🚀 go-pugleaf

Although I see much debate on the "purity" problem/non-problem of
logic programming language, I think one important aspect is missing in
the discussion.

In article <1990Jun18.221058.11331@chaos.cs.brandeis.edu> dsmith@chaos.cs.brandeis.edu (Donald A. Smith) writes:

> 5. Because of its single-assignment property, Prolog uses lots of memory. 
> In C we can write an in-place quicksort.   In any Prolog I know of, the
> input list would be copied.  (Has anyone figured out how to make quicksort 
> in-place in Prolog?  Is there a workable implementation of declarative 
> vectors?)

The worst thing about single-assignment property of Prolog is _not_
that it consumes up lots of memory.  Garbage collection, at most,
takes time proportional to the time required to pile the garbage.  A
more serious problem is that random access memory that computer
hardware provides (or, at least, tries to do so) is not available from
the programming language level.  This means that certain algorithms,
including dynamically updated hash tables, cannot be expressed in
standard Prolog with the same computational complexity.

This, however, can be solved by certain implementation technique
without disturbing the single-assignment semantics.  See, for example:

	author = "L. H. Eriksson and M. Rayner",
	title = "Incorporating Mutable Arrays into Logic Programming",
	booktitle = "Proceedings of the Second International
	Conference on Logic Programming",
	pages = "76--82",
	year = "1984"

Takashi Chikayama