From: gabriele renzi <rff_rff@...>
Date: 2004-11-12T01:08:29+09:00
Subject: Re: Thinking About Java Interfaces In Ruby

James Edward Gray II ha scritto:
> I'm currently reading "Holub on Patterns", an excellent volume on Object 
> Oriented Programming Voodoo.  The language of the book is Java and it 
> makes extensive use of Java's Interfaces to avoid "The Fragile Base 
> Class Problem".
> 
> This has me thinking a lot about proper Ruby coding techniques.  What is 
> Ruby's equivalent to a Java Interface?
> 
> module Interface
>     def some_method; end
>     def another_method; end
> end
> 
> That doesn't really require you to provide those methods in a class that 
> mixes them in.  Perhaps:
> 
> module Interface
>     def some_method
>         raise NoMethodError, "Must be overriden!"
>     end
>     def another_method
>         raise NoMethodError, "Must be overriden!"
>     end
> end
> 
> Of course, I guess the only point of something like this is type 
> checking, which Java needs when it declares a variable, but Ruby does 
> not. 

notice that there is a Interface  module that allows you to have 
interfaces a-la java. See here:
http://ruby-miscutils.sourceforge.net/interface.txt
(yeah ruby can become most anything :)

> Given that, client code can easily check interface:
> 
> def my_method( interface )
>     interface.respond_to?(:some_method) or raise ArgumentError, "Invalid 
> interface."
>     interface.respond_to?(:another_method) or raise ArgumentError, 
> "Invalid interface."
> 
>     # ...
> end
> 
> But that probably doesn't gain you much over just trying to call the 
> methods and throwing exceptions that way, I guess.  Which brings us full 
> circle to doing nothing at all.
>

Imo it gives you both a crash-early and a documentation hint, wich are 
valuable things on their own.  I'd like to see optional type hints in 
ruby, even because I'd love to have generic methods or predicate dispatch.
The problem is that either they're checked at runtime, thus slowing down 
the code or they're handled with some complex partial evaluation at 
compile time (I think some scheme/lisp do this)



> Does Ruby combat The Fragile Base Class Problem with philosophy alone?  
> I'm referring to Duck Typing here, of course.  Let them pass in what 
> they want and if it responds to all the right messages, we don't care 
> what it is.  In this scenario, we're not supposed to check "kind_of?" at 
> all, right?  In that case, how do you document a method like this?
> 
> class DatabaseTable
>     # ...
> 
>     def export( exporter )
>         exporter.start_table
>         exporter.store_metadata(@name, @width, @column_names)
>         @rows.each { |row| exporter.store_row(row) }
>         exporter.end_table
>     end
> end
> 
> You don't really just say, "Pass in an object that responds to X, Y, and 
> Z." in the RDoc, do you?
> The "exporter" argument should probably be a type, it seems to me.  I 
> guess in this case a module with empty methods is a fine choice.  Then 
> you can just override what you need.  But wait, isn't that just a "Base 
> Class"???  Might as well use a class then...

maybe you can define a module Exporter or an AbstractExporter class and 
document that, but it's not a real solution, I think.
But it does document things, and whenever a user of DatabaseTable look 
at it he can just figure out 'oh, well, I have to define this, no need 
to import.. '


> In case it isn't obvious, this message is mostly just me thinking out 
> loud.  However, I'm very interested in the opinions of others, which is 
> why I'm thinking out loud on Ruby Talk.


Everubody come to think of this sooner or later :)
I even wrote an rcr wich is related : http://www.rcrchive.net/rcr/show/280



> I figure I'm probably just too much in Java mode right now (sorry, it's 
> the day job) so that I can't see the Ruby answers.  If you're reading my 
> babble and thinking "You wouldn't do it like that in Ruby!", criticize 
> away.  You won't hurt my feelings.

I hope not :)
anyway, I think problems such "I will break stuff whenever I modify 
something and don't have a clear documented interface!" is mostly solved 
from Test::Unit. You're writing tests anyway, don't you? :)


> James Edward Gray II
> 
> P.S.  Book writers, are you listening?!  I'm crying out for a Ruby OO 
> Voodoo book!!!

+1