From: Trans <transfire@...>
Date: 2005-08-02T07:36:04+09:00
Subject: Re: Selective mixins; who can improve on this?


Kirk Haines wrote:
> So, on #ruby-lang an hour ago, Daniel Berger asked for ideas to make some
> non-working code of his work.  He's trying to come up with a way to
> selectively mixin methods from a module into a class.
>
> After some tinkering, I came up with this working, but ugly and definitely
> improveable code to do it:
>
> class Module
>   def use(mod, *methods)
>     c = Class.new
>     # Make a singleton that mixes everything in, so that we can cherrypick.
>     eval("class << c; include #{mod}; end")
>
>     methods.each do |m|
>       # There has to be a more elegant way to do this.
>       m_sym = m.to_sym
>       mc_sym = m.to_s.upcase.to_sym
>       um = c.method(m_sym)
>       self.const_set(mc_sym,um)
>       self.class_eval("def #{m.to_s}; #{mc_sym.to_s}.call; end")
>     end
>   end
> end
>
>
> module Foo
>   def a; 'abc'; end
>   def z; 'xyz'; end
> end
>
> class Bing
>   use(Foo, :a, 'z')
> end
>
> x = Bing.new
> p x.a
> p x.z
>
>
> There has to be a more elegant way to do this.  What is it?
>

I'm afraid that is not doing what you expect it too. Try:

  def a; p self; end

and you will see that self is not a Bing.

There is no _simple_ solution to this. Ruby is picky about where its
methods come from and where they can thus go to. Try playing with
#method, Class#instance_method and #bind and you'll see what I mean.
BUT I have done what you ask, as I have/am considering using something
like this for Ruby Facets. Off the top of my head (so forgive any
little bugs) like:

  class Module
    METHINDEX = {}

    def index_def( msym, &mdef )
      METHINDEX[[self,:"#{msym}"]] = mdef
    end

    def use(mod, *methods)
      methods.each { |msym| self.class_eval
&METHINDEX[[mod,:"#{msym}"]] }
    end
  end

  module Foo
    index_def( :a ) {
    def a; p self ; end
    }

    index_def( :z ) {
    def z; p "z" ; end
    }
  end

   class Bing
    use Foo, :a, 'z'
  end

The block is needed to protect the method defintion from binding to an
unusable location.

Am interested in other people's solutions too.

T.