From: Gregory Seidman <gsslist+ruby@...>
Date: 2007-01-12T01:33:30+09:00
Subject: Re: Style question: where to put additions to standard classes?

On Fri, Jan 12, 2007 at 12:55:09AM +0900, Greg Hurrell wrote:
[...]
> Specifically, just say I add some methods to an existing standard class
> like Array, what would be the best convention for choosing where to
> store the added code? Here's a concrete example: let's say I want to
> add a "eol?" method  to the StringScanner class?
> 
> class StringScanner
>   def eol?
>     return true if self.eos? or self.match?(/[\n\r]/)
>     false
>   end
> end

In many cases it is not necessary to reopen existing classes at all, and is
often preferable not to. For your example, I would guess that you are doing
some sort of parsing, maybe something like (pseudo-Ruby, because I don't
really know StringScanner):

scanner = StringScanner.new(open(filename))
scanner.each do |token|
  if scanner.eol?
    #do stuff
  else
    #do other stuff
  end
end

You need scanner to respond to #eol?, but you don't need to modify
StringScanner to do it. Consider using a module:

module StringScannerEol
  def eol?
    # cleaned up a little
    self.eos? or self.match?(/[\n\r]/)
  end
end

Now we change the first line of the code above to:

scanner = StringScanner.new(open(filename)).extend(StringScannerEol)

Now scanner will respond to #eol? and all is well.

> At the moment I have stuck it inside another file (for one of the
> classes that needs to use this method). But what if I want to keep it
> in a separate file? What would be the conventional name for such a file
> (evidently not "strscan.rb" because that would most likely cause
> confusion with the StringScanner implementation when doing a "require
> 'strscan'").

The module I gave above can be appropriately included in the file with the
code that uses it.

> Likewise I had a similar doubt when I wanted to added some assertions
> of my own in addition to those which come with Test::Unit::Assertions.
[...]

Again, a good opportunity for a module. Your test classes can include a
module with the additional assertions.

--Greg