Statemonad.cs

using System;

namespace ConsoleApp28
{
    public readonly struct Unit
    {
        public static Unit Instance { get; } = new Unit();
    }

    public class State<S, A>
    {
        public Func<S, (A, S)> Run { get; }

        public State(Func<S, (A, S)> run)
        {
            Run = run;
        }

        public static State<S, A> Return(A a) => new State<S, A>(s => (a, s));
        public State<S, B> Bind<B>(Func<A, State<S, B>> g) => new State<S, B>(s1 =>
        {
            var (a, s2) = Run(s1);
            return g(a).Run(s2);
        });
    }

    public static class State
    {
        public static State<S, S> Get<S>() => new State<S, S>(s => (s, s));
        public static State<S, Unit> Set<S>(S s) => new State<S, Unit>(_ => (Unit.Instance, s));
    }

    public static class XState
    {
        public static State<S, B> Select<S, A, B>(this State<S, A> value, Func<A, B> map) =>
            value.Bind(a => State<S, B>.Return(map(a)));

        public static State<S, TResult> SelectMany<S, A, B, TResult>(this State<S, A> value, Func<A, State<S, B>> bind, Func<A, B, TResult> project) =>
            value.Bind(a => bind(a).Bind(b => State<S, TResult>.Return(project(a, b))));
    }

    class Program
    {
        static State<int, Unit> Foo(int a) =>
            from x in State.Get<int>()
            let valueToAdd = a > 0 ? 1 : a == 0 ? 2 : 3
            from _ in State.Set(x + valueToAdd)
            select Unit.Instance;

        static State<string, Unit> ConcatStates(State<string, Unit> a, State<string, Unit> b) =>
            from __ in a
            from aValue in State.Get<string>()
            from _ in b
            from bValue in State.Get<string>()
            from ___ in State.Set<string>(aValue + bValue)
            select Unit.Instance;

        static void Main(string[] args)
        {
            var initial = State<int, Unit>.Return(Unit.Instance);
            var next = Foo(0);
            var next2 = Foo(-1);
            Console.WriteLine(next.Bind(_ => next2).Run(100));

            var a = State.Set("Hello ");
            var b = State.Set("World!");
            Console.WriteLine(ConcatStates(a, b).Run(""));
        }
    }
}