Kansas City on Rails

1. The Basics
2. Unary Ampersand Operator
3. Booleans
4. Filters
5. New Collections
6. Aggregates

You’re probably familiar with Ruby’s Enumerable module, even if you don’t know it by that name. It adds neat methods to arrays, like map, inject, select, and so on. You might have thought (like I did for a long time) that these methods were *array* methods, but they’re not.

Ruby’s arrays use Enumerable, and you can as well, in any class you want. Of course, the class should represent a collection of things, or all the iterative methods in Enumerable wouldn’t make much sense. Let’s say we have a Team class, that manages a group of members:

class Team
  include Enumerable
  
  attr_accessor :members
  
  def initialize
    @members = []
  end
  
  def each &block
    @members.each{|member| block.call(member)}
  end
end

Enumerable requires that your class contain an each method that serves up the items in the collection. All the other Enumerable methods rely on this. Now we can use the map method, for instance:

irb(main):001:0> require 'team.rb'
=> true
irb(main):002:0> team = Team.new
=> #<Team:0x100391088 @members=[]>
irb(main):003:0> team.members = ['joshua', 'gabriel', 'jacob']
=> ["joshua", "gabriel", "jacob"]
irb(main):004:0> team.map{|member| member.capitalize}
=> ["Joshua", "Gabriel", "Jacob"]

Now we can call any Enumerable methods on our team itself, and it will assume we want to work with the members array within. Enumerable can be a powerful mix-in to your own classes. We can even take this a step further, and clean up our call to map:

team.map(&:capitalize)

If you haven’t seen this before, it’s called the unary ampersand operator, and it’s the subject of my next post.

Tags: ampersand, array, block, enumerable, Rails, Ruby, ruby on rails

This entry was posted on November 30, 2010 at 8:00 am and is filed under Rails, Ruby. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.