Question

I am attempting to make a module that extends the functionality of the FileUtils class.

require 'fileutils'

module FileManager
  extend FileUtils
end

puts FileManager.pwd

if I run this, I’ll get a private method 'pwd' called for FileManager:Module (NoMethodError)
error

Update:
Why are these class methods included privately and how can I expose all of them without having to manually include each method as public class methods in the FileManager module?

Answer 1

It seems like the instance methods on FileUtils are all private (as mentioned in another answer here, that means they can only be called without an explicit receiver). And what you get when you include or extend is the instance methods. For example:

require 'fileutils'

class A
  include FileUtils
end
A.new.pwd #=> NoMethodError: private method `pwd' called for #<A:0x0000000150e5a0>

o = Object.new
o.extend FileUtils
o.pwd #=> NoMethodError: private method `pwd' called for #<Object:0x00000001514068>

It turns out all the methods we want on FileUtils are there twice, as private instance methods and also as public class methods (aka singleton methods).

Based on this answer I came up with this code which basically copies all the class methods from FileUtils to FileManager:

require 'fileutils'

module FileManager
  class << self
    FileUtils.singleton_methods.each do |m|
      define_method m, FileUtils.method(m).to_proc
    end
  end
end

FileManager.pwd #=> "/home/scott"

It’s not pretty, but it does the job (as far as I can tell).