How can I create and manipulate abstract syntax trees (ASTs) using ANTLR4?

Creating an AST with ANTLR4: A Practical Guide

The concept of an abstract syntax tree (AST) can be daunting, but ANTLR4 provides an effective mechanism to construct them. This article will guide you through the process of creating an AST from a custom grammar, including using visitors to extract and manipulate data.

Building the Grammar

First, define your grammar. For this example, we'll create a simple math grammar in Java:

grammar Math;

compileUnit
    :   expr EOF
    ;

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

Creating the AST Nodes

Next, define the AST nodes. These custom nodes will represent the structure of our math expressions:

abstract class ExpressionNode {}
abstract class InfixExpressionNode extends ExpressionNode { ExpressionNode left, right; }
class AdditionNode extends InfixExpressionNode {}
class SubtractionNode extends InfixExpressionNode {}
class MultiplicationNode extends InfixExpressionNode {}
class DivisionNode extends InfixExpressionNode {}
class NegateNode extends ExpressionNode { ExpressionNode innerNode; }
class FunctionNode extends ExpressionNode { Func<Double, Double> function; ExpressionNode argument; }
class NumberNode extends ExpressionNode { Double value; }

Converting CST to AST

Visitors allow us to transform the concrete syntax tree (CST) generated by ANTLR into an AST. The BuildAstVisitor class will perform this transformation:

class BuildAstVisitor extends MathBaseVisitor<ExpressionNode> {
    @Override
    public ExpressionNode visitCompileUnit(MathParser.CompileUnitContext context) { return visit(context.expr()); }

    @Override public ExpressionNode visitNumberExpr(MathParser.NumberExprContext context) { ... }
    @Override public ExpressionNode visitParensExpr(MathParser.ParensExprContext context) { ... }
    @Override public ExpressionNode visitInfixExpr(MathParser.InfixExprContext context) { ... }
    @Override public ExpressionNode visitUnaryExpr(MathParser.UnaryExprContext context) { ... }
    @Override public ExpressionNode visitFuncExpr(MathParser.FuncExprContext context) { ... }
}

Working with the AST

Once the AST is constructed, we can use a visitor to traverse and evaluate it. The EvaluateExpressionVisitor will perform this task:

class EvaluateExpressionVisitor extends AstVisitor<Double> {
    @Override
    public Double visitAdditionNode(AdditionNode node) { ... }
    @Override
    public Double visitSubtractionNode(SubtractionNode node) { ... }
    @Override
    public Double visitMultiplicationNode(MultiplicationNode node) { ... }
    @Override
    public Double visitDivisionNode(DivisionNode node) { ... }
    @Override
    public Double visitNegateNode(NegateNode node) { ... }
    @Override
    public Double visitFunctionNode(FunctionNode node) { ... }
    @Override
    public Double visitNumberNode(NumberNode node) { ... }
}

Putting It All Together

Finally, the Main class ties everything together and prompts the user for input:

class Main {
    public static void main(String[] args) {
        while (true) {
            String exprText = ...; // Read input from the user

            // Parse the input and use the visitors to create and manipulate the AST

            Double value = ...; // Evaluate the AST using the `EvaluateExpressionVisitor`

            System.out.println("= " + value);
        }
    }
}

By following these steps, you can effectively create and manipulate ASTs using ANTLR4. This powerful feature provides a robust foundation for building complex language processors and other applications.

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