# Monad tutorials timeline

This is a comprehensive timeline of monad tutorials and related articles.

Please update this list as it becomes outdated! If you find a tutorial, article, post, comment, or message that stands on its own as an explanation of monads, then please take a moment to paste the link somewhere on this page (register a throwaway account, if you prefer). The date, author, and blurb can be added later. This will greatly help others who are using this list as a resource for learning about monads.

## 1 before 2000

• 1992-08 Monads for Functional Programming (pdf) - Phil Wadler, a designer of Haskell [9100 words]
"Shall I be pure or impure?" ... "A monad is a triple (M; unit; *) consisting of a type constructor M and two operations of the given polymorphic types."
• 1995-05 Monadic IO in Haskell 1.3 - Andrew D. Gordon and Kevin Hammond
"We describe the design and use of monadic I/O in Haskell 1.3"
• 1999-02 What the hell are Monads? - Noel Winstanley [2800 words]
"Once upon a time, people wrote their Haskell programs by sequencing together operations in an ad-hoc way." ... "For our purposes, a monad is a triple of a type and then> & return operators defined over it so that the following laws apply: ..."
• 1999-Spring Monads for the working Haskell Programmer - Theodore S. Norvell [4200 words, Haskell98, Gofer]
"...monads can be used to implement several other programming features including: consuming input, producing output, exceptions and exception handling, nondeterminisim."

## 2 year 2002

• 2002 Yet Another Haskell Tutorial (Chapter: Monads) - Hal Daumé III
"The definition of a monad is a slightly trimmed-down version of our Computation class. The Monad class has four methods ..."

## 3 year 2003

• 2003-08 All about Monads - Jeff Newbern [22600 words]
A comprehensive introduction to monads, covering also "advanced" topics like monad transformers and use of some common monads. There is an appendix which presents monads as assembly lines.
"A monad is a way to structure computations in terms of values and sequences of computations using those values."

## 4 year 2004

• 2004-07 A Schemer's Introduction to Monads - Dave Herman [1700 words, Lisp, Scheme]
"The main insight of monads is that all side effects, from mutation to I/O to non-termination, have one thing in common: order of evaluation matters." ... "So monads are about talking about effects in the context of a pure semantics."
• 2004-07 Monads as Containers (Russian translation) - Cale Gibbard [2900 words]
"A monad is a container type together with a few methods defined on it. Monads model different kinds of computations." ... "it is more natural sometimes to begin with map (fmap), return and join"
• 2004-08 Monads in Perl - Greg Buchholz [2200 words, Perl]
"Essentially a monad is a hidden data structure (Fig. 1) which automatically passes state around for us. "

## 5 year 2005

• 2005-07 Monads in Ruby - MenTaLguY
Presents monads in a friendly language, starting from Identity and building on up
• 2005-11 Of monads and space suits - Eric Kow
Functions are space stations, parameters are astronauts and monads are space suits that let us safely travel from one function to another.

## 6 year 2006

• 2006-03 Understanding Monads - Eric Kow
Monads as nuclear waste containers, an adaptation of monads as space suits with a new metaphor suggested by Paul Johnson
"What we need is some way to capture the pattern 'do X and then do Y, where Y may be affected by X'. Monads are the way we do this." ... "In a sense, each monad is its own little minilanguage specially suited for its particular task."
• 2006-07 The Monadic Way - Andrea Rossato
A two-part tutorial. The first part shows you how build a simple evaluator, and the second part shows you how to "take the complexity" out of it by using techniques such as monad transformers
Revised 2006-09 to include Meet Bob The Monadic Lover: "what monads look like and what they are useful for, from the perspective of a ... lover."
• 2006-08 You could have invented monads! (and maybe you already have) - Dan Piponi
"Writing introductions to monads seems to have developed into an industry," Dan (sigfpe) observes. He argues that monads are not "something esoteric in need of explanation", but walks you through the process of reinventing monads to solve some very basic and practical problems.
"So now I can reveal what a monad is. The triple of objects (m,unit,bind) is the monad, and to be a monad they must satisfy a bunch of laws such as the ones you've been proving."
• 2006-10 Monad Transformers Step by Step - Martin Grabmüller
Monad transformers are rarely covered in introductory tutorials. This "is not a paper about implementing transformers, but about using them to write elegant, clean and powerful programs in Haskell". Available as a 12 page PDF or .lhs file.
• 2006-11 There's a Monster in my Haskell! Andrew Pimlott
This delightful "tutorial" presents monads as monsters which devour values, use them to feed other monsters and regurgitate them when slain.

## 7 year 2007

• 2007-01 Think of a monad - Don Stewart (reposted on Eric Kow's blog)
Don integrates some pre-existing monadic metaphors, shedding light on monads in a truly comprehensive manner (illustration by Eric)
• 2007-02 Understanding Monads. For Real - Karsten Wagner
A monad is like a macro
• 2007-02 Crash Course in Monads Monads for Mathematicians
Author's Description: This crash course starts with an EASY! introduction to categories and functors, then we define a monad, then give some basic examples of monads in categories, then present monadic terminology as used in programming languages.
Then I lie down in a dark room with a warm wet cloth over my eyes.
• 2007-04 The Real Monad Transformer - Henning Thielemann
Not a tutorial either, but an important aid in demystifying monads
• 2007-03 Monads in 15 Minutes - Eric Kidd
Eric boils monads down to 15 minutes, using backtracking and Maybe as motivating examples. Eric uses
join
, which seems quite rare for monad tutorials (cf Cale's Monads as containers)
• 2007-07 Monads! (and why monad tutorials are all awful) - Colin Gordon?
Csgordon reports that monad tutorials tend to "get horribly botched" and says "[they] either bored me to tears, struck me as completely inane, or simply confused me". He uncovers the early Phil Wadler's paper, Monads for Functional Programming, which not only gives a clear explanation but provides non-trivial motivating examples
• 2007-08 Monads as computation - Cale Gibbard
A very straightforward presentation of monads. Notable for its "The whole point" section, which conveys why we bother with all this monad business.
• 2007-08 Understanding Monads (2) - Apfelmus
Wikibook rewrite of the original monads tutorial. Less fluff, more pedagogy. [In progress at the time of this writing].
• 2007-08 Monad (sans metaphors) - Claus Reinke
From a discussion about monad tutorials on Haskell Café (the name is due to haskellwiki user 'Green tea').
• 2007-11 How to do IO in Haskell - Toby Goodwin
In some ways, a non-monad tutorial, describes Haskell IO (with lots and lots of examples) with an emphasis on types, rather than monad theory

## 8 year 2008

• 2008-06 Monads (in Russian, English translation) - Eugene Kirpichov [8200 words ru, 10000 en]
"A monad is a triple (m, return, >>=), where: ..." ... "As we progressed, we saw that monads are most commonly used for two different purposes: structuring the control flow and describing imperative effectful computations (IO, State, IndentIO), and structuring data flow (Maybe, List, Dist)."

## 10 year 2010

• 2010-03 State Monad Tutorial - Byron Johnson
"My goal is to teach others to help them understand the State monad. ... After this point, you should be able to easily understand the other monads."
• 2010-07 I come from Java and want to know what monads are in Haskell - Tim Carstens [Java]
Translates a simple Java class into a stack of monad transformers, with a metaphor about how monads are like conversations, and why this idea should be familiar to OO programmers.
"What I am going to talk about is how to use monads to do something in Haskell that is easy to do in Java." ... "This is what different monads do: each comes with its own set of operations that are legal within the context that the monad is modeling."
• 2010-08 Yet Another Monad Tutorial, part 1 (2, 3, 4, 5, 6, 7, 8) - mvanier [7100, 5400, 3100, 5600, 5500, 5800, 6700, 5800 words]
"Monads are a generalization of functions, function application, and function composition to allow them to deal with richer notions of computation than standard functions."

## 11 year 2011

• 2011-01 Monads for the Curious Programmer: Part 1, Part 2, Part 3, and Monads in C++ - Bartosz Milewski [3400, 4300, 3400, and 5200 words]
"Monads are hard to describe because they don’t correspond to anything in our everyday experience" ... "A monad is an endofunctor together with two special families of morphisms, both going vertically, one up and one down"
• 2011-04 Understanding Haskell Monads - Ertugrul Söylemez [12400 words]
"A monad is a wrapper type around another type (the inner type), which adds a certain structure to the inner type and allows you to combine computations of the inner type in a certain way."

## 12 year 2012

• 2012-04 Why Do Monads Matter? - Chris Smith
"... The category that they form is called a Kleisli category, and it’s basically another way of looking at monads."