From: dblack@...
Date: 2006-11-13T03:01:21+09:00
Subject: Re: McGovern Likes JRuby...

Hi --

On Sun, 12 Nov 2006, David Vallner wrote:

> Running on Java would of course do bugger all for that. However, I am
> one of the proponents of -some- (optional) ahead-of-time contract
> checking for Ruby. Static typing is one method of contract checking, and
> an easy one to implement, and in optional form wouldn't be rubbing me
> the wrong way. YMMV, and it's a vapourware proposition anyway - IIRC, it
> was pondered for Ruby 2.0, but there's no sign of that in any material
> describing Ruby 1.9 changes, so I'm not holding my breath.
>
> Duck typing fans will probably howl over the above comment, but if I've
> seen code behaviour do -any- type checking (mostly in the standard lib),
> it's based on #is_a?, and not on #respond_to?, so apparently most duck
> typing preachers really mean "I don't want to bother with any checks".

I can't speak to the preaching and howling, neither being much in my
line; but just in terms of the technical stuff: "duck typing" is a
name for a style of programming that does not branch on an object's
class/module ancestry and which may, but does not always, branch on an
object's type, generally using #respond_to? when it does so.
#respond_to? is not the heart of duck typing, though, nor
consubstantial with it.  It's a useful duck-typing tool, sometimes,
but that's about all.

In the past I've described the difference between using and not using
#respond_to? as the difference between "soft" and "hard" duck typing.
By that, I mean that the real business of duck typing is to live as
close to the object, and to the present-time of the sending of the
message, as possible; and respond_to? is a step away from that moment,
therefore arguably a "softening" of the practice.  But it's still duck
typing, in the sense that the step away is still directly related to
the goal of sending the object a message.

The flip side of this is that the absence of #respond_to? does not
mean the presence of duck typing.


David

-- 
                   David A. Black | dblack@rubypal.com
Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org