// closures in C#
using System;
using System.Collections.Generic;

// type A -> B in C# is Func<A,B>.
// Func(A,B,C) is type (A,B) -> C (not curried).
// Func is called a "delegate" in C#.

// A function that returns void is called an Action: Action<A> is A -> void

/*
// use inheritance to solve problem of the "transfer" method

interface transaction {}

using account = Func<string,transaction>;

class doubletrans : transaction {
  public Action<double> f;
  public doubletrans(Action<double> f) { this.f=f; }
}
class voidtrans : transaction {
  public Action f;
  public doubletrans(Action f) { this.f=f; }
}
class othertrans : transaction {
  public Action<double,account> f;
  public doubletrans(Action<double,account> f) { this.f=f; }  
}
*/

public class closures {

   // I , K and S combinators
   public static a I<a>(a x) => x;  // same as { return x; }

   public static Func<b,a> K<a,b>(a x) => (b y) => x; // a->b->a

   public static Func<Func<a,b>,Func<a,c>> S<a,b,c>(Func<a,Func<b,c>> x) =>
      (y) => (z) => x(z)(y(z)); //(a->b->c)->(a->b)->a->c  

   // accumulator
   static Func<int,int> make_accumulator() {
      int x = 0;
      return y => {
         x +=y;
	 return x;
      };
   }//make_accumulator

   // bank accounts
   static Func<string,Action<double>> make_account(double init) {
     double balance = init;
     Action<double> deposit = (double amt) => {
        if (amt>0) balance += amt;
     };
     Action<double> withdraw = (double amt) => {
        if (amt>0 && amt<balance) balance -= amt;
     };
     Action<double> inquiry = (x) => {
        Console.WriteLine("your balance is "+balance);
     };

     return (string request) => {
        switch (request) {
           case "withdraw" : return withdraw;
	   case "deposit"  : return deposit;
	   case "inquiry"  : return inquiry;
	   default: return (x) => Console.WriteLine("invalid request");
        }//switch
     };
   }//make_account

  public static void Main() {

     var a1 = make_accumulator();
     var a2 = make_accumulator();
     Console.WriteLine(a1(2));
     Console.WriteLine(a1(2));
     Console.WriteLine(a1(2));
     Console.WriteLine(a2(2));

     var acc = make_account(200);
     acc("withdraw")(30);
     acc("deposit")(100);
     Action inquiry = () => acc("inquiry")(0);
     inquiry();
  }//main
}