Unexpected results with Clojure core.logic (using CLP/FD)

Question

I am trying to understand why the following product-pluso function is returning unexpected results. Did I find a bug or am I misunderstanding how this works? I am using core.logic with CLP/FD. I am still learning, so I could just be doing it wrong. The function should take two factors, a number, and a sum. The sum should be the product of the factors plus the number. It works great, unless both of the factors are fresh. Then I get strange results. This is happening with core.logic v0.8.2.

(ns strang-result
  (:refer-clojure :exclude [==])
  (:use
    clojure.test
    [clojure.core.logic :exclude [is]])
  (:require
    [clojure.core.logic.fd :as fd]))

(defn product-pluso [factor1 factor2 number sum]
  (fd/eq (= sum (+ number (* factor1 factor2)))))

(run* [x y]
  (fd/in x y (fd/interval 1 38))
  (product-pluso x y 2 40))

;=> ([1 38] [2 19] [3 13] [4 10] [5 8] [6 7] [7 6] [8 5] [9 5] [10 4] [11 4] 
;    [12 4] [13 3] [14 3] [15 3] [16 3] [17 3] [18 3] [19 2] [38 1])

Answer 1

This just appears to be a bug. Something strange is happening with fd/+ constraint that is not being checked. I'm one of the lead developers, I've filed a ticket for this:

http://dev.clojure.org/jira/browse/LOGIC-126

UPDATE: Please try 0.8.3, the issue has been resolved there and you should see only 4 results. Thanks for the report.

Answer 2

You can solve the issue using project, without project it seems to somehow ORing the constrains rather then ANDing them.

(defn product-pluso [factor1 factor2 number sum]
  (fresh [product]
         (fd/+ product number sum)
         (project [product]
                  (fd/* factor1 factor2 product))))