How can I create Abstract Syntax Trees (ASTs) with ANTLR4 using Visitors?

Creating Abstract Syntax Trees (ASTs) with ANTLR4

Introduction

Constructing ASTs is not an inherent feature of ANTLR4 as it was in ANTLR3. Instead, it is suggested to employ visitors to achieve this functionality. This article aims to demonstrate the practical implementation of AST creation using visitors in ANTLR4.

Building Math ASTs with ANTLR4

To illustrate the process, let's work with a simple math grammar. After defining the grammar, we can focus on creating custom AST nodes:

internal abstract class ExpressionNode
{
    // Abstract expression node base class
}

For this example, we'll have specific nodes for addition, subtraction, multiplication, division, negation, functions, and numbers.

Converting Parse Trees to ASTs

The MathBaseVisitor class will serve as the foundation for our AST construction visitor:

internal class BuildAstVisitor : MathBaseVisitor<ExpressionNode>
{
    // Override visitor methods to create AST nodes based on parse tree nodes
}

Working with the AST

To interact with the AST, we'll define an abstract AstVisitor base class that supports specific visitor methods for each of our AST node types:

internal abstract class AstVisitor<T>
{
    // Visitor methods for each AST node type
}

Expression Evaluation

As a practical application, let's implement an expression evaluation visitor:

internal class EvaluateExpressionVisitor : AstVisitor<double>
{
    // Override visitor methods to evaluate expressions using AST nodes
}

Main Program

Finally, the Main method serves as the entry point for our program:

internal class Program
{
    public static void Main()
    {
        // Prompt user for math expression and construct AST
        var ast = new BuildAstVisitor().VisitCompileUnit(parser.compileUnit());

        // Evaluate AST and print result
        var value = new EvaluateExpressionVisitor().Visit(ast);
        Console.WriteLine("= {0}", value);
    }
}

By following these steps, you can construct custom ASTs using visitors in ANTLR4 and perform operations on them, such as mathematical evaluations.

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