IO Monad

iomonad.cs

using NUnit.Framework;
using System;

namespace Monads
{
   
    public class Unit{

        private Unit(){
           
        }

        public static Unit Instance(){
            return value.Value;
        }
            
        private static Lazy<Unit> value = new Lazy<Unit>(() => new Unit() );
    }

    public interface IIO<out T>{
        T PerformUnsafeIO();
    }

    public class IO<T> : IIO<T>{

        public IO(Func<T> function){
            this.function = function;
        }

        public T PerformUnsafeIO(){
            return function();
        } 

        private Func<T> function;
    }

    public static class IOExtensions{

        public static IIO<T> Unit<T>(this Func<T> f){
            return new IO<T>(f);
        }

        public static IIO<U> Select<T,U>(this IIO<T> io, Func<T,U> map){
            return new IO<U>(() => map(io.PerformUnsafeIO()));
        }

        public static IIO<U> SelectMany<T,U>(this IIO<T> io, Func<T, IIO<U>> mapMany){
            return new IO<U>(() =>
                {
                    return mapMany(io.PerformUnsafeIO()).PerformUnsafeIO();
                });
        }

        public static IIO<V> SelectMany<T,U,V>(this IIO<T> io, Func<T, IIO<U>> mapMany, Func<T,U,V> map){
            return new IO<V>(() =>
                {
                    var a = io.PerformUnsafeIO();
                    var b = mapMany(a).PerformUnsafeIO();
                    return map(a,b);
                });
        }
    }

    public interface IConsoleWriter : IIO<Unit>{

    }

    public class ConsoleWriter : IConsoleWriter{

        public ConsoleWriter(string text){
            this.text = text;
        }

        public Unit PerformUnsafeIO()
        {
            Console.WriteLine(text);
            return Unit.Instance();
        }

        private string text;
    }


    [TestFixture]
    public class MonadTests
    {

        public IIO<Unit> WriteWorld(Unit unit){
            return new ConsoleWriter("World");
        }


        [Test]
        public void IOMonadSample()
        {
            var a = new ConsoleWriter("Hello").SelectMany(WriteWorld).PerformUnsafeIO();

            var r =  from x in new ConsoleWriter("Hello")
                     from z in WriteWorld(x)
                     select z;

            r.PerformUnsafeIO();
        }
    }
}