From: Brian Mitchell <binary42@...>
Date: 2005-09-29T04:39:01+09:00
Subject: Re: Python to Ruby: Two puzzlements...

A fixed error:

On 9/28/05, Brian Mitchell <binary42@gmail.com> wrote:
> On 9/28/05, Elf M. Sternberg <elf@drizzle.com> wrote:
> > I'm afraid that I'm coming from Python, a B&D language where I'm used to
> > everything be spelled out cleanly, and although I programmed in Perl for
> > many years that was also many years ago.  Ruby is baffling the heck out
> > of me.
> >
> > I've been looking at two examples, and I was wondering if someone could
> > explain to me what the Hell is going on.  The first is from
> > http://redhanded.hobix.com/bits/hyperextended.html, and my question is
> > about the 'extend' keyword there.
>
> To start off, it is not a keyword. Rather it is a method that is found
> on an instance of Module (Class is a subclass of Module -- see
> Class.ancestors).
>
Scratch that. Object. Sorry.

> >  Where does that came from?
>
> I explained where it is defined but the extend keyword is also a bit
> trickier. It could be thought of as running include on the singleton
> class of an object. Particular gotchas are the order in which instance
> methods are looked up and called.
>
> class A
>   def foo
>     p 'A'
>   end
> end
>
> module M
>   def foo
>     p 'M'
>   end
> end
>
> class B
>   include M
>   def foo
>     p 'B'
>   end
> end
>
> a = A.new
> b = B.new
>
> puts 'class'
> a.foo
> b.foo
>
> puts 'extend'
> a.extend M
> a.foo
>
> b.extend M
> b.foo
>
> class << a
>   def foo
>     p 'a'
>   end
> end
>
> # another way to do the above.
> def b.foo
>   p 'b'
> end
>
> puts 'singleton class'
> a.foo
> b.foo
>
> >  In what
> > object is it defined?
>
> Module is where extend is. The methods are in the singleton class. You
> can check this out by running at the end of the above script.
>
> C = Class.new
> c = C.new
> c.extend M
>
> class << c
>   p self.ancestors
> end
>
> >  It's just hanging there in space.  In the same
> > breath, what does the 'super' keyword do in the append_features()
> > method, and why does it need a 'self' in front of it?
>
> ri Module#append_features will talk about how the default
> implementation adds constants, methods, and module variables. To
> maintain this it calls the overridden method explicitly using super.
> super is an example of a magic keyword. It will pass the arguments
> given to the current by default.
>
> >  I've tried
> > reading ruby-docs without finding much illumination.
> >
>
> That is ok. This is what this mailing list is for. It is good to hear
> that some people try before they post still though...
>
> > module Mix
> >    def inst_meth
> >      puts 'inst_meth'
> >    end
> >
> >    module ClassMethods
> >      def class_meth
> >        puts 'class_meth'
> >      end
> >    end
> >
> >    extend ClassMethods
> >
> >    def self.append_features(klass)
> >      super
> >      klass.extend(ClassMethods)
> >    end
> >  end
> >
>
> This code, from what it looks like. Takes a module and appends the
> instance methods along with class methods (which usually don't
> follow). I haven't run it but it seems the ClassMethods module makes
> this more obvious. This is a useful technique which quite a few ruby
> projects have made use of (I think rails uses this). Read the comments
> on the RedHanded post has there are many other interesting
> improvements that can be made.
>
> >
> > Second, I've seen the following constructions:
> >
> > validates :url :with => %r{^http:.+\.(gif|jpg|png)$}i
> >
>
> %r{} or %r[] or %r|| etc... are just another nice way of defining
> regular expressions.
>
> >         and
> >
> > check_associations %w(friends downloads links)
> >
>
> %w just creates an array from a space delliminated list. The escaping
> rules follow that of '. The other version is %W which escapes like "
> does.
>
> > I'm guessing that the "%r" and "%w" are the ruby equivalents of perl's
> > qr{} and qw{} constructs (does this mean there's a %qq() and %qx() as
> > well?), but I can't find documentation to that effect inside the
> > references available on-line.  (And before someone says "Buy the
> > Pickaxe!" I'll just say that I spent my book budget this month on the
> > Rails book and any other purchases will have to wait until my next
> > paycheck.)
>
> Well, in due time... I will, however, tell you that it is worth the
> money for most people.
>
> Brian.
>