13 lines of confusion

I can’t for the life of me figure out why the following code makes Ruby
puke when I try to execute it:

    module Rlang
      module Error
        class RlangException < Exception ; end
      end
      module Utility
        def e8(value) ; [value].pack('C') ; end
      end
      module External
        include Rlang::Error
        include Rlang::Utility
        TAG_CACHED_ATOM = e8(67)
      end
    end

The precise error I get is “junk.rb:11: undefined method `e8’ for
Rlang::External:Module (NoMethodError)” which is the line with
“TAG_CACHED_ATOM = …” in it. I’ve tried the following changes with
the following results:

 1. Completely specify e8 by using "Rlang::Utility.e8" or
    "Rlang::Utility::e8".  This produces no change to the message
    whatsoever.
 2. Renaming bits and pieces under the assumption that I've got a
    naming clash with something deep.  This only changes the name of
    the method it claims to not be able to find.
 3. Moving the definition of e8 to be in the External module.  This
    is where brain damage sets in as the message doesn't change in
    the slightest.  (I am well and truly baffled by this stage.)
 4. Moving the definition of e8 to be in the Rlang module.  This
    results in, you guessed it!, no change to the message at all.
 5. Moving the definition of e8 to be outside of any module at all.
    This results in ... it works.

#5 is a solution but it is a truly suboptimal one in my opinion. I do
not want to clutter up the global namespace of my clients with cryptic
little utility functions (in the real code there’s about a dozen of
these). So while I could live with #5, I would rather not if there was
a better way to do this. Something deep about Ruby’s module system is
eluding me here, I think, and I’d rather understand it than just cover
it over and inconvenience my clients’ coding in the process.

Oops. Forgot to mention: I’ve also tried putting each module into a
separate file which is then required. Same result.

On Tue, May 20, 2008 at 10:02 PM, Michael T. Richter
[email protected] wrote:

module External
include Rlang::Error
include Rlang::Utility
TAG_CACHED_ATOM = e8(67)
end
end

The precise error I get is “junk.rb:11: undefined method `e8’ for
Rlang::External:Module (NoMethodError)” which is the line with
“TAG_CACHED_ATOM = …” in it.

the include Rlang::Utility causes the External module to define the e8
method for classes which include the External module either directly
or indirectly (e.g. through an included module which includes
External) but the line

TAG_CACHED_ATOM = e8(67)

is evaluated in the context of the module itself, self is External,
and the External object doesn’t have this method.

Now if you change

include Rlang::Utility
to
extend Rlang::Utility

the file will run without error.

If you want classes which include External to have e8 as an instance
method as well you can both include and extend Rlang::Utility


Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/

On Wed, 2008-05-21 at 11:34 +0900, Rick DeNatale wrote:

If you want classes which include External to have e8 as an instance
method as well you can both include and extend Rlang::Utility

While I’m not sure I understand the explanation (time to hit the books
again), the solution you propose works. Time for me to brush up on
Ruby’s concept of modules again (which is, as is often the case,
entirely different from everybody else’s take on the topic!:D)

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