Ruby Best Practices

Higher-Order Procedures

Throughout this whole chapter, we’ve been taking something for granted that not all languages can: in Ruby, a Proc is just another object. This means we can sling these chunks of code around as if they were any other value, which results in a whole lot of powerful functionality.

This feature is so ingrained in Ruby development, and so well integrated into the system, that it’s easy to overlook. However, I could not in good conscience wrap up a chapter on functional programming techniques without at least showing a simple example of higher-order procedure.

A function is said to be a higher-order function if it accepts another function as input or returns a function as its output. We see a lot of the former in Ruby; basically, we’d see this any time we provide a code block to a method. But functions that return functions might be a bit less familiar to those who are new to the language.

Through closures, we can use a function to essentially build up customized procedures on the fly. I could show some abstract examples or academically exciting functionality such as Proc#curry, but instead, I decided that I wanted to show you something I use fairly frequently.

If you used Rails before Ruby 1.9, you probably were familiar with the “innovation” of Symbol#to_proc. What some smart folks realized is that Ruby’s &block mechanism actually calls a hook on the underlying object, which could be self-defined. Though Symbol#to_proc exists in Ruby 1.9 by default, let’s look at ...