From: dblack@...
Date: 2003-09-13T10:29:52+09:00
Subject: Re: Duck Typing

Hi --

On Sat, 13 Sep 2003, Jim Weirich wrote:

> In the Method Redefinition thread, this explanation of Duck Typing is
> offered ...
>
> > What Duck typing is based mostly on realising what sort of operations
> > you want to do with the object and testing for those operations,
> > rather than testing for the class. As Dave is fond of saying: type and
> > class aren't the same.
>
> This is slightly different than my understanding of Duck Typing.  I
> would phrase it more like this:
>
>   Duck typing is based mostly on realising what sort of operations
>   you want to do with the object and just doing them, rather than
>   worrying if the object inherits from the proper base class or
>   interface.
>
> I've heard others also explain duck typing in terms of explicitly
> testing for particular methods and I feel that leaves the wrong
> impression.  If we say Ruby supports duck typing, then newcomers are
> left with the impression that you need to do a lot of testing for
> particular methods (which you don't).

Or even, more generally, left with the impression that there's an
on/off switch: if you say "is_a?" a lot, then you're not doing duck
typing, whereas if you say "respond_to?" a lot, then you are -- when
in fact it's really Ruby that's doing the duck typing, and you're
just deciding how to react to it.

I think I remember the phrase "duck-typed language" from one of your
posts a while ago, which I thought really summed it up (though it may
be a harder phrase to break people in with :-) It captures the idea
that the inferential, dynamic concept of type resides in the language
itself.


David

-- 
David Alan Black
home: dblack@superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav