🚀 go-pugleaf

In article <5500005@uiucdcsm>, mccaugh@uiucdcsm.cs.uiuc.edu writes:

> I submit that "prolog" - the so-called "logic-programming" language - is
> (among other disclaimers) NOT altogether the "declarative" non-procedural
>  language it is advertised to be. Case in point:
>	sum(0,0).
>	sum(N,M) :- N1 = N-1, sum(N1,M1), M = M1+N.
>  In a truly non-procedural (i.e. (I anticipate controversy here) non-
>  deterministic) language, the test: N>0 should not be necessary, and yet
>  (as we know), it must appear to avoid stack-overflow (in CPROLOG, UNSW-
>  PROLOG, anyway).

The conclusion is valid, but the argument is bogus.  {What's with this
N1 = N-1 business, anyway?  In C Prolog the code as written will  never
succeed for any first argument other than 0.  E.g.
sum(1, X) -> sum(1-1, M1) -> sum(1-1-1, M1') -> sum(1-1-1-1, M1") -> ...}

Claim:	there is no "declarative" language permitting recursive declarations
	currently existing in which control issues can be ignored.
	(Lazy ML, Miranda, even the relational algebra, support this claim.)

While in this case it is straightforward to show that for sum/2 to succeed
its first argument must be a non-negative integer, in general it is at least
as hard as solving the halting problem (insert standard trivial proof here).

"Non-procedural" and "non-deterministic" are orthogonal concepts.
E.g. ICON is non-deterministic but non-procedural, ML is non-procedural
but not non-deterministic, Pascal is neither.