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From: ech@spuxll.UUCP (Ned Horvath)
Newsgroups: net.lang
Subject: Re: levelheight: automatic "nil pointer" handling
Message-ID: <618@spuxll.UUCP>
Date: Mon, 4-Feb-85 10:52:48 EST
Article-I.D.: spuxll.618
Posted: Mon Feb  4 10:52:48 1985
Date-Received: Wed, 6-Feb-85 01:33:06 EST
References: <6304@boring.UUCP>
Organization: AT&T Information Systems, South Plainfield NJ
Lines: 27

Your example of "static checks" of nil dereferencing make, rather than
refute, the argument that nil-dereferencing cannot be checked for statically.
What you have accomplished is to enforce the convention that any potentially
nil-valued pointer must be explicitly AND DYNAMICALLY checked for a nil-value
prior to dereferencing it.  This reduces to the same thing; what difference
does it make if the compiler forces the programmer to insert nil-checking
code, or does it automatically?  Either way, the check MUST be done
dynamically, and that is all the reductio ad absurdum (via TM halting) was
meant to show.

Strong typing is not at all the same thing; it is possible to rewrite your
example so that 'insert' and 'print' take only non-empty trees as arguments,
for example.  If we did that without modifying the main program, that would
correctly generate a compiler (or linker) exception due to type mis-match.
Static checking for a type mismatch is eminently feasible.

To summarize: static analysis suffices to determine WHERE TO PUT code to
check for things like nil values and array-index-out-of-bounds, but the
checks themselves MUST, in general, be made at run time.

There really isn't anything to argue about here: can a compiler recognize
(and enforce or generate protection against) potentially inconvenient run-time
conditions?  Sure, no argument.  Is it in the nature of some of those
conditions that they MUST be checked for at run time?  I think we have
established that as well.

=Ned=