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

public static class ReaderMonadExt
{
	public static Func<T, C> SelectMany<T, A, B, C>(this Func<T, A> t_a, Func<A, Func<T, B>> a_t_b, Func<A, B, C> ab_c)
	{
		return t =>
		{
			var a = t_a(t);
			return ab_c(a, a_t_b(a)(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(config);
		Console.WriteLine(ret);
	}

	//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 => Console.WriteLine("!" + str)),
		};
	}

	public static Func<Config, Int32> GetIntFromDB()
	{
		return 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 Func<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 Func<Config, Guid> GetGuidWithAuth()
	{
		return config =>
		{
			config.LogMethod("Getting a GUID");
			return new Guid();
		};
	}


	public static Func<Config, Boolean> WriteStuffToDisk(int i, string str)
	{
		return 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;
		};
	}	
}

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