From: Austin Ziegler <austin@...>
Date: 2003-11-20T06:25:49+09:00
Subject: Re: "stereotyping" (was: Re: Strong Typing (Re: Managing metadata  about attribute types)

On Thu, 20 Nov 2003 05:17:10 +0900, Sean O'Dell wrote:
> On Wednesday 19 November 2003 11:47 am, Weirich, James wrote:
>> Can you give an example of where someone would NEED to check.
> Okay, you got it. Here's an example of how this sort of type checking 
would
> be useful:
[...]

> He decides to build the full HTTP request as a text file, and pipe the
> file to the CGI method through my library by opening it with the File
> object and passing a File object where I was expecting a Socket object.

  irb(main):002:0> require 'socket'
  => true
  irb(main):003:0> Socket.ancestors
  => [Socket, BasicSocket, IO, File::Constants, Enumerable, Object, Kernel]
  irb(main):004:0> File.ancestors
  => [File, IO, File::Constants, Enumerable, Object, Kernel]
  irb(main):005:0> Socket.methods - File.methods
  => ["pack_sockaddr_in", "do_not_reverse_lookup", "getservbyname",
  "do_not_reverse_lookup=", "unpack_sockaddr_in", "socketpair",
  "gethostbyname", "getaddrinfo", "pair", "getnameinfo", "gethostname",
  "sockaddr_in", "gethostbyaddr"]
  irb(main):006:0> Socket.instance_methods - File.instance_methods
  => ["bind", "sysaccept", "recvfrom", "accept", "listen", "connect"]

Aside from the above methods, as long as your HTTP server doesn't depend on
those items in the place where Bob would send the request (you *have* 
designed
your code with appropriate separation of concerns, haven't you?), then I see
no problem.

> The code goes BOOM. The library is trying to make calls to methods that
> don't exist. Bob sees what methods are being requested, but he has no
> reference to them. Nowhere does he see "I expected a Socket."

Since Bob is a smart programmer, he'll see in his error log:

  NoMethodError: undefined method `accept' for #<File:3>
        from ...

Well, this means that his File object doesn't know the method #accept.
Therefore, he can open the source to your HTTP library and see "oh! this 
guy's
expecting a Socket and has put accept in the wrong place!" If Bob's a really
smart programmer, he'd already know that #accept is a Socket method.

I've found bugs in libraries and bugs in my own code -- with the software
working *just as it does already*. Your scenario doesn't stand.

-austin
--
austin ziegler    * austin@halostatue.ca * Toronto, ON, Canada
software designer * pragmatic programmer * 2003.11.19
                                         * 16.13.28