Ruby: Monkey patching String class

metaprogramming, monkeypatching, ruby

Solution

File.send(method, self)

This `self` will be evaluated when that dynamically generated method will be called. At that point it will be set to an instance of String. ("asdf" in your example).

It's actually equivalent to opening `String` class and writing all those methods manually.

class String
  def readlink
    File.send :readlink, self
  end

  def expand_path
    File.send :expand_path, self
  end
end

Problem

I am a little confused why the following piece of code actually works: ``` String.instance_eval do # self is set to String [:readlink, :symlink?, :expand_path].each do |method| # self is still String define_method(method) do # self is still String File.send(method, self) # what exactly is this self? end end end "asdf".expand_path # => "C:/users/some_user/asdf" ``` I don't understand why the last line works as it does. When each method is defined isn't the body of the method equivalent to `File.send(method, String)`? None of the above blocks actually change `self`. The only line that changes `self` is `String.instance_eval` and it changes `self` to `String`.

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