C# Maybe Sketch

gistfile1.cs

public abstract class Maybe<A> {
  public abstract A Value { get; }
  public abstract Maybe<B> Map<B>(Func<A, B> f);
  
  private Maybe() { }
  
  public static Maybe<A> nothing() {
    return new Nothing();
  }
  
  public static Maybe<A> just(A value) {
    return new Just(value);
  }
  
  private class Nothing<A> : Maybe<A> {
    private Nothing() : base() {}
    
    public override A Value {
      get { throw new Exception(); }
    }
    
    public override Maybe<B> Map<B>(Func<A, B> f) {
      return nothing();
    }
  }
  
  private class Just<A> : Maybe<A> {
    private readonly A _value;
    
    private Just(A value) : base() {
      _value = value;
    }
    
    public override A Value {
      get { return _value; }  
    }
    
    public override Maybe<B> Map<B>(Func<A, B> f) {
      return just(f(_value));
    }
  }
}