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I want to define an instance method Date#next which returns the next day. So I made a DateExtension module, like this:

module DateExtension def next(symb=:day) dt = DateTime.now {:day => Date.new(dt.year, dt.month, dt.day + 1), :week => Date.new(dt.year, dt.month, dt.day + 7), :month => Date.new(dt.year, dt.month + 1, dt.day), :year => Date.new(dt.year + 1, dt.month, dt.day)}[symb] end end

Using it:

class Date include DateExtension end

Calling the method d.next(:week) makes Ruby throw an error ArgumentError: wrong number of arguments (1 for 0). How can I override the default next method from Date class with the one declared in DateExtension module?

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In Ruby 2.0 and later you can use Module#prepend:

class Date prepend DateExtension end

Original answer for older Ruby versions is below.

The problem with include (as shown in the following diagram) is that methods of a class cannot be overridden by modules included in that class (solutions follow the diagram): Ruby Method Lookup Flow

Solutions

  1. Subclass Date just for this one method:

    irb(main):001:0> require 'date'; module Foo; def next(a=:hi); a; end; end #=> nil irb(main):002:0> class MyDate < Date; include Foo; end #=> MyDate irb(main):003:0> MyDate.today.next(:world) #=> :world

  2. Extend just the instances you need with your own method:

    irb(main):001:0> require 'date'; module Foo; def next(a=:hi); a; end; end #=> nil irb(main):002:0> d = Date.today; d.extend(Foo); d.next(:world) #=> :world

  3. When including your module, perform a gross hack and reach inside the class and destroy the old 'next' so that yours gets called:

    irb(main):001:0> require 'date' #=> true irb(main):002:0> module Foo irb(main):003:1> def self.included(klass) irb(main):004:2> klass.class_eval do irb(main):005:3* remove_method :next irb(main):006:3> end irb(main):007:2> end irb(main):008:1> def next(a=:hi); a; end irb(main):009:1> end #=> nil irb(main):010:0> class Date; include Foo; end #=> Date irb(main):011:0> Date.today.next(:world) #=> :world

    This method is far more invasive than just including a module, but the only way (of the techniques shown so far) to make it so that new Date instances returned by system methods will automatically use methods from your own module.

  4. But if you're going to do that, you might as well skip the module altogether and just go straight to monkeypatch land:

    irb(main):001:0> require 'date' #=> true irb(main):002:0> class Date irb(main):003:1> alias_method :_real_next, :next irb(main):004:1> def next(a=:hi); a; end irb(main):005:1> end #=> nil irb(main):006:0> Date.today.next(:world) #=> :world

  5. If you really need this functionality in your own environment, note that the Prepend library by banisterfiend can give you the ability to cause lookup to occur in a module before the class into which it is mixed.

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9 Comments

RUBY_DESCRIPTION => "ruby 1.8.7 (2011-02-18 patchlevel 334) [i386-mingw32]" I just realized something. My code is using the DateTime class, so I need to require 'date' (according to this). Using exactly your example after this instruction, Ruby throws the error. I posted a gist because I couldn't post the whole code here. Thanks!
@kaos12 Thanks, I've edited my answer with various options available to you.
@kaos12 There is no change in behavior. Methods from included modules always go 'above' the class they are included into. Try this: class MyClass; end; [Object,Class,Module,MyClass].each{ |k| k.class_eval{ define_method(:foo){ puts "#{k}#foo"; super() unless k==Object }}}; MyClass.new.foo Instance methods on a class with no included modules go straight to instance methods on Object.
In the future, Module#prepend might be worth looking into.
Module#prepend is already available in Ruby 2.0
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The next method for Date is defined in the Date class and methods defined in a class take precedence over those defined in an included module. So, when you do this:

class Date include DateExtension end

You're pulling in your version of next but the next defined in Date still takes precedence. You'll have to put your next right in Date:

class Date def next(symb=:day) dt = DateTime.now {:day => Date.new(dt.year, dt.month, dt.day + 1), :week => Date.new(dt.year, dt.month, dt.day + 7), :month => Date.new(dt.year, dt.month + 1, dt.day), :year => Date.new(dt.year + 1, dt.month, dt.day)}[symb] end end

From the the Programming Ruby chapter on Classes and Objects:

When a class includes a module, that module's instance methods become available as instance methods of the class. It's almost as if the module becomes a superclass of the class that uses it. Not surprisingly, that's about how it works. When you include a module, Ruby creates an anonymous proxy class that references that module, and inserts that proxy as the direct superclass of the class that did the including.

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