What is Monad and how it is useful

[Nishan Patel]

Hi All,

I am learning Scala and I came across Monad, I have gone through it but I didn't understand it. So here I have question what is Monad in Scala and other Functional Programming and how it is useful?

[chris webster]

Short answer? I don't know, but I've decided not to worry about it!

Long answer? Well, there seems to be a whole industry around explaining monads to functional programmers. My limited understanding is that it's a set of rules for functional behaviour, a bit like an OO pattern but based on mathematical theory. If your "foo" function obeys these rules then it's a monad and certain assumptions can be made about it e.g. about how you can combine it with other functions. I think one common use of monads is to encapsulate functions that mutate state so you know when your code may have side effects. So monads (and monoids) occur in other functional programming languages, not just Scala, because they all need to manage state changes in a controlled fashion.

But don't rely on my vague ramblings - this guy can explain it all much better:
http://channel9.msdn.com/Shows/Going+Deep/Brian-Beckman-Dont-fear-the-Monads

[Nishan Patel]

chris webster wrote:Short answer? I don't know, but I've decided not to worry about it!

Long answer? Well, there seems to be a whole industry around explaining monads to functional programmers. My limited understanding is that it's a set of rules for functional behaviour, a bit like an OO pattern but based on mathematical theory. If your "foo" function obeys these rules then it's a monad and certain assumptions can be made about it e.g. about how you can combine it with other functions. I think one common use of monads is to encapsulate functions that mutate state so you know when your code may have side effects. So monads (and monoids) occur in other functional programming languages, not just Scala, because they all need to manage state changes in a controlled fashion.

But don't rely on my vague ramblings - this guy can explain it all much better:
http://channel9.msdn.com/Shows/Going+Deep/Brian-Beckman-Dont-fear-the-Monads

Thanks for your reply.. It means a lot to me.

But here question is what set of rules functions has to satisfy? Only these three, 1) Associativity. 2) Left identity. and 3) Right identity.

Or functions have also follow other rule as well?

[chris webster]

You could try tomorrow's webinar what have the monads ever done for us from Typesafe.

[Jesper de Jong]

My advice is: Unless you're an academic who wants to know all the theoretical details, don't worry about what the theoretical explanation of what a monad is exactly. Understanding the theoretical, mathematical definition of monads is not necessary for practical programming.

This is my own practical idea: Monads are a common functional programming design pattern. A monad is a data type that acts as a "box" for some data. The box has a method that you can give a function, and it will execute that function of the content of the box, giving you another box with the result of the function. That method is often called "map" or "flatMap".

Examples of monads in Scala are Option and List. These are both "boxes" that contain data. An Option contains either zero (None) or one (Some) element; a List can contain zero or more elements. Both Option and List have a flatMap method. When you call flatMap and pass it a function, you'll get back another Option or List with that function applied to the element(s) contained by the Option or List.

[chris webster]

Nice practical explanation from Jesper there.

If you really want to start digging into this pure FP stuff, have a look at Functional Programming In Scala by Paul Chiusano and Runar Bjarnason. I just got my hard copy of this a couple of days ago (after waiting 4 months!), and it looks like a great way to learn about pure FP and improve your Scala at the same time.

[Joe San]

Here is a very good graphical explanation of these concepts!

http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html

Don't I deserve a Cow for this?

[Nishan Patel]

Thanks to all. Now I can imagine what Monad is..