Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!decwrl!sun!pitstop!sundc!seismo!uunet!mcvax!ukc!icdoc!cdsm
From: cdsm@doc.ic.ac.uk (Chris Moss)
Newsgroups: comp.lang.prolog
Subject: Re: Committed Choice
Message-ID: <733@gould.doc.ic.ac.uk>
Date: 14 Mar 89 14:26:04 GMT
References: <11500012@hpldola.HP.COM>
Reply-To: cdsm@doc.ic.ac.uk (Chris Moss)
Organization: Dept. of Computing, Imperial College, London, UK.
Lines: 34

In article <11500012@hpldola.HP.COM> patch@hpldola.HP.COM (Pat Chkoreff) writes:
>It seems to me that the if-then-else construct in Prolog completely
>eliminates the need for the cut, and is a far superior mechanism for
>enforcing committed choice.  Am I right?

There are 2 reasons for choice of control structures in Prolog:
efficiency and clarity. Certainly from a theoretical point of view
(the "power" of the system) if then-else is adequate, but there are
lots of reasons why it's given problems. See for instance the appendix
in Ehud Shapiro's book "Algorithmic Program debugging" which uses 
conditionals far more than his later book "Art of Prolog". (He had an 
argument with David Warren and lost!). (Art of Prolog has very few cuts
too). 

Richard O'Keefe put a powerful counterargument in his "Treatment of Cuts
in Prolog Source-Level tools" paper in IEE Symposium on LP, 1985.
But even he advocates cuts in some places on stylistic and efficiency
grounds.

>Books on Prolog tend to emphasize the nuances of using the cut, but
>rarely discuss if-then-else.  Is this due to historical reasons?

Well for one thing there are different variations even of if-then-else!
For example, does an if-then without an else fail or succeed. It appears
that the Europeans on the standardization committee think it should
succeed: in "Edinburgh" systems it fails.

If we were to start Prolog again, I'd back Gert Smolka's proposals in
"Making Control and Data Flow in Logic Programs Explicit" in the 1984
ACM Functional Programming Conference. 

But of course we aren't going to!

Chris Moss.