実用重視のOptionMonad"""モドキ""" in C#

Option.cs

//直和を表現しきれない点を気にしたら負け
public interface IOption<out T> : IEnumerable<T>
{
    bool HasValue { get; }//== this.Any()
}
sealed public class Some<T> : IOption<T>
{
    private readonly T val;
    public bool HasValue { get { return true; } }

    public Some(T v) {
        val = v;
    }

    public IEnumerator<T> GetEnumerator() {
        yield return val;
    }
    IEnumerator IEnumerable.GetEnumerator() {
        return this.GetEnumerator();
    }
}
sealed public class None<T> : IOption<T>
{
    public bool HasValue { get { return false; } }

    public None() { }

    public IEnumerator<T> GetEnumerator() {
        yield break;
    }
    IEnumerator IEnumerable.GetEnumerator() {
        return this.GetEnumerator();
    }
}

static public class Option
{
    static public Some<T> Some<T>(T v) {
        return new Some<T>(v);
    }
    static public None<T> None<T>() {
        return new None<T>();
    }
    static public IOption<T> Return<T>(T v) {
        if (v != null)
            return new Some<T>(v);
        else
            return new None<T>();
    }
    static public IOption<U> Bind<T, U>(this IOption<T> _, Func<T, IOption<U>> f) {
        return _.HasValue ?
            f(_.First()) :
            Option.None<U>();
    }
    static public IOption<T> ToOption<T>(this IEnumerable<T> _) {
        if (_.Any())
            return Option.Some<T>(_.First());
        else
            return Option.None<T>();
    }
    static public U Match<T, U>(this IOption<T> _, Func<T, U> Some, Func<U> None) {
        return _.HasValue ?
            Some(_.First()) :
            None();
    }
    static public void Match<T>(this IOption<T> _, Action<T> Some, Action None) {
        if (_.HasValue)
            Some(_.First());
        else
            None();
    }
}

Validation.cs

var x = 10;
var m = Option.Return<int>(x);
Func<int, IOption<int>> f = v => Option.Some(v + 1);
Func<int, IOption<int>> g = v => Option.Some(v * 3);

//左恒等性(m00 == m01)
var m00 = Option.Return(x).Bind(f);
var m01 = f(x);

//右恒等性(m10 == m11)
var m10 = m.Bind(Option.Return);
var m11 = m;

//結合則(m21 == m21)
var m20 = m.Bind(f).Bind(g);
var m21 = m.Bind(v => f(v).Bind(g));