state.cs

public class State<TS,TR>
	{
		public State(Func<TS, Tuple<TS, TR>> f){
			Run = f;
		}

		public State<TS,TN> Map<TN>(Func<TR,TN> f){
			return new State<TS,TN>( x => {
				var t = Run(x);
				return Tuple.Create(t.Item1, f(t.Item2));
			});
		}

		public State<TS,TN> FlatMap<TN>(Func<TR, State<TS,TN>> f){
			return new State<TS,TN>( x => {
				var t = Run(x);
				var u = f(t.Item2).Run(t.Item1);
				return Tuple.Create(u.Item1, u.Item2);
			});
		}

		public readonly Func<TS,Tuple<TS,TR>> Run;

	}

	public static class State{
		public static State<S,V> ToState<S,V>(V value){
			return new State<S, V>(x => Tuple.Create(x,value));
		}
	}