// Using phantom types in GC#
// This program requires .Net version 2.0
// Peter Sestoft (sestoft@itu.dk) * 2001-10-26 v 0.2

// A typed (int, bool) embedding of object expressions as in
// Leijen and Meijer 1999 section 5.1.

// The `phantom' types parameters are actually used in the types of
// methods in the objects representing the expressions; not `phantom'.

// Representing object language *variables* in a typed manner seems
// much harder, unless variables of different types are distinguished,
// and we have distinct environments for every type of variable when
// evaluating the expression.

using System;

abstract class E<R> {		// R = result type
  abstract public R eval(); 
}

class Lit<R> : E<R> {
  private readonly R v;

  public Lit(R v) {
    this.v = v;
  }

  public override R eval() {
    return v;
  }
}

abstract class Bin<S,R> : E<R> { // S = subexpr type; R = result type
  protected readonly E<S> e1, e2;

  public Bin(E<S> e1, E<S> e2) {
    this.e1 = e1; this.e2 = e2;
  }
}

class Add : Bin<int,int> {
  public Add(E<int> e1, E<int> e2) : base(e1, e2) { }

  public override int eval() {
    return e1.eval() + e2.eval();
  }
}

class Eq<S> : Bin<S,bool> {	// S = subexpr type
  public Eq(E<S> e1, E<S> e2) : base(e1, e2) { }

  public override bool eval() {
    return e1.eval().Equals(e2.eval());
  }
}

class And : Bin<bool,bool> {
  public And(E<bool> e1, E<bool> e2) : base(e1, e2) { } 

  public override bool eval() {
    return e1.eval() && e2.eval();
  }
}

// Exercising the encoding

class Phantom {
  // Build the expression n + ... + 1 + 0
  static E<int> sum(int n) {
    if (n == 0)
      return new Lit<int>(0);
    else 
      return new Add(new Lit<int>(n), sum(n-1));
  }

  static void Main() {
    E<int> e1 = sum(40);
    E<bool> e1Eq55 = new Eq<int>(new Lit<int>(55), e1);
    E<bool> e1Ne55 = new Eq<bool>(new Lit<bool>(false), e1Eq55);
    Console.WriteLine(e1.eval() + " " + e1Eq55.eval() + " " + e1Ne55.eval());
    E<string> e2 = new Lit<string>("foo");  // typesafe, no confusion possible
  }
}