Ruby Singleton, module vs class - ruby

I want to get in difference of implementation singleton pattern in ruby vs class and vs module.I'm talking about singleton with class methods only and no instances. As for me, it is logical to use
module Foo
def self.foo= other
##foo=other
end
def self.foo
##foo
end
end
but very often I see in others code class Foo;....;end and I want to understand why?If there is no instances and no sub classes Module is more convenient. Or may be I miss something?
The question is what is the diff between module and class in singleton pattern implementation?

The simplest way to get an object in Ruby is to use, well, an object:
class << Foo = Object.new
attr_accessor :foo
end
Using either a module or a class is overkill, both have features you don't need.

Singletons are objects whose class ensures that it is instantiated only once, and same instance is shared by all clients.
Ruby's Singleton module helps in achieving this in convenient manner.
A module cannot be considered singleton as it cannot be instantiated. Look at Can a Ruby module be described as a singleton class? for more details.
To make a class singleton, its better to use Singleton module than re-inventing the wheel.

Related

Ruby inheritance and typing

I am having trouble with some fundamental concepts in Ruby, specifically the interchangeability of a subclass for the superclass.
According to the Ruby documentation on classes, "Class" inherits from "Module".
https://ruby-doc.org/core-2.5.3/Class.html
class MyClassTest end
MyClassTest.is_a? Module # => true
However, when trying to use the module keyword to reopen a class defined with the keyword class, you get a TypeError that the class is not a module.
class MyClassTest end
module MyClassTest end # => TypeError: MyClassTest is not a module
This SO question has some excellent discussion surrounding subclasses vs subtypes, but I think it has lead me to more questions:
Why can't classes be used as modules?
Generally, since Ruby is dynamically typed, I am confused by the existence of TypeErrors.
Specifically, in this case, I am extra confused as to how Ruby inheritance can result in a TypeError where the subclass cannot be substituted for the superclass. In my mind, subclassing is equivalent to subtyping in Ruby since the subclass will inherit the interface (methods and public attributes) of the superclass.
My current guess is that TypeError's are raised by the core Ruby library when certain assertions fail, and these TypeErrors don't necessarily have anything to do with Ruby's dynamic typing system, which is to say that typing is not a first-class concept in Ruby. The linked SO question raises excellent points about the diamond problem with multiple class inheritance, so it makes sense that Ruby would prevent the interchangeable usage of modules and classes when using the module or class keyword. Still, it feels like there are inconsistencies in my understanding of Ruby.
How can a "Class" input result in a TypeError when a "Module" object is expected?
Basic assertions are
Class is a Class (Class.is_a?(Class) #=> true)
Class is a Module (Class.is_a?(Module) #=> true)
An instance of the class Class is a Class (Class.new.is_a?(Class) #=> true)
An instance of the class Class is a Module (Class.new.is_a?(Module) #=> true)
Module is a Class (Module.is_a?(Class) #=> true)
By virtue Module is a Module (Module.is_a?(Module) #=> true)
An instance of the class Module is a Module (Module.new.is_a?(Module) #=> true)
However An instance of the class Module is not a Class (Module.new.is_a?(Class) #=> false)
an instance of the class Class is an instance of Class but not and instance of the class Module (Class.new.instance_of?(Module) #=> false)
module is a declaration for an instance of the class Module just as class is a declaration for an instance of the class Class.
If this were a method it might look like
def module(name,&block)
raise TypeError if defined?(name) && !const_get(name).instance_of?(Module)
obj = defined?(name) ? const_get(name) : const_set(name, Module.new)
obj.instance_eval(&block)
end
TypeError exists to prevent ambiguity.
As in your case by using class MyClassTest you have created an instance of the class Class and that instance is called MyTestClass.
If you were also allowed to use module MyTestClass, in the same global context, then during usage I would be unaware if when calling MyClassTest I would be calling the Class or the Module.
The basic (very basic) difference is a Class can be instantiated (have instances) however a Module cannot.
For instance
Class.new.new #creates an instance of the anonymous class created by Class.new
Module.new.new # results in NoMethodError
I think the first point of confusion is the distinction between usage and definition.
The following code defines a class:
class C; end
If I see the above code, I expect to be able to instantiate C later:
C.new
However, imagine that C was already defined as a module:
# In one file:
module C; end
# Later in a different file:
class C; end # C is actually a module
C.new # fails
Ruby surfaces the problem (conflicting definitions of C) at the point where C is redefined as a class, rather than allowing the program to continue to where C is used.
The benefit of surfacing the problem earlier is generally the earlier an error is identified, the easier it is to find and fix its root cause (in this example, perhaps C is supposed to be a class after all, and thus the real issue is the module C definition).
Your second question is, I think, why a class cannot always be used as a module, for example why the following is prohibited:
class C; end
class A
include C
end
The answer, I think, is the programming language starts with concepts which are then implemented using various constructs. We could describe classes and modules as follows:
A class represents an object which has data and behavior (classic OOP definition).
A module is a collection of behavior.
The include keyword extends a class with a module's behavior. In principle it is possible to take just a class's methods and add them to another class. But this operation doesn't make sense because a class is an object and its behavior together. Taking just the behavior goes against the concept of the class.
There are other programming languages which take different positions on this issue. For example, in JavaScript any function can be taken out of any class and invoked with an instance of any other class. This can be convenient in some situations and difficult to debug in others.

How to define module methods like those in module Math?

The methods in Math can be invoked like a class method:
Math.cos(0)
but also can be include-d like instance method:
include Math
cos(0)
In contrast, the following modules can be invoked in one way but not the other:
module Foo
def bar
end
end
Foo.bar() # NoMethodError for this call
include Foo
bar() # but this call is fine
Singleton method:
module Foo
def self.bar
end
end
Foo.bar() # this call is fine
include Foo
bar() # but not this one
Any idea how to write a module like Math?
There're a few ways to get singleton methods, so I'm going to go over those first. We'll get to the part that lets the include Math work in a minute. So, first, if you're in a module or class body, you can define a singleton method as a method of self, like so:
module Foo
# Define bar as a method on self (the Foo module), thereby making
# it a singleton method.
def self.bar
"baz"
end
end
Alternatively, you can define them as methods on a module or class's singleton class:
module Foo
# Opens the singleton class of self (the Foo module). This makes
# bar a singleton method (see Module#define_singleton_method for
# some more on that).
class <<self
def bar
"baz"
end
end
end
include Math, Having Your Methods, and Eating Them Too
Thirdly, if you want methods as both instance and singleton methods, you can use extend. This allows you to include the module somewhere and call its methods without qualification or at least with differing qualification, depending on where you include the module (sort of -- that's beyond the scope of this, though). You can also extend self or extend using another module (containing instance methods) to add them as singleton methods when in a module or class body. This might sound more complicated than it probably looks:
module Foo
def bar
"baz"
end
# Extending self will add the instance methods of self as
# methods on the object self -- which happens to be a module,
# so you basically get class methods from the instance methods.
extend self
end
This last case allows you to also include the module in another module or class and gain bar as an instance method as well, so what you do depends on what you need. In general, I prefer the first route if I'm just defining a singleton method and it's all I'll need. The second option is more or less equivalent, but also allows you to use alias_method and so on. Qualified access is next to godliness, as far as I'm concerned.
The third option, however, — using extend self — is good for doing what you're asking about with include Math, where you want to be able to both call a function as a singleton method (Math.cos(0)) and include the module to access and call the methods without qualifying them with the module name (cos(0)). If you want that, you can do one of the following:
Define the method twice, both as a singleton method and as an instance method. This is not preferrable.
Define them in another module and both include and extend using that module. This is handy if you want to use the module in multiple places.
extend self. Extending using self is probably the best choice here, since it's simple, reduces duplicate code, and it's sufficient for the purpose of the question.
So there you go, instance methods and singleton methods living side-by-side in harmony, just like Holan and Hamlet.
That's what Module#module_function is for.

why are metaclasses created in ruby?

I m trying to understand the Ruby Object Model. I understood that the instance methods are saved in the class rather than in the objects of the class because it removes redundancy. I read that whenever a class is created, a metaclass is created too for the newly created class. the metaclass stores the class methods. ie the singleton methods of the class are located in the metaclass. eg
class MyClass
def hi
'hi object'
end
def self.bye
'bye singleton method'
end
end
for the above MyClass, a metaclass (say #MyClass) is created too. now the method 'hi' is an instance level method and can be called on all the objects of MyClass. method 'bye' is a singleton method of MyClass and it resides in the #MyClass. the reason (i think so) why 'hi' is saved in MyClass rather than all the objects of MyClass is because it avoids redundancy. But we cant have more than one classes named MyClass. So why not store 'bye' in MyClass rather than in #MyClass, since we cant have more than one MyClass. I have absolutely no idea why this is the way it is and i m just trying to understand the reason behind it.
-----UPDATE----
metaclass store the class information like the singleton methods and other stuff. But since a class is a singleton object(its an instance of class Class and is alone of its type) then why not save all the information in the class itself rather than the metaclass.
Just to be super duper clear.
Here is a quick ruby script that explains the question:
#!/usr/bin/env ruby
puts ObjectSpace.count_objects[:T_CLASS] #>> 471
class X
def self.foo
end
def bar
end
end
puts ObjectSpace.count_objects[:T_CLASS] #>> 473
This is what the OP meant by "ObjectSpace.count_objects[:T_CLASS] increments the count by 2." Let's call the extra class the singleton class of X, because that appears to be what Ruby calls it internally.
irb> X
=> X
irb> X.singleton_class
=> <Class: X>
Notice that the #foo method is an instance method of X.singleton_class, not X.
irb> X.instance_methods(false)
=> [:baz]
irb> X.singleton_class.instance_methods(false)
=> [:foo]
So why is :foo stored in X.singleton_class instead of X? Isn't there only ever going to be one X?
I believe the main reason is for consistency. Consider the following, simpler scenario concerning plain instance objects.
car = Car.new
def car.go_forth_and_conquer
end
As #mikej explained superbly, this new method is stored in car's singleton class.
irb> car.singleton_class.instance_methods(false)
=> [:go_forth_and_conquer]
Classes are Objects
Now, classes are objects too. Each class is an instance of Class. Thus, when a class (say, X) is defined, ruby is really creating an instance of Class, and then adding methods to the instance (similar to what we did to car above.) For example, here is an alternative way to create a new class
Car = Class.new do
def go_forth_and_conquer
puts "vroom"
end
end
Car.new.go_forth_and_conquer
Therefore, it is much easier to just reuse the code and do it the same way (i.e. keep foo in X.singleton_class.) This probably requires less effort and will lead to fewer surprises, so no one will ever need to write code to handle Class instances differently from other instances.
Probably Doesn't Matter
You might be thinking that if Ruby did not have singleton classes for instances of Class, there could be some memory savings. However, it sounds to me that where bar is actually stored is an implementation detail that we probably shouldn't count on. Rubinius, MRI, and JRuby could all store methods and instances differently, as long as the behavior is consistent. For all we know, there could be a reasonable implementation of Ruby that doesn't eagerly create singleton classes for class objects, for the exact same reasons you outlined, as long as the overall behavior conforms to the ruby spec. (E.g. an actual singleton class doesn't exist until the #singleton_class method is first invoked.)
This isn't quite an answer to your question, but it might be useful. Two things to think about that might help:
metaclass is not really a good name for what's going on here when you think of how the meta prefix is used in other scenarios. eigenclass which you will see used in other documentation is probably a better name, meaning "an object's own class"
It's not just classes that have an eigenclass, every object does
The eigenclass is used to store methods that are specific to a particular object. e.g. we can add a method to a single String object:
my_string = 'Example'
def my_string.example_method
puts "Just an example"
end
This method can only be called on my_string and not on any other String object. We can see that it is stored in my_string's eigenclass:
eigenclass = class << my_string; self; end # get hold of my_string's eigenclass
eigenclass.instance_methods(false) # => [:example_method]
Remembering that classes are objects, in this context, it makes sense that the methods specific to a particular class should be stored in that class's eigenclass.
Update: actually, there is an eigenclass for the eigenclass. We can see this more easily if we add eigenclass as a method to Object:
class Object
def eigenclass
class << self
self
end
end
end
and then we can do:
irb(main):049:0> my_string.eigenclass
=> #<Class:#<String:0x269ec98>>
irb(main):050:0> my_string.eigenclass.eigenclass
=> #<Class:#<Class:#<String:0x269ec98>>>
irb(main):051:0> my_string.eigenclass.eigenclass.eigenclass # etc!
=> #<Class:#<Class:#<Class:#<String:0x269ec98>>>>
whilst this seemingly creates an infinite regress, this is avoided because Ruby only creates the eigenclasses on as they are needed. I think the name "metaclass" really is a source of part your confusion because you are expecting a "metaclass" to hold some kind of information that it actually doesn't.
The reason really boils down to what self is. For any given method, self is an instance of the object that the method is defined one.
In an instance method, stored on MyClass, self will be the instance of MyClass that you called #hi from. In a class method, self will be the instance of the metaclass (that is, the class itself). Doing it with the metaclass means that the concept of self is unchanged, even when operating on a class, which is itself just a singleton instance of its metaclass.
As per The Ruby Programming Language the class methods, are infact singleton methods on an instance of the class that got same name as the class.
Class Foo
def self.bar
"Am a class method"
end
end
here method self.bar can be depicted as a singleton method on an instance Foo of type Class Foo.
#the following code is just to explain on what actually are class methods called
Foo.bar #=> "Am a class method"
#the call is done on an instance of class Foo which got ref name Foo
We can go on adding more class/singleton/metaclass methods on Foo by
class<<Foo
def self.another_bar
"Am another class method"
end
end
Foo.another_bar #=>"Am another class method"
More formally singleton methods are defined as instance methods of an anonymous eigenclass/meta class.
Though conceptually wrong we can assume classes are objects in this context, so as to have a better grasp.
This concept is there to bring in true Object Oriented - ness in all levels of the language. Objective-C implements class methods in a similar fashion.
In Obj-C metaclasses bails out as classes which contain information about the classes it meta. And the principles of inheritance do apply for meta-classes as well, there super class is its classe's superclasse's metaclass and climbs up so until it reaches the base object, whoe's metaclass is the metaclass itself. More reading on this can be done here.

Using class << self, when to use classes or modules?

Is there a difference in usage between
class Helper
class << self
# ...
end
end
and
module Helper
class << self
# ...
end
end
When would you use one over the other?
The class<<self seems to be a red herring, as the only difference here is a class versus a module. Perhaps you're asking "I want to create an object that I do not intend to instantiate, but which exists only as a namespace for some methods (and possibly as a singleton with its own, global, state)."
If this is the case, both will function equally well. If there is any chance that you might want to create a derivative (another object inheriting the same methods) then you should use a class as it slightly is easier to write:
class Variation < Helper
instead of
module Helper
module OwnMethods
# Put methods here instead of class << self
end
extend OwnMethods
end
module Variation
extend Helper::OwnMethods
However, for just namespacing I would generally use a module over a class, as a class implies that instantiation will occur.
The difference between a Module and a Class is that you can make an instance of a Class, but not a Module. If you need to create an instance of Helper (h = Helper.new) then it should be a class. If not, it is probably best to remain a module. I'm not sure how the rest of your code is relevant to the question; whether you have class methods on a Module or a Class is not relevant to whether you need to create instances of that object.

Using Class vs Module for packaging code in Ruby

Let's say I have a bunch of related functions that have no persistent state, say various operations in a string differencing package. I can either define them in a class or module (using self) and they can be accessed the exact same way:
class Diff
def self.diff ...
def self.patch ...
end
or
module Diff
def self.diff ...
def self.patch ...
end
I can then do Diff.patch(...). Which is 'better' (or 'correct')?
The main reason I need to group them up is namespace issues, common function names are all used elsewhere.
Edit: Changed example from matrix to diff. Matrix is a terrible example as it does have state and everyone started explaining why it's better to write them as methods rather than answer the actual question. :(
In your two examples, you are not actually defining methods in a Class or a Module; you are defining singleton methods on an object which happens to be a Class or a Module, but could be just about any object. Here's an example with a String:
Diff = "Use me to access really cool methods"
def Diff.patch
# ...
end
You can do any of these and that will work, but the best way to group related methods is in a Module as normal instance methods (i.e. without self.):
module Diff
extend self # This makes the instance methods available to the Diff module itself
def diff ... # no self.
def patch ...
end
Now you can:
use this functionality from within any Class (with include Diff) or from any object (with extend Diff)
an example of this use is the extend self line which makes it possible to call Diff.patch.
even use these methods in the global namespace
For example, in irb:
class Foo
include Diff
end
Foo.new.patch # => calls the patch method
Diff.patch # => also calls Diff.patch
include Diff # => now you can call methods directly:
patch # => also calls the patch method
Note: the extend self will "modify" the Diff module object itself but it won't have any effect on inclusions of the module. Same thing happens for a def self.foo, the foo won't be available to any class including it. In short, only instance methods of Diff are imported with an include (or an extend), not the singleton methods. Only subclassing a class will provide inheritance of both instance and singleton methods.
When you actually want the inclusion of a module to provide both instance methods and singleton methods, it's not completely easy. You have to use the self.included hook:
module Foo
def some_instance_method; end
module ClassMethods
def some_singleton_method; end
end
def self.included(base)
base.send :extend, ClassMethods
end
def self.will_not_be_included_in_any_way; end
end
class Bar
include Foo
end
# Bar has now instance methods:
Bar.new.some_instance_method # => nil
# and singleton methods:
Bar.some_singleton_method # => nil
The main difference between modules and classes is that you can not instantiate a module; you can't do obj = MyModule.new. The assumption of your question is that you don't want to instantiate anything, so I recommend just using a module.
Still you should reconsider your approach: rather than using arrays of arrays or whatever you are doing to represent a Matrix, it would be more elegant to make your own class to represent a matrix, or find a good class that someone else has already written.
Ruby Modules are used to specify behaviour, pieces of related functionality.
Ruby Classes are used to specify both state and behaviour, a singular entity.
There is a maxim in software design that says that code is a liability, so use the less code possible. In the case of Ruby, the difference in code lines is cero. So you can use either way (if you don't need to save state)
If you want to be a purist, then use a Module, since you won't be using the State functionality. But I wouldn't say that using a class is wrong.
As a trivia info: In Ruby a Class is a kind of Module.
http://www.ruby-doc.org/core-1.9.3/Class.html
The following also works
Matrix = Object.new
def Matrix.add ...
def Matrix.equals ...
That's because so-called "class methods" are just methods added to a single object, and it doesn't really matter what that object class is.
As a matter of form, the Module is more correct. You can still create instances of the class, even if it has only class methods. You can think of a module here as a static class of C# or Java. Classes also always have the instance related methods (new, allocate, etc.). Use the Module. Class methods usually have something to do with objects (creating them, manipulating them).

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