Ruby Tidbits: Cutting out the Block with an &proc

I find one of the most useful ‘tricks’ in Ruby is to use the ‘&’ character to convert Ruby Proc objects into a block. What exactly does this mean? Well, the best way to explain it is with a couple of examples.

words = %w(one two three)
words.collect!{|word| word.upcase }
puts words.inspect
=> ["ONE", "TWO", "THREE"]

This is a pretty simple block of code, all we’re doing is converting an array of words to uppercase. Now say we’re good ruby programmers and want to keep our code readable and concise, how does this stand up? Well, it is a simple block so it is fairly readable, but how about we take a look at the same piece of code using the ‘&’ method to convert a proc into a block.

words = %w(one two three)
words.collect!(&:upcase)
puts words.inspect
=> ["ONE", "TWO", "THREE"]

These two blocks do the exact same thing, but I find the second method to dramatically improve the readability even in this simple example.

So, what’s really going on here? Well Ruby sees the ‘&’ so it wants to convert the Proc to a block. Since we have a Symbol on the other side Ruby type converts it to a proc by calling to_proc on the Symbol which returns a Proc that takes one argument and calls the method named by the symbol on that object. The implementation of to_proc on the Symbol class might look something like this:

def to_proc(arg)
  Proc.new {|arg| arg.send(self) }
end

The actual implementation is a little more complicated, allowing multiple arguments etc, but you get the idea.

You can also use this method with a normal Proc object instead of just with a Symbol, making something like this possible:

numbers = [1,2,3,4]
proc = Proc.new {|num| num*num}
numbers.collect!(&proc)
puts numbers.inspect
=> [1, 4, 9, 16]

Pretty cool isn’t it? I’ve been using the Symbol technique for awhile now to keep my code concise and readable, I never even thought about using a Proc until doing a little bit of research for this article. If you want to know more about all of this check out Understanding Ruby blocks, Procs and methods by Eli Bendersky, I based some of my examples off of his article.