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[LOGIC-91] nom/tie and spurious reification of predc constraint Created: 02/Jan/13  Updated: 28/Jul/13  Resolved: 02/Jan/13

Status: Closed
Project: core.logic
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Defect Priority: Blocker
Reporter: Nada Amin Assignee: David Nolen
Resolution: Completed Votes: 0
Labels: None

(is (= (run* [q]
           (nom/fresh [a]
             (fresh [x]
               (predc x number? `number?)
               (== x 1)
               (== (nom/tie a [a x]) q))))
        [(nom/tie 'a_0 '(a_0 1))]))
  (is (= (run* [q]
           (nom/fresh [a]
             (fresh [x]
               (== x 1)
               (predc x number? `number?)
               (== (nom/tie a [a x]) q))))
        [(nom/tie 'a_0 '(a_0 1))])) ;; false b/c of extra :- number? constraint

Comment by Nada Amin [ 02/Jan/13 5:07 AM ]

Implementing -force-ans in Tie

  (-force-ans [v x]
    (force-ans (:body v)))

resolves the test above, but not this one:

(is (= (run* [q]
           (nom/fresh [a b c]
             (fresh [x y]
               (== x 1)
               (predc x number? `number?)
               (== (nom/tie b (nom/tie a [a x])) (nom/tie c q)))))
        [(nom/tie 'a_0 '(a_0 1))])) ;; still false b/c of extra :- number? constraint
Comment by David Nolen [ 02/Jan/13 8:15 AM ]

This issue actually impacts any "single shot" constraint that implements `-relevant?` as simply returning `true`. The problem is that the `cgoal` constraint wrapper checks `runnable?`, runs the constraint, and then checks `relevant?` and if that's true adds the constraint even though it may very well may be entailed!

There are 6 constraints (1 of them the `defc` macro) which implement `-relevant?` as returning `true`. I think we should have more support (a protocol) for constraints which are essentially "single shot" and don't need to bother with implementing the `IRelevant` protocol.

For some background - the CLP(FD) constraints benefit most from the `IRelevant` protocol, where the constraints involve up to 3 terms and a considerable amount of propagation work may be avoided by doing some checking up front.

Comment by Nada Amin [ 02/Jan/13 8:19 AM ]

Hi David,

I have a fix proposed in pull request https://github.com/clojure/core.logic/pull/15

The idea there is that to honor single-shot constraints, we only need to honor the (remcg this) that they contain. This is only possible if they have proper ids. That's what I fix.

Let me know what you think.

Comment by David Nolen [ 02/Jan/13 8:46 AM ]

fixed, http://github.com/clojure/core.logic/commit/020f730429d71315f752ea51abad20dca896c8b0

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