Clojure Q&A - Recent questions tagged core.logic

clojure.core.logic/project returning LVar instead of bound value

Wed, 05 May 2021 07:18:08 +0000

I have a packing problem somewhat adjacent to the "make change" problem described and solved in this blog post by Taylor Wood. Here is the code, transcribed with 1 addition:

(ns blah
  (:require [clojure.core.logic :refer :all :as l]
            [clojure.core.logic.fd :as fd]))

(defn productsumo [vars dens sum]
  (fresh [vhead vtail dhead dtail product run-sum]
    (conde
     [(emptyo vars) (== sum 0)]
     [(conso vhead vtail vars)
      (conso dhead dtail dens)
      (project [vhead]
        (do (println vhead) succeed))  ;;<--- Just a lame goal that should print out the head..
      (fd/* vhead dhead product)
      (fd/+ product run-sum sum)
      (productsumo vtail dtail run-sum)])))

(defn change [amount denoms]
  (let [dens (sort > denoms)
        vars (repeatedly (count dens) lvar)]
    (run* [q]
      ;; we want a map from denominations to their quantities
      (== q (zipmap dens vars))
      ;; prune problem space...
      ;; every var/quantity must be 0 <= n <= amount
      (everyg #(fd/in % (fd/interval 0 amount)) vars)
      ;; the real work
      (productsumo vars dens amount))))

user> (change 14 #{1 5 10})
<lvar:81678>
<lvar:81679>
<lvar:81680>
({10 0, 5 0, 1 14} {10 1, 5 0, 1 4} {10 0, 5 1, 1 9} {10 0, 5 2, 1 4})

When run, instead of getting the projected or current value of the LVar vhead, as I expect and have normally seen when I use project, I get the Lvar. Are there any ideas why this is the case?

My specific use case is a modification of productsumo, where I have a map of key->weight that I need to use in liue of the coll dens that is here. My initial approach - which seems reasonable - is to use project as above to unify a cost LVar based off the current value of vhead, and then use that in the product computation. Currently I keep getting back LVars instead of values.

Any ideas?

Is there a way to save a core.logic logic db to a file (non-datomic)?

Sat, 20 Feb 2021 18:20:31 +0000

I looked around and didn't see anything obvious. But, I am relatively new to Clojure and core.logic. Just want see if there is some simple way to store a logic db to a file. I want to store/retract facts in a logic db and save the entire db off occasionally for recovery purposes. Then, later, possibly restore that db to memory.

Thanks in advance for any help.

Memoization in core.logic

Sun, 29 Nov 2020 08:07:28 +0000

I'm solving a sub-part of my problem using core.logic. I'm anticipating that the declarative nature of describing the problem would lead to poor performance on large inputs.

But the setup I have is that the inputs to core.logic rarely change, but I query them a lot. So naturally I think that memoization would speed up the performance.

I could only memoize the results from top-level (l/run* form, but again that's less efficient than memoizing at every l/defne or at every call to conde, because the latter can give the same speedups as dynamic-programming does.

But I do not understand what kind of values (like LVars introduced by fresh) pass between the logical clauses. They are different from regular clojure values, and I have never even called one of the clauses (like l/== or l/membero) "normally". They are always called indirectly by l/run*.

So, do I need to do anything special if I want to memoize such defnes?

Can core.logic reason about sets?

Fri, 27 Nov 2020 19:57:39 +0000

l/membero lets you relate list-items and lists, but I did not find an equivalent method in core.logic for contains?.

I'm currently converting my set to a list to use with l/membero, but I was just wondering -- is it possible to reason about sets in general with core.logic? e.g. relating two sets and their union, and other common operations?

Can core.logic ask questions?

Fri, 28 Aug 2020 23:08:51 +0000

Can core.logic ask a question, ala prolog:

commits(Person, Crime) :-

crime(Crime),
format('is ~w ?', [commits(Person, Crime)]),
read(yes).

As seen on stackoverflow.

Constrain results from core.logic query?

Thu, 16 Apr 2020 11:36:31 +0000

I am trying to model a graph using core.logic and would like to match all nodes that have edges to two other nodes. I have come up with the below but it returns two results instead of the desired one, as [2 3 4] and [2 4 3] are equivalent. How would I constrain the query to only return the desired result?

(use ' clojure.core.logic.pldb)

(db-rel edge a b)

(def g
  (db
   [edge 1 2]
   [edge 2 3]
   [edge 3 4]
   [edge 2 4]))

(with-db g
  (run* [q]
    (fresh [x y z]
      (edge x y)
      (edge x z)
      (!= y z)
      (== q [x y z]))))