From: Austin Ziegler <austin@...>
Date: 2003-11-20T06:03:54+09:00
Subject: Re: "stereotyping" (was: Re: Strong Typing (Re: Managing metadata about attribute types)

On Thu, 20 Nov 2003 03:52:30 +0900, Sean O'Dell wrote:
> On Wednesday 19 November 2003 10:27 am, David A. Black wrote:
>> A Ruby module or class is neither a contract, nor a fixed
>> collection. I really don't think, for example, that what Betrand
>> Meyer meant by "design by contract" was Ruby classes :-)
> A contract is any interface, and a Ruby class is an interface.
> True, a Ruby class isn't the definition of a contract, but any OO
> class serves the function well enough. Despite Ruby's flexibility,
> at any given point, a class implements an interface. It can be
> extended or changed, but it definitely describes an interface.

A class defines a collection of methods that could be called an
interface. However, an instance of a Ruby class need not implement
that interface. Again, a pathological example:

  f = File.open("foo.txt")
  class << f
    undef write
  end

The object referenced by f *is* a File, but it doesn't support
#write.

>> What you're doing is pressing the closest thing Ruby has to
>> something that *looks* like the kind of type-checking you want
>> (i.e., kind_of?) into service as a type check, which it isn't.
> Yes, but no one cares if an object being passed in is really of a
> certain type, all they care about is if it has the interface their
> method(s) need. I don't want a type check, I just want some
> enforcement that an interface exists where I need it.

Which #kind_of? does *not* do. *Why* do you want or need that
enforcement? When I first used dynamic languages, I wanted them to
act like the static languages that I worked with, but I quickly grew
out of it. I briefly experimented with #kind_of? checking in Ruby,
but found few cases where I really needed it.

-austin
--
austin ziegler    * austin@halostatue.ca * Toronto, ON, Canada
software designer * pragmatic programmer * 2003.11.19
                                         * 15.53.42