From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-07-28T23:35:38+09:00
Subject: Re: What's so special about operators, built-in classes and modules?

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On Thu, 28 Jul 2005, Daniel Brockman wrote:

>> i'd say __all__ code depends on that.  otherwise
>>
>>    module M
>>      def self::new
>>        raise
>>      end
>>    end
>>    class C
>>      include M
>>    end
>>
>>    c =3D C::new
>
> Surely you are not suggesting that all code defines
> singleton =E2=80=98new=E2=80=99 methods on modules?
>
> Considering that this doesn't work,
>
>   class X
>     def self.new
>       raise
>     end
>   end
>
>   class Y < X
>   end
>
>   x =3D X.new
>
> why do you expect your example to work?

well - you have to actually __use__ the method - i was pointing to the fact
that they are inherited in the case of subclasses and are not in the case o=
f
mixins ;-)

   harp:~ > cat a.rb
   class X
     def self.new
       raise
     end
   end

   class Y < X
     def self.new; super; end
   end

   x =3D X.new

   harp:~ > ruby a.rb
   a.rb:3:in `new': unhandled exception
           from a.rb:11


> I don't get why Y's singleton class inherits from X's,
> while C's *doesn't* inherit from M's.

because otherwise including module could clobber class methods which genera=
lly
include hooks to create instances like 'new', 'instance', 'parse', etc.  if=
 you
replace the set of methods responsible for stamping out instances you are n=
ot
'mixing-in' functionality to those instances - but changing what type those
will be.  there is a fundemental difference here.

> Why are you defining M.new?  Do you have actual code that > does this?

tons.  just yesterday i was writing a parser.  all the code is wrapped in a
module to protect namespaces.  each line of the file i am parsing is a comm=
and
sent to a satelite.  i have a factory which produced command objects of a
given type that's arranged like

   module Command
     class Abstract
       ...
     end
     class Load < Abstract
       ...
     end
     class Sub < Abstract
       ...
     end
     class Add < Abstract
       ...
     end
     class Store < Abstract
       ...
     end
     def self::new line
       orbit_normal_time, command, register, value =3D parse line
       klass =3D
         case command
           when /load/
             Load
           when /sub/
             Sub
           when /add/
             Add
           when /store/
             Store
         end
       klass::new(orbit_normal_time, register, value)
     end
   end

however, i actually use this class in another like

   class PayloadActivationMessage < AbstractMessage
     include Command
   end

and this would change the notion of calling 'super' if what you are suggest=
ing
were true.  eg - if 'some_method' were defined as a class method in
PayloadActivationMessage, AbstractMessage, and Command, then a definition l=
ike

   def PayloadActivationMessage::some_method
     super + 42
   end

would seem to call AbstractMessage::some_method (due to inheritence) but wo=
uld
actually call Command::some_method if class methods were automatically pull=
ed
in by 'include' statements.  there are cases where you want this and that's
what 'append_features' is for - but to have this for the normal case would =
be
quite suprising for most applications i think.  not to mention changing the
notion of super is, be definition, a very strong form on inheritence.

> No, it would not be possible to retain full backwards compatibility, but =
I
> cannot think of a real-world case in which actual problems would result.

added multiple inheritence semantics for module inclusion would not be
backwards compatibile imho.

>> inheritence needs to be addressed specifically for it to make sense.
>
> Why do you say that?

well - because that's what you are suggesting!?


pine, mutt, or mail.

but even the web interface shows the problem:

   http://www.ruby-talk.org/cgi-bin/scat.rb/ruby/ruby-talk/149703

look at the line with 'const char' on it.

cheers.

-a
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| email :: ara [dot] t [dot] howard [at] noaa [dot] gov
| phone :: 303.497.6469
| My religion is very simple.  My religion is kindness.
| --Tenzin Gyatso
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