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Subject: Re: Purity and Laziness
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Date: 30 May 90 09:22:09 GMT
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In-reply-to: brian@comp.vuw.ac.nz's message of 28 May 90 02:03:25 GMT

In article <1990May28.020325.18011@comp.vuw.ac.nz> brian@comp.vuw.ac.nz (Brian Boutel) writes:

   It would be highly desirable for Miranda and Haskell to be sufficiently
   lazy to guarantee that they would never loop/die/bottom while matching
   equations if there is just one equation which matches the actual
   arguments. Unfortunately,  examples like

   . . .

   The only ways to avoid this are
   a) to outlaw programs like the example, which seem to be otherwise
   reasonable, or
   b) to go to parallel evaluation, which is expensive to simulate on a
   sequential machine.

   So, if the goal is to define a useful language for real computation, and
   to have lazy evaluation, it seems that some compromise is necessary.

um... I may be way off base here, but I seem to recall some discussion
of this at a recent APL conference (don't remember the ACM SIGAPL
volume, but it was the one in New York).  Essentially, they were using
the parallel boolean functions of the language... there is a class of
problems that serial unification does not handle correctly.. this
parallel algorithm dependended on all information being available up
front (self referent problems, such as paradoxes could not be
represented).

A big question was whether there were useful self-referent problems.
The answer to this isn't clear.

Sorry I don't remember the specific reference.. I'll try to find it
and post it.

raulmill@usc.edu