question about parameter passing in Ruby

Comparing the following two code snippets:

class Logger 
  def self.add_logging(id_string)
    define_method(:log) do |msg| 
      now = Time.now.strftime("%H:%M:%S")
      STDERR.puts "#{now}-#{id_string}: #{self} (#{msg})"
    end 
  end
end 

class Song < Logger
  add_logging "Tune" 
end

song = Song.new 
song.log("rock on")

class开发者_开发百科 Logger
  def self.add_logging(id_string)
    def log(msg)
      now = Time.now.strftime("%m")
      puts "#{now}-#{id_string}: #{self}(#{msg})"
    end
  end
end

class Song < Logger
  add_logging "Tune"
end

s = Song.new

s.log("can't smile with you")
#=> NameError: undefined local variable or method `id_string' for #<Song:0x000001018aad70>

I can't figure out why the second case gets the NameError error, and why the id_string can't be passed to it.

A def creates a new scope; a block does not. A new scope cuts off the visibility of the surrounding variables. ruby has two other 'new scope creators': class and module.

x = 10

3.times do |i|
  puts i * x
end

def do_stuff
  puts x * 10
end

do_stuff  

--output:--
0
10
20
`do_stuff': undefined local variable or method `x' 

id_string is local to method add_logging. In your latter implementation, log-method can not see it, hence the error. In the former implementation, you dynamically define log-method within add_logging.

In other words, local variable is visible within the scope it is defined in (in this case, a method). In that latter implementation, you have nested scopes (=a method declaration within a method), and inner scope can not access variables that are local to outer scope.

As suggested in answer by @stef, you might get around this my widening the scope of the variable. I would recommend keeping variable scopes as 'tight' as possible, and therefore prefer your first implementation.

Try this with a class variable?

class Logger 
  def self.add_logging(id_string)
    @@my_id = id_string
    define_method(:log) do |msg| 
      now = Time.now.strftime("%H:%M:%S")
      STDERR.puts "#{now}-#{@@my_id}: #{self} (#{msg})"
    end 
  end
end 

Class variables should be avoided in ruby due to their problematic nature. The ruby way is to use 'class instance variables' instead.