From: "Mauricio Fernández" <batsman.geo@...>
Date: 2003-01-22T20:06:36+09:00
Subject: Re: accessors for module instance vars

On Wed, Jan 22, 2003 at 06:00:03PM +0900, Phil Tomson wrote:
> In article <Pine.LNX.4.33.0301220531480.26588-100000@eli.fsl.noaa.gov>,
> ahoward  <ahoward@fsl.noaa.gov> wrote:
> >On 22 Jan 2003, Phil Tomson wrote:
> >> And if I leave it as a module it could be mixed into a class later (though
> >> that's not how I'm using it now) and all of the 'module instance variables'
> >> would also end up in the class it was mixed into....
> >
> >i don't think it works that way :
> >
> >  module M
> >    attr_accessor :x
> >    @x = 42
> >  end
> >
> >  class Foo; include M; end
> >
> >  p Foo.new.x  # >> nil
> >
> >the instance variable STAY in M, that's what i was referring to when i said i
> >was unsure of the semantics.

See my other post.
 
> this works:
> 
>  module M
>   def x
>      if not defined? @x
>        @x = 0
>      end
>      @x
>    end
> 
>    def x=(x)
>      @x=x
>    end
>  end
> 
>  class Foo; include M; end
> 
>  Foo.new.x #-> 0

But you're not using the module instance variable of M!

> However, those are instance methods (x and x=) defined in module M and 
> since M can't be instantiated they are only callable after M is mixed into 
> a class and an instance of that class is created.
> 
> I wonder if there is a way to mixin module methods so that they become 
> instance methods of a class?
> 
> module M
>   def M.x
>     if not defined? @x
>       @x=0
>     end
>     @x
>   end
> end

This one is.

> class MyM
>   include_module_methods_of M #somehow
> end
> 
> MyM.new.x #-> 0

If that worked, it'd mean that you're extending one object with the
singleton class of M, which is AFAIK impossible since it's a virtual
class. You cannot 'transfer' singleton methods.

>> module M
>> def M.x
>>  @x
>> end
>> def M.x= a
>>  @x = a
>> end
>> end
=> nil
>> M.x
=> nil
>> M.x = 1
=> 1
>> M.x
=> 1
>> singleton_class = class << M; self; end
=> Module
>> class Foo < singleton_class
>> end
TypeError: can't make subclass of virtual class
        from (irb):13
>> singleton_class.instance_methods
=> ["x", "x="]
>> Foo.extend singleton_class
TypeError: wrong argument type Class (expected Module)
        from (irb):24:in `extend'
        from (irb):24
>> M.singleton_methods
=> ["x", "x="]
>> Foo.extend M
=> Foo
>> Foo.x
NameError: undefined method `x' for Foo:Class
        from (irb):35
>> class Foo
>> include M
>> end
=> Foo
>> Foo.x
NameError: undefined method `x' for Foo:Class
        from (irb):40
>> Foo.new.x
NameError: undefined method `x' for #<Foo:0x4022e908>
        from (irb):41

> Anyone know of a way to do this?  (I have no pressing need for this, just 
> curious)

Just wait for Guy Decoux' post to correct me and find a way or confirm
it cannot be done ;-)

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