PartitionBy.clj

;; Bear in mind this is a literal translation of the Haskell partitionBy.
;; This might not be the most idiomatic clojure.


(defn partition-by'' [f head tail]
  (if (empty? tail)
    (cons [head] tail)
    (let [generator (partition-by'' f (first tail) (next tail))]
      (if (= (f head) (f (first tail)))
        (cons (cons head (first generator)) (next generator))
        (cons [head] generator)))))

(defn partition-by' [f coll]
 (lazy-seq
  (when-let [s (seq coll)]
    (partition-by'' f (first s) (next s)))))


;; Translated from

;; partitionBy :: Eq b => (a -> b) -> [a] -> [[a]]
;; partitionBy f [] = []
;; partitionBy f [x] = [[x]]
;; partitionBy f l = partitionBy' f (head l) (tail l)

;; partitionBy' _ x [xs] = [x : [xs]]
;; partitionBy' f x xs@(x':_)
;;  | (f x) == (f x') = (x : (head generator)) : (tail generator)
;;  | otherwise = [x] : generator
;; where
;;  generator = partitionBy' f (head xs) (tail xs)