How can I implement if/else statements in ANTLR using Visitors?

If/else Statements in ANTLR using Listeners

By default, ANTLR 4 generates listeners. However, using the -visitor command line parameter, you can instead have ANTLR produce visitor classes. Visitors offer more control over which sub-trees are visited, making them a better choice for implementing if/else statements than listeners.

Example Implementation

Here's an example implementation of an IfVisitor class for evaluating if/else statements:

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

    // if_stat overrides
    @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;
                // evaluate this block whose expr==true
                this.visit(condition.stat_block());
                break;
            }
        }

        if (!evaluatedBlock && ctx.stat_block() != null) {
            // evaluate the else-stat_block (if present == not null)
            this.visit(ctx.stat_block());
        }

        return Value.VOID;
    }
}</code>

In this implementation:

• The visitIf_stat method iterates through the condition blocks in an if/else statement.
• If the expression for a condition block evaluates to true, its corresponding statement block is executed, and the evaluation stops.
• If all condition blocks evaluate to false and there is an else block (non-null stat_block), the else block is executed.

Conclusion

Using visitors instead of listeners allows for more granular control over the traversal of an ANTLR parse tree, making it well-suited for implementing if/else statements. This approach is particularly useful when you need to conditionally execute different sections of your code based on the evaluation of expressions.

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