From: Michael Guterl <mguterl@...>
Date: 2006-04-23T22:09:23+09:00
Subject: Re: Metaprogramming is fun!

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Thanks David!  I removed the evals, cleaned up self.open, and ran my tests
and everything works.

I really like the fact that I don't have to use the last_position variable
and I can use String#slice! to incrementally chop it down.

Any time you feel like reflecting on the need for class variables, my ears
are open.

Thanks again!

Michael Guterl


On 4/23/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
>
> Hi --
>
> On Sun, 23 Apr 2006, Michael Guterl wrote:
>
> > I finally decided to dive in and give metaprogramming in Ruby a
> shot.  I'm
> > not sure that my example is exactly practical, but it seemed useful at
> the
> > time.  There's a few areas I would like to hear others suggestions on:
> >
> > 1. Does the syntax appear to be in line with the community standards?
>
> You're indenting one space instead of two on the first indent, but
> generally it looks good.
>
> > 2. Are my uses of class_eval and instance_eval okay / is there a better
> way?
>
> See below.
>
> > require 'rubygems'
> > require 'facets'
> > require 'dictionary'
> >
> > class FixedLength
> >
> >  def self.structure(ordered_hash)
> >
> >    class_eval do
> >      @@structure =3D ordered_hash
> >    end
> >
> >    keys =3D @@structure.keys
> >    instance_eval do
> >      attr_accessor *keys
> >    end
>
> I'm not sure why you're using all these *_eval calls.  Try this:
>
>    def self.structure(ordered_hash)
>      @@structure =3D ordered_hash
>      attr_accessor *@@structure.keys
>    end
>
> >  end
> >
> >  # this entire method could probably be a lot cleaner
> >
> >  def self.open(file_name)
> >    class_eval do
>
> I haven't tested it but I don't see any reason for that one either.
>
> >      data =3D IO.read(file_name)
> >      records =3D []
> >      data.each_line do |line|
> >        last_position =3D 0
> >        record =3D self.new
> >        @@structure.each_pair do |name, length|
> >          record.instance_variable_set( "@#{name.to_s}",
>
> The #{} thing does an automatic to_s.  Also, since you've gone to the
> trouble of creating accessors, why not do:
>
>    record.send("#{name}=3D"), line.slice...
>
> > line.slice(last_position,
> > length.to_i).strip)
> >          last_position +=3D length.to_i
> >        end
> >        records << record
> >      end
> >      return records
> >    end
> >  end
> >
> > end
>
> You could tighten that method up a bit. Here's an untested rewrite;
> see if this is of any use:
>
>    def self.open(file_name)
>      records =3D []
>      File.open(file_name) do |fh|
>        fh.each_line do |line|
>          record =3D new
>          @@structure.each do |name,len|
>            record.send("#{name}=3D", line.slice!(0,len))
>          end
>          records << record
>        end
>      end
>      return records
>    end
>
> (I'll save my reflections on the likelihood of class variables being
> necessary for another time :-)
>
>
> David
>
> --
> David A. Black (dblack@wobblini.net)
> Ruby Power and Light, LLC (http://www.rubypowerandlight.com)
>
> "Ruby for Rails" PDF now on sale!  http://www.manning.com/black
> Paper version coming in early May!
>
>

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