// basic implementation of Option monad in C#

public abstract class Option<T> {
  public abstract Option<U> map<U>( Func<T,U> fun);
  public abstract U match<U>( Func<T,U> somef, Func<U> nonef );
  public T unwrap_or(T default_val) {
    return match(some: v => v,
                 none: () => default_val
                );
  }
  // this functor is also called "bind" or "flatmap":
  public Option<U> and_then<U>( Func<T,Option<U>> flatfun ) =>
    match(some: v => flatfun(v),
          none: () => Option<U>.Nothing
         );

  public static readonly Option<T> Nothing = new None<T>(); //1 time constant
  public static Option<T> Make(T x) {   // includes null-testing
    if (x!=null) return new Some<T>(x); else return Nothing;
  }
}//Option abstract type

// concrete, non-public sealed classes (can't be further extended)
sealed class Some<T> : Option<T> {
   private T val;
   public Some(T v) {val=v;}
   public override Option<U> map<U>(Func<T,U> fun) => Option<U>.Make(fun(val));
   public override U match<U>( Func<T,U> somef, Func<U> nonef ) => somef(val);
}

sealed class None<T> : Option<T> {
   public override Option<U> map<U>(Func<T,U> fun) => Option<U>.Nothing;
   public override U match<U>( Func<T,U> somef, Func<U> nonef ) => nonef();
}