How to Create Abstract Syntax Trees (ASTs) using ANTLR4 and C#/Java?

Creating Abstract Syntax Trees (ASTs) in ANTLR4

Introduction

ANTLR4 doesn't directly create ASTs like its predecessor, ANTLR3. Instead, it generates concrete syntax trees (CSTs), which can be transformed into ASTs. Visitors are commonly used to achieve this, but understanding how to apply them can be challenging. This article aims to provide a practical explanation on how to create ASTs using ANTLR4 and C#/Java through a simplified math example.

Building AST Nodes

Custom AST nodes represent the structural elements of your expression language. In our math example, we define nodes for expressions, operators, and functions.

Transforming CST to AST

A visitor is used to transform CST nodes into AST nodes. The BuildAstVisitor iterates over the CST and creates appropriate AST nodes based on the grammar rules.

AST Visitation

Once the AST is constructed, you can traverse it using an AST visitor. We define an AstVisitor base class with methods to visit each AST node type.

Expression Evaluation

To demonstrate the power of ASTs, we create an EvaluateExpressionVisitor that evaluates a given expression. By overriding the visitor methods for different AST node types, we implement the evaluation logic.

Main Program

The main program reads input expressions, parses them using ANTLR4, builds an AST, evaluates it, and prints the result.

Conclusion

Creating ASTs in ANTLR4 involves transforming CST nodes into custom AST nodes using visitors. This allows for flexible manipulation and evaluation of expressions. The provided code example demonstrates the practical application of this process, providing a step-by-step guide to building and using ASTs in ANTLR4.

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