The Definitive ANTLR 4 Reference, 2nd Edition

Building a Calculator Using a Visitor

To get the previous arithmetic expression parser to compute values, we need to write some Java code. ANTLR v4 encourages us to keep grammars clean and use parse-tree visitors and other walkers to implement language applications. In this section, we’ll use the well-known visitor pattern to implement our little calculator. To make things easier for us, ANTLR automatically generates a visitor interface and blank visitor implementation object.

Before we get to the visitor, we need to make a few modifications to the grammar. First, we need to label the alternatives of the rules. (The labels can be any identifier that doesn’t collide with a rule name.) Without labels on the alternatives, ANTLR generates ...