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Metaprogramming Ruby
This PDF file contains pages extracted from Metaprogramming Ruby, published by the
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Note: This extract contains some colored text (particularly in code listing). This is
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Pagination might vary between the online and printer versions; the content is otherwise
identical.
Copyright ? 2009 The Pragmatic Programmers, LLC.
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Copyright ? 2010 Paolo Perrotta.
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmit-
ted, in any form, or by any means, electronic, mechanical, photocopying, recording, or
otherwise, without the prior consent of the publisher.
Printed in the United States of America.
ISBN-10: 1-934356-47-6
ISBN-13: 978-1-934356-47-0
Printed on acid-free paper.
P1.0 printing, January 2010
Version: 2010-1-29
Whenever someone says they have “a cool trick,” take them
outside and slap them up.
Jim Weirich
Appendix C
Spell Book
This appendix is a “spell book”—a quick reference to all the “spells” in
the book, in alphabetical order. Most of these spells are metaprogram-
ming related (but the ones from Appendix A, on page 242, are arguably
not that “meta”).
Each spell comes with a short example and a reference to the page
where it’s introduced. Go to the associated pages for extended examples
and the reasoning behind each spell.
C.1 The Spells
Argument Array
Collapse a list of arguments into an array.
def my_method(*args)
args.map {|arg| arg.reverse }
end
my_method('abc' , 'xyz' , '123' ) # => ["cba", "zyx", "321"]
For more information, see page 248.
Around Alias
Call the previous, aliased version of a method from a redefined method.
class String
alias :old_reverse :reverse
def reverse
"x#{old_reverse}x"
end
end
T HE S PELLS
"abc".reverse # => "xcbax"
For more information, see page 157.
Blank Slate
Remove methods from an object to turn them into Ghost Methods (75).
class C
def method_missing(name, *args)
"a Ghost Method"
end
end
obj = C.new
obj.to_s # => "#<C:0x357258>"
class C
instance_methods.each do |m|
undef_method m unless m.to_s =~ /method_missing|respond_to?|^__/
end
end
obj.to_s # => "a Ghost Method"
For more information, see page 86.
Class Extension
Define class methods by mixing a module into a class’s eigenclass (a
special case of Object Extension (153)).
class C; end
module M
def my_method
'a class method'
end
end
class << C
include M
end
C.my_method # => "a class method"
For more information, see page 153.
Class Extension Mixin
Enable a module to extend its includer through a Hook Method (183).
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259
T HE S PELLS
module M
def self.included(base)
base.extend(ClassMethods)
end
module ClassMethods
def my_method
'a class method'
end
end
end
class C
include M
end
C.my_method # => "a class method"
For more information, see page 187.
Class Instance Variable
Store class-level state in an instance variable of the Class object.
class C
@my_class_instance_variable = "some value"
def self.class_attribute
@my_class_instance_variable
end
end
C.class_attribute # => "some value"
For more information, see page 129.
Class Macro
Use a class method in a class definition.
class C; end
class << C
def my_macro(arg)
"my_macro(#{arg}) called"
end
end
class C
my_macro :x # => "my_macro(x) called"
end
For more information, see page 138.
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260
T HE S PELLS
Clean Room
Use an object as an environment in which to evaluate a block.
class CleanRoom
def a_useful_method(x); x * 2; end
end
CleanRoom.new.instance_eval { a_useful_method(3) }
# => 6
For more information, see page 109.
Code Processor
Process Strings of Code (165) from an external source.
File.readlines("a_file_containing_lines_of_ruby.txt" ).each do |line|
puts "#{line.chomp} ==> #{eval(line)}"
end
# >> 1 + 1 ==> 2
# >> 3 * 2 ==> 6
# >> Math.log10(100) ==> 2.0
For more information, see page 166.
Context Probe
Execute a block to access information in an object’s context.
class C
def initialize
@x = "a private instance variable"
end
end
obj = C.new
obj.instance_eval { @x } # => "a private instance variable"
For more information, see page 107.
Deferred Evaluation
Store a piece of code and its context in a proc or lambda for evaluation
later.
class C
def store(&block)
@my_code_capsule = block
end
def execute
@my_code_capsule.call
end
end
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261
T HE S PELLS
obj = C.new
obj.store { $X = 1 }
$X = 0
obj.execute
$X # => 1
For more information, see page 110.
Dynamic Dispatch
Decide which method to call at runtime.
method_to_call = :reverse
obj = "abc"
obj.send(method_to_call) # => "cba"
For more information, see page 66.
Dynamic Method
Decide how to define a method at runtime.
class C
end
C.class_eval do
define_method :my_method do
"a dynamic method"
end
end
obj = C.new
obj.my_method # => "a dynamic method"
For more information, see page 70.
Dynamic Proxy
Forward to another object any messages that don’t match a method.
class MyDynamicProxy
def initialize(target)
@target = target
end
def method_missing(name, *args, &block)
"result: #{@target.send(name, *args, &block)}"
end
end
obj = MyDynamicProxy.new("a string" )
obj.reverse # => "result: gnirts a"
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262
T HE S PELLS
For more information, see page 80.
Flat Scope
Use a closure to share variables between two scopes.
class C
def an_attribute
@attr
end
end
obj = C.new
a_variable = 100
# flat scope:
obj.instance_eval do
@attr = a_variable
end
obj.an_attribute # => 100
For more information, see page 105.
Ghost Method
Respond to a message that doesn’t have an associated method.
class C
def method_missing(name, *args)
name.to_s.reverse
end
end
obj = C.new
obj.my_ghost_method # => "dohtem_tsohg_ym"
For more information, see page 75.
Hook Method
Override a method to intercept object model events.
$INHERITORS = []
class C
def self.inherited(subclass)
$INHERITORS << subclass
end
end
class D < C
end
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263
T HE S PELLS
class E < C
end
class F < E
end
$INHERITORS # => [D, E, F]
For more information, see page 183.
Kernel Method
Define a method in module Kernel to make the method available to all
objects.
module Kernel
def a_method
"a kernel method"
end
end
a_method # => "a kernel method"
For more information, see page 53.
Lazy Instance Variable
Wait until the first access to initialize an instance variable.
class C
def attribute
@attribute = @attribute || "some value"
end
end
obj = C.new
obj.attribute # => "some value"
For more information, see page 246.
Mimic Method
Disguise a method as another language construct.
def BaseClass(name)
name == "string" ? String : Object
end
class C < BaseClass "string" # a method that looks like a class
attr_accessor :an_attribute # a method that looks like a keyword
end
obj = C.new
obj.an_attribute = 1
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# a method that looks like an attribute
264
T HE S PELLS
For more information, see page 243.
Monkeypatch
Change the features of an existing class.
"abc".reverse # => "cba"
class String
def reverse
"override"
end
end
"abc".reverse # => "override"
For more information, see page 35.
Named Arguments
Collect method arguments into a hash to identify them by name.
def my_method(args)
args[:arg2]
end
my_method(:arg1 => "A" , :arg2 => "B" , :arg3 => "C" ) # => "B"
For more information, see page 247.
Namespace
Define constants within a module to avoid name clashes.
module MyNamespace
class Array
def to_s
"my class"
end
end
end
Array.new # => []
MyNamespace::Array.new # => my class
For more information, see page 43.
Nil Guard
Override a reference to nil with an “or.”
x = nil
y = x || "a value" # => "a value"
For more information, see page 246.
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265
T HE S PELLS
Object Extension
Define Singleton Methods by mixing a module into an object’s eigen-
class.
obj = Object.new
module M
def my_method
'a singleton method'
end
end
class << obj
include M
end
obj.my_method # => "a singleton method"
For more information, see page 153.
Open Class
Modify an existing class.
class String
def my_string_method
"my method"
end
end
"abc".my_string_method # => "my method"
For more information, see page 33.
Pattern Dispatch
Select which methods to call based on their names.
$x = 0
class C
def my_first_method
$x += 1
end
def my_second_method
$x += 2
end
end
obj = C.new
obj.methods.each do |m|
obj.send(m) if m.to_s =~ /^my_/
end
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266
T HE S PELLS
$x # => 3
For more information, see page 69.
Sandbox
Execute untrusted code in a safe environment.
def sandbox(&code)
proc {
$SAFE = 2
yield
}.call
end
begin
sandbox { File.delete 'a_file' }
rescue Exception => ex
# => #<SecurityError: Insecure operation `delete' at level 2>
ex
end
For more information, see page 174.
Scope Gate
Isolate a scope with the class, module, or def keyword.
a = 1
defined? a # => "local-variable"
module MyModule
b = 1
defined? a # => nil
defined? b # => "local-variable"
end
defined? a
defined? b
# => "local-variable"
# => nil
For more information, see page 102.
Self Yield
Pass self to the current block.
class Person
attr_accessor :name, :surname
def initialize
yield self
end
end
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267
T HE S PELLS
joe = Person.new do |p|
p.name = 'Joe'
p.surname = 'Smith'
end
For more information, see page 250.
Shared Scope
Share variables among multiple contexts in the same Flat Scope (105).
lambda {
shared = 10
self.class.class_eval do
define_method :counter do
shared
end
define_method :down do
shared -= 1
end
end
}.call
counter
3.times { down }
counter
# => 10
# => 7
For more information, see page 106.
Singleton Method
Define a method on a single object.
obj = "abc"
class << obj
def my_singleton_method
"x"
end
end
obj.my_singleton_method # => "x"
For more information, see page 135.
String of Code
Evaluate a string of Ruby code.
my_string_of_code = "1 + 1"
eval(my_string_of_code) # => 2
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268
T HE S PELLS
For more information, see page 165.
Symbol To Proc
Convert a symbol to a block that calls a single method.
[1, 2, 3, 4].map(&:even?)
# => [false, true, false, true]
For more information, see page 253.
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269
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