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From: mario@cs.man.ac.uk (Mario Wolczko)
Newsgroups: comp.object
Subject: Re: A Hard Problem for Static Type Systems
Message-ID: <2500@m1.cs.man.ac.uk>
Date: 7 May 91 17:58:44 GMT
References: <1991Apr20.010347.28984@leland.Stanford.EDU> <554@eiffel.UUCP> <1991Apr26.203642.17387@leland.Stanford.EDU> <556@eiffel.UUCP> <1146@tetrauk.UUCP> <2479@m1.cs.man.ac.uk> <1158@tetrauk.UUCP>
Sender: news@cs.man.ac.uk
Reply-To: mario@cs.man.ac.uk (Mario Wolczko)
Organization: Department of Computer Science, University of Manchester
Lines: 37

In article <1158@tetrauk.UUCP>, rick@tetrauk.UUCP (Rick Jones) writes:
[long defence of global type analysis, followed by critique of my 6
line program fragment]

But now you're moving the goalposts.  Your original posting stated
quite unequivocally that global type analysis only rejected programs
that were not type-safe, and my example showed that this was false.
(The example was not meant to be particularly meaningful, only to make
the point.)

Describing such a program as "inherently unreliable" is similarly
bogus --- by appropriate substitution I could make it into a program
that was provably correct.

Having retreated from the earlier claim, you now claim that only
well-designed programs will be amenable to global type analysis, a
much more subjective claim.  

I feel sure that we could argue until blue in the face as to what
constitutes a well-designed program.  My guess is that your bottom
line will be that any program which is not amenable to global type
analysis is not well-designed!

I will make only two further observations:
1. Eiffel's "reverse assignment attempt" is a run-time type test
   anyway, and
2. The most common "type error" in Smalltalk is sending a message to
   nil, ie forgetting to initialize a variable.  Static typing doesn't
   help here at all.

Mario Wolczko

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