This is an example of using the Reader Monad for Dependency Injection using LINQ in C#. I learned about this from Tony Morris and Rúnar Bjarnason's video here: http://www.youtube.com/watch?v=ECPGTUa1WAI To figure out the (slightly weird) SelectMany peculiarities I found http://mikehadlow.blogspot.com/2011/01/monads-in-c-4-linq-loves-monads.html

ReaderMonad.cs

    //Reader Monad and its extension class to give it SelectMany(bind/flatMap) capabilities for use in LINQ queries
    public static class ReaderMonadExt
    {
        public static ReaderMonad<T, C> SelectMany<T, A, B, C>(this ReaderMonad<T, A> rm, Func<A, ReaderMonad<T, B>> bindf, Func<A, B, C> select)
        {
            return new ReaderMonad<T, C>(t =>
            {
                var a = rm.Run(t);
                return select(a, bindf(a).Run(t));
            });
        }
    }

    public class ReaderMonad<T, R>
    {
        private Func<T, R> _run;

        public ReaderMonad(Func<T, R> run)
        {
            _run = run;
        }

        public R Run(T t)
        {
            return _run(t);
        }
    }


    class ReaderMonadUsageExample
    {
        public static void Main(String[] args)
        {
            var config = GetConfig();
            //Notice how LINQ gives us nice syntax opposed to nested calls to SelectMany.
            var readerMonad =
                from intDb in GetIntFromDB()
                from netstr in GetStrFromNetwork()
                from writeSuccess in WriteStuffToDisk(intDb, netstr)
                select (writeSuccess ? "We wrote " + intDb + " and " + netstr + " to disk" : "error!");

            var ret = readerMonad.Run(config);
            Console.WriteLine(ret);
        }

        public static ReaderMonad<Config, Int32> GetIntFromDB()
        {
            return new ReaderMonad<Config, Int32>(config =>
            {
                config.LogMethod("Getting an int from the DB using the credentials" + config.AuthInfo);
                //some db code logging in with authinfo
                return 4;
            });
        }

        //this method shows nested composition through the magic of SelectMany
        public static ReaderMonad<Config, String> GetStrFromNetwork()
        {
            return (from aDbInt in GetIntFromDB()
                    from aGuid in GetGuidWithAuth()
                    //another method call returning a ReaderMonad that would go out over the network and get some data
                    select (aDbInt + "|" + aGuid + "|"));
        }


        private static ReaderMonad<Config, Guid> GetGuidWithAuth()
        {
            return new ReaderMonad<Config,Guid>(config =>
                {
                    config.LogMethod("Getting a GUID");
                    return new Guid();
                });
        }
                
    
        public static ReaderMonad<Config, Boolean> WriteStuffToDisk(int i, string str)
        {
            return new ReaderMonad<Config, Boolean>(config =>
            {
                config.LogMethod("attempting to write this to disk =" + i + str + " with the credentials " + config.AuthInfo);
                //write to disk if all goes well
                return true;
            });
        }

        public static void MyCustomLog(String str)
        {
            Console.WriteLine("!" + str);
        }
        //What we are trying to Inject into all our methods.  In this example we have authorization and a logging method
        //Likely you could use this for sql connections, transactions, auth credentials, a pool of resources, etc.
        public static Config GetConfig()
        {
            return new Config()
            {
                AuthInfo = "vmarquez",
                LogMethod = (str => MyCustomLog(str)),
            };
        }
    }

    class Config
    {
        public String AuthInfo { get; set; }
        public Action<String> LogMethod { get; set; }
    }