[antlr-interest] conditional processing

Tue Nov 27 05:34:44 PST 2007

Thanks for taking the time to review this and thanks for the design
feedback! I'll at least safe-guard the global variables.

What I really hoping for was a way to let the rule for simpleStatement
execute the intended action instead of simply recording the details
about it for later execution. I thought maybe there was a way in the
rewrite rule for the 'IF' to avoid evaluating the simpleStatement rule
unless the condition rule evaluated to true. It seems that there's no
way around what I'll call the bookkeeping approach. Is that right?

On 11/27/07, Harald Mueller <harald_m_mueller at gmx.de> wrote:
> Hi
>
> after a quick scanning of your code, I'd say this should work.
>
> However, I don't like the design that much: global variables are per se a bad decision - passing your knowledge around (in single parameters; or, when that gets too cumbersome, in my beloved "context object") is simply more state-of-the-art.
>
> Still, I confess that this argument only is valid if your language will grow one day - then a better design will scale better ... (wouldn't you (or someone) at some time like (or need) to have nested ifs or somewhat more expressive expressions? - all little languages I saw one day wanted to do such things).
>
> At least, you could safe-guard your global variables so that you can only assign a value once to them; then do an evaluation which clears the variables; etc. (in other words, define a clean state machine for them; and implement it - probably in some small classes). That way, any modification of your grammar or your evaluation logic which invalidates your implicit assumptions about when a variable is written and when it is read will at least fly into your face immediately ...
>
> Regards
> Harald M.
>
> -------- Original-Nachricht --------
> > Datum: Tue, 27 Nov 2007 06:50:22 -0600
> > Von: "Mark Volkmann" <r.mark.volkmann at gmail.com>
> > An: antlr-interest at antlr.org
> > Betreff: Re: [antlr-interest] conditional processing
>
> > On 11/27/07, Harald Mueller <harald_m_mueller at gmx.de> wrote:
> > > Hi -
> > >
> > > You need a context that keeps knowledge about the world - variable
> > values, but also results of evaluations e.g. for ifs, returns etc. Usually the
> > context is a stack - see any text on fundamentals of progam language
> > semantics. The context is threaded through all your calls (as a simple parameter),
> > and your calls - e.g. "simpleStatement" - can then react to it:
> > >
> > > if (boolStack.Top) {
> > >     ...do evaluation...
> > > }
> > >
> > > Just imagine what data a virtual machine would need to do what you want
> > - exactly these data are what you need in your context.
> >
> > >>From what you described, it sounds like my current solution might be
> > using the right approach. I don't have to worry about nested
> > conditionals, so I probably don't need stacks now. Here's what I have.
> > Note how the rule for simpleStatement just remembers what it saw but
> > doesn't execute the statement. Also note how the rule that matches
> > "IF" executes the statement if the condition evaluates to true. Does
> > this seem like a reasonable way to implement this?
> >
> > tree grammar MathScriptTree;
> >
> > options {
> >   ASTLabelType = CommonTree;
> >   tokenVocab = MathScript;
> >   output = template;
> > }
> >
> > @members {
> >   enum Operation { Add, Subtract, Print }
> >   Operation operation;
> >   int operand;
> >   int value;
> >
> >   private void execute() {
> >     switch (operation) {
> >       case Add: value += operand; break;
> >       case Subtract: value -= operand; break;
> >       case Print: System.out.println(value); break;
> >     }
> >   }
> >
> >   private static int toInt(CommonTree node) {
> >     return Integer.parseInt(node.getText());
> >   }
> > }
> >
> > script: ^(SCRIPT statement*);
> >
> > statement
> >   : simpleStatement { execute(); }
> >   | ^('IF' c=condition simpleStatement) { if ($c.result) execute(); }
> >   ;
> >
> > simpleStatement
> >   : ^('ADD' n=NUMBER) { operation = Operation.Add; operand = toInt(n); }
> >   | ^('SUBTRACT' n=NUMBER) { operation = Operation.Subtract; operand =
> > toInt(n); }
> >   | 'PRINT' { operation = Operation.Print; }
> >   ;
> >
> > condition returns [boolean result]
> >   : 'POSITIVE' { $result = value > 0; }
> >   | 'NEGATIVE' { $result = value < 0; }
> >   | ^('<' n=NUMBER) { $result = value < toInt(n); }
> >   | ^('>' n=NUMBER) { $result = value > toInt(n); }
> >   | ^('=' n=NUMBER) { $result = value == toInt(n); }
> >   ;
> >
> > > -------- Original-Nachricht --------
> > > > Datum: Mon, 26 Nov 2007 21:02:46 -0600
> > > > Von: "Mark Volkmann" <r.mark.volkmann at gmail.com>
> > > > An: antlr-interest at antlr.org
> > > > Betreff: [antlr-interest] conditional processing
> > >
> > > > Part of my AST looks like this.
> > > >
> > > > ^('if' condition simpleStatement)
> > > >
> > > > In my tree grammar, the rule for "condition" evaluates the matching
> > > > content to true or false. The rule for "simpleStatement" executes the
> > > > statement, for example, it could be a simple print statement. I only
> > > > want simpleStatement to be executed if condition evaluates to true. I
> > > > haven't been able to figure out how to make this work because when
> > > > this part of my AST is matched, the rules for both condition and
> > > > simpleStatement are fired.
> > > >
> > > > I'd appreciate any hints on how to achieve this.
> > > >
> > > > --
> > > > R. Mark Volkmann
> > > > Object Computing, Inc.
> > >
> > > --
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> >
> >
> > --
> > R. Mark Volkmann
> > Object Computing, Inc.
>
> --
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-- 
R. Mark Volkmann
Object Computing, Inc.