redefining a single ruby method on a single instance with a lambda

Question

In Ruby, is there a way to redefine a method of a particular instance of a class using a proc? For example:

class Foo
  def bar()
    return "hello"
  end
end

x = Foo.new
y = Foo.new

(Something like):

y.method(:bar) = lambda { return "goodbye" }

x.bar
y.bar

Producing:

hello
goodbye

Thanks.

score 53 · Accepted Answer · edited May 23 '17 at 12:25

53

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

edited May 23 '17 at 12:25

Community

1
1

answered Apr 29 '09 at 15:54

John Douthat

40,711
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wondering why 'probably doesn't fit your needs' for the last solution? – Abdul Rahman K Jun 02 '20 at 14:59
Both: ``` class Foo end obj = Foo.new class << obj def method1; end end def obj.method2; end – Abdul Rahman K Jun 02 '20 at 15:05
1

@AbdulRahmanK because he asked how to do it with a lambda, and that way doesn't use a lambda, but I wanted to include it anyway – John Douthat Jun 03 '20 at 00:36

score 31 · Answer 2 · answered May 19 '13 at 05:39

31

I'm not sure what version of Ruby this was added in (at least 1.8.7), but there seems to be an even simpler way of doing this:

str1 = "Hello"
str2 = "Goodbye"
def str1.to_spanish
  "Hola"
end
puts str1 # => Hello
puts str1.to_spanish # => Hola
puts str2 # => Goodbye
puts str2.to_spanish # => Throws a NoMethodError

Learnt about this whilst reading the Ruby Koans (about_class_methods.rb lesson). I'm still not entirely sure what the purpose of this is since it seems a bit dangerous to me.

answered May 19 '13 at 05:39

Patrick

2,769
2
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29

Thank you for posting this (more) up-to-date answer ! – Cyril Duchon-Doris Mar 29 '15 at 23:15
1

Assuming `str.to_spanish` was already defined and I wanted to “redefine” it, how would I call the “original” `str.to_spanish` from within the newly defined one? – Daniel A. R. Werner Jul 29 '16 at 22:34
If `str` is your instance object and `String` is your class, you can call the class's version of the method by `String.to_spanish` – BRPocock Aug 26 '16 at 17:20
1

God I love Ruby – Benjineer Sep 29 '18 at 03:35

score 18 · Answer 3 · answered Apr 29 '09 at 20:09

18

You can use the syntax class <<object to get an object's "singleton class" (that's a special parent class belonging only to that object) and define methods only for that instance. For example:

str1 = "Hello"
str2 = "Foo"

class <<str1
  def to_spanish
    'Hola'
  end
end

Now if you do str1.to_spanish, it will return "Hola", but str2.to_spanish will give you a NoMethodFound exception.

answered Apr 29 '09 at 20:09

Chuck

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if he has an externally defined proc in a simple variable, it's difficult to get it into the new scope created by class< – John Douthat Apr 29 '09 at 20:13

redefining a single ruby method on a single instance with a lambda

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