So I am trying out mixins and some metaprogramming in ruby and can't get this to work for me. I want it to print "Baboon"
module A
def self.included(base)
base.extend ClassMethods
end
module ClassMethods
def install_A
include InstanceMethods
end
end
module InstanceMethods
class B
def testB
#What goes here???
A.monkey
end
end
attr_accessor :monkey
def initialize()
#monkey = "baboon"
end
def test
b = B.new
puts b.testB
end
end
end
class Chimp
include A
install_A
end
c = Chimp.new
c.test
B is its own self-contained class. It is not associated or connected with any of the other classes or modules except to say that an instance of class B happens to be created inside of InstanceMethods::test. Declaring class B inside of the definition for module InstanceMethods limits the scope of B such that it's not visible outside of InstanceMethods, but it doesn't connect B and InstanceMethods in any way.
What you need is to make the module's variable visible inside B. You can implement an initialize method for B that takes a parameter, and InstanceMethods::test can use b = B.new(#monkey) to pass the value to B (make sure B::initialize stores the value as an instance variable).
Related
How do you make class methods defined within a nested series of modules propagate up the module mixin tree?
Consider the following:
module A
def self.included(base)
base.extend(ClassMethods)
end
def foo; end
module ClassMethods
def bar; end
end
end
module B
include A
end
class C
include B
end
puts "B class methods: #{(B.methods-Module.methods).inspect}"
puts "B instance methods #{B.instance_methods.inspect}"
puts "C class methods: #{(C.methods-Class.methods).inspect}"
puts "C instance methods #{(C.instance_methods-Class.instance_methods).inspect}"
Class C does not inherit the class methods defined in A, even though it includes B.
B class methods: [:bar]
B instance methods [:foo]
C class methods: []
C instance methods [:foo]
Is there a neat way of ensuring the the class methods from A are propagated upwards into C (so I could call C.bar)? I'm looking for a nice generic method that doesn't involve specifically calling out and extending C with every inherited module.
Kind regards
Steve
If you don't want to manually extend ClassMethods and you want anyone including B having its class interface extended by ClassMethods you could do that:
module B
include A
def self.included(base)
base.extend(ClassMethods)
end
end
Or you could have B defined as a class and have C inherit from it. I guess it all depends on your design and what exactly you are trying to achieve.
OK - so I figured it out. It is a slight hack, because it depends on the module hierarchy always using the module name ClassMethods to hold any Class Methods.
However, the following works (in A) - it basically detects whether the base module has ClassMethods defined. If so, it adds A's Classmethods to the base ClassMethods. If not it creates a ClassMethods module in the base (cloning A's class methods)
def self.included(base)
extend ClassMethods
if base.kind_of? Module
if base.include? ClassMethods
base::ClassMethods.extend(ClassMethods)
else
base.const_set(:ClassMethods, ClassMethods)
base.module_eval('def self.included(klass);klass.extend(ClassMethods.clone);end')
end
end
end
I have a module like this
module A
module ClassMethods
def a
"a"
end
def b
a
end
end
def self.included(klass)
klass.extend ClassMethods
end
end
I want to factor b into it's own module because Bs are clearly not As. The b method depends on A, and I would like the methods defined in A to be available to B.
Doing this:
module B
module ClassMethods
extend A
def b
a
end
end
def self.included(klass)
klass.extend(ClassMethods)
end
end
It does not work. Nor does include. I tried moving include or extend as well as the how constants are references. What am I missing here?
I hope I understood what you wanted. I split up module A and module B. When I include those in a new class Klass, I have access to both method a and method b, whereas method b from module B depends on method a from module A:
module A
module ClassMethods
def a
"a"
end
end
def self.included(klass)
klass.extend(ClassMethods)
end
end
module B
module ClassMethods
# include A::ClassMethods here and you can do only 'include B' inside Klass
def b
"Called from B: %s" % a
end
end
def self.included(klass)
klass.extend(ClassMethods)
end
end
class Klass
include A, B
end
p Klass.a # => "a"
p Klass.b # => "Called from B: a"
As I understand you can define the same Module in different places, no need to say include, extend or anything. As you see neither Module A nor Module B uses those keywords, they both just define ClassMethods.
Of course, you will have to include module A wherever you use method b of module B or else the method call to a will fail. You can also do include A::ClassMethods inside B::ClassMethods, that way you only need to include B inside Klass.
Add the extend A action in the included callback of the module B instead of adding it in the ClassMethod definition, like the following code does:
module A
module ClassMethods
def a
"a"
end
end
def self.included(klass)
klass.extend ClassMethods
end
end
module B
module ClassMethods
def b
a
end
end
def self.included(klass)
klass.extend(ClassMethods)
klass.extend(A)
end
end
class C
include B
end
C.b
# => "a"
It is known that in Ruby, class methods get inherited:
class P
def self.mm; puts 'abc' end
end
class Q < P; end
Q.mm # works
However, it comes as a surprise to me that it does not work with mixins:
module M
def self.mm; puts 'mixin' end
end
class N; include M end
M.mm # works
N.mm # does not work!
I know that #extend method can do this:
module X; def mm; puts 'extender' end end
Y = Class.new.extend X
X.mm # works
But I am writing a mixin (or, rather, would like to write) containing both instance methods and class methods:
module Common
def self.class_method; puts "class method here" end
def instance_method; puts "instance method here" end
end
Now what I would like to do is this:
class A; include Common
# custom part for A
end
class B; include Common
# custom part for B
end
I want A, B inherit both instance and class methods from Common module. But, of course, that does not work. So, isn't there a secret way of making this inheritance work from a single module?
It seems inelegant to me to split this into two different modules, one to include, the other to extend. Another possible solution would be to use a class Common instead of a module. But this is just a workaround. (What if there are two sets of common functionalities Common1 and Common2 and we really need to have mixins?) Is there any deep reason why class method inheritance does not work from mixins?
A common idiom is to use included hook and inject class methods from there.
module Foo
def self.included base
base.send :include, InstanceMethods
base.extend ClassMethods
end
module InstanceMethods
def bar1
'bar1'
end
end
module ClassMethods
def bar2
'bar2'
end
end
end
class Test
include Foo
end
Test.new.bar1 # => "bar1"
Test.bar2 # => "bar2"
Here is the full story, explaining the necessary metaprogramming concepts needed to understand why module inclusion works the way it does in Ruby.
What happens when a module is included?
Including a module into a class adds the module to the ancestors of the class. You can look at the ancestors of any class or module by calling its ancestors method:
module M
def foo; "foo"; end
end
class C
include M
def bar; "bar"; end
end
C.ancestors
#=> [C, M, Object, Kernel, BasicObject]
# ^ look, it's right here!
When you call a method on an instance of C, Ruby will look at every item of this ancestor list in order to find an instance method with the provided name. Since we included M into C, M is now an ancestor of C, so when we call foo on an instance of C, Ruby will find that method in M:
C.new.foo
#=> "foo"
Note that the inclusion does not copy any instance or class methods to the class – it merely adds a "note" to the class that it should also look for instance methods in the included module.
What about the "class" methods in our module?
Because inclusion only changes the way instance methods are dispatched, including a module into a class only makes its instance methods available on that class. The "class" methods and other declarations in the module are not automatically copied to the class:
module M
def instance_method
"foo"
end
def self.class_method
"bar"
end
end
class C
include M
end
M.class_method
#=> "bar"
C.new.instance_method
#=> "foo"
C.class_method
#=> NoMethodError: undefined method `class_method' for C:Class
How does Ruby implement class methods?
In Ruby, classes and modules are plain objects – they are instances of the class Class and Module. This means that you can dynamically create new classes, assign them to variables, etc.:
klass = Class.new do
def foo
"foo"
end
end
#=> #<Class:0x2b613d0>
klass.new.foo
#=> "foo"
Also in Ruby, you have the possibility of defining so-called singleton methods on objects. These methods get added as new instance methods to the special, hidden singleton class of the object:
obj = Object.new
# define singleton method
def obj.foo
"foo"
end
# here is our singleton method, on the singleton class of `obj`:
obj.singleton_class.instance_methods(false)
#=> [:foo]
But aren't classes and modules just plain objects as well? In fact they are! Does that mean that they can have singleton methods too? Yes, it does! And this is how class methods are born:
class Abc
end
# define singleton method
def Abc.foo
"foo"
end
Abc.singleton_class.instance_methods(false)
#=> [:foo]
Or, the more common way of defining a class method is to use self within the class definition block, which refers to the class object being created:
class Abc
def self.foo
"foo"
end
end
Abc.singleton_class.instance_methods(false)
#=> [:foo]
How do I include the class methods in a module?
As we just established, class methods are really just instance methods on the singleton class of the class object. Does this mean that we can just include a module into the singleton class to add a bunch of class methods? Yes, it does!
module M
def new_instance_method; "hi"; end
module ClassMethods
def new_class_method; "hello"; end
end
end
class HostKlass
include M
self.singleton_class.include M::ClassMethods
end
HostKlass.new_class_method
#=> "hello"
This self.singleton_class.include M::ClassMethods line does not look very nice, so Ruby added Object#extend, which does the same – i.e. includes a module into the singleton class of the object:
class HostKlass
include M
extend M::ClassMethods
end
HostKlass.singleton_class.included_modules
#=> [M::ClassMethods, Kernel]
# ^ there it is!
Moving the extend call into the module
This previous example is not well-structured code, for two reasons:
We now have to call both include and extend in the HostClass definition to get our module included properly. This can get very cumbersome if you have to include lots of similar modules.
HostClass directly references M::ClassMethods, which is an implementation detail of the module M that HostClass should not need to know or care about.
So how about this: when we call include on the first line, we somehow notify the module that it has been included, and also give it our class object, so that it can call extend itself. This way, it's the module's job to add the class methods if it wants to.
This is exactly what the special self.included method is for. Ruby automatically calls this method whenever the module is included into another class (or module), and passes in the host class object as the first argument:
module M
def new_instance_method; "hi"; end
def self.included(base) # `base` is `HostClass` in our case
base.extend ClassMethods
end
module ClassMethods
def new_class_method; "hello"; end
end
end
class HostKlass
include M
def self.existing_class_method; "cool"; end
end
HostKlass.singleton_class.included_modules
#=> [M::ClassMethods, Kernel]
# ^ still there!
Of course, adding class methods is not the only thing we can do in self.included. We have the class object, so we can call any other (class) method on it:
def self.included(base) # `base` is `HostClass` in our case
base.existing_class_method
#=> "cool"
end
As Sergio mentioned in comments, for guys who are already in Rails (or don’t mind depending on Active Support), Concern is helpful here:
require 'active_support/concern'
module Common
extend ActiveSupport::Concern
def instance_method
puts "instance method here"
end
class_methods do
def class_method
puts "class method here"
end
end
end
class A
include Common
end
You can have your cake and eat it too by doing this:
module M
def self.included(base)
base.class_eval do # do anything you would do at class level
def self.doit #class method
##fred = "Flintstone"
"class method doit called"
end # class method define
def doit(str) #instance method
##common_var = "all instances"
#instance_var = str
"instance method doit called"
end
def get_them
[##common_var,#instance_var,##fred]
end
end # class_eval
end # included
end # module
class F; end
F.include M
F.doit # >> "class method doit called"
a = F.new
b = F.new
a.doit("Yo") # "instance method doit called"
b.doit("Ho") # "instance method doit called"
a.get_them # >> ["all instances", "Yo", "Flintstone"]
b.get_them # >> ["all instances", "Ho", "Flintstone"]
If you intend to add instance, and class variables, you will end up pulling out your hair as you will run into a bunch of broken code unless you do it this way.
Say there are three classes: A, B & C. I want each class to have a class method, say self.foo, that has exactly the same code for A, B & C.
Is it possible to define self.foo in a module and include this module in A, B & C? I tried to do so and got an error message saying that foo is not recognized.
Yep
module Foo
def self.included(base)
base.extend(ClassMethods)
end
module ClassMethods
def some_method
# stuff
end
end
end
One possible note I should add - if the module is going to be ALL class methods - better off just using extend ModuleName in the Model and defining the methods directly in the module instead - rather than having a ClassMethods module inside the Module, a la
module ModuleName
def foo
# stuff
end
end
module Common
def foo
puts 'foo'
end
end
class A
extend Common
end
class B
extend Common
end
class C
extend Common
end
A.foo
Or, you can extend the classes afterwards:
class A
end
class B
end
class C
end
[A, B, C].each do |klass|
klass.extend Common
end
Rails 3 introduced a module named ActiveSupport::Concern which has the goal of simplifying the syntax of modules.
module Foo
extend ActiveSupport::Concern
module ClassMethods
def some_method
# stuff
end
end
end
It allowed us to save a few lines of "boilerplate" code in the module.
This is basic ruby mixin functionality that makes ruby so special.
While extend turns module methods into class methods, include turns module methods into instance methods in the including/extending class or module.
module SomeClassMethods
def a_class_method
'I´m a class method'
end
end
module SomeInstanceMethods
def an_instance_method
'I´m an instance method!'
end
end
class SomeClass
include SomeInstanceMethods
extend SomeClassMethods
end
instance = SomeClass.new
instance.an_instance_method => 'I´m an instance method!'
SomeClass.a_class_method => 'I´m a class method'
Just wanted to extend Oliver's answer
Define Class methods and instance methods together in a module.
module Foo
def self.included(base)
base.extend(ClassMethods)
end
module ClassMethods
def a_class_method
puts "ClassMethod Inside Module"
end
end
def not_a_class_method
puts "Instance method of foo module"
end
end
class FooBar
include Foo
end
FooBar.a_class_method
FooBar.methods.include?(:a_class_method)
FooBar.methods.include?(:not_a_class_method)
fb = FooBar.new
fb.not_a_class_method
fb.methods.include?(:not_a_class_method)
fb.methods.include?(:a_class_method)
I need to define the constant in the module that use the method from the class that includes this module:
module B
def self.included(base)
class << base
CONST = self.find
end
end
end
class A
def self.find
"AAA"
end
include B
end
puts A::CONST
But the compiler gives the error on the 4th line.
Is there any other way to define the constant?
The more idiomatic way to achieve this in Ruby is:
module B
def self.included(klass)
klass.class_eval <<-ruby_eval
CONST = find
ruby_eval
# note that the block form of class_eval won't work
# because you can't assign a constant inside a method
end
end
class A
def self.find
"AAA"
end
include B
end
puts A::CONST
What you were doing (class << base) actually puts you into the context of A's metaclass, not A itself. The find method is on A itself, not its metaclass. The thing to keep in mind is that classes are themselves objects, and so have their own metaclasses.
To try to make it clearer:
class Human
def parent
# this method is on the Human class and available
# to all instances of Human.
end
class << self
def build
# this method is on the Human metaclass, and
# available to its instance, Human itself.
end
# the "self" here is Human's metaclass, so build
# cannot be called.
end
def self.build
# exactly the same as the above
end
build # the "self" here is Human itself, so build can
# be called
end
Not sure if that helps, but if you don't understand it, you can still use the class_eval idiom above.
In your specific case.
module B
def self.included(base)
base.const_set("CONST", base.find)
end
end
class A
def self.find
"AAA"
end
include B
end
puts A::CONST
Despite it works, it's a little bit messy. Are you sure you can't follow a different way to achieve your goal?
module B
def self.included(base)
class << base
CONST = self.find
end
end
end
class A
class << self
def self.find
"AAA"
end
end
include B
end
then the compiler error is fixed, pls try.