Common Lisp で 1 を返す関数 lambda constantly *

;; 次の3つは 0 を返す関数
(lambda () 1)
(constantly 1)
#'*

;; 昨日の↓は
(subseries (scan-fn t (lambda () 1) #'1+) 0 10)

;; こう書ける
(subseries (scan-fn t #'* #'1+) 0 10)

ただし (constantly 1) 以外は引数なしでコールしなければならない。 (constantly 1) はどんな引数を渡しても1が返る。

SBCL での constantly の定義がこれ。

(defun constantly (value)
  #!+sb-doc
  "Return a function that always returns VALUE."
  (lambda ()
    ;; KLUDGE: This declaration is a hack to make the closure ignore
    ;; all its arguments without consing a &REST list or anything.
    ;; Perhaps once DYNAMIC-EXTENT is implemented we won't need to
    ;; screw around with this kind of thing. -- WHN 2001-04-06
    (declare (optimize (speed 3) (safety 0)))
    value))

すごいよね？

constantly の存在を今日まで知らなくて g000001 に教えてもらった。