A function that injects a class level function

Question

I write the same function repeatedly in my classes. Is there a way to declare a function once, and just reference the function declaration in each relevant class to have it become part of the class? I want to do something like:

def add_func
  expand self.new_func
    puts "hello"
  end
end

class A
  import "add_func"
end

A.new_func
# >> hello world

I'm looking to make a class method, not an instance method, but I'd be interested in knowing how to do both. What is the Ruby construct I am looking for?

[This answer](https://stackoverflow.com/a/10693010/4537254) describes what you are looking for. — André Guimarães Sakata, Nov 04 '18 at 20:12
It does not seem to make sense. Where do you expect `"world"` to come from? — sawa, Nov 05 '18 at 05:01

André Guimarães Sakata · Accepted Answer · 2018-11-05T15:46:05.813

3

You can extend the methods like this:

module SomeModule
  def foo
    true
  end
end

class SomeClass
  extend SomeModule
end

SomeClass.foo

edited Nov 05 '18 at 15:46

answered Nov 04 '18 at 20:15

André Guimarães Sakata

1,189
8
16

1

This is almost right, def foo instead of def self.foo is what extends the class, please confirm – user1130176 Nov 04 '18 at 20:51
2

You can just `extend SomeModule` instead of `include SomeModule` and remove the `included` callback. – Stefan Nov 05 '18 at 10:04
2

What @Stefan said because right now `foo` is added as an instance method and again as a class instance method. e.g. `SomeClass.foo == SomeClass.new.foo` in this case I guess that works but in most real world cases extend and include serve very different purposes – engineersmnky Nov 05 '18 at 14:20

score 0 · Answer 2 · answered Nov 04 '18 at 20:36

You cann package individual methods into their own modules and extend them:

module MyModule
  module NewFunc
    def new_func
      puts "hello"
    end
  end
end

class A
  extend MyModule::NewFunc
end

A.new_func

With some metaprogramming / monkeypatching you could make a way to extend only certain methods of a module, but I think the approach I have shown works good enough.

If you wanted to make it so individual methods can be imported, or you can import all of them, you can do it like so:

module MyModule
  module NewFunc
    def new_func
      puts "hello"
    end
  end

  module NewFunc2
    def new_func2
      puts "hello2"
    end
  end

  include NewFunc, NewFunc2 # this adds them to MyModule directly
end

class A
  extend MyModule
end

A.new_func
A.new_func2

A function that injects a class level function

2 Answers2