POODR Highlights Part 1

Practical Object-Oriented Design in Ruby or POODR is clearly a book about Ruby development, however the odd aspect is much of the concepts apply to other languages. In fact I've taken these ideas and used them both before and after reading the book in other dynamic languages and even static languages such as C#. In summary the book is well worth a read, even if you don't do Ruby development full-time.

A few of the highlights for me will be spread out across the following posts.


The author takes a firm stance on dependencies. Anything that cannot be controlled by the class itself should be protected from change. In other words a message sent to self/this is preferred than directly interacting with a dependency.

I've followed this pattern in the past, but the seeing the justifications for the benefit of this has made me realise the importance of such a practice. In the first example the publish method directly knows about the twitter feed it must interact with. In the second example the class sends a message to itself, while the class internally will still know how to interact with the dependency this is hidden. The private method has this responsibility.

With a single use you could argue there is not much difference, but the PostPublished method is a nice seam for both testing and changes. We could easily add assertions or make changes within the PostPublished method without fear of changing anything else. Finally if the PostPublished method is used in multiple places this abstraction pays for itself straight away.


Arguments are another key area that can change. Just like dependencies, the book focuses on the idea that making small changes up front can lead to flexible code that can handle change in the future. While you could argue that the order of arguments changing in the future may never happen, using named arguments has a great side effect on readability.

In static languages your IDE will most likely have a automated method of adding these in, so the C# example below can easily add named arguments with the press of a keyboard shortcut.

Named arguments provide increased readability with very little effort. Tests often benefit from the use of named arguments as you can remove the need for temporary variables, and instead in-line them to the location of use. While the third example is more “wordy”, they can safely be re-ordered without fear of compilation or runtime errors.


  • Wrap dependencies even if they are only used once. A message to self/this is preferred. Easier to change and provides seams for future work.
  • Use named arguments for improved readability and the ability to reduce temporary variables. Named variables can be dropped if there is only one argument or the variable is well named.