From: "David A. Black" <dblack@...>
Date: 2005-04-27T07:33:16+09:00
Subject: Re: Announcing Reg 0.4.0

Hi --

On Wed, 27 Apr 2005, vikkous wrote:

> I wrote:
>
>> I'm not sure about Christian, but I think itsme213 meant
>> sort of the opposite: how does your system relate to the
>> duck-typing environment of Ruby, where type != class
>> (i.e., an object's capabilities and its class's instance
>> methods are not necessarily the same thing)?  Are you
>> thinking of extending the system so that it could match,
>> for example,  "an array of objects that respond to '[]'"
>
> Oh! Ok, well that's easy, it would be something like:
>
> +[ proceq{|x| x.respond_to? :[] }+1 ]
>
> Being forced to use a proceq (gawd that's a terrible name) here is a
> little ugly. Eventually, I want to be able to pass arguments to
> property matchers, so the above could be:
>
> +[ -{ [:respond_to?, :[]]=>true }+1 ]

I wonder whether one could combine this with tests for class ancestry,
by matching a class/module name (#is_a?) and, at the same time,
verifying that the object's capabilities are a match for its ancestry.
Or something.  I haven't puzzled it through very deeply.

>> duck typing per se, as I understand it, really means just
>> requesting action from objects without a lot of preliminary
>> querying and measuring (whether it be is_a?, respond_to?,
>>  or whatever).
>
> If you're doing something complicated, you occasionally have to
> explicity request the type (whether duck- or class-) of objects you're
> working with, in order to do the right thing with it. I know this isn't
> polymorphic, but sometimes it is the right way...

I'd tend to say: if it's the type, it's not the class, and if you have
to explicitly request it, it's not duck-typing :-)  But yes, I think
there are occasions for all these things, in various combinations.
As others have often pointed out, respond_to? != duck typing, but I do
tend to think of respond_to?-based logic as a kind of "weak duck
typing", if that isn't too arcane.  It falls short of the true "Here,
do this!" spirit of true duck typing, but undeniably (in my view) has
a foot in the same camp, in that it factors in the dynamism of type.


David

-- 
David A. Black
dblack@wobblini.net