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From: bimbart@kulcs.cs.kuleuven.ac.be (Bart Demoen)
Newsgroups: comp.lang.prolog
Subject: Re: '->' operator
Message-ID: <1471@n_kulcs.cs.kuleuven.ac.be>
Date: 14 Jan 91 08:13:23 GMT
Sender: news@cs.kuleuven.ac.be
Organization: Dept. of Computer Science (K.U.Leuven)
Lines: 42
Originator: bimbart@kulcs.cs.kuleuven.ac.be

Thomas Sjoeland <1991Jan11.170214.3756@sics.se> writes:

> Should we take it (->;) out ?

In the course of the standardisation process, it has been in and out and very
recently in again.

It was out before, because of its syntax, because of different behaviour in
different systems and because taking it out at some moment, was the right
compromise to keep the standardisation process going.

It was put back in, because most Prolog systems have it, because it would break
too many programs by taking it out, because the differences between actual
implementations were less important than previously thought and because the most
recent meeting was the right moment to put it back in. (The last meeting was
memorable for other decisions as well, e.g. about modules.)

So, sorry for those disappointed: if-then-else is IN right now and in the
DEC-10 form and with DEC-10 meaning - mostly.

> The standard committee should think hard about these things.

It has. And not all problems are solved with if-then-else. Perhaps people want to
give their opinion about the following:

	should ! in the condition be allowed ? (QP doesn't - since when ?)
	if yes, what should be its scope ?
			(local to condition or up to head included ?)

	should ?- X = (true -> write(1)) , call((X ; write(2))) . have the
	same answer as ?- X = (true -> write(1)) , (X ; write(2)) .
	and as ?- true -> write(1) ; write(2) .


> Why not sort out the overloading of commas and the list syntax while you're at it ?

The standard committee has been critisized a lot for designing a new language
instead of standardising an old one, and that's why certain things can't be
done, however much we can regret this.


Bart Demoen