How to Implement If/Else Statements in ANTLR 4 using Visitors?

If/else Statements with ANTLR 4

ANTLR 4 uses listeners by default, but it also supports visitors. Visitors provide more control over the traversal of the parse tree, making them more suitable for implementing if/else statements. To enable visitors, run the following command:

java -cp antlr-4.0-complete.jar org.antlr.v4.Tool Mu.g4 -visitor

This will generate a class called MuBaseVisitor, which you can extend to implement your desired functionality. Below is an example of an EvalVisitor class:

<code class="java">public class EvalVisitor extends MuBaseVisitor<Value> {

    // Override visit methods for each rule that needs to be implemented

    // Example: visitIf_stat for handling if/else statements
    @Override
    public Value visitIf_stat(MuParser.If_statContext ctx) {
        List<MuParser.Condition_blockContext> conditions = ctx.condition_block();
        boolean evaluatedBlock = false;

        for (MuParser.Condition_blockContext condition : conditions) {
            Value evaluated = this.visit(condition.expr());
            if (evaluated.asBoolean()) {
                evaluatedBlock = true;
                this.visit(condition.stat_block()); // Evaluate the true block
                break;
            }
        }

        if (!evaluatedBlock && ctx.stat_block() != null) {
            this.visit(ctx.stat_block()); // Evaluate the else block
        }

        return Value.VOID;
    }
}</code>

Here, we iterate through the conditions and evaluate the first true one. If no condition is true and an else block is present, we evaluate that instead.

To use this visitor, create a Main class to parse and evaluate the input:

<code class="java">public class Main {
    public static void main(String[] args) throws Exception {
        MuLexer lexer = new MuLexer(new ANTLRFileStream("test.mu"));
        MuParser parser = new MuParser(new CommonTokenStream(lexer));
        ParseTree tree = parser.parse();
        EvalVisitor visitor = new EvalVisitor();
        visitor.visit(tree); // Start the evaluation process
    }
}</code>

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