Generate AST using ANTLR4 generated visitor

Question

I am converting an ANTLR3 grammar to an ANTLR4 grammar, this means that I had to remove all tree rewriting rules because they now have to be implemented in code and by visiting ParseTree with a custom Visitor it's possible to generate an AST.

The old grammar had rewrite rules that prevented ambiguities and automatically generated the wanted tree. I couldn't find any useful resource online showing how this can be done with the visitor/listener paradigm.

A Visitor needs a type that is used as return type for all the visit* functions. If I need to generate an AST, what return type should I use? ANTLR3 used CommonTree objects.
When entering a visit* node, I can create a node of the tree, but how should I track its parent node?
Is there any example on how to convert a tree rewrite rule to a method generating a node of the AST? If not, would it be possible to have an example for a rule like the one below?

Here the rule for (3):

ctor_initializer: '::'? identifier '(' expr? ')' -> ^(CTOR_INITIALIZER^(INITIALIZER_ID '::'? identifier) ^(CTOR_EXPR expr?) );

Community · Accepted Answer · 2017-05-23 11:44:00Z

2

Idiomatically, Antlr4 creates and supports use of parse-trees. There is no direct support for creating and modifying ASTs.

Some of the motivating factors for preferring parse-trees over ASTs are summarized here.

Antlr4 does not preclude constructing ASTs -- the structure can be defined in any manner desired. Some examples are here and here.

edited May 23, 2017 at 11:44

CommunityBot

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answered Jul 17, 2015 at 19:29

GRosenberg

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2 Comments

Vektor88 Over a year ago

Thank you for the links, however (3) is not covered in the other questions because they all assume to create a new grammar instead of converting antlr3 grammar rules.

GRosenberg Over a year ago

Yes. The lack of examples anywhere on the 'net along the lines of (3) gives a telling indication of how frequently that path is taken.

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