[antlr-interest] AST rewrites for recursive rules

[Phil Goodwin]

I'm building a grammar for a language that is loosely based on the Java
example grammar in v3. I liked the way methodDeclaration was defined
recursively so I wrote a toy parser to see if I could use it to build an
AST. I hunted around in the documentation and examples but I couldn't find
the syntax I needed in order to access the tree returned by a referenced
rule, so I decided to try and see if I could implement it using actions. I
started with a recognizer:

methodDeclaration
    :    TYPE IDENTIFIER ':' methodDeclaratorRest
    ;
methodDeclaratorRest
    :    formalParameters?
    ;
formalParameters
    :    TYPE formalParameterDeclsRest
    ;
formalParameterDeclsRest
    : IDENTIFIER (',' formalParameters)*
    ;
TYPE
    : '<type''0'..'9''>'
    ;
IDENTIFIER
    : '<Identifier''0'..'9''>'
    ;


When I started writing the AST code the first thing I thought was that I'd
need to make the parse iterative instead of recursive, but I'm a stubborn
sort and I liked the recusive algorithm. So I set out to just add the AST
generation code. What I ended up with takes this input:
<type0><Identifier0>:<type1><Identifier1>,<type2><Identifier2>,<type3><Identifier3>
and generates a tree that looks like this:
tree: (METHOD <Identifier0> <type0> (PARAMETERS (<type1> <Identifier1>)
(<type2> <Identifier2>) (<type3> <Identifier3>)))

I've pasted the code below. Here's my question: is there a better way to
access and manipulate the tree generated by a referenced rule? The AST
generation code below mirrors the elegance of the original recognizer but
that is lost amid the brutality done to the returned trees.

methodDeclaration
    :    t=TYPE i=IDENTIFIER ':' r=methodDeclaratorRest
    ->    ^(METHOD $i $t $r)
    ;
methodDeclaratorRest
    :    formalParameters?
    ->    ^(PARAMETERS formalParameters?)
    ;
formalParameters
    :    t=TYPE r=formalParameterDeclsRest
    ->    {((Tree)r.getTree()).getChildCount() > 1}? ^($t
{((BaseTree)r.getTree()).deleteChild(0)}) $r
    ->    ^($t $r)
    ;
formalParameterDeclsRest
    :    i=IDENTIFIER
        (',' f=formalParameters -> $i $f
        |            -> $i
        )
    ;
TYPE
    :    '<type''0'..'9''>'
    ;
IDENTIFIER
    :    '<Identifier''0'..'9''>'
    ;
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