[antlr-interest] Implementing "break" statement in antlr-based interpreter

[Juancarlo Añez]

Michael,

Simplest, is best.

One option is to split the rules for statement sequences into two: those
allowed within loops, and everything else.

The other is to allow a "break" in any statement sequence, and deal with
it's validity later.

There's a lot of semantic nuances to a programming language that are very
difficult to solve at the syntactic level.

-- Juanca

On Tue, Oct 23, 2012 at 3:16 PM, Michael Cooper <tillerman35 at yahoo.com>wrote:

> Say you have your basic "for" or "while" loop, e.g.
>
> for(i=0; i<10; i++) {
>   print i
> }
>
>
> In the "pie" example, the author has a while loop that uses a "defer"
> parameter to indicate that the interpreter will do the job of evaluating
> the expr that determines if the loop proceeds.
>
>
>    |   'while' expr[true] slist[true]
>         {if (!defer) interp.whileloop($expr.start, $slist.start);}
>
> I would like to be able to break out of a "for" or "while" loop, as is
> done in many programming languages, e.g.
>
> //Read rows from a cursor and print out the contents of field 1 until it
> says "break"
> while cursor.hasrows() {
>   cursor.getrow()
>   if cursor.getfield[1] = "stop" then break  //<-- When field one says
> "break" we should exit the "while" loop.
>   print cursor.getfield[1]
> }
>
> What I think I need to do is add a boolean to my interpreter that
> indicates that a break statement has been encountered, and then test for
> that condition = true in each rule action.
>
> so in my "statement" rule, I would add an alternative:
>
> | 'break' { if(!defer && !interp.breakfound) interp.break = true; }
>
> and then add the "&& !interp.breakfound)" into every rule.  That way, the
> parser would not execute any interpreter functions until the breakfound
> condition was re-set.
>
> I would also need to save the break condition prior to entering break-able
> constructs (loops and functions are the only ones I can think of) so that I
> could restore it after the end of the construct.  That way the break
> statement only exits the loop it executes in.
>
> e.g. in class Interpreter:
>
> public whileloop(Token expr_start, Token slist_start) {
>   boolean saved_breakfound = this.breakfound;
>     ...Handle the loop stuff here...
>   this.breakfound = saved_breakfound;
> }
>
> Does this make sense?  Is there a better way that I'm missing?  I imagine
> the same idea could be used to implement a "return(value)" statement in a
> function as well.  The only difference would be that a return statement
> would exit the function no matter how deep it was into loops.  For that,
> I'd need some kind of tri-state indicator, with values like 0 => continue,
> 1=> break out of loop, 2 => return from function.
>
> Any thoughts?
>
> Thanks!
>
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-- 
Juancarlo *Añez*