Passing blocks into nested method within class_eval in Ruby?

Question

I want to be able to define a block, and later evaluate that block from within a dynamically generated module/class. It seems like I could accomplish this somehow using eval and block.binding, but I haven't figured it out.

I have this as the base:

def define_module(name, &block)
  name = name.to_s.gsub(/\/(.?)/) { "::#{$1.upcase}" }.gsub(/(?:^|_)(.)/) { $1.upcase }
  parts = name.split("::")
  parts.each_with_index do |part, index|
    sub_name = parts[0..index].join("::")
    eval("module #{sub_name}; end")
  end
  clazz = eval(name)
  clazz.class_eval(&block) if block_given?
  clazz
end

def add_module(name, &block)
  module_block = block
  define_module(name).class_eval <<-EOF
    def self.included(base)
      base.class_eval do
        # something like this, I'm stuck
        instance_eval(&#{module_block})
      end
    end
  EOF
end

And I want to use it like this:

add_module("My::Library") do
  def a_method
    "added 'a_method'"
  end
end

class ::User
  include My::Library
end

user = ::User.new

assert_equal "added 'a_method'", user.a_method

Is there any way to do something like that?

ok, don't use string based evals, use block based evals, that way you can close over block — horseyguy
– horseyguy, Commented Sep 11, 2010 at 4:59

horseyguy · Accepted Answer · 2010-09-11 06:05:50Z

2

This works:

def add_module(name, &block)
    define_module(name).class_eval do
        class << self; self; end.send(:define_method, :included) { |base|
            base.class_eval(&block)
        }
    end
end

add_module("My::Library") do
    def a_method
        "added 'a_method'"
    end
end

class ::User
    include My::Library
end

user = ::User.new
user.a_method #=> "added a_method"

EDIT:

Why don't you just do this instead? Much simpler, and it's actually the job of a module:

def add_module(name, &block)
    define_module(name).class_eval(&block)
end

edited Sep 11, 2010 at 6:05

answered Sep 11, 2010 at 5:57

horseyguy

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1 Comment

horseyguy Over a year ago

though the whole idea seems weird -- why not just class_eval the block on the module itself rather than waiting until the include ? Seems like you're unnecessarily rewriting the entire idea of a module using the included hook???

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