[antlr-interest] Flattening lists?

[Gary R. Van Sickle]

> From: Jim Idle
> 
> On Mon, 2008-12-15 at 08:44 -0600, Gary R. Van Sickle wrote: 
> 
> 	
> 	Hi all,
> 	
> 	Not sure if this is something that should be obvious or 
> not, but is there a
> 	way, completely in ANTLR 3.x (C backend FWIW), to 
> flatten a list of
> 	"things"?  What I mean is, say I have a language I'm 
> trying to parse with
> 	constructs something like this:
> 	
> 		// Declare a bunch of variables.
> 		int varA, varB, varC;
> 	
> 	Now, it's pretty straightforward to convert this via 
> lexer->parser->tree
> 	parser into:
> 	
> 		(VAR_DECL int (INITIALIZER varA) (INITIALIZER 
> varB) (INITIALIZER
> 	varC))
> 	
> 	But is there a way to get this instead?:
> 	
> 		(VAR_DECL int varA) (VAR_DECL int varB) 
> (VAR_DECL int varC)
> 	
> 	Of course once I have the AST I can traverse it with C 
> &&/|| C++, but it
> 	would sure be nice if ANTLR was able to flatten these 
> sort of constructs for
> 	me.
> 	
> 	Thanks,
> 	
> 
> 
> 
> rule : INT i+=ID (COMMA i+=ID)  ->^(VAR_DECL INT $i)+
>   ;
> 
> 
> Jim 

Thanks Jim, but what I'm looking at is a bit more complicated.  What I have
currently for a parser grammer looks in part like this:


[...]
declaration
    : ts=type_specifier idl=init_declarator_list ';' -> ^(VAR_DECL $ts $idl)
    ;

init_declarator_list
    : init_declarator (',' init_declarator)* -> ^(init_declarator)+
    ;

init_declarator
    : declarator ('=' initializer)? -> ^(INITIALIZER declarator
initializer?)
    ;
[...]

(i.e. a slightly simplified version of the C variable declaration syntax)


I suppose I could flatten my *grammar* by eliminating the
init_declarator_list entirely and doing something like this:


[...]
declaration
    : ts=type_specifier idl+=init_declarator (',' idl+=init_declarator)* ';'
-> ^(VAR_DECL $ts $idl)+
    ;
[...]


But this seems like a step backwards, and something that either the rewrite
rule itself should be able to handle without flattening the grammar, or
something the tree parser should be able to handle (i.e. match the VAR_DECL
subtree and then 'iterate' over the children in the init_declarator_list and
taking some action for each one).

Is there really no other way to do this other than flattening the grammar
itself?  I can especially see that as being unworkable if one's language
supported "lists of lists" and you wanted to do this sort of flattening.

Thanks again,

-- 
Gary R. Van Sickle