How do I fix this NameError?

Question

I want to use the value v inside of an instance method on the metaclass of a particular object:

v = ParserMap[kind][:validation]   # We want to use this value later.
s = ParserMap[kind][:specs]
const_set(name, lambda {
  p = Parser.new(&s)

  # This line starts a new scope...
  class << p
    define_method :validate do |opts|
      v.call(self, opts)  # => NameError! The `class` keyword above
                          #    has started a new scope and we lost
                          #    old `v`.
    end
  end
  p
})

Unfortunately, the class keyword starts a new scope, so I lose the old scope and I get a NameError. How do I fix this?

Answer 1

Replace class << p with class << p; self end.class_eval do and it will work.

class << p; self end will return the metaclass of p, so you can call class_eval on it. The block given to class_eval will then execute in the context of the metaclass (same as it did before), but without starting a new scope.

Answer 2

Your first inclination might be to use class_eval on p, like this:

p.class_eval {
  ...
}

Alas, that won't work, because class_eval is a method defined on Module, not on Object. Since p is an instance of an object, not of a Module or Class, it doesn't have a class_eval method.

The trick is to get p's singleton class first, and then run class_eval on that. Since that is a Class (and by extension, a Module), it has a class_eval method. If you're in 1.9.2 or later, there's a singleton_class method you can use:

p.singleton_class.class_eval {
  ...
}

Otherwise, you can just get the singleton class directly:

(class << p; self; end).class_eval {
  ...
}

---

As Jorg points out, you can also use define_singleton_method:

p.define_singleton_method :validate { |opts|
  v.call(self, opts)
}

But note that if you do this, the resulting validate method will be private, which may not be what you want.

Answer 3

Just for kicks, here's how it would look like in Ruby 1.9.2:

v = ParserMap[kind][:validation]
s = ParserMap[kind][:specs]
const_set(name, ->{
  Parser.new(&s).tap {|p|
    p.define_singleton_method :validate do |opts| v.(self, opts) end
  }
})

• replace the explicit return of p at the end with the K combinator (Object#tap) introduced in Ruby 1.8.7 and 1.9.0
• replace the lambda method call with a proc literal introduced in Ruby 1.9.0
• replace obj.call(args) with obj.(args) introduced in Ruby 1.9.0
• most importantly: use Object#define_singleton_method introduced (or more precisely: going to be introduced) in Ruby 1.9.2