Tree of variable definitions with and without initializations - antlr

Question

my language to parse contains statements like
public var a, b = 42, c;
I.e. the .g file looks something like:
statements
: (introduction | expression ';'! | ... )+
;
introduction
: head single+ -> ^(head single)+
;
single
: Name ('='^ expression)?
;
head
: modifiers* v='var' -> ^(VARIABLE[$v] modifiers*)
;
Generating a tree like that would be easy, but mostly useless (for me):
----------statements----------
/ | \
variable variable variable
/ \ / \ / \
'public' 'a' 'public' '=' 'public' 'c'
/ \
'b' expr
I would like to have the the '=' on top of the middle node:
----------statements----------
/ | \
variable '=' variable
/ \ / \ / \
'public' 'a' variable expr 'public' 'c'
/ \
'public' 'b'
but I can't find the rewrite rule to do that.

Answer 1

That is not easly done with the way you've set up your rules.
Here's a way it is possible:
grammar T;
options {
output=AST;
ASTLabelType=CommonTree;
}
tokens {
STATEMENTS;
VARIABLE;
DEFAULT_MODIFIER;
}
declaration
: modifier 'var' name[$modifier.tree] (',' name[$modifier.tree])* ';' -> ^(STATEMENTS name+)
;
modifier
: 'public'
| 'private'
| /* nothing */ -> DEFAULT_MODIFIER
;
name [CommonTree mod]
: ID '=' expression -> ^('=' ^(VARIABLE {new CommonTree(mod)} ID) expression)
| ID -> ^(VARIABLE {new CommonTree(mod)} ID)
;
// other parser & lexer rules
which produces the following AST:
for the input:
public var a, b = 42, c;
And produces:
for the input:
var a, b = 42, c;