The ELC Community Blog

A knowledge exchange on Ruby on Rails and Agile Development

Blocks are to Ruby...

by josh on November 06, 2007

What 'ba' is to bash.

Without blocks, ruby would be sh...

Say I need to verify a user is old enough to perform a certain action or run a specific method. I could do:

if user.old_enough?(13)
  user.must_be_13_method
else
  puts "User is not old enough" # where puts is most likely a logger or flash[:error]
  return false
end

However, it quickly becomes tedious to have if/else statements like this all over the place. Wouldn't it be nice if you could put that error message somewhere else?

Good news. You can!

You can use a block in the old_enough? method of the User class:

def old_enough?(age_in_years, &block) # &block specifies that I plan to send a block
  if Time.parse(born_on) < Time.today-age_in_years.years
    # block.call executes the block.  
    # It's the same as yield, except I can pass parameters or call it multiple times.
    block.call(self)
  else
    puts "Sorry. You must be #{age_in_years} years old."
  end
end

Then you can do this:

user.old_enough?(13) do |user|
  user.must_be_13_action
end

Here's a script you can use and run to test this out.

#!/usr/bin/env ruby
require 'rubygems'
require 'active_support'

class User

  attr_accessor :born_on

  def initialize(attributes = {})
    self.born_on = attributes[:born_on]
  end

  def old_enough?(age_in_years, &block)
    if Time.parse(born_on) < Time.today-age_in_years.years
      block.call(self)
    else
      puts "Isn't it past your bedtime?"
    end
  end

  def must_be_13_action
    puts "Teenie Boppers Welcome"
  end

end

user = User.new(:born_on => '1992-01-01')

user.old_enough?(13) do |user|
  user.must_be_13_action
end

user.old_enough?(18) do |user|
  user.must_be_18_action
end

Because I am initializing the user with a birthdate in 1992 (between the ages of 13 and 18 as of this entry's publish date), the output will be:

Teenie Boppers Welcome
Isn't it past your bedtime?