Emacs で with-cache を実装してみた

test-with-cache.el

;;; -*- lexical-binding: t; -*-

;; 括弧への異常な愛情 (https://www.slideshare.net/m2ym/common-lisp-9838434)
;; からの引用

(require 'cl-lib)

(defmacro with-cache (key &rest body)
  (declare (indent 1))
  (cl-once-only (key)
    (cl-with-gensyms (cache)
      `(let ((,cache (cl-load-time-value
                      (make-hash-table :test 'equal))))
         (or (gethash ,key ,cache)
             (setf (gethash ,key ,cache)
                   (cl-locally ,@body)))))))

(defun fib (n)
  (if (< n 2)
      n
    (+ (fib (- n 2)) (fib (1- n)))))

(benchmark-progn
  (print (fib 38)))

(defun fib (n)
  (with-cache n
    (if (< n 2)
        n
      (+ (fib (- n 2)) (fib (1- n))))))

(benchmark-progn
  (print (fib 38)))

;; 実行方法
;;
;; $ emacs --batch -f batch-byte-compile test-with-cache.el
;; $ emacs --script test-with-cache.elc

;; 実行結果
;;
;; 39088169
;;
;; Elapsed time: 4.469694s
;;
;; 39088169
;;
;; Elapsed time: 0.000939s