redefining a single ruby method on a single instance with a lambda
def define_singleton_method_by_proc(obj, name, block)
metaclass = class << obj; self; end
metaclass.send(:define_method, name, block)
end
p = proc { "foobar!" }
define_singleton_method_by_proc(y, :bar, p)
or, if you want to monkey-patch Object to make it easy
class Object
# note that this method is already defined in Ruby 1.9
def define_singleton_method(name, callable = nil, &block)
block ||= callable
metaclass = class << self; self; end
metaclass.send(:define_method, name, block)
end
end
p = proc { "foobar!" }
y.define_singleton_method(:bar, p)
#or
y.define_singleton_method(:bar) do
"foobar!"
end
or, if you want to define your proc inline, this may be more readable
class << y
define_method(:bar, proc { "foobar!" })
end
or,
class << y
define_method(:bar) { "foobar!" }
end
this is the most readable, but probably doesn't fit your needs
def y.bar
"goodbye"
end
This question is highly related
Calling original method from a redefined one
If some_object.some_method
is not a singleton method, then you can just call super
in your redefined method.
def some_object.some_method
super
do_something_else
end
If some_object.some_method
is a singleton method, then
Ruby >= 2.0.0
You can define that method in a module
module SomeModule
def some_method
super
do_something_else
end
end
And then prepend it to the singleton class of the object
some_object.singleton_class.prepend(SomeModule)
Ruby < 2.0.0
You have to make an alias then redefine, since there is no Module#prepend
.
class << some_object # open the singleton class of some_object
alias some_method_original some_method
def some_method
some_method_original
do_something_else
end
end
Is there a way to create methods just for the instance of a Ruby class from inside that instance?
You need to grab a reference to the instance's singleton class, the class that holds all the instance specific stuff, and define the method on it. In ruby 1.8, it looks a little messy. (if you find a cleaner solution let me know!)
Ruby 1.8
class Example
def initialize(test='hey')
singleton = class << self; self end
singleton.send :define_method, :say_hello, lambda { test }
end
end
Ruby 1.9 however, provides a much easier way in.
Ruby 1.9
class Example
def initialize(test='hey')
define_singleton_method :say_hello, lambda { test }
end
end
Is there a way to override a method of an instance variable
I would personally go for hash_value_to_xml
route. But since you insist, here's an idea that might work (haven't tested that)
class ProductVersion < ActiveRecord::base
serialize :hash_value, Hash
alias_method :old_hash_value, :hash_value
def hash_value
h = old_hash_value
h.define_singleton_method(:to_xml) do
# your custom xml logic here
end
h
end
end
The idea is that you intercept value returned from hash_value
and patch it on the fly.
Is there a method for in place modification on Object in ruby?
This is impossible. A method that modifies any object can only know how to manipulate the state which is common to all objects, and that is none.
A method that can replace any object in Ruby with a new one is impossible since object references cannot change their class in Ruby.
It sounds like you are a Smalltalk programmer, and you are looking for an equivalent to become:
? That is impossible in Ruby.
Ruby reflection composition: call original method from redefined method
In my opinion, this approach is way too complex, and an inappropriate use of Module
s.
I recommend thinking about a simpler way to implement this.
One simple way is to just include all the methods in the Phone class.
Or, you could use a hash as a lookup table for ring strategies:
class Phone
attr_accessor :ring_strategy
RING_STRATEGIES = {
ringtone: -> { ring_with_tone },
discreet: -> { ring_quietly },
screening: -> { ring_with_tone; ring_screening_too }
# ...
}
def initialize(ring_strategy = :ringtone)
@ring_strategy = ring_strategy
end
def ring
RING_STRATEGIES[:ring_strategy].()
end
end
How to override a lambda variable from a subclass in Ruby?
Setting your own Hashify
in your subclass doesn't help, because the Ruby interpreter will use the constant defined in Redis
, as the methods that call Hashify.call
are defined there, too.
You can override Redis::Hashify
, though. This will result in your lambda being used for all Redis connections and in warning: already initialized constant Redis::Hashify
require 'redis'
Redis::Hashify = lambda do |array|
Hash.new.tap do |hash|
array.each_slice(2) do |field, value|
hash[field.to_sym] = value
end
end
end
Please note writing code that produces warnings is considered bad style by many developers. It might be better to modify the response of Redis#hgetall
after you received it. If you don't mind using ActiveSupport, you can use it's Hash#symbolize_keys
, for instance.
I want to add a singleton method with a closure to a Ruby object
You can define a method with a block using define_method
.
Example:
class Object
def eigenclass
class <<self; self end
end
end
a = "Hello"
other_word = "World"
a.eigenclass.class_eval do
define_method(:cliche) {"#{self} #{other_word}"}
end
a.cliche # => "Hello World"
"Goodbye".cliche # => NoMethodError: undefined method `cliche' for "Goodbye":String
Here is an implementation of a define_singleton_method
method:
class Object
def define_singleton_method(name, &block)
eigenclass = class<<self; self end
eigenclass.class_eval {define_method name, block}
end
end
When monkey patching an instance method, can you call the overridden method from the new implementation?
EDIT: It has been 9 years since I originally wrote this answer, and it deserves some cosmetic surgery to keep it current.
You can see the last version before the edit here.
You can’t call the overwritten method by name or keyword. That’s one of the many reasons why monkey patching should be avoided and inheritance be preferred instead, since obviously you can call the overridden method.
Avoiding Monkey Patching
Inheritance
So, if at all possible, you should prefer something like this:
class Foo
def bar
'Hello'
end
end
class ExtendedFoo < Foo
def bar
super + ' World'
end
end
ExtendedFoo.new.bar # => 'Hello World'
This works, if you control creation of the Foo
objects. Just change every place which creates a Foo
to instead create an ExtendedFoo
. This works even better if you use the Dependency Injection Design Pattern, the Factory Method Design Pattern, the Abstract Factory Design Pattern or something along those lines, because in that case, there is only place you need to change.
Delegation
If you do not control creation of the Foo
objects, for example because they are created by a framework that is outside of your control (like ruby-on-rails for example), then you could use the Wrapper Design Pattern:
require 'delegate'
class Foo
def bar
'Hello'
end
end
class WrappedFoo < DelegateClass(Foo)
def initialize(wrapped_foo)
super
end
def bar
super + ' World'
end
end
foo = Foo.new # this is not actually in your code, it comes from somewhere else
wrapped_foo = WrappedFoo.new(foo) # this is under your control
wrapped_foo.bar # => 'Hello World'
Basically, at the boundary of the system, where the Foo
object comes into your code, you wrap it into another object, and then use that object instead of the original one everywhere else in your code.
This uses the Object#DelegateClass
helper method from the delegate
library in the stdlib.
“Clean” Monkey Patching
Module#prepend
: Mixin Prepending
The two methods above require changing the system to avoid monkey patching. This section shows the preferred and least invasive method of monkey patching, should changing the system not be an option.
Module#prepend
was added to support more or less exactly this use case. Module#prepend
does the same thing as Module#include
, except it mixes in the mixin directly below the class:
class Foo
def bar
'Hello'
end
end
module FooExtensions
def bar
super + ' World'
end
end
class Foo
prepend FooExtensions
end
Foo.new.bar # => 'Hello World'
Note: I also wrote a little bit about Module#prepend
in this question: Ruby module prepend vs derivation
Mixin Inheritance (broken)
I have seen some people try (and ask about why it doesn’t work here on StackOverflow) something like this, i.e. include
ing a mixin instead of prepend
ing it:
class Foo
def bar
'Hello'
end
end
module FooExtensions
def bar
super + ' World'
end
end
class Foo
include FooExtensions
end
Unfortunately, that won’t work. It’s a good idea, because it uses inheritance, which means that you can use super
. However, Module#include
inserts the mixin above the class in the inheritance hierarchy, which means that FooExtensions#bar
will never be called (and if it were called, the super
would not actually refer to Foo#bar
but rather to Object#bar
which doesn’t exist), since Foo#bar
will always be found first.
Method Wrapping
The big question is: how can we hold on to the bar
method, without actually keeping around an actual method? The answer lies, as it does so often, in functional programming. We get a hold of the method as an actual object, and we use a closure (i.e. a block) to make sure that we and only we hold on to that object:
class Foo
def bar
'Hello'
end
end
class Foo
old_bar = instance_method(:bar)
define_method(:bar) do
old_bar.bind(self).() + ' World'
end
end
Foo.new.bar # => 'Hello World'
This is very clean: since old_bar
is just a local variable, it will go out of scope at the end of the class body, and it is impossible to access it from anywhere, even using reflection! And since Module#define_method
takes a block, and blocks close over their surrounding lexical environment (which is why we are using define_method
instead of def
here), it (and only it) will still have access to old_bar
, even after it has gone out of scope.
Short explanation:
old_bar = instance_method(:bar)
Here we are wrapping the bar
method into an UnboundMethod
method object and assigning it to the local variable old_bar
. This means, we now have a way to hold on to bar
even after it has been overwritten.
old_bar.bind(self)
This is a bit tricky. Basically, in Ruby (and in pretty much all single-dispatch based OO languages), a method is bound to a specific receiver object, called self
in Ruby. In other words: a method always knows what object it was called on, it knows what its self
is. But, we grabbed the method directly from a class, how does it know what its self
is?
Well, it doesn’t, which is why we need to bind
our UnboundMethod
to an object first, which will return a Method
object that we can then call. (UnboundMethod
s cannot be called, because they don’t know what to do without knowing their self
.)
And what do we bind
it to? We simply bind
it to ourselves, that way it will behave exactly like the original bar
would have!
Lastly, we need to call the Method
that is returned from bind
. In Ruby 1.9, there is some nifty new syntax for that (.()
), but if you are on 1.8, you can simply use the call
method; that’s what .()
gets translated to anyway.
Here are a couple of other questions, where some of those concepts are explained:
- How do I reference a function in Ruby?
- Is Ruby’s code block same as C♯’s lambda expression?
“Dirty” Monkey Patching
alias_method
chain
The problem we are having with our monkey patching is that when we overwrite the method, the method is gone, so we cannot call it anymore. So, let’s just make a backup copy!
class Foo
def bar
'Hello'
end
end
class Foo
alias_method :old_bar, :bar
def bar
old_bar + ' World'
end
end
Foo.new.bar # => 'Hello World'
Foo.new.old_bar # => 'Hello'
The problem with this is that we have now polluted the namespace with a superfluous old_bar
method. This method will show up in our documentation, it will show up in code completion in our IDEs, it will show up during reflection. Also, it still can be called, but presumably we monkey patched it, because we didn’t like its behavior in the first place, so we might not want other people to call it.
Despite the fact that this has some undesirable properties, it has unfortunately become popularized through AciveSupport’s Module#alias_method_chain
.
An aside: Refinements
In case you only need the different behavior in a few specific places and not throughout the whole system, you can use Refinements to restrict the monkey patch to a specific scope. I am going to demonstrate it here using the Module#prepend
example from above:
class Foo
def bar
'Hello'
end
end
module ExtendedFoo
module FooExtensions
def bar
super + ' World'
end
end
refine Foo do
prepend FooExtensions
end
end
Foo.new.bar # => 'Hello'
# We haven’t activated our Refinement yet!
using ExtendedFoo
# Activate our Refinement
Foo.new.bar # => 'Hello World'
# There it is!
You can see a more sophisticated example of using Refinements in this question: How to enable monkey patch for specific method?
Abandoned ideas
Before the Ruby community settled on Module#prepend
, there were multiple different ideas floating around that you may occasionally see referenced in older discussions. All of these are subsumed by Module#prepend
.
Method Combinators
One idea was the idea of method combinators from CLOS. This is basically a very lightweight version of a subset of Aspect-Oriented Programming.
Using syntax like
class Foo
def bar:before
# will always run before bar, when bar is called
end
def bar:after
# will always run after bar, when bar is called
# may or may not be able to access and/or change bar’s return value
end
end
you would be able to “hook into” the execution of the bar
method.
It is however not quite clear if and how you get access to bar
’s return value within bar:after
. Maybe we could (ab)use the super
keyword?
class Foo
def bar
'Hello'
end
end
class Foo
def bar:after
super + ' World'
end
end
Replacement
The before combinator is equivalent to prepend
ing a mixin with an overriding method that calls super
at the very end of the method. Likewise, the after combinator is equivalent to prepend
ing a mixin with an overriding method that calls super
at the very beginning of the method.
You can also do stuff before and after calling super
, you can call super
multiple times, and both retrieve and manipulate super
’s return value, making prepend
more powerful than method combinators.
class Foo
def bar:before
# will always run before bar, when bar is called
end
end
# is the same as
module BarBefore
def bar
# will always run before bar, when bar is called
super
end
end
class Foo
prepend BarBefore
end
and
class Foo
def bar:after
# will always run after bar, when bar is called
# may or may not be able to access and/or change bar’s return value
end
end
# is the same as
class BarAfter
def bar
original_return_value = super
# will always run after bar, when bar is called
# has access to and can change bar’s return value
end
end
class Foo
prepend BarAfter
end
old
keyword
This idea adds a new keyword similar to super
, which allows you to call the overwritten method the same way super
lets you call the overridden method:
class Foo
def bar
'Hello'
end
end
class Foo
def bar
old + ' World'
end
end
Foo.new.bar # => 'Hello World'
The main problem with this is that it is backwards incompatible: if you have method called old
, you will no longer be able to call it!
Replacement
super
in an overriding method in a prepend
ed mixin is essentially the same as old
in this proposal.
redef
keyword
Similar to above, but instead of adding a new keyword for calling the overwritten method and leaving def
alone, we add a new keyword for redefining methods. This is backwards compatible, since the syntax currently is illegal anyway:
class Foo
def bar
'Hello'
end
end
class Foo
redef bar
old + ' World'
end
end
Foo.new.bar # => 'Hello World'
Instead of adding two new keywords, we could also redefine the meaning of super
inside redef
:
class Foo
def bar
'Hello'
end
end
class Foo
redef bar
super + ' World'
end
end
Foo.new.bar # => 'Hello World'
Replacement
redef
ining a method is equivalent to overriding the method in a prepend
ed mixin. super
in the overriding method behaves like super
or old
in this proposal.
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