CLAZY であるべきか

元は SICP です。

CLAZY: Lazy Calling in Common Lisp

お試しのソースコード：

(defmacro delay (expr) `(lambda () ,expr))
(defun force (thunk) (funcall thunk))
(defmacro cons-stream (head tail) `(cons ,head (delay ,tail)))
(defun head (lazy-stream) (car lazy-stream))
(defun tail (lazy-stream) (force (cdr lazy-stream)))

(defun make-plus-stream (x y)
  (cons-stream
   x
   (make-plus-stream (+ x y) y)))

(defun take-and-prn (stm b)
  (defun iter (s i)
    (if (>= i b) 'done
    (progn
      (format t "~A~%" (head s))
      (iter (tail s) (+ i 1)))))
  (iter stm 0))

こんな風に無限で遊べます：

(setq stm (make-plus-stream 1 1))
=>(1 . #<lexical-closure: (anonymous)>)
(take-and-prn stm 5)
1
2
3
4
5
=>done
(setq stm (make-plus-stream 1 2))
=>(1 . #<lexical-closure: (anonymous)>)
(take-and-prn stm 5)
1
3
5
7
9
=>done