From: dblack@...
Date: 2006-04-23T21:15:54+09:00
Subject: Re: Metaprogramming is fun!

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 = ordered_hash
>    end
>
>    keys = @@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 = 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 = IO.read(file_name)
>      records = []
>      data.each_line do |line|
>        last_position = 0
>        record = 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}="), line.slice...

> line.slice(last_position,
> length.to_i).strip)
>          last_position += 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 = []
     File.open(file_name) do |fh|
       fh.each_line do |line|
         record = new
         @@structure.each do |name,len|
           record.send("#{name}=", 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)

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