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ruby--ruby/doc/syntax/refinements.rdoc

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= Refinements
Due to Ruby's open classes you can redefine or add functionality to existing
classes. This is called a "monkey patch". Unfortunately the scope of such
changes is global. All users of the monkey-patched class see the same
changes. This can cause unintended side-effects or breakage of programs.
Refinements are designed to reduce the impact of monkey patching on other
users of the monkey-patched class. Refinements provide a way to extend a
class locally. Refinements can modify both classes and modules.
Here is a basic refinement:
class C
def foo
puts "C#foo"
end
end
module M
refine C do
def foo
puts "C#foo in M"
end
end
end
First, a class +C+ is defined. Next a refinement for +C+ is created using
Module#refine.
Module#refine creates an anonymous module that contains the changes or
refinements to the class (+C+ in the example). +self+ in the refine block is
this anonymous module similar to Module#module_eval.
Activate the refinement with #using:
using M
c = C.new
c.foo # prints "C#foo in M"
== Scope
You may activate refinements at top-level, and inside classes and modules.
You may not activate refinements in method scope. Refinements are activated
until the end of the current class or module definition, or until the end of
the current file if used at the top-level.
You may activate refinements in a string passed to Kernel#eval. Refinements
are active until the end of the eval string.
Refinements are lexical in scope. Refinements are only active within a scope
after the call to +using+. Any code before the +using+ statement will not have the
refinement activated.
When control is transferred outside the scope, the refinement is deactivated.
This means that if you require or load a file or call a method that is defined
outside the current scope the refinement will be deactivated:
class C
end
module M
refine C do
def foo
puts "C#foo in M"
end
end
end
def call_foo(x)
x.foo
end
using M
x = C.new
x.foo # prints "C#foo in M"
call_foo(x) #=> raises NoMethodError
If a method is defined in a scope where a refinement is active, the refinement
will be active when the method is called. This example spans multiple files:
c.rb:
class C
end
m.rb:
require "c"
module M
refine C do
def foo
puts "C#foo in M"
end
end
end
m_user.rb:
require "m"
using M
class MUser
def call_foo(x)
x.foo
end
end
main.rb:
require "m_user"
x = C.new
m_user = MUser.new
m_user.call_foo(x) # prints "C#foo in M"
x.foo #=> raises NoMethodError
Since the refinement +M+ is active in <code>m_user.rb</code> where
<code>MUser#call_foo</code> is defined it is also active when
<code>main.rb</code> calls +call_foo+.
Since #using is a method, refinements are only active when it is called. Here
are examples of where a refinement +M+ is and is not active.
In a file:
# not activated here
using M
# activated here
class Foo
# activated here
def foo
# activated here
end
# activated here
end
# activated here
In a class:
# not activated here
class Foo
# not activated here
def foo
# not activated here
end
using M
# activated here
def bar
# activated here
end
# activated here
end
# not activated here
Note that the refinements in +M+ are *not* activated automatically if the class
+Foo+ is reopened later.
In eval:
# not activated here
eval <<EOF
# not activated here
using M
# activated here
EOF
# not activated here
When not evaluated:
# not activated here
if false
using M
end
# not activated here
When defining multiple refinements in the same module inside multiple +refine+ blocks,
all refinements from the same module are active when a refined method
(any of the +to_json+ methods from the example below) is called:
module ToJSON
refine Integer do
def to_json
to_s
end
end
refine Array do
def to_json
"[" + map { |i| i.to_json }.join(",") + "]"
end
end
refine Hash do
def to_json
"{" + map { |k, v| k.to_s.dump + ":" + v.to_json }.join(",") + "}"
end
end
end
using ToJSON
p [{1=>2}, {3=>4}].to_json # prints "[{\"1\":2},{\"3\":4}]"
== Method Lookup
When looking up a method for an instance of class +C+ Ruby checks:
* If refinements are active for +C+, in the reverse order they were activated:
* The prepended modules from the refinement for +C+
* The refinement for +C+
* The included modules from the refinement for +C+
* The prepended modules of +C+
* +C+
* The included modules of +C+
If no method was found at any point this repeats with the superclass of +C+.
Note that methods in a subclass have priority over refinements in a
superclass. For example, if the method <code>/</code> is defined in a
refinement for Numeric <code>1 / 2</code> invokes the original Integer#/
because Integer is a subclass of Numeric and is searched before the refinements
for the superclass Numeric. Since the method <code>/</code> is also present
in child +Integer+, the method lookup does not move up to the superclass.
However, if a method +foo+ is defined on Numeric in a refinement, <code>1.foo</code>
invokes that method since +foo+ does not exist on Integer.
== +super+
When +super+ is invoked method lookup checks:
* The included modules of the current class. Note that the current class may
be a refinement.
* If the current class is a refinement, the method lookup proceeds as in the
Method Lookup section above.
* If the current class has a direct superclass, the method proceeds as in the
Method Lookup section above using the superclass.
Note that +super+ in a method of a refinement invokes the method in the
refined class even if there is another refinement which has been activated in
the same context.
== Methods Introspection
When using introspection methods such as Kernel#method or Kernel#methods refinements are not honored.
This behavior may be changed in the future.
== Refinement inheritance by Module#include
When a module X is included into a module Y, Y inherits refinements from X.
For example, C inherits refinements from A and B in the following code:
module A
refine X do ... end
refine Y do ... end
end
module B
refine Z do ... end
end
module C
include A
include B
end
using C
# Refinements in A and B are activated here.
Refinements in descendants have higher precedence than those of ancestors.
== Further Reading
See https://bugs.ruby-lang.org/projects/ruby-trunk/wiki/RefinementsSpec for the
current specification for implementing refinements. The specification also
contains more details.