From: Mathieu Bouchard <matju@...>
Date: 2005-01-27T01:06:42+09:00
Subject: Re: Self and Ruby Comparisons


On Tue, 25 Jan 2005, Robert Klemme wrote:

> > Actually, Perl has it better, because you can change the class of an
> > object, and you can change the list of superclasses of any class.
> And if a copy is sufficient you can do this even in pure Ruby:

No, a copy is _not_ sufficient.

> Do you have a link or explanation that demonstrates the merits of the
> capability to change an instance's class at runtime?  Currently I don't
> have a clue in which situations I would want this.  Thx!

Just like almost any other programming concept invented to improve
structured programming, it's about replacing if/case expressions by tables
functionpointers such that it creates a "separation of concerns". (i mean
all the various forms of polymorphisms)

Now, for that feature in particular, think of a File which may be open or
closed, and you have to write at the start of each damn method:

  if not @file then raise "file not open" end

well, this could be solved by changing the set of all methods of that
class.

In general an object that has a few major cases of behaviour like that,
can benefit from switching methodtables.

In Ruby, my way to solve this problem is instead to use one object
representing a File that may be open or not, and one object that
represents a File guaranteed to be open, and then use message-forwarding
using Object#method_missing.

_____________________________________________________________________
Mathieu Bouchard -=- Montr�al QC Canada -=- http://artengine.ca/matju