[antlr-interest] Creating a simple expression language

[Ivar Refsdal]

>
> Hi Ivar,
>
> Thanks for the response. The Dragon book is good, isn't it? I skimmed
> it last year, but probably need to go through it again and do more of
> the exercises this time. Ditto Programming Language Pragmatics by
> Michael Scott and Essentials of Programming Languages. I am doing the
> latter at the moment, exercises and all, but it's Scheme, so it's
> great, but not immediately applicable to using ANTLR.
>
> I think I'm fairly happy with the grammar in terms of it parsing my
> expected input. Doubtless I will refine it as my understanding grows.
> Where I'm stuck is how to evaluate my AST; i.e. Eval.g is where I'm
> not sure how to procee, rather than Expr.g.
>
> I think k=1 is the right choice. My understanding is that you want the
> smallest possible value of k for performance reasons. You can use a
> higher value (ideally scoped by production) to resolve ambiguities,
> etc; but you should always strive to resolve that by using
> left-factoring, semantic or syntactic predicate. Similarly for
> backtracking; it's powerful and useful, but there are other options.
>
> So in summary, I'm still stuck and looking for an example of how to
> register and execute my functions. I intuitively feel that my
> tree-walking approach is wrong, but I'm not sure what the idiomatic,
> best-practice for ANTLR would be.
>
> Cheers,
>
> James
>
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>   


Hi James,

Thanks for your response.

I had a second go (without any tree things as I'm not familiar with
that yet):
http://pastie.org/pastes/346265

this input:	
square(add( add(5,5,5), add(2,2,2)))

should give this output:	
method: add [5, 5, 5] -> 15
method: add [2, 2, 2] -> 6
method: add [15, 6] -> 21
method: square [21] -> 441


The functions look like this:

class PublicFunctions {
	public Integer square(Integer x) { return x*x; }
	public Integer addfive(Integer x) { return x+5; }
	
	public Integer add(Integer...arguments) {
		Integer sum = new Integer(0);
		for (Integer x : arguments) {
			sum+= x;
		}
		return sum;
	}
}



It's just a small modification of this grammar:
http://www.antlr.org/wiki/display/ANTLR3/Expression+evaluator

	fn=ID  '(' (fexpr=expr { args.add( new Integer($fexpr.value)); } 		// is there a better way to do this?
	              (',' aexpr=expr { args.add( new Integer($aexpr.value)); } )*	// is there a better way to do this?
	           )? ')'
	{

I tried using var+=expr, but couldn't get it to work... Any tips?

Any tips on Java coding style is also appreciated... :)

Good luck and happy holidays :-)


Ivar