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From: broome@brl-smoke.ARPA (Paul Broome )
Newsgroups: comp.lang.prolog
Subject: Re: Badness
Message-ID: <6669@brl-smoke.ARPA>
Date: Thu, 12-Nov-87 16:16:30 EST
Article-I.D.: brl-smok.6669
Posted: Thu Nov 12 16:16:30 1987
Date-Received: Sat, 14-Nov-87 19:35:13 EST
References: <8711090842.AA20806@ucbvax.Berkeley.EDU> <2385@mulga.oz> <4815@burdvax.PRC.Unisys.COM>
Reply-To: broome@brl.arpa (Paul Broome (CTAB) <broome>)
Organization: Ballistic Research Lab (BRL), APG, MD.
Lines: 27
Keywords: declarative programming
Summary: IF-THEN-ELSE is not required

It's not clear that IF-THEN-ELSE is essential for programming.  In fact it's 
often hazardous!  The ELSE part of an IF-THEN-ELSE is too all-enclosing.  

A small example that immediately comes to mind is factorial, often written
something like

	fac(N) = if N=0 then  1
		 	else  N*fac(N-1).

The danger comes from permitting an unbounded recursion for negative N.
It's much better, in this case at least, to say explicitly what cases
can be handled and fail for those outside.  E.g. something like

	fac(N) = if N=0 then 1
		 if N>0 then N*fac(N-1).

It would be interesting to see a small example that *requires* IF-THEN-ELSE?

			     Paul  <Broome@brl.mil>, <Broome@brl-smoke.uucp>

p.s.  This was prompted by the following:

In article <4815@burdvax.PRC.Unisys.COM> blenko@burdvax.PRC.Unisys.COM (Tom Blenko) writes:

>Programming declaratively, in the sense of pure PROLOG (without call(),
>not(), and cut()) is not even powerful enough to express IF-THEN-ELSE,
>which I am willing to assume is a _sine_qua_non_ for programming.