// -> http://tomasp.net/blog/2015/csharp-pattern-matching/

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace PatternMatching
{
    class Program
    {
        static void Main(string[] args)
        {
            var expr = new Multiply
            {
                Left = new Variable { Name = "x"},
                Right = new Add
                {
                    Left = new Constant { Value = 1 },
                    Right = new Constant { Value = 2}
                }
            };

            Console.WriteLine(Format(expr));

            Console.WriteLine(Evaluate(expr, new Dictionary<string, int> { { "x", 4 } }));
            Console.ReadLine();
        }

        public static string Format(Exception e)
        {
            try
            {
                throw e;
            }
            catch (Constant n)
            {
                return n.Value.ToString();
            }
            catch (Variable v)
            {
                return v.Name;
            }
            catch (Add a)
            {
                return $"({Format(a.Left)} + {Format(a.Right)})";
            }
            catch (Multiply a)
            {
                return $"{Format(a.Left)} * {Format(a.Right)}";
            }
        }

        public static int Evaluate(Exception e, IDictionary<string, int> vars)
        {
            int res;
            try { throw e; }
            catch (Constant n) { return n.Value; }
            catch (Variable v) when (vars.TryGetValue(v.Name, out res)) { return res; }
            catch (Variable _) { throw new ArgumentException("Variable not found!"); }
            catch (Add a) { return Evaluate(a.Left, vars) + Evaluate(a.Right, vars); }
            catch (Multiply a) { return Evaluate(a.Left, vars) + Evaluate(a.Right, vars); }
        }
    }

    public class Variable : Exception
    {
        public string Name { get; set; }
    }

    public class Constant : Exception
    {
        public int Value { get; set; }
    }

    public class Add : Exception
    {
        public Exception Left { get; set; }
        public Exception Right { get; set; }
    }

    public class Multiply : Exception
    {
        public Exception Left { get; set; }
        public Exception Right { get; set; }
    }
}