string length constraint

strlenc.clj

(ns strlenc.core
  (:refer-clojure :exclude [==])
  (:require [clojure.core.logic :as l :refer [run* fresh ==]]
            [clojure.core.logic.protocols :as lp])
  (:import []))

(defn -strlenc
  ([str len]
   (reify
     lp/IConstraintStep
     (-step [this s]
       (reify
         clojure.lang.IFn
         (invoke [_ s]
           (let [str (lp/walk s str)]
             (when (<= (count str) len)
              ((l/remcg this) s))))
         lp/IRunnable
         (-runnable? [_]
           (l/ground-term? str s))))
     lp/IConstraintOp
     (-rator [_]
       `strlenc)
     (-rands [_]
       [str len])
     lp/IReifiableConstraint
     (-reifyc [_ v r s]
       `(strlenc ~str ~len))
     lp/IConstraintWatchedStores
     (-watched-stores [this] #{::subst}))))

(defn strlenc
  [str len]
  (l/cgoal (-strlenc str len)))