ContStateInCSharp.cs

using System;

namespace Monad.Extensions
{
    public interface ContState<TState, TValue> : Cont<Unit, State<TState, TValue>> { }

    internal readonly struct PureContState<TState, TValue> : ContState<TState, TValue>
    {
        readonly Func<Func<State<TState, TValue>, Unit>, Unit> function;
        internal PureContState(Func<Func<State<TState, TValue>, Unit>, Unit> function) => this.function = function;

        public Unit Run(Func<State<TState, TValue>, Unit> observer)
        {
            return function(observer);
        }
    }

    public static partial class ContState
    {
        public static ContState<TState, TValue> Pure<TState, TValue>(Func<Func<State<TState, TValue>, Unit>, Unit> function)
            => new PureContState<TState, TValue>(function);

        public static ContState<TState, TValue> Lift<TState, TValue>(this Cont<Unit, TValue> cont)
            => Pure<TState, TValue>(observer => cont.Run(value => observer(State.Return<TState, TValue>(value))));

        public static ContState<TState, TValue> Lift<TState, TValue>(this State<TState, TValue> state)
            => Pure<TState, TValue>(observer => observer(state));

        public static ContState<TState, TValue> GetSelect<TState, TValue>(Func<TState, Cont<Unit, TValue>> generator)
            =>
            from state in State.Get<TState>().Lift()
            from value in generator(state).Lift<TState, TValue>()
            select value;
                

        public static ContState<TState, TSelectedValue> Select<TState, TSourceValue, TSelectedValue>(this ContState<TState, TSourceValue> source, Func<TSourceValue, TSelectedValue> selector)
            => Pure<TState, TSelectedValue>(observer => source.Run(state => observer(state.Select(selector))));

        public static ContState<TState, TSelectedValue> SelectMany<TState, TSourceValue, TSelectedValue>(
            this ContState<TState, TSourceValue> source, 
            Func<TSourceValue, ContState<TState, TSelectedValue>> selector)
            => Pure<TState, TSelectedValue>(observer => source.SelectMany(sourceValue => selector(sourceValue)).Run(observer));

        public static ContState<TState, TProjectedValue> SelectMany<TState, TSourceValue, TSelectedValue, TProjectedValue>(
            this ContState<TState, TSourceValue> source,
            Func<TSourceValue, ContState<TState, TSelectedValue>> selector,
            Func<TSourceValue, TSelectedValue, TProjectedValue> projector)
            => Pure<TState, TProjectedValue>(observer => source.SelectMany(sourceValue => selector(sourceValue), projector).Run(observer));
    }
}