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.
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. Refinements only modify classes, not modules so the argument must be a class.
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 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 m_user.rb
where MUser#call_foo
is defined it is also active when main.rb
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
# 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 a refine block all refinements from the same module are active when a refined method 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 /
is defined in a refinement for Integer 1 / 2
invokes the original Fixnum#/ because Fixnum is a subclass of Integer and is searched before the refinements for the superclass Integer.
If a method foo
is defined on Integer in a refinement, 1.foo
invokes that method since foo
does not exist on Fixnum.
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.
Indirect Method Calls
When using indirect method access such as Kernel#send, Kernel#method or Kernel#respond_to? refinements are not honored for the caller context during method lookup.
This behavior may be changed in the future.
Further Reading
See bugs.ruby-lang.org/projects/ruby-trunk/wiki/RefinementsSpec for the current specification for implementing refinements. The specification also contains more details.