Notes on using monads in Ruby for an upcoming ATLRUG talk
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Video for this talk is now available on Vimeo.

Monads in Ruby

Monads are typically encountered when working in a functional language like Haskell or Scala, where they provide exactly the right abstraction to be able to shuttle functions around into different contexts without losing composability. While monads are great at easing the pain of things that just aren't that painful to begin with in Ruby (like mutating state, possibly unfortunately), they can still be a useful pattern to understand.

Playing along at home

This repo contains the source code used as part of a presentation that I will be giving at the ATLRUG in August, but can also be used as something like monad koans. If you'd like to try it out:

$ git clone
$ cd rb-monads
$ git checkout moore
$ bundle
$ bundle exec guard

The moore branch has several implementations removed, which causes most of the specs to fail. guard will start watching for changes to the files and rerunning the specs - your job is to get them all to pass.


The scripts/ directory contains two scripts which use the constructed monads to solve a "real" problem. knight_moves deals with a knight traversing a chess board, while flip touches on computing chains of probabilities for flipping weirdly-behaved coins.


Special thanks to Learn You a Haskell, in general for being a fantastic book and specifically for posing the problems addressed by the two included scripts. Also thanks to MenTaLguY's blog and pzol's monadic library for providing examples of "useful" monads in Ruby.


  • Video accompanying presentation and post links
  • Work on a useable implementation of the Dev monad for e.g. chaining decorators