Xref: utzoo comp.object:3020 comp.lang.misc:7260
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!hsdndev!cmcl2!kramden.acf.nyu.edu!brnstnd
From: brnstnd@kramden.acf.nyu.edu (Dan Bernstein)
Newsgroups: comp.object,comp.lang.misc
Subject: Re: Type Systems and Dynamic Binding
Message-ID: <11479:Apr319:53:2191@kramden.acf.nyu.edu>
Date: 3 Apr 91 19:53:21 GMT
References: <3776@ssc-bee.ssc-vax.UUCP>
Organization: IR
Lines: 75

In article <3776@ssc-bee.ssc-vax.UUCP> dmg@ssc-vax.uucp (David M Geary) writes:
>In article <7689:Mar2623:28:5091@kramden.acf.nyu.edu>> (Dan Bernstein) writes:
>>>In article <48805@nigel.ee.udel.edu> new@ee.udel.edu (Darren New) writes:
>>>Hmm... How about a window with a hetrogeneous collection of buttons, sliders,
>>>text displays, etc, all of which respond to "redraw" and "is the mouse
>>>over you"?
>>struct {any object; void (*redraw)(); int (*ismouseover)();} *whatsinwindow();
 [ ... ]
>NO.  This is *not* an example of dynamic typing implemented on top of C.

What a load of... uh, what I mean is that this is a perfect example of
the fact that dynamic typing is just a programming technique. (``A state
of mind'' is perhaps too fluffy for such a serious discussion as this.)

With appropriate macros, I can use the above structure in the same way
as someone using a dynamically typed language. In fact, with a
reasonable preprocessor like AT&T's C++, I can declare the structure in
the same way. I can get all the same type-checking (at run-time, too!).

You can shout all you want about it not *really* being dynamic typing.
But it obviously is. Why does it so offend you that dynamic typing can
be implemented so effectively outside the compiler?

> A void* is not a "polymorphic type", and C++ provides much more than
> "merely syntactic sugar".

Uh-huh, whatever you say, boss.

> > On the occasions when I want dynamically typed variables, I
> >can use them without trouble in C.
> No you cannot;  C has no support whatsover for "dynamically typed variables."

It has a macro facility that is adequate to let me use a variable, x,
that can take on any value I can express in the language, and that
forces me to use x as the type it is currently holding. What else do you
want out of types, other than the syntax and the checking?

> One cannot "implement
> dynamic typing on top of C", without writing some kind of preprocessor,

In case you haven't noticed, C has a preprocessor. It's not great, but
it works. But even if it didn't, I would happily use m4. As I've said
before, you cannot change the fundamental characteristics of a language
by adding a library---or a preprocessor.

>   Here, one is adding arbitrary elements to a collection.  There is 
> *no restriction whatsoever* as to the type of things that can be
> added to the collection.  Note that this differs from the C++ example,
> where all elements added to the list *must* be linkables (or
> descendents of class linkable).  

Nobody has come up with any examples of why anyone would want to use
such a list---i.e., a list which does not have a definite type, and to
which you cannot meaningfully apply any particular operations. Do you
have a brilliant example that everyone else has missed?

>   Lastly, note that neither of the two examples given above may be
> implemented in C, without the programmer *at one time or another*
> explicitly identifying the types of all objects being operated upon.

C++ is a counterexample.

> Here, BTW, are some good references.  I would suggest that Dan read
> some of these before providing any more erroneous information in the
> tradition of his previous postings:

Why, thank you, ma'am.

Here, BTW, is an excellent reference. I strongly suggest that you read
it---and work at least 10% of the exercises---before you post anything,
again, anywhere, ever.

Art of Computer Programming, Donald E. Knuth. 3 volumes.

---Dan