型だけ合わせてみたC#でStateモナド

StateMonad.cs

using System;

namespace StateMonad
{
    // sample:
    //     var proc =
    //         from initVal in State.Get<int>()
    //         let x = initVal + 10
    //         from _ in State.Put(x * 2)
    //         select (x / 2).ToString();
    //     var result = proc.Run(100);
    //     Console.WriteLine("Result Value: " + result.Value);
    //     Console.WriteLine("Last State:   " + result.State);
    public class State<TState, T>
    {
        public class Data
        {
            public readonly T Value;
            public readonly TState State;

            public Data(T value, TState state)
            {
                this.Value = value;
                this.State = state;
            }
        }

        internal readonly Func<TState, Data> RunState;

        public State(Func<TState, Data> runState)
        {
            this.RunState = runState;
        }

        public State<TState, U> Select<U>(Func<T, U> f)
        {
            return new State<TState, U>(state =>
            {
                var res = this.RunState(state);
                var value = res.Value;
                var newState = res.State;
                return new State<TState, U>.Data(f(value), newState);
            });
        }

        public State<TState, U> SelectMany<U>(Func<T, State<TState, U>> f)
        {
            return new State<TState, U>(state =>
            {
                var res = this.RunState(state);
                var value = res.Value;
                var newState = res.State;
                return f(value).RunState(newState);
            });
        }

        public State<TState, V> SelectMany<U, V>(Func<T, State<TState, U>> f, Func<T, U, V> g)
        {
            return new State<TState, V>(state =>
            {
                var res = this.RunState(state);
                var value = res.Value;
                var newState = res.State;
                var newRes = f(value).RunState(newState);
                return new State<TState, V>.Data(g(value, newRes.Value), newRes.State);
            });
        }

        public Data Run(TState initialState)
        {
            return this.RunState(initialState);
        }
    }

    public enum Unit { _ }

    public static class State
    {
        public static State<T, T> Get<T>()
        {
            return new State<T, T>(state => new State<T, T>.Data(state, state));
        }

        public static State<TState, Unit> Put<TState>(TState state)
        {
            return new State<TState, Unit>(_ => new State<TState, Unit>.Data(Unit._, state));
        }
    }
}