Module visibility

ruby-talk
1

Hi all,

Visibility issues within a module are not entirely clear to me. I was
writing a little Ruby module in the process of learning the language,
but found myself facing with a behaviour I considered quite strange.
The guts of my problem is summarized by the following case study:

module A1
def f; puts 'A1::f'; end
module_function :f
end

module A2
def f; g; end;
def g; puts 'g'; end
module_function :f
end

module A3
def f; g; end;
private
def A3.g; puts 'A3.g'; end
module_function :f
end

Now, if I call A1::f, I get 'A1::f' printed on output, and that's OK.
Problems arise whenever I call A2::f and A3.g! The call to A2::f fails
because a NameError is raised, telling me about an undefined local
variable or method g for A2 module, which, ahem, it seems to me it's
plain false, since a g function is definetely defined in module A2. The
call to A3.g, then, prints 'A3.g' on output, but that should not happen
because I defined that function to be private in the module.

So, what's really happening? I'm puzzled!
Thanks in advance,
Giulio Piancastelli.

2

module A1
def f; puts 'A1::f'; end
module_function :f
end

module A2
def f; g; end;
def g; puts 'g'; end

     ^^^
  def self.g or better module_function :g too

module_function :f
end

module A3
def f; g; end;

        ^^^
        call with no explicit receiver, private methods work OK

···

On Mon, Oct 04, 2004 at 02:04:55AM +0900, Giulio Piancastelli wrote:

private
def A3.g; puts 'A3.g'; end
module_function :f
end

--
Running Debian GNU/Linux Sid (unstable)
batsman dot geo at yahoo dot com

3

"Giulio Piancastelli" <giulio.piancastelli@gmail.com> schrieb im
Newsbeitrag news:1096822941.974872.80190@h37g2000oda.googlegroups.com...

Hi all,

Visibility issues within a module are not entirely clear to me. I was
writing a little Ruby module in the process of learning the language,
but found myself facing with a behaviour I considered quite strange.
The guts of my problem is summarized by the following case study:

module A1
def f; puts 'A1::f'; end
module_function :f
end

module A2
def f; g; end;
def g; puts 'g'; end
module_function :f
end

module A3
def f; g; end;
private
def A3.g; puts 'A3.g'; end
module_function :f
end

Now, if I call A1::f, I get 'A1::f' printed on output, and that's OK.
Problems arise whenever I call A2::f and A3.g! The call to A2::f fails
because a NameError is raised, telling me about an undefined local
variable or method g for A2 module, which, ahem, it seems to me it's
plain false, since a g function is definetely defined in module A2.

g is defined as an *instance method* in A2, but not as a module singleton
method. By doing module_function :f in A2 you do basically the same as
"def A2.f() ...", i.e. you define a method of the instance A2. f tries to
invoke self.g while self is the module instance. But since you did not do
"def A2.g..." or "module_function g:" it's not there.

The
call to A3.g, then, prints 'A3.g' on output, but that should not happen
because I defined that function to be private in the module.

Well, the private relates to instance methods defined thereafter, but "def
A3.g ..." defines a method of the instance A3, i.e. not instances of A3
but the module itself. If you want to make it private you can do this:

module A3
class <<self
private
def g; puts 'A3.g'; end
end
end

A3.g

NoMethodError: private method `g' called for A3:Module
        from (irb):9

A3.instance_eval { g }

A3.g
=> nil

So, what's really happening? I'm puzzled!

Hope I could clear the fog a bit.

Regards

    robert

4

Mauricio Fernández ha scritto:

def g; puts 'g'; end

     ^^^
  def self.g or better module_function :g too

why better module_function ?

PS

giulio, your name sounds italian, if you are not aware of
the italian ruby user group you may like to visit
ada2.unipv.it/ruby and join the mailing list

5

Mauricio Fernández wrote:

> module A3
> def f; g; end;
        ^^^
        call with no explicit receiver, private methods work OK

Yes, but the problem is that I was able to call A3.g from outside the
module even if it was declared private. Why is that happening? Private
symbols in a module are private in respect to... what?
Regards,
Giulio Piancastelli

6

so that
  include A2
  f

works

···

On Mon, Oct 04, 2004 at 03:34:55AM +0900, gabriele renzi wrote:

Mauricio Fernández ha scritto:

>>def g; puts 'g'; end
>
> ^^^
> def self.g or better module_function :g too

why better module_function ?

--
Running Debian GNU/Linux Sid (unstable)
batsman dot geo at yahoo dot com

7

Why is it needed or why is module_function a better way to define it
than "def self."? I can answer the first: if you don't do one or the
other of them the module won't be able to see "g" (and thus f will fail
with a name error) since g will only be defined for instances of classes
that include the module.
     As for the second, I'd have to guess that he was expressing a
stylist preference, but that is only a guess.

   -- MarkusQ

···

On Sun, 2004-10-03 at 11:34, gabriele renzi wrote:

Mauricio Fernández ha scritto:

>>def g; puts 'g'; end
>
> ^^^
> def self.g or better module_function :g too

why better module_function ?

8

Giulio Piancastelli wrote:

Yes, but the problem is that I was able to call A3.g from outside the
module even if it was declared private. Why is that happening? Private
symbols in a module are private in respect to... what?

Interesting -- this is another difference between module_function without arguments and extend self:

module A3
   extend self
   def f; g; end
   private
   def g; "A3.g"; end
end

A3.g # raises NoMethodError: private method `g' called for A3:Module
A3.f # => "A3.g"

I think the behavior of extend self is more appropriate than the module_function one.

Regards,
Giulio Piancastelli

More regards,
Florian Gross

9

Talk about your Too Many Ways!

  module X
    extend self
    private
    # ...
  end

  module X
    module_function
    # ...
  end

  module X
    class << self
      def ameth
    end
    private
    include self
  end

  module X
    class << self
      def ameth
    end
    private
    def ameth
  end

  module X
    def self.ameth
    private
    def ameth
  end

  module X
    def X.ameth
    private
    def ameth
  end

  module X
    def X.ameth
    def ameth
    priavte :ameth
  end

T.

···

On Sunday 03 October 2004 05:14 pm, Florian Gross wrote:

Giulio Piancastelli wrote:
> Yes, but the problem is that I was able to call A3.g from outside the
> module even if it was declared private. Why is that happening? Private
> symbols in a module are private in respect to... what?

Interesting -- this is another difference between module_function
without arguments and extend self:

module A3
   extend self
   def f; g; end
   private
   def g; "A3.g"; end
end

A3.g # raises NoMethodError: private method `g' called for A3:Module
A3.f # => "A3.g"

I think the behavior of extend self is more appropriate than the
module_function one.