ANTLR Visitors

Parse-tree visitor

In the ANTLR parse-tree listener mechanism, we don’t have to walk trees ourselves. We don’t need to know that the runtime is walking a tree to call our methods. We know only that our listener is notified at the start and end of phrases tied to grammar rules. Unlike the parse-tree listener, a parse-tree visitor is a powerful mechanism that enables controlled traversal of the parse tree generated by the parser. This approach provides developers with a structured and flexible way to perform actions on different types of nodes within the parse tree.

Working principle of parse-tree visitors

The parse-tree visitor allows developers to define methods corresponding to different types of nodes in the parse tree. These methods are then invoked during the traversal, enabling the execution of specific actions at each node.

Understanding of parse-tree nodes

In the parse tree, each node corresponds to a specific element in the input language:

Terminal nodes represent the actual tokens in the input, while non-terminal nodes represent higher-level constructs derived from the grammar rules. Parse-tree visitors provide a means to traverse these nodes, and execute custom actions at each node type.

Anatomy of a parse-tree visitor class

When you define visitor methods for your grammar rules, ANTLR automatically generates a base visitor class. Developers extend this class to create a custom parse-tree visitor. The generated class contains visit methods corresponding to each rule in your grammar. By overriding these methods, developers can inject their logic for processing specific parse-tree nodes.

Controlling the traversal

The visit methods in the parse-tree visitor class control the traversal of the parse tree. Developers can decide whether to visit child nodes, skip certain branches, or perform custom actions based on the characteristics of the ...