This page contains automated test results for code from O'Reilly's Ruby Cookbook. If this code looks interesting or useful, you might want to buy the whole book.

Avoiding Boilerplate Code with Metaprogramming
CodeExpectedActual
class Fetcher
  def fetch(how_many)
    puts "Fetching #{how_many ? how_many : "all"}."
  end
  def fetch_one
    fetch(1)
  end
  def fetch_ten
   fetch(10)
  end
  def fetch_all
    fetch(nil)
  end
end
class GeneratedFetcher
  def fetch(how_many)
    puts "Fetching #{how_many ? how_many : "all"}."
  end
  [['one', 1], ['ten', 10], ['all', nil]].each do |name, number|
    define_method("fetch_#{name}") do
      fetch(number)
    end
  end
end
GeneratedFetcher.instance_methods - Object.instance_methods
 ["fetch_one", "fetch", "fetch_ten", "fetch_all"] ["fetch_one", "fetch", "fetch_ten", "fetch_all"] 
GeneratedFetcher.new.fetch_one
 
Fetching 1.
 
Fetching 1.
 
GeneratedFetcher.new.fetch_all
 
Fetching all.
 
Fetching all.
 
class Numeric
  [['add', '+'], ['subtract', '-'], ['multiply', '*',],
   ['divide', '/']].each do |method, operator|
    define_method("#{method}_2") do
      method(operator).call(2)
    end
  end
end
4.add_2
 6 6 
10.divide_2
 5 5 
define_method 'call_with_args' do |*args, &block|
  block.call(*args)
end
call_with_args(1, 2) { |n1, n2| n1 + n2 }
 
3
 
Error! (Exception?) Here's stdout:
SyntaxError: compile error
(irb):38: parse error, unexpected ',', expecting '|'
define_method 'call_with_args' do |*args, &block|
                                         ^
(irb):40: parse error, unexpected kEND, expecting $
	from (irb):40
NoMethodError: undefined method `call_with_args' for main:Object
	from (irb):41

call_with_args 'mammoth' { |x| x.upcase }
 
"MAMMOTH"
 
Error! (Exception?) Here's stdout:
SyntaxError: compile error
(irb):42: parse error, unexpected '{', expecting $
call_with_args 'mammoth' { |x| x.upcase }
                          ^
	from (irb):42