ruby confusing -- local variable or instance_method?

Question

I have the following program.

module C
  def self.included(base)
    base.extend(ClassMethods)

  end

  module ClassMethods
    def test_for
      class_eval <<-DEFINECLASSMETHODS
        def self.my_method(param_a)
          puts "SELF is: #{self.inspect}"
          puts param_a
          puts "#{param_a}"
        end
      DEFINECLASSMETHODS
    end
  end
end

class A
  include C
end

class B < A
  test_for
end

when I run B.new.my_method("aaa"), I got this error

NameError: undefined local variable or method `param_a' for B:Class

I am quite confused.

I define param_a as a local variable in class method my_method,

puts param_a

runs good, and will output the "aaa".

however,

puts "#{param_a}"

output that error.

why?

Can anyone explain this?

sepp2k · Accepted Answer · 2010-05-14 10:15:20Z

3

You get that error because the #{} doesn't interpolate param_a into the string passed to puts - it interpolates it into the string passed to class_eval. It will work when you escape it, i.e.

puts "\#{param_a}"

You can also disable interpolation inside the heredoc by using <<-'DEFINECLASSMETHODS' instead of <<-DEFINECLASSMETHODS. This will also allow you to use other meta characters without having to escape them.

answered May 14, 2010 at 10:15

sepp2k

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2 Comments

Jörg W Mittag Over a year ago

Damn! I totally missed that! +1 for eagle-eyes! Personally, I have the rule that I always use single-quoted strings, unless I am 100% sure that I absolutely, positively want string interpolation to occur. This is just one more example why that is a good idea.

Chuck Over a year ago

This is one of the many reasons why I always recommend using the block form of class_eval.

S. Christoffer Eliesen · Accepted Answer · 2010-05-14 16:37:10Z

2

Try using "class_eval do; end" instead, like this:

def test_for
  class_eval do
    def self.my_method(param_a)
      puts "SELF is: #{self.inspect}"
      puts param_a
      puts "#{param_a}"
    end 
  end 
end

This way, no code escaping is necessary.

answered May 14, 2010 at 16:37

S. Christoffer Eliesen

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Comments

Jörg W Mittag · Accepted Answer · 2010-05-14 11:52:25Z

1

Those are some majorly complex hoops you are jumping through, to achieve basically this:

module C
  def test_for
    define_singleton_method :my_method do |param_a|
      puts "SELF is: #{inspect}"
      p param_a
    end
  end
end

class A
  extend C
end

class B < A
  test_for
end

B.my_method 'foo'
# => SELF is: B
# => "foo"

EDIT: I just realized that the solution above is still much more complicated than it needs to be. In fact, we do not need any metaprogramming at all:

module C
  module D
    def my_method(param_a)
      puts "SELF is: #{inspect}"
      p param_a
    end
  end
  def test_for
    extend D
  end
end

class A
  extend C
end

class B < A
  test_for
end

B.my_method 'foo'
# => SELF is: B
# => "foo"

edited May 14, 2010 at 11:52

answered May 14, 2010 at 9:46

Jörg W Mittag

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