How to Build Abstract Syntax Trees (ASTs) with Visitors in ANTLR4?

How to Create AST with ANTLR4 Using Visitors

While ANTLR4 no longer automatically constructs ASTs (Abstract Syntax Trees) like ANTLR3, it provides mechanisms for creating ASTs using visitors. Visitors allow you to traverse parse trees and perform custom actions on each encountered node.

Building an AST with Visitors

1. Define AST Nodes: Create custom AST nodes that represent the structure and semantics of your grammar.
2. Create Visitor: Extend the MathBaseVisitor class and override methods for each node type. Extract relevant information from each CST node and construct the corresponding AST node.
3. Traverse CST: Use the visitor to traverse the parse tree created by ANTLR. For each node, call the visitor's method for that node type. The visitor will create and populate the AST.

Example (Math Expression)

Grammar:

expr
    :   '(' expr ')'                         # parensExpr
    |   op=('+'|'-') expr                    # unaryExpr
    |   left=expr op=('*'|'/') right=expr    # infixExpr
    |   func=ID '(' expr ')'                 # funcExpr
    |   value=NUM                            # numberExpr
;

AST Nodes:

internal abstract class ExpressionNode { }

internal class InfixExpressionNode : ExpressionNode { }

internal class AdditionNode : InfixExpressionNode { }
internal class SubtractionNode : InfixExpressionNode { }
internal class MultiplicationNode : InfixExpressionNode { }
internal class DivisionNode : InfixExpressionNode { }

internal class NegateNode : ExpressionNode { }

internal class FunctionNode : ExpressionNode { }
internal class NumberNode : ExpressionNode { }

Visitor (BuildAstVisitor):

internal class BuildAstVisitor : MathBaseVisitor<ExpressionNode>
{
    // Visit each node type and create the corresponding AST node.
    //...
}

AST Visitor (EvaluateExpressionVisitor):

internal class EvaluateExpressionVisitor : AstVisitor<double>
{
    // Implement visit methods for each AST node type to evaluate the expression.
    //...
}

Main Program:

internal class Program
{
    // Process input expression.
    //...
}

By using the visitor pattern with ANTLR4, you can create custom ASTs that accurately represent the structure and semantics of your grammar, allowing for further analysis, evaluation, and transformation tasks.

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