I come from Java and want to know what monads are in Haskell

This is an introduction to monads. There are many of these. My goal today is to show how a simple class written in Java could be translated into equivalent functionality in Haskell using some monads, without getting into any of the theory stuff.

Hopefully some people coming from a non-Haskell background will get something out of this, though the Haskell syntax is likely to be very WTF-inducing for those who haven’t seen it before.

I should begin with a few things that this guide is not about:

  • Categories. The etymology of the word “monad” is a red herring. Trust me. Knowing a lot about category theory will make you a better programmer in the same way that playing a lot of checkers will make you better at chess: there’s probably some benefit, but it’s not a good way to get up and running with the basics.

    We will be treating monads as a design pattern instead of monoids in the category of endofunctors. The latter perspective is interesting to the people who like to design languages; the former perspective is interesting to people who like to write code.

  • The full generality of monads in programming. Lots of things are monads, and I will be ignoring most of them. I’m going to focus on how monads can be used to translate a particular Java class into Haskell, and what it looks like as we add functionality to both versions of the code.
  • Haskell evangelism. I’ve already written about the things in Haskell that I love. I love Haskell, and I use it to make my dreams come true. On the other hand, I’m not at all invested in whether or not you like Haskell.

    Though if you want to learn more about this language, I’d like to help you along your journey.

What I am going to talk about is how to use monads to do something in Haskell that is easy to do in Java.