[Issue 8622] New: Allow labeled breaks to work on *any* block

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8622

           Summary: Allow labeled breaks to work on *any* block
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: monarchdodra at gmail.com


--- Comment #0 from monarchdodra at gmail.com 2012-09-05 06:35:59 PDT ---
D gives the possibility of multiple-breaking out (nested) while/for blocks
using labeled breaks.

I think it would be beneficial to extend this to being able to break out of any
"block", as long as it is labeled:

----
void main()
{
    label: {
      // ..code..
      writefln("code1");
      writefln("break..");
      break label;  // jumps to "here:"
      // ..code..
      writefln("code2");
    }
    // break lands here
    writefln("here");
}
----

The main use case here would actually be to replace the "triangle if" pattern:

----
    if(condition1)
    {
        if(condition2)
        {
            if(condition3)
            {
                if(condition4)
                {
                    //DO SOMETHING
                }
            }
        }
    }
----

Which would become

----
    conditional_block:
    {
        if(!condition1) break conditional_block;
        if(!condition2) break conditional_block;
        if(!condition3) break conditional_block;
        if(!condition4) break conditional_block;
        //DO SOMETHING
    }
----

The usual "workaround" to the "triangle if" pattern is to move it to it's own
function, and *return* from it. The disadvantage is that it burdens the
developer with a whole extra function. The "breakable block" has the advantage
of not breaking the logic flow of the current function.

Another workaround, the "do_while_false" is also possible:
----
do:
{
    if(!condition1) break;
    if(!condition2) break;
    if(!condition3) break;
    if(!condition4) break;
    //DO SOMETHING
}while(false);
----
However, my experience is that this pattern tends to be confusing for readers
that expected a loop: The actual condition "while(false)" is only documented
near the end.

Further more, given both the above workarounds, neither scale should you want
to nest a double break.

Conclusion: Generic breakable labeled blocks. It works better and is
clearer/more verbose.

Thank you/

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