Ruby Symbol#to_proc

What does &: mean in Ruby? In this post, I’ll explain.

Explore a Rails app, and you might see something like this:

def self.names
  find(:all).collect(&:name)
end

What is that &:, you might be wondering? It’s shorthand for this:

def self.names
  find(:all).collect { |p| p.name }
end

Often in Ruby, we are using a method that invokes a given block. The methods map, collect, and select are just three examples. To employ one of these methods, you might write something like this:

[1,2,3,4,5].select { |num| num.odd? } #=> [1,3,5]

This is fine, but there is a more concise expression, known as the Ruby Symbol#to_proc method. From the Ruby 2.1.3 documentation, the to_proc method ‘returns a Proc object which respond to the given method by sym.’ Here it is in action:

[1,2,3,4,5].select(&:odd?) #=> [1,3,5]

This takes the method named odd? as a symbol :odd? and invokes it as a Proc via the ampersand (&). It’s more concise, and gains value as you chain block-invoking methods. Here is a contrived example to illustrate:

[1,2,3,4,5].select { |num| num.odd? }.map { |num| num.to_f }.map { |num| num.to_s }
=> ['1.0', '3.0', '5.0']

[1,2,3,4,5].select(&:odd?).map(&:to_f).map(&:to_s)
=> ['1.0', '3.0', '5.0']

Try integrating this technique into your Ruby app.

I was surprised to find out that this was a Ruby method rather than a Rails method. Although my examples above are pure Ruby, this method gets really useful in Rails when you start chaining ActiveRecord associations. To me, that’s where its power is fully realized.

As it turns it, this was actually a Rails method first, starting with 1.1.1 and deprecated after 2.3.8. It was then merged into Ruby itself.

This type of cross-pollination between Rails and Ruby is part of why both technologies remain relevant. It takes communication between core teams and responsiveness to users to make the right call about where a feature belongs. I think this was successful, because Symbol#to_proc is valuable and should be available to any Ruby framework, not just Rails.

• Ruby