Path: utzoo!attcan!uunet!husc6!bloom-beacon!corwin.ccs.northeastern.EDU!wand From: wand@corwin.ccs.northeastern.EDU (Mitchell Wand) Newsgroups: comp.lang.scheme Subject: Floyd-Hoare Verification Harmful?? Message-ID: <8805211818.AA02999@corwin.CCS.Northeastern.EDU> Date: 21 May 88 18:18:15 GMT References: <19880520165408.4.HES@MERLIN.SCRC.Symbolics.COM> Sender: daemon@bloom-beacon.MIT.EDU Organization: The Internet Lines: 37 Date: Thu, 19 May 88 10:35:23 EDT From: Mitchell Wand One ought not to say things like: "F(G(C)) := D ought to ensure that F(G(C)) = D afterwards." too blithely. Consider the array assignment: A[A[1]] := 2 in a two element array A, where initially A[1]=A[2]=1 . This sort of thing had program verifiers confused for a good while in the early 70's. Date: Fri, 20 May 88 12:54 EDT From: Howard Shrobe I wrote a paper that was distributed by hand to friends in the late '70s called "Floyd-Hoare Verifiers Considered Harmful" that pointed this ought. It was somewhat tongue in cheek but was based on catching Vaughn Pratt making exactly this kind of mistake. I just moved my office and found copies of the paper. Sussman would remember it well. That seems like an awfully rash conclusion to draw from this kind of bug. It would be more reasonable to say something like "Unrestricted Pointer Manipulation Considered Harmful" or "Hiding Nasty Pointer Manipulation in Innocuous-Looking Array Manipulation Considered Harmful" or... :-) Mitchell Wand College of Computer Science Northeastern University 360 Huntington Avenue #161CN Boston, MA 02115 CSNet: wand@corwin.ccs.northeastern.edu