CODEWITHSUNDEEP
parsingantlrgrammarabstract-syntax-tree
David James Ball
David James Ball

Reputation: 913

ANTLR Variable List AST

I'm trying to allow my grammar to support variable declarations where you don't have to repeat the variable type such as int i = 3, j = 4, k;

The problem I have is with my generated tree. This is my rule...

varDeclaration
:   type ID (ASSIGN expression)? (COMMA ID (ASSIGN expression)?)* SEMICOLON -> ^(VAR_DECL type ID expression?)+;

It successfully splits apart the declaration into separate variable declarations, but it is repeating the expression tree for all of them.

So for int x = 4, y = 5, they both have value 4 in the AST.

Any help with an operator or something I can use would be appreciated.

Upvotes: 1

Views: 1146

Answers (1)

Bart Kiers
Bart Kiers

Reputation: 170158

ANTLR cannot make a distiction between the expressions. AFAIK, you'll have to create a "helper" rule.

A quick demo:

options {
  output=AST;
  ASTLabelType=CommonTree; // <- important, otherwise `$t.tree` returns an Object instead of a CommonTree
}

tokens {
  VAR_DECLS;
  VAR_DECL;
}

// ...

varDeclaration 
 : t=type assign[$t.tree] (COMMA assign[$t.tree])* SEMICOLON -> ^(VAR_DECLS assign+)
 ;

assign[CommonTree type]
 : ID (ASSIGN expression)? -> ^(VAR_DECL {type} ID expression?)
 ;

// ...

Now your input will produce the following AST:

enter image description here

EDIT

Note that the type-node will be the same for all VAR_DECL nodes. This might not be an issue (since this node is most probably not going to change), but if you want each VAR_DECL node to have its own instance of a type node, do something like this:

assign[CommonTree type]
 : ID (ASSIGN expression)? -> ^(VAR_DECL {new CommonTree(type)} ID expression?)
 ;

Or something similar. You can use plain target code inside the { and } in the rewrite rule.

Upvotes: 2

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