package org.apidesign.visitor;

public final class Language {

    private Language() { }

    public static abstract class Expression {

        Expression() {}

        public abstract void visit(Visitor v);

    }

    public static final class Plus extends Expression {

        private final Expression first;

        private final Expression second;

        public Plus(Expression first, Expression second) {

            this.first = first;

            this.second = second;

        }

        public Expression getFirst() { return first; }

        public Expression getSecond() { return second; }

        @Override

        public void visit(Visitor v) { v.visitPlus(this); }

    }

    public static final class Number extends Expression {

        private final int value;

        public Number(int value) { this.value = value; }

        public int getValue() { return value; }

        @Override

        public void visit(Visitor v) { v.visitNumber(this); }

    }

    public interface Visitor {

        public void visitUnknown(Expression e);

        public void visitPlus(Plus s);

        public void visitNumber(Number n);

    }

}