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What is a good online reference to learn about "Monads" for beginners? Someone posted a reference here on HN sometime ago but I lost it. Edit: The book was enj

What is a good online reference to learn about "Monads" for beginners? Someone posted a reference here on HN sometime ago but I lost it. Edit: The book was enjoyable and a delight to read, but I started struggling to keep up from Chapter 8. as it went into Monoids and Monads, I feel like I need to read other tutorials before coming back to it.

TazeTSchnitzel 12y ago
Well, Learn You a Haskell's explanation of Monads in Haskell seems alright.
danem 12y ago
There was quite a long discussion about this on the Haskell subreddit this past week[1], and indeed this topic seems to always be something newcomers get caught up with. My advice is to use the language until you gain an intuition for them. Despite being rather simple, I found it was difficult to fully appreciate them without a decent amount of exposure. Really though, monads aren't what Haskell is all about, and don't let an incomplete understanding of them stop you from diving in. [1] http://www.reddit.com/r/haskell/comments/2vm0ed/is_there_a_good_succinct_metaphorfree_monad/ http://www.reddit.com/r/haskell/comments/2vm0ed/is_there_a_g...
Retra 12y ago
I recommend this to anybody who wants to learn about Haskell's important data types: https://wiki.haskell.org/Typeclassopedia https://wiki.haskell.org/Typeclassopedia
- bstamour 12y ago
  I found the typeclassopedia to be really helpful after I'd been programming in Haskell for a few years. It really helped tie all of the types together, but a part of me thinks that it might be a bit overkill for someone who's just dipping their toes in for the first time.
  - Retra 12y ago
    I like that it builds up to monads by introducing functor and applicative first, as those are far easier stones to leap to.
fleshweasel 12y ago
I have seen a few monad tutorials and found this series far more comprehensible than others-- it starts with the real-world monads many programmers use unknowingly and works back to the definition. http://ericlippert.com/2013/02/21/monads-part-one/ http://ericlippert.com/2013/02/21/monads-part-one/
cstrahan 12y ago
I highly recommend watching Tom Stuart's talk "Refactoring Ruby with Monads": https://www.youtube.com/watch?v=J1jYlPtkrqQ https://www.youtube.com/watch?v=J1jYlPtkrqQ
- tomstuart 12y ago
  Thanks! There’s a transcript at http://codon.com/refactoring-ruby-with-monads http://codon.com/refactoring-ruby-with-monads.
mietek 12y ago
The original monad papers, by Philip Wadler. Accept no substitutes. http://homepages.inf.ed.ac.uk/wadler/topics/monads.html http://homepages.inf.ed.ac.uk/wadler/topics/monads.html http://homepages.inf.ed.ac.uk/wadler/ http://homepages.inf.ed.ac.uk/wadler/ Alternatively, if you really are allergic to LaTeX, or beards, the recently Oscared sigfpe’s “You Could Have Invented Monads! (And Maybe You Already Have.)” http://blog.sigfpe.com/2006/08/you-could-have-invented-monads-and.html http://blog.sigfpe.com/2006/08/you-could-have-invented-monad... There are no other monad tutorials. Just forget about them.
- codygman 12y ago
  I highly recommend "You could have invented Monads", it helped me a lot. I also recommend the Haskell Wikibook: http://en.wikibooks.org/wiki/Haskell/Understanding_monads http://en.wikibooks.org/wiki/Haskell/Understanding_monads
- nathell 12y ago
  Yes there are. What made monads eventually "click" for me was the incredibly detailed, eight-part Mike Vanier's tutorial on monads in general and in Haskell in particular. First part: http://mvanier.livejournal.com/3917.html http://mvanier.livejournal.com/3917.html
- tensorproduct 12y ago
  You Could Have Invented Monads is what really made monads "click" for me. There are better tutorials for how to use monads in the real world, but I've never read a better description of why you would want to, if you don't know what they are.
Mikeb85 12y ago
You don't need to really understand monads to use them. Not at first anyway. They're basically just containers that allow you to use side effects, hold state, etc... https://acm.wustl.edu/functional/io-monad.jpg https://acm.wustl.edu/functional/io-monad.jpg
- wyager 12y ago
  >They're basically just containers that allow you to use side effects, hold state, etc... No, that is one use for Monads. A Monad is just something that has a bind (>>=) function and a return function.
  - dllthomas 12y ago
    The something has to be a single argument type constructor, and bind and return need to abide by some laws. I don't mean that as any sort of "gotcha", just some additional clarification - especially if people read your "just" too strong.
  - vertex-four 12y ago
    > A Monad is just something that has a bind (>>=) function and a return function. You realise that this is the most useless description ever? While I'm sure it's right, it doesn't actually mean anything. Why's it a good abstraction for doing <X>, better than any other abstraction? If it abstracts over a gazillion different things, why not have special-purpose abstractions like other languages (and thus probably make the language a lot simpler to use)? Your description is on the same level as "programming languages are structured text".
    - thedufer 12y ago
      The point, I think, is that the original description leaves out the easiest monads, which makes learning about them a lot more difficult. See: List, Maybe, etc.
      - the_af 12y ago
jdeisenberg 12y ago
First, you are not alone. I also "hit the wall" when the discussion turned to functors, monoids, and monads. Now, let me be the token heretic here. I took an online Haskell course where the instructor said, "Now that you have learned about monads, please don't write yet another monad tutorial!" Perhaps the reason that there are so many tutorials about them is because the really are NOT easy to grasp.
bcbrown 12y ago
If you don't mind abstract mathematics, the category theory lectures from https://www.youtube.com/playlist?list=PL8Ky8lYL8-Oh7awp0sqa82o7Ggt4AGhyf https://www.youtube.com/playlist?list=PL8Ky8lYL8-Oh7awp0sqa8... were what made monads finally click for me.
platz 12y ago
Why do we need monads?: http://stackoverflow.com/a/28141248/176876 http://stackoverflow.com/a/28141248/176876
akurilin 12y ago
Theory is generally wasted on me until I bang my head against the code for a while. If you're like me, you'll probably find it easiest to try to use monads in real code over and over, mostly based on existing examples, and then it will finally click and you'll be able to go back and grok the underlying theory.
exDM69 12y ago
They don't teach you number theory before they teach you addition and subtraction in primary school. Don't read the "Monad tutorials" and especially do not stop programming in Haskell until you "learn monads". Find one of the respected books, read it and do the exercises. You do not need to thoroughly understand monads in order to do some IO programming just like a violinist does not have to understand the physics of vibrating strings or you don't need to be an auto mechanic to drive to the grocery store. Write the "Guess the Number" game in Haskell using IO and you should understand the basics and that gets you far. Reading and writing to/from files and network sockets, etc works pretty much the same as you're used to from imperative programming languages. "Guess the number" contains just enough control flow (repetition, conditionals) to get you started. I've written tens of thousands of lines of Haskell code, including production code at work, and I still don't know what a Monoid is. I have only a faint theoretical understanding of monads, but I am fluent writing IO code and applying monads to other problems like parsing and error handling. You should only study monads when you're fluent doing some basic imperative programming in Haskell or if you are a math student and have a keen interest in category theory.
- tomjakubowski 12y ago
  > Reading and writing to/from files and network sockets, etc works pretty much the same as you're used to from imperative programming languages. I agree completely that it's not necessary (and maybe even detrimental) to learn monads to use Haskell's IO, but I think this claim is dubious. Not too many imperative languages have lazy IO and this feature can be a surprising hurdle for the Haskell beginner who is versed in other languages.
- dllthomas 12y ago
  "I still don't know what a Monoid is." That is surprising. They show up all over in fairly common libraries - for instance, Writer collects a monoidal value. The very simplest notion is "monoid is for things you can combine". Taken from math, a monoid is a set of things (a type) for which you have an operation (called mappend in Haskell) which you can combine associatively, where the set contains an element (mempty in Haskell) that is an identity with respect to that operation. Some common monoids are: Anything list-like: mappend appends two lists mempty is the empty list Integers over addition (newtype Sum) mappend adds two integers mempty is zero Tuple of monoids: mappend is componentwise mappend mempty is (mempty, mempty) Functions where the domain and range are the same: mappend is function composition mempty is id
kaeluka 12y ago
Both monads and monoids: The only thing you need to learn about them is practice. Screw tutorials and books. Read LYAH's intro chapter once (you won't understand everything, but that's fine) and then write code.
louthy 12y ago
If you want to start right at the concept level before jumping into code, this is a great intro to functors, applicatives, and monads. http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html http://adit.io/posts/2013-04-17-functors,_applicatives,_and_...
sinkasapa 12y ago
The best explanation I ever had was, "It is just a type class." In other words, don't worry about some deep metaphor. A Monad will have a type constructor, bind and the badly named return. There are laws that say how they'll work. The intuitions can come later.
domenzain 12y ago
Stephan Boyer teaches about them in a peculiarly pragmatic way. Haskell programmers often abstract application-specific computations into monads and use them as a sort of interface—a functional API. http://www.stephanboyer.com/post/9/monads-part-1-a-design-pattern http://www.stephanboyer.com/post/9/monads-part-1-a-design-pa...

TazeTSchnitzel 12y ago

Well, Learn You a Haskell's explanation of Monads in Haskell seems alright.

danem 12y ago

There was quite a long discussion about this on the Haskell subreddit this past week[1], and indeed this topic seems to always be something newcomers get caught up with. My advice is to use the language until you gain an intuition for them. Despite being rather simple, I found it was difficult to fully appreciate them without a decent amount of exposure. Really though, monads aren't what Haskell is all about, and don't let an incomplete understanding of them stop you from diving in. [1] http://www.reddit.com/r/haskell/comments/2vm0ed/is_there_a_good_succinct_metaphorfree_monad/ http://www.reddit.com/r/haskell/comments/2vm0ed/is_there_a_g...

Retra 12y ago

I recommend this to anybody who wants to learn about Haskell's important data types: https://wiki.haskell.org/Typeclassopedia https://wiki.haskell.org/Typeclassopedia

bstamour 12y ago
I found the typeclassopedia to be really helpful after I'd been programming in Haskell for a few years. It really helped tie all of the types together, but a part of me thinks that it might be a bit overkill for someone who's just dipping their toes in for the first time.
- Retra 12y ago
  I like that it builds up to monads by introducing functor and applicative first, as those are far easier stones to leap to.

fleshweasel 12y ago

I have seen a few monad tutorials and found this series far more comprehensible than others-- it starts with the real-world monads many programmers use unknowingly and works back to the definition. http://ericlippert.com/2013/02/21/monads-part-one/ http://ericlippert.com/2013/02/21/monads-part-one/

cstrahan 12y ago

I highly recommend watching Tom Stuart's talk "Refactoring Ruby with Monads": https://www.youtube.com/watch?v=J1jYlPtkrqQ https://www.youtube.com/watch?v=J1jYlPtkrqQ

tomstuart 12y ago
Thanks! There’s a transcript at http://codon.com/refactoring-ruby-with-monads http://codon.com/refactoring-ruby-with-monads.

mietek 12y ago

The original monad papers, by Philip Wadler. Accept no substitutes. http://homepages.inf.ed.ac.uk/wadler/topics/monads.html http://homepages.inf.ed.ac.uk/wadler/topics/monads.html http://homepages.inf.ed.ac.uk/wadler/ http://homepages.inf.ed.ac.uk/wadler/ Alternatively, if you really are allergic to LaTeX, or beards, the recently Oscared sigfpe’s “You Could Have Invented Monads! (And Maybe You Already Have.)” http://blog.sigfpe.com/2006/08/you-could-have-invented-monads-and.html http://blog.sigfpe.com/2006/08/you-could-have-invented-monad... There are no other monad tutorials. Just forget about them.

codygman 12y ago
I highly recommend "You could have invented Monads", it helped me a lot. I also recommend the Haskell Wikibook: http://en.wikibooks.org/wiki/Haskell/Understanding_monads http://en.wikibooks.org/wiki/Haskell/Understanding_monads
nathell 12y ago
Yes there are. What made monads eventually "click" for me was the incredibly detailed, eight-part Mike Vanier's tutorial on monads in general and in Haskell in particular. First part: http://mvanier.livejournal.com/3917.html http://mvanier.livejournal.com/3917.html
tensorproduct 12y ago
You Could Have Invented Monads is what really made monads "click" for me. There are better tutorials for how to use monads in the real world, but I've never read a better description of why you would want to, if you don't know what they are.

Mikeb85 12y ago

You don't need to really understand monads to use them. Not at first anyway. They're basically just containers that allow you to use side effects, hold state, etc... https://acm.wustl.edu/functional/io-monad.jpg https://acm.wustl.edu/functional/io-monad.jpg

wyager 12y ago
>They're basically just containers that allow you to use side effects, hold state, etc... No, that is one use for Monads. A Monad is just something that has a bind (>>=) function and a return function.
- dllthomas 12y ago
  The something has to be a single argument type constructor, and bind and return need to abide by some laws. I don't mean that as any sort of "gotcha", just some additional clarification - especially if people read your "just" too strong.
- vertex-four 12y ago
  > A Monad is just something that has a bind (>>=) function and a return function. You realise that this is the most useless description ever? While I'm sure it's right, it doesn't actually mean anything. Why's it a good abstraction for doing <X>, better than any other abstraction? If it abstracts over a gazillion different things, why not have special-purpose abstractions like other languages (and thus probably make the language a lot simpler to use)? Your description is on the same level as "programming languages are structured text".
  - thedufer 12y ago
    The point, I think, is that the original description leaves out the easiest monads, which makes learning about them a lot more difficult. See: List, Maybe, etc.
    - the_af 12y ago

jdeisenberg 12y ago

First, you are not alone. I also "hit the wall" when the discussion turned to functors, monoids, and monads. Now, let me be the token heretic here. I took an online Haskell course where the instructor said, "Now that you have learned about monads, please don't write yet another monad tutorial!" Perhaps the reason that there are so many tutorials about them is because the really are NOT easy to grasp.

bcbrown 12y ago

If you don't mind abstract mathematics, the category theory lectures from https://www.youtube.com/playlist?list=PL8Ky8lYL8-Oh7awp0sqa82o7Ggt4AGhyf https://www.youtube.com/playlist?list=PL8Ky8lYL8-Oh7awp0sqa8... were what made monads finally click for me.

platz 12y ago

Why do we need monads?: http://stackoverflow.com/a/28141248/176876 http://stackoverflow.com/a/28141248/176876

akurilin 12y ago

Theory is generally wasted on me until I bang my head against the code for a while. If you're like me, you'll probably find it easiest to try to use monads in real code over and over, mostly based on existing examples, and then it will finally click and you'll be able to go back and grok the underlying theory.

exDM69 12y ago

They don't teach you number theory before they teach you addition and subtraction in primary school. Don't read the "Monad tutorials" and especially do not stop programming in Haskell until you "learn monads". Find one of the respected books, read it and do the exercises. You do not need to thoroughly understand monads in order to do some IO programming just like a violinist does not have to understand the physics of vibrating strings or you don't need to be an auto mechanic to drive to the grocery store. Write the "Guess the Number" game in Haskell using IO and you should understand the basics and that gets you far. Reading and writing to/from files and network sockets, etc works pretty much the same as you're used to from imperative programming languages. "Guess the number" contains just enough control flow (repetition, conditionals) to get you started. I've written tens of thousands of lines of Haskell code, including production code at work, and I still don't know what a Monoid is. I have only a faint theoretical understanding of monads, but I am fluent writing IO code and applying monads to other problems like parsing and error handling. You should only study monads when you're fluent doing some basic imperative programming in Haskell or if you are a math student and have a keen interest in category theory.

tomjakubowski 12y ago
> Reading and writing to/from files and network sockets, etc works pretty much the same as you're used to from imperative programming languages. I agree completely that it's not necessary (and maybe even detrimental) to learn monads to use Haskell's IO, but I think this claim is dubious. Not too many imperative languages have lazy IO and this feature can be a surprising hurdle for the Haskell beginner who is versed in other languages.
dllthomas 12y ago
"I still don't know what a Monoid is." That is surprising. They show up all over in fairly common libraries - for instance, Writer collects a monoidal value. The very simplest notion is "monoid is for things you can combine". Taken from math, a monoid is a set of things (a type) for which you have an operation (called mappend in Haskell) which you can combine associatively, where the set contains an element (mempty in Haskell) that is an identity with respect to that operation. Some common monoids are: Anything list-like: mappend appends two lists mempty is the empty list Integers over addition (newtype Sum) mappend adds two integers mempty is zero Tuple of monoids: mappend is componentwise mappend mempty is (mempty, mempty) Functions where the domain and range are the same: mappend is function composition mempty is id

kaeluka 12y ago

Both monads and monoids: The only thing you need to learn about them is practice. Screw tutorials and books. Read LYAH's intro chapter once (you won't understand everything, but that's fine) and then write code.

louthy 12y ago

If you want to start right at the concept level before jumping into code, this is a great intro to functors, applicatives, and monads. http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html http://adit.io/posts/2013-04-17-functors,_applicatives,_and_...

sinkasapa 12y ago

The best explanation I ever had was, "It is just a type class." In other words, don't worry about some deep metaphor. A Monad will have a type constructor, bind and the badly named return. There are laws that say how they'll work. The intuitions can come later.

domenzain 12y ago

Stephan Boyer teaches about them in a peculiarly pragmatic way. Haskell programmers often abstract application-specific computations into monads and use them as a sort of interface—a functional API. http://www.stephanboyer.com/post/9/monads-part-1-a-design-pattern http://www.stephanboyer.com/post/9/monads-part-1-a-design-pa...