From: Rich Kilmer <rich@...>
Date: 2002-02-02T11:08:52+09:00
Subject: RE: Newbie question



> -----Original Message-----
> From: Phil Tomson [mailto:ptkwt@shell1.aracnet.com]
> Sent: Friday, February 01, 2002 8:55 PM
> To: ruby-talk ML; undisclosed-recipients:
> Subject: Re: Newbie question
>
>
> In article <ad2f1094.0202011555.2887f94d@posting.google.com>,
> andrew queisser <andrew_queisser@hp.com> wrote:
> >Oliver Mensinger <cyo@gmx.net> wrote in message
> >news:<Pine.LNX.4.21.0201301341560.312-100000@bogus.trumper>...
> >Thanks, that almost gets me there. I wasn't clear enough in my
> >original post but the "words" could consist of any characters other
> >than comma. They include spaces, dashes, etc. I'm sure the regexp
> >could be modified to account for that.
> >
> >Your post brings up another question: I noticed you're adding a
> >function to the String class. My first thought was (my primary
> >language is C++) "shouldn't we derive a new class and add the function
> >there." Then I realized that I have a loop in my program that does a
> >File.each_line and the extended string class dovetails nicely into the
> >loop. I just call your new function on the block parameter (which is a
> >String) like so:
> >
> >  infile.each_line do |line|
> >    ofile.puts line.getCommaElements(4)
> >  end
> >
> >Very nice. Then I started thinking about extending well-known classes
> >in general and I'm wondering whether injecting new functions is such a
> >good idea. After all, I could change my loop to this:
> >
> >  infile.each_line do |line|
> >    ofile.puts CommaElementSearcher(line, 4)
> >  end
> >
> >where CommaElementSearcher is a new function (or class) that handles
> >the same functionality.
> >
> >So, what are the feelings about cost/benefit of modifying existing
> >classes in this way? Are there hidden dangers or is it a good thing?
>
> I think it tends to be a cultural thing.  Apparently in SmallTalk this is
> also a common practice.  I can see why it would be problematic in the C++
> world because it really isn't as easy to do in C++ (actually, is it even
> possible in C++ at all to modify an existing class and have all current
> objects of that class respond accordingly? - I doubt it).
>
> You probably should use caution in changing the way an existing method
> of a class works, though.  For example, it probably wouldn't be a good
> idea to change Array#each to do something different than it currently
> does.  But adding methods to a built-in class, as in your example, is
> fairly common practice in the Ruby community.
>
> Phil

This is fine especially if you are writing a script that simply needs
to redefine the class while its running.  If you are producing a library
that others are going to use and your modification is going to be part
of their runtime environment, that may pose a larger problem.  For
example, someone else's library may make use of adding a method to
String with the same name as yours, but with different functionality.
In that case it may make more sense to build a module to contain the
function "getCommaElements(line)" which allows you to isolate the
funtion away from others.

-rich