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From: dmg@ssc-vax.uucp (David M Geary)
Newsgroups: comp.object,comp.lang.misc
Subject: Re:  Type Systems and Dynamic Binding
Message-ID: <3864@ssc-bee.ssc-vax.UUCP>
Date: 17 Apr 91 19:52:42 GMT
Sender: news@ssc-vax.UUCP
Organization: Boeing Aerospace & Electronics
Lines: 73
Originator: dmg@ssc-vax

]   Dan Bernstein
]]  David Geary
]]] Darren New

]] Dan Bernstein claims to be able to implement dynamic typing on top of C:
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

] David, there is a big difference between claiming that a *particular*
] example of dynamic typing can be implemented (directly) in C, and
] claiming that *all* examples of dynamic typing can be implemented
] (directly) in C. As you will realize if you reread your article, I have
] claimed the former, for several particular examples. I have not claimed
] the latter ...

] For each EXAMPLE of dynamic typing that people put up, I showed how it
] would typically be implemented (directly) in C. I hope you can see the
] difference between this and what you're talking about.

  If C had dynamic typing, we would be able to do this:

  void  doFooToThingsInList(list)
    list_t*  list;
  {
    void*  nextThingInList;

    while(nextThingInList = list->getNextThing())
      nextThingInList->foo(&nextThingInList);
  }

  However, this is not possible, due to the fact that a void* in C must
have a cast applied to it before it can be dereferenced.  Therefore, we 
must do something like the following:

((someType*)nextThingInList)->foo(&nextThingInList);

In other words, the *programmer* is responsible for ensuring that the statement
((someType*)nextThingInList)->foo() is valid for the type someType.

  With dynamic typing, however, this requirement (that the programmer ensures
that all statements are type-safe) is no longer a requirement at all.  The
responsiblity for type-safety is no longer left to the programmer, as there
is no requirement that all statements be type-safe.


]]] Am I crazy, or isn't (void *) impossible to indirect?  Don't you have
]]] to type-cast it first?  Isn't this dynamic typing implemented on top of
                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
]]] C?
   ^^^

]YES. People are tossing around the term ``dynamic typing'' as if it were
 ^^^
]some truly important feature. ``Yeah, Bob, not only does the language
]have *loops*, but it has *dynamic typing*!'' ``Wow! *Dynamic typing*?
]Really?''

  NO.  It was *not* dynamic typing implemented on top of C.  It was 
an example of what a programmer in a statically typed language has to do
in order to accomplish what is (more simply) done with a dynamically typed
language.  

  Dynamic typing *is* a truly important feature.  It allows a greater
degree of code reuse.  Dan, if you would take the time to understand the
scenario I proposed where one developer provides another developer with
classes of objects in a window, the benefits of dynamic typing would be
apparent.

]You can implement all real-world EXAMPLES---or, at least, every example
]that's come up in this group---of dynamic typing directly in C. That's

  You cannot implement the real-world example that I posted earlier
directly in C.  If you wish, I will post a different real-world example
of dynamic typing that cannot be implemented directly in C.