Module methods in Object Ruby

Question

I have troubles with understanding the global area of visibility in Ruby, so, I know that you cant use Module methods in your own class for example:

module Mod
   def self.meth
      “module method”
   end
end

class Klass
   include Mod
end

p Klass.meth

# Error

but when i knew that you can do such thing:

include Math

p sin 2
#0.909....

I was confused, because i thought you cant use module methods in any class without calling the method name. Also i had a supposition, that module Math has instance methods, like Kernel, but, unfortunately, no. Now i am doubting, that I understood such methods like extend and include correctly, so, could you please explain to me this thing and what will happen if we will change include to extend

BroiSatse · Accepted Answer · 2021-08-03 12:13:03Z

1

You have encounter a weirdness of module_function: https://apidock.com/ruby/Module/module_function/

module Foo
  def foo # this is (irb) 2
  end
end

Foo.singleton_methods #=> []
Foo.instance_methods #=> [:foo]
Foo.instance_method(:foo).source_location #=> ["(irb)", 2]

module Foo
  module_function :foo # this is (irb) 9
end

Foo.singleton_methods #=> [:foo]
Foo.singleton_method(:foo).source_location #=> ["(irb)", 2]
Foo.instance_methods #=> []
Foo.private_instance_methods #=> [:foo]
Foo.instance_method(:foo).source_location #=> ["(irb)", 2]

So, module_function takes an instance method of the module, makes it private and copies it onto a singleton class.

module_function can also be used without the method name, which works a bit like private method, modifying all the future methods added to this module.

Math is a full module_function module, meaning that the methods are defined both as singletons and private instance methods, which is why you can use it both ways.

answered Aug 3, 2021 at 12:13

BroiSatse

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1 Comment

Rcinos Over a year ago

thanks a lot. Talking about why module#instance_methods doesn’t show foo, that is why module_function defines method foo as private.

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