refinements - Documentation for Ruby 4.0
Description: refinements: Refinements Due to Ruby's open classes you can redefine or add functionality to existing classes. This is called a "monkey patch".
Keywords: ruby,documentation,refinements

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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 `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 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:

- The refinements of `C`, in reverse order of activation

- 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 `Numeric` `1 / 2` invokes the original `Integer#/` because `Integer` is a subclass of `Numeric` and is searched before the refinements for the superclass `Numeric`. Since the method `/` 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, `1.foo` 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. This is only true for `super` in a method of a refinement, it does not apply to `super` in a method in a module that is included in a refinement.

## 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 github.com/ruby/ruby/wiki/Refinements-Spec for the current specification for implementing refinements. The specification also contains more details.