Here's my attempt at implementing an example of a "does not exist" relation
in core.logic:
(ns diplomacy.vote
(:refer-clojure :exclude [==])
(:use [clojure.core.logic])
(:require [clojure.core.logic.pldb :as pldb]))
;; `voter-name` voted for `candidate`
(pldb/db-rel vote voter-name candidate)
(def voter-db (pldb/db
[vote "Peter" :blue]
[vote "Sarah" :blue]
[vote "Larry" :red]
[vote "Steve" :red]
[vote "Amy" :yellow]))
(defn candidate-received-no-votes
"Relation where candidate `candidate` received no votes"
[candidate]
(fresh [voter candidate-with-votes]
(conda
[(vote voter candidate)
(!= candidate candidate-with-votes)]
;; No candidate received a vote, so `candidate` received no votes
[succeed])))
diplomacy.vote> (run-db* voter-db [q]
(diplomacy.vote/candidate-received-no-votes
:blue))
((_0 :- (!= (_1 :blue))) (_0 :- (!= (_1 :blue))))
This result makes sense because unification succeeds as long as
`candidate-with-votes` is not `:blue` (though I'm not sure why there's two
results). However, this is not what I wanted: the run should 'fail' because
the candidate `:blue` received at least one vote.
The only other thing in the core.logic API
<http://clojure.github.io/core.logic/> that looked promising was `nafc`:
(defn candidate-received-no-votes
"Relation where candidate `candidate` received no votes"
[candidate]
(fresh [voter]
(nafc vote voter candidate)))
diplomacy.vote> (run-db* voter-db [q]
(diplomacy.vote/candidate-received-no-votes
:blue))
((_0 :- (clojure.core.logic/nafc #function[clojure.lang.AFunction/1] _1
:blue)))
This constraint this produces makes sense, but I believe the only correct
answer to my query is getting no results: ()
I'm struggling to think of a correct implementation of
`candidate-received-no-votes`. Can anyone provide a correct implementation,
or point me in the right direction?
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