David Siegel dvdsgl

A monad is a fancy word for a generic type of the form MyMonad<T> (a generic type of arity 1).

A monad is special because it adds 'special powers' to the T that it wraps. These 'special powers' won't sound very special to an imperative programmer, so you have to squint to see them but bear with me.

IEnumerable<T> is a monad that gives values of type T the special power of nondeterminism, or the ability to 'be' multiple values at once.
Nullable<T> is a monad that gives values of type T the special power of nullability, or the ability to be absent.
Task<T> is a monad that gives values of type T the special power of asynchronicity, or the ability to be used before they are computed.

The trick with monads comes when you want to play with the T values, because they are inside another type. C# introduced language changes to make dealing with values inside these monads easier:

	using System;
	using System.Linq;
	using System.Collections.Generic;
	using System.Text.RegularExpressions;

	namespace Gibberish
	{
	public static class LoremIpsum
	{
	const string Corpus = @"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Praesent ornare id erat a pellentesque. Integer tincidunt velit odio, molestie varius lorem volutpat at. Sed ultricies ligula orci, congue condimentum libero sollicitudin non. Morbi eget nisl facilisis, semper enim in, lacinia ligula. Etiam felis leo, malesuada non consequat ut, dignissim vitae arcu. Quisque a turpis sodales, dignissim nunc posuere, ornare risus. Nunc vitae ligula porttitor, consectetur odio ut, condimentum ipsum.

	module MonkeyBot.Plugins.WhatsForLunch

	open System
	open DDay.iCal
	open MonkeyBot

	let ZeroCaterId = "..."

	type LunchResponse =
	\| Lunch of string

	using System.Drawing;
	using MonoTouch.Foundation;
	using MonoTouch.UIKit;

	namespace HexagonApp {
	[Register ("AppDelegate")]
	class AppDelegate : UIApplicationDelegate {
	UIWindow window;

	public override bool FinishedLaunching (UIApplication app, NSDictionary options) {

	open System
	open MonoTouch.UIKit
	open Cirrious.FluentLayouts.Touch

	type PhotoViewController() =
	inherit UIViewController()

	let imageViewSize = 200.0f
	let imageView =
	let view = UIImageView(

	Device.OnPlatform <\| function
	\| iOS, Tablet ->
	this.Padding <- Thickness(0, 0, 0, 0)
	\| _ -> ()

	let trace format = flip Printf.ksprintf format <\| fun line ->
	if Environment.GetEnvironmentVariable "DEBUG" = "true" then
	Console.WriteLine line

	type Writer<'Result, 'Output> =
	\| W of 'Result * seq<'Output>

	type WriterBuilder() =
	member this.Zero() = W ((), Seq.empty)
	member this.Yield(x) = W (x, Seq.empty)
	member this.Return(x) = this.Yield(x)
	member this.Bind(W (a, xs), k) =
	let (W (b, ys)) = k a
	W (b, Seq.append xs ys)

	module Trello
	( BoardId()
	, Credentials()
	, Trello()
	, runTrello
	, Board()
	, getBoard
	) where

	import Data.Maybe

	type ExpressM a = forall e. Eff (express :: Express \| e) a
	data HandlerM a = HandlerM (Request -> Response -> ExpressM Unit -> ExpressM a)

	instance monadEffHandlerM :: MonadEff eff HandlerM where
	liftEff act = HandlerM \_ _ _ -> unsafeInterleaveEff act