How do you build an Abstract Syntax Tree (AST) with ANTLR4?

Building an AST with ANTLR4

Understanding AST Creation

ANTLR4 does not generate ASTs like ANTLR3. Instead, visitors are utilized to convert Concrete Syntax Trees (CSTs) into Abstract Syntax Trees (ASTs).

Generating Parse Trees

You can use ANTLR4 to create parse trees from your input text. When creating parse trees, you will typically use rules like expr that handle various constructs (e.g., parentheses expressions, unary expressions, infix expressions, function expressions, numbers).

Creating AST Node Classes

Define custom AST node classes that will hold your abstract syntax. For example, for a math language, you could have classes like ExpressionNode, InfixExpressionNode, AdditionNode, NumberNode, and more.

Building the AST

Use a MathBaseVisitor to traverse CST nodes and create corresponding AST nodes. For instance, you would create an AdditionNode instance when encountering infxExpr CST nodes with the operator.

Math Expression Evaluation

Once the AST is built, you can create a visitor to perform semantic actions, such as evaluating expressions. For example, an EvaluateExpressionVisitor can traverse the AST, performing addition, subtraction, multiplication, division, and function calls.

Putting It Together

In your main program, you combine these components. You parse the user's input to generate a CST, create an AST using a visitor, and finally evaluate the AST using another visitor. This allows you to perform high-level computations and computations on your syntax.

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