From: Joel VanderWerf <vjoel@...>
Date: 2004-06-06T11:08:45+09:00
Subject: Re: How to ducktype a Hash?

Lloyd Zusman wrote:

> Duck typing cannot work unless methods with the same names and
> signatures behave in the same way in the various classes that implement
> them.  If the ability to do duck typing is deemed to be an important and
> necessary feature of an object-oriented language, then the standard
> libraries that are written for that language _must_ provide different
> names and/or signatures for methods that perform different functions.

But "same" and "different" are on a continuum. Sometimes Array#[] and 
Hash#[] are close enough that sharing their method name makes sense 
(e.g., using hashes as sparse arrays).

> Since [] and []= have certain semantics for one set of classes
> (array-like classes) and different semantics for other classes
> (hash-like classes), then we must conclude that Ruby was not designed
> with the duck typing in mind.

[] and []= don't even have the same semantics from hash to hash, because 
of default_procs, singletons, etc. An object may respond to all the 
usual Hash methods, and the object may be a Hash in the sense of 
obj.is_a?(Hash), but it still doesn't behave as expected.

Of course, this may be an extreme example that is irrelevant to a given 
application. But it reinforces the point that it's really hard to encode 
semantics within interface, in any language. Hence unit tests.