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From: micha@ecrcvax.UUCP (Micha Meier)
Newsgroups: comp.lang.prolog
Subject: Re: Committed Choice
Keywords: cut if-then-else garbage collection
Message-ID: <711@ecrcvax.UUCP>
Date: 6 Apr 89 08:53:08 GMT
References: <11500012@hpldola.HP.COM> <733@gould.doc.ic.ac.uk> <18798@adm.BRL.MIL> <192@aucsv.UUCP> <751@gould.doc.ic.ac.uk>
Reply-To: micha@ecrcvax.UUCP (Micha Meier)
Organization: ECRC, Munich 81, West Germany
Lines: 28

In article <751@gould.doc.ic.ac.uk> cdsm@doc.ic.ac.uk (Chris Moss) writes:
>What I'd like to know is: is there EVER any good reason for using a
>repeat loop, with current Prolog technology?
>
>Repeat loops are basically of value for side effect programs such as
>file readers, compilers etc. They don't do any global binding of variables
>so they can be represented by parameterless procedures (or procedures that
>take only input bindings). Thus they do not produce garbage on the stack
>if the top level is a tail recursive procedure.

	Chris, why do you think repeat loops are of value only for
	file readers, compilers etc.? When such programs
	do not produce garbage on the global stack then of course
	the repeat loops are *not* of value for them, you can use
	tail recursion instead, which is faster. The repeat loops
	are useful for programs which do create garbage on the global stack
	because the garbage can be disposed of without the garbage collector.

	It is certainly not necessary to use repeat loops if your
	Prolog system has a garbage collector, but failure is a very
	simple and efficient way of a partial garbage collection.
	It is also possible to write a preprocessor that adds
	to your tail-recursive program control primitives that
	make the garbage collection faster (garbage cut of Jonas
	Barklund), but the failure loops will still remain
	as a valid alternative, I think.

--Micha