[ruby-talk:13443] Re: Using Antlr for Ruby? (was RE: Re: why won't "( a) = 1" parse?)

On Mon, 2 Apr 2001, hipster wrote:

> ANTLR grammars are written in a kind of EBNF, which makes them very
> readable:
> 
> rule
> 	:	TOKEN
> 	|	nonterminal
> 	|	(optional)?
> 	|	(zero_or_more)*
> 	|	(one_or_more)?
> 	;
> 
Yeah, its great. Do you know if it is general (ie. you can apply operators
(?/+/*) to sequences of symbols and nest them or if they only applies to
individual symbols? (rockit has the latter but not the former)

> semantic_predicates
> 	:	{ conditional_code; } => rule
> 	|	{ conditional_code; } => rule
> 	|	foo
> 	;
Ok, and you have access to parsing/lexing info when writing the
conditional_code? What kind of info? Can you access the AST being built?

> automatic Abstract Syntax Tree generation with grammar annotations (^):
> 
> mult_expr
> 	:	add_expr ((STAR^ | SLASH^) add_expr)
> 	;
> 
> add_expr
> 	:	atom ((PLUS^ | MINUS^) atom)
> 	;
> 
> so 1 + 2 * 3 gives (+ 1 (* 2 3)) (LISPy representations)
> 
Interesting. In rockit I've used a different but similar thing:

mult_expr:    add_expr STAR  add_expr        [Mult:  left, _, right]
         |    add_expr SLASH add_expr        [Div:   nom, _, denom]
add_expr:     atom PLUS  atom                [Plus:  left, _, right]
         |    atom MINUS atom                [Minus: left, _, right]
atom:         NUMBER                         [^]

but as you see it's not as terse even though its more flexible. Maybe
should learn from ANTLR here...

Thanks for the intro,

Robert