gistfile1.clj

; This works as intended
(defn bfs [from]
  (letfn [(bfs-inner [frontier seen]
            (lazy-seq 
              (println frontier seen)
              (let [artist-and-similar (first frontier)
                    similar (similar-with-parent (second artist-and-similar))
                    new-frontier (concat (rest frontier) (remove #(contains? seen (second %)) similar))
                    new-seen (clojure.set/union seen (set (flatten similar)))]
                (cons artist-and-similar (bfs-inner new-frontier new-seen)))))]

    (bfs-inner [(list nil from)] #{})))

; This doesn't
(defn bfs2 [from]
  (letfn [(bfs-inner [frontier seen]
              (println frontier seen)
              (let [artist-and-similar (first frontier)
                    similar (similar-with-parent (second artist-and-similar))
                    new-frontier (concat (rest frontier) (remove #(contains? seen (second %)) similar))
                    new-seen (clojure.set/union seen (set (flatten similar)))]
                (cons artist-and-similar (bfs-inner new-frontier new-seen))))]

    (lazy-seq (bfs-inner [(list nil from)] #{}))))