.

[danrubel]

Added this to the Later milestone.

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[sethladd]

Added NotPlanned label.

[clayberg]

Added this to the 1.6 milestone.