using System;

​

​

/// DATA STRUCTURES

​

interface IExprVisitor {

    void On(Num num);

    void On(Plus plus);

}

​

​

interface IExprVisitor<T> {

    T On(Num num);

    T On(Plus plus);

}

​

​

abstract class Expr {

    public abstract void Accept(IExprVisitor v);

    public abstract T Accept<T>(IExprVisitor<T> v);

}

​

class Num: Expr {

    public double Value { get; set; }

    public override void Accept(IExprVisitor v) { v.On(this); }

    public override T Accept<T>(IExprVisitor<T> v) { return v.On(this); }

}

​

​

class Plus: Expr {

    public Expr Left { get; set; }

    public Expr Right { get; set; }

    public override void Accept(IExprVisitor v) { v.On(this); }

    public override T Accept<T>(IExprVisitor<T> v) { return v.On(this); }

}

​

////  OPERATIONS

​

​

class Printer: IExprVisitor<string> {

    public string On(Plus plus) {

        return plus.Left.Accept(this) + " + " + plus.Right.Accept(this);

    }

    public string On(Num num) {

        return num.Value.ToString();

    }

}

​

class Interpreter: IExprVisitor<double> {

    public double On(Plus plus) {

        return plus.Left.Accept(this) + plus.Right.Accept(this);

    }

    public double On(Num num) {

        return num.Value;

    }

}

​

class Compiler: IExprVisitor<string> {

    public string On(Plus plus) {

        return plus.Left.Accept(this) + plus.Right.Accept(this) + "add\n";

    }

    public string On(Num num) {

        return "push " + num.Value + "\n";

    }

}

​

​

public class Program

{

    public static void Main()

    {

        Expr e = new Plus(){ 

            Left = new Plus() {

                Left = new Num() { Value = 1 },

                Right = new Num() { Value = 2 }

            },

            Right = new Num() { Value = 42 }

        };

        var printer = new Printer();

        var evaluator = new Interpreter();

        var compiler = new Compiler();

        Console.WriteLine("PRINT = {0}", e.Accept(printer));

        Console.WriteLine("EVAL = {0}", e.Accept(evaluator));

        Console.WriteLine("CODE = \n---------\n{0}---------", e.Accept(compiler));

    }

}