sum-tco.clj

; (sum 10) ;-> (+ 10 (sum 9))
; (sum 9)  ;-> (+ 9 (sum 8))
; ...
; (sum 1)  ;-> (+ 1 0)
(defn sum [n] ; foldr
  (case n
    1 1 ;(+ 1 0) ; 0 is initial value
    (+ n (sum (dec n)))))

; [i s] -> [10 0] -> [9 10] -> [8 19] -> ... -> [1 54] -> [0 55]
(defn sum-loop-recur [n]
  (loop [i n
         s 0]
    (case i
      0 s
      (recur (dec i) (+ s i)))))

; (f 10 0) -> (recur 9 10) -> (recur 8 19) -> ...
; ... -> (recur 1 54) -> (recur 0 55)
(defn sum-recur [n]
  (letfn [(f [i s]
            (case i
              0 s
              (recur (dec i) (+ s i))))]
    (f n 0)))

; ref. プログラミング Clojure 第 2 版 P. 90