I'm working through Zed Shaw's Learn Ruby The Hard Way and I'm having an issue including a module in IRB. In exercise 25, we define a new module Ex25, require it in IRB, and then can use its various methods through the namespace of that module, e.g. Ex25.break_words(sentence). In the Extra Credit, it's stated that typing include Ex25 will basically add the methods from the module into the current "space" (not sure what to call it) and you can then call them without explicitly referring to the module, e.g. break_words(sentence). When I do this, however, I'm getting an "undefined method" error. Any help / explanation would be greatly appreciated, thanks!
That's an error in the book. The methods in Ex25 are class methods. include adds instance methods to the "current space." Remove self from the method definitions and it'll work:
module Ex25
def break_words(stuff)
stuff.split(' ')
end
end
include Ex25
break_words 'hi there' # => ["hi", "there"]
If you're curious, here's some more detail about what's going on: The place where the methods are included—the "current space"—is the Object class:
Object.included_modules # => [Ex25, Kernel]
All Object instances gain the included methods ...
Object.new.break_words 'how are you?' # => ["how", "are", "you?"]
... and the top-level is just an Object instance:
self.class # => Object
But wait. If the top-level is an Object instance, why does it respond to include? Isn't include an instance method of Module (and its subclass, Class)? The answer is that the top-level has a singleton method ...
singleton_methods.include? "include" # => true
... which we can assume forwards to the Object class.
Related
Today I came across the Pathname class in Ruby and noticed that you could directly call the class itself as a method (which would basically return a new instance):
Pathname("some/path")
# => #<Pathname:some/path>
I've been trying to replicate the same thing with my CustomClass but haven't been successful. I don't know what these methods are called and I can't find any Ruby code that gives me an idea on how to do this. My Question is how do I use the Class name as method?
Things I've tried so far:
Defining self.self()
Defining self.class()
Using the class << self syntax
Googling - But it just returns comparisons of class methods vs instance methods
This isn't using the class itself. This is calling a method in Kernel with the same name as the class. It's generally discouraged to do it yourself as you pollute almost all objects with new methods and leads to confusion (as you already see).
Here is the documentation for the method. There are a few others like Array, Hash, String, etc.
What you're looking for is a conversion method to coerce the input to the instance of the class.
It is not a method of the class itself, but a method in Kernel module. So in order to be able to use the form of MyClass(value) you should add the method to Kernel module:
module Kernel
def Foo(value)
# you can implement any logic here
value.is_a?(Foo) ? value : Foo.new(value)
end
module_function :Foo
end
class Foo
def initialize(bar)
#bar = bar
end
end
baz = Foo('bar')
#=> #<Foo:0x007fd4e5070370 #bar="bar">
Foo(baz)
#=> #<Foo:0x007fd4e5070370 #bar="bar">
baz == Foo(baz)
#=> true
This is not a class call, but a shortcut. And the trickiest part - it was defined for a Kernel module to be available everywhere in the form as you just specified.
Please proceed to the link of the official docs. There you can see, that requiring a Pathname module, it extend Kernel module to add the method of the same name.
To be honest, I strongly recommend against extending Kernel with your own method. Or at least to use refinements
From the Module
Module#append_features(mod) → mod => When this module is included in another, Ruby calls append_features in this module, passing it the receiving module in mod. Ruby’s default implementation is to add the constants, methods, and module variables of this module to mod if this module has not already been added to mod or one of its ancestors.
Module#prepend_features(mod) → mod => When this module is prepended in another, Ruby calls prepend_features in this module, passing it the receiving module in mod. Ruby’s default implementation is to overlay the constants, methods, and module variables of this module to mod if this module has not already been added to mod or one of its ancestors.
Can anyone help me to understand the below questions:
What more features of Module are defined as append and prepend except those default?
How they differ functionally?
When to use append_features and when prepend_features?
what is the difference between two bold lines as above?
What features of Module are defined as append and prepend?
As specified in the text you quoted:
the constants, methods, and module variables
How they differ functionally?
Both add methods of the mixed-in module to the passed module (class). The difference is in the lookup order of these methods, in case that the target class already has them defined:
include behaves as if the target class inherited mixed-in module:
module FooBar
def say
puts "2 - Module"
end
end
class Foo
include FooBar
def say
puts "1 - Implementing Class"
super
end
end
Foo.new.say # =>
# 1 - Implementing Class
# 2 - Module
prepend makes the methods from the mixed in module "stronger" and executes them first:
module FooBar
def say
puts "2 - Module"
super
end
end
class Foo
prepend FooBar
def say
puts "1 - Implementing Class"
end
end
Foo.new.say # =>
# 2 - Module
# 1 - Implementing Class
The example kindly ripped off from here: http://blog.crowdint.com/2012/11/05/3-killer-features-that-are-coming-on-ruby-2-0.html
When to use append_features and when prepend_features?
Use prepend when you want to keep methods of the target module (class) at the end of the method lookup chain.
Some real-world examples can be found by searching SO for ruby, module and prepend:
Overriding method by another defined in module
When monkey patching a method, can you call the overridden method from the new implementation?
Ruby: Module, Mixins and Blocks confusing?
(Note: I am mentioning only methods, as they are easiest to picture when it comes to inheritance and mixing-in, but the same applies to other features.)
I thought to add it as a comment to a good answer which #Mladen Jablanovic has already made but I couldn't due to my low reputation point.
I've found a more concise, clearer and more descriptive answer on a post here - Ruby modules: Include vs Prepend vs Extend and I post it here just in case someone needs it and could get it with less effort.
Direct quotes:
Though include is the most common way of importing external code into a class, Ruby provides also two other ways to achieve that: extend and prepend. However, they don’t have the same behavior at all, and these differences are often misunderstood by Ruby developers.
To understand how to use them, we must first have a deeper look into how Ruby is resolving methods to execute at runtime, using something called the ancestors chain.
When a Ruby class is created, it holds a list of constant names which are its ancestors. They are all the classes that the class inherits from, and the modules they include. For example, by calling ancestors on the String class, we get the list of its ancestors:
String.ancestors
=> [String, Comparable, Object, PP::ObjectMixin, Kernel, BasicObject]
include is the most used and the simplest way of importing module code. When calling it in a class definition, Ruby will insert the module into the ancestors chain of the class, just after its superclass.
Available since Ruby 2, prepend is a bit less known to Rubyists than its two other friends. It actually works like include, except that instead of inserting the module between the class and its superclass in the chain, it will insert it at the bottom of the chain, even before the class itself.
I would suggest reading the post to get a better understanding as it comes with examples.
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).
It looks like const_missing is an instance method of Object. If so, why doesn't this code work?
module Extensions
def const_missing(c)
puts c
end
end
class Object
include Extensions
end
NonExistent.new
In order to get it to function correctly, I have to change def const_missing to def Object.const_missing. Why?
This is just a consequence of the way method calls are resolved in Ruby.
First, singleton methods are checked. Then instance methods of the class, followed by the ancestors (which will be the included modules, then superclasses with their included modules).
So you could define Object.const_missing directly, or include your Module in the singleton class of Object:
class << Object
include Extensions
end
NonExistent # => prints "NonExistent"
You could also monkeypatch Module#const_missing.
What I'm trying to find out is whether there is some sort of equivalence to what I see in Groovy as ExpandoMetaClasses. I've been reading about Open Classes but I can't quite see what level of scoping Ruby allows of the class modifications.
Borrowing an example from the blog above, in Groovy, I could modify Java's String class and add a method to it like so:
String.metaClass.shout = {->
return delegate.toUpperCase()
}
println "Hello MetaProgramming".shout()
// output
// HELLO METAPROGRAMMING
And I think that Ruby would have you redefine the class and possibly alias it (please help clarify my misunderstandings at this point):
class String
def foo
"foo"
end
end
puts "".foo # prints "foo"
In Groovy, there are ways to scope the redefinition of core Java library methods to single instances or to a group of instances using Categories, which feel similar to what I would define as mixins in Ruby.
What are the ways to scope open classes to specific instances or to subsets of modules?
If I were to install a gem that had redefined some core class, would only that module be affected, or would any .rb file I include that gem with be affected with it?
Apologies in advance for making some possible assumptions on both Ruby and Groovy, I'm new to both but have been trying to find equivalence between the two.
Ruby's classes are never "closed". So when you say:
class String
def omg!
self.replace "OMG"
end
end
You are defining the omg! method on the String class. Unlike in Groovy, which requires the usage of a special metaclass concept, Ruby classes are always open, period.
If you wanted to modify a particular set of Strings, you could do this:
module Magic
def presto
puts "OMG A HAT!"
end
end
class Array
include Magic
end
x = "Hello".extend(Magic)
puts x #=> Hello
x.presto #=> OMG A HAT!
[].presto #=> OMG A HAT!
def x.really?
true
end
x.really? #=> true
Effectively, a module is a collection of methods that can be added to a class or specific instances.
So you can either open a class directly or add new methods to a class using a module. You can also open an instance directly or add new methods to an instance using a module. That's because a class is just an instance of Class ;) Pretty nifty!
In addition to what Yehuda said, instances in Ruby also have metaclasses (technically called "singleton classes"), accessed with class <<whatever. For example, to redo Yehuda's Magic example with a singleton class:
x = "Hello"
class <<x
include Magic
def magical?
true
end
end
x.presto #=> OMG A HAT!
x.magical? #=> true
"Something else".magical? #=> NoMethodError
There's no scoping on modifications to classes. As soon as a class is modified, the modified class is accessible to all later requires and following code.