Trying to explain monads in java

1 minute read

A few days ago, a colleague currently taking the coursera course about reactive programming in scala, asked me to explain him what monads are. It’s always a tough question, and I rarely manage to give un understandable answer simply. This time though, I kind of managed to pass him some understanding of monads :

  1. When modelising a statefull data structure with immutable constructs, one has to pass in and return the data structure state of every function
  2. This results in a lot repeated code to pass this state around
  3. With a monad, you can factorize this glue code and only write the ‘real’ code

I thought it might be a good subject for a java kata ! This is what I tried to do in java-monads-kata. Here is some sample monadic code from the kata itself :

@Test public void
pops_objects_in_reverse_push_order() {
  monad = monad.


  assertEquals(B, monad.value);

  monad = monad.bind(pop());
  assertEquals(A, monad.value);

  assertEquals(empty(), monad.stack);

You can have a look at all the final code, or go through the whole history to get the ‘kata’ feeling. It’s a shame Github does not offer a nice chronological repo history slideshow, for better experience, I recommend that you use Chrome with Github improved : this allows to view diffs right from the Github history.

The resulting code is still quite far from a Haskell monad :

  • Functions are not first class objects in Java. It is written in Java 7, without lambda, which does not help neither.
  • Java does not have Haskell type class polymorphism, but only supports OO polymorphism and very little covariance
  • The whole monad thing, designed at simulating/isolating side effects has a WTF feel in Java where side effects are just everywhere

I’d love to hear some feedback about it.