(defmacro fnc
  "Define an anonymous constraint that can be used with the unifier:

     (let [oddc (fnc [x] (odd? x))]

       (unifier {:a '?a} {:a 1} :when {'?a oddc})
         ;;=> {:a 1}

       (unifier {:a '?a} {:a 2} :when {'?a oddc})
         ;;=> nil
     )

  Note, the constraint will not run until all arguments are fully ground.

  Use defnc to define a constraint and assign a toplevel var."
  [args & body]
  (let [name (symbol (gensym "fnc"))]
    `(fn ~args
       (letfn [(~name [~@args]
                 (reify
                   ~'clojure.lang.IFn
                   (~'invoke [this# a#]
                     (let [[~@args :as args#] (map #(clojure.core.logic/walk* a# %) ~args)
                           test# (do ~@body)]
                       (when test#
                         ((clojure.core.logic/remcg this#) a#))))
                   clojure.core.logic.protocols/IConstraintOp
                   (~'rator [_#] '~name)
                   (~'rands [_#] (filter clojure.core.logic/lvar? (flatten ~args)))
                   clojure.core.logic.protocols/IReifiableConstraint
                   (~'reifyc [_# _# r# a#]
                     (list '~name (map #(clojure.core.logic/-reify r# %) ~args)))
                   clojure.core.logic.protocols/IRunnable
                   (~'runnable? [_# s#]
                     (clojure.core.logic/ground-term? ~args s#))
                   clojure.core.logic.protocols/IConstraintWatchedStores
                   (~'watched-stores [_#] #{:clojure.core.logic/subst})))]
         (cgoal (~name ~@args))))))