Re: [eclipse-clp-users] testing NOT existence

From: Kish Shen <kisshen_at_...5...>
Date: Wed, 14 Apr 2010 18:50:43 +0100
Christian Wirth wrote:
> i have a conditon NOT(pred(A)) and a fact pred(1). The existenc check 
> \+(\+(not(pred(A))) should succeed, because it's possible that A is not 
> bound to 1, but it fails because A gets bound to 1 by the test.
> How can i test if a NOT(condition) is solvable ?

Hi Christian,

not/1 and \+/1 behave exactly the same -- I believe \+/1 was added 
because not/1 does not do negation as some people understand it, so \+/2 
is considered by some to be a better name for the way Prolog (and thus 
ECLiPSe) implements negation. Specifically, negation as failure (with 
closed world assumption).

\+(A) and not(A) succeeds if A fails, and fails if A succeeds.
Thus \+( \+( not(A))) (which is what you have) has exactly the same 
semantics as \+(A) (and not(A)), except it does more work.

What exact semantics do you want? Note that if you want pred(A) to 
behave like a constraint (i.e. it can delay if it is not immediately 
entailed), then you cannot use the \+(\+(A)) trick, because if A delays, 
A will appear to succeed, and \+(A) will fail.



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Received on Wed Apr 14 2010 - 17:50:50 CEST

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