From: Mark Hubbart <discordantus@...>
Date: 2004-10-06T06:22:46+09:00
Subject: Re: Extending existing classes

On Wed, 6 Oct 2004 03:55:12 +0900, Henrik Horneber <ryco@gmx.net> wrote:
> Edgardo Hames wrote:
> 
> > On Wed, 6 Oct 2004 03:29:35 +0900, Henrik Horneber <ryco@gmx.net> wrote:
> >
> >>>>- extending String so you can tell it to remove cpp comments /* remove
> >>>>me */
> >>>
> >>>Extending string with something like remove_cpp_comments seems awkward
> >>>to me. In any usecase for this I'd rather have a class CppCode that I
> >>>can ask to remove_comments. (Or even better, to give me the source
> >>>without comments.)
> >>>
> >>>So my personal heuristic tells me not to extend in this case.
> >>
> >>Now, the reason I was immediatly reminded of that remove_cpp_comments
> >>example is, that I wrote a small script some time ago, which used
> >>exactly this case. First, I implemented remove_cpp_comments in its own
> >>class (which was not called CppCode, but had the same intend), but out
> >>of curiosity (and because in ruby I can, dang it! :) ) I moved the
> >>remove_cpp_comments method to String. Turns out, the method got shorter
> >> and somewhat clearer, at least to me. And I could write something like
> >>
> >>source_code = IO.readlines(file).join("\n")
> >>source_code.strip_comments!
> >># instead of source_code = CppCode.strip_comments( source_code)
> >>
> >>.... you get the idea.
> >>
> >>Even though the code became clearer on that level, I still have to agree
> >>with Brian that it is somehow dirty/awkward on a different level to
> >>extend String with such a method.
> >>
> >
> >
> > I usually try not to do that kind of things. Then, for each new
> > lanaguage you are going to support, you should now add a
> > remove_#{lang}_comments to String. But then, when reading a given
> > source file you could remove comments that are not so in that
> > language.
> >
> > Regards,
> > Ed
> 
> In my case there is never going to be a different language to support,
> but in general, that is a good reason not to do it. I'd like to get away
> from my example and to a little wider discussion of when to extend
> existing classes. I could have written my own subclass of String called
> CppSource, so I could have written
> 
> source_code = CppSource.new( IO.readlines(file).join("\n") )
> source_code.strip_comments!
> 
> which might actually be a better solution than to put strip_comments!
> into String. On the other hand (again stealing from the other thread),
> writing
> 
> time_to_restart = TimeNumeric.new(5).hours + TimeNumeric.new(15).minutes
> 
> is just what we would like to avoid. So, as a guideline, should you only
> extend an existing class when you use lots of literals in your code?

imho, there are two occasions for extending a class:
1. When the added methods are useful for that class in general (ie,
String#rot13, Integer#factors)
2. When you're just hacking :)

If the data that you are working with is sort of an extended version
of the class, that's when I would subclass. I think the common text
markup modules are excellent examples of this; redcloth and bluecloth
both act just like strings, but the have a few important methods that
treat them as specific data types.

In this vein, TimeNumeric might be subclassed to use like this:
# TimeNumeric < Float
... where the float value is the number of seconds.

time_to_shutdown = TimeNumeric.new(5*60*60 + 15*60)
or better yet:
time_to_shutdown = TimeNumeric.hours(5.25)
or even:
time_to_shutdown = TimeNumeric["5:15:00"] # calls "#to_time_numeric"
on the string

cheers,
Mark