Why is my define_method content being evaluated in the class scope?

Question

I'm doing some (too) fancy meta programming, and I have a hard time understanding why the scope is different in the following two cases:

Case 1:

class TesterA

  def the_method
    puts "I'm an instance_method!"
  end

  def self.the_method
    puts "I'm a class_method!"
  end

  def self.define_my_methods *method_names
    method_names.each do |name|
      define_method("method_#{name}") do
        the_method
      end
    end
  end

  define_my_methods :a, :b, :c
end

t = TesterA.new
t.method_a #=> I'm an instance_method!
t.method_b #=> I'm an instance_method!
t.method_c #=> I'm an instance_method!

Case 2

class TesterB

  def the_method
    puts "I'm an instance_method!"
  end

  def self.the_method
    puts "I'm a class_method!"
  end

  def self.define_the_method attr
    define_method("method_#{attr}") do
      begin
        yield
      rescue
        raise $!, "method_#{attr} was called: #$!", $@
      end
    end
  end

  def self.define_my_methods *method_names
    method_names.each do |name|
      define_the_method(name) do
        the_method
      end
    end
  end

  define_my_methods :a, :b, :c
end

t = TesterB.new
t.method_a #=> I'm a class_method!
t.method_b #=> I'm a class_method!
t.method_c #=> I'm a class_method!

I the second example I introduce a kind of "helper-mothod" define_the_method which I use for defining the methods rather than define_method it self. Reason for that is, That I want to append the name of the dynamic method to any exception messages that might occur inside those methods. The problem however is, that the content (when using the latter case) seems to be evaluated in the class-scope.

Why is this, and how can I make it get evaluated in the instance-scope?

score 4 · Answer 1 · answered Aug 30 '17 at 10:54

4

This block:

do
  the_method
end

it defined and scoped in class-scope. There is no reason to expect it’ll be somehow injected into instance-scope. To fix the issue simply pass the receiver explicitly:

yield(self)

and:

def self.define_my_methods *method_names
  method_names.each do |name|
    define_the_method(name) do |receiver|
      receiver.the_method
    end
  end
end

answered Aug 30 '17 at 10:54

Aleksei Matiushkin

119,336
10
100
160

Buuuuut, it's the same in the first snippet. no? (the scope) – Sergio Tulentsev Aug 30 '17 at 10:55
which is present (and called from a class method) in both snippets – Sergio Tulentsev Aug 30 '17 at 10:56
Ah, I see. I'll go have more coffee – Sergio Tulentsev Aug 30 '17 at 10:57
Thanks for the answer, I went with the `instance_eval` one instead, since I had to change a lot of code, if I needed to explicitly call all methods on the scope. – Niels Kristian Aug 30 '17 at 11:45
“I needed to explicitly call all methods on the scope”—I doubt I get why do you need to explicitly call _all_ methods on the scope. – Aleksei Matiushkin Aug 30 '17 at 11:52

potashin · Answer 2 · 2017-08-30T11:12:00.393

4

It happens because you are providing the block in the scope of the class method self.define_my_methods, where self is a class, not an instance. So what you can do is yield the scope of the define_method itself:

def self.define_the_method attr
  define_method("method_#{attr}") do
    begin
      yield self
    rescue
      raise $!, "method_#{attr} was called: #$!", $@
    end
  end
end

def self.define_my_methods *method_names
  method_names.each do |name|
    define_the_method(name) do |scope|
      scope.send(:the_method)
    end
  end
end

edited Aug 30 '17 at 11:12

answered Aug 30 '17 at 10:54

potashin

44,205
11
83
107

“you are `yield`ing in the scope of the class method”—nope, it’s `yield`ing in the instance scope, since `define_method` uses `instance_eval` under the hood. The block is class-scoped, that is why. – Aleksei Matiushkin Aug 30 '17 at 11:02
@mudasobwa: I am sorry, this is what I meant in the first place, I have not been answering for a while and the articulation is kinda rusty. – potashin Aug 30 '17 at 11:09
Thanks for the answer, I went with the `instance_eval` one instead, since I had to change a lot of code, if I needed to explicitly call all methods on the scope. – Niels Kristian Aug 30 '17 at 11:45

Danil Speransky · Accepted Answer · 2017-08-30T11:03:28.827

2

That's because proc defined with define_method will be called by using instance_eval.

In the first sample it is:

do
  the_method
end

In second:

do
  begin
    yield
  rescue
    raise $!, "method_#{attr} was called: #$!", $@
  end
end

But yield will have parent scope from the place it was defined.

Here is my suggestion:

def self.define_the_method attr, &block
  define_method("method_#{attr}") do
    begin
      instance_eval(&block)
    rescue
      raise $!, "method_#{attr} was called: #$!", $@
    end
  end
end

edited Aug 30 '17 at 11:03

answered Aug 30 '17 at 10:57

Danil Speransky

29,891
5
68
79

I like this one! However in my real world use case, I need to be able to ALSO pass a param to the methods so I just ended up using `instance_exec` instead. Thanks for the tip! – Niels Kristian Aug 30 '17 at 11:43

Why is my define_method content being evaluated in the class scope?

3 Answers3