On pages 36 and 37 of The Ruby Way (2nd Edition) Hal talks about “Coding at Runtime.” He specifically mentions how while
ifdef directives can be used in C to change how things are defined depending on conditions set prior to the compile, you can just do the same at runtime in Ruby.
For example, let’s say your app is designed to run on multiple platforms, but a certain action has to be executed in a totally different way for a couple of platforms (a common situation where Windows is involved). You could define your logic like this:
if platform == Windows action1 elsif platform == Linux action2 else default_action end
To this sort of arrangement Hal says: “Of course, there is a small runtime penalty for coding in this way since the flag may be tested many times in the course of execution.” He then presents a technique where a method itself is defined at run-time based upon the condition:
if platform == Windows def my_action action 1 end elsif platform == Linux def my_action action2 end else def my_action default_action end end
To this, Hal says: “In this way, the same result is achieved, but the flag is only evaluated once; when the user’s code calls my_action, it will already have been defined appropriately.”
Personally I’m not entirely convinced by Hal’s specific example in The Ruby Way, but it’s a handy reminder of how dynamic Ruby is. I’m sure there are some great examples of this technique in the wild.