[antlr-interest] Tree grammars & optional AST nodes

[Nicolas Rouquette]

Thanks for the answer!

Terence Parr <parrt at ...> writes:

> Hi :)
> 
> You mean a parser grammar that builds trees, right?

yes

> expression
> options {backtrack=true;}
>   : e1=conditionalExpression op=assignmentOperator e2=expression
>     -> ^($op $e1 $e2)
>   | conditionalExpression
>   ;
> 
> OR, do this:
> 
> expression
>   : conditionalExpression (assignmentOperator^^ expression)?
>   ;
> 
> :)
> 
> Hard to beat tree operators for expression stuff.

An ANTLR afficionado colleague suggested 
adding a simple semantic predicate 
to guide the tree construction:

@init { boolean flag=false; }
  : e1=conditionalExpression (op=assignmentOperator e2=expression {flag=true;})?
    -> {flag} ? ^($op $e1 $e2)
    -> $e1
  ;

I tried this w/ the ANTLRWorks debugger on a grammar
with several nested expression-like rules. Works very nicely!

I've used the same options as the java grammar
(although my language isn't java at all).

Adding the semantic predicate obfuscates a bit the grammar
but the price is really worth the result IMHO.

-- Nicolas. 

> Ter