一般原则
假设你有以下range函数：
(defun range (start end &optional (step 1))
(loop for x from start below end by step collect x))
您可以接受另一个参数，一个函数，并为每个元素调用它：
(defun range-generator (callback start end &optional (step 1))
(loop for x from start below end by step do (funcall callback x)))
这使调用者可以控制迭代过程：
(block root
(range-generator (lambda (v)
(print v)
(when (>= v 10)
(return-from root)))
0 300))
0
1
2
3
4
5
6
7
8
9
10
请参阅RETURN、BLOCK。
排列
如果您想避免分配过多的内存，您可以安排代码分配中间数据结构一次，并在每次调用回调时重复使用它们。这是一个带注释的示例：
(defun permutations% (list callback)
(when list
(let* (;; Size of input list
(size (length list))
;; EMPTY is a sentinel value which is guaranteed to
;; never be equal to any element from LIST.
(empty (gensym "SENTINEL"))
;; Working vector containing elements from LIST, or
;; EMPTY. This vector is mutated to remember which
;; element from the input LIST was already added to the
;; permutation.
(items (make-array size :initial-contents list))
;; Working vector containing the current
;; permutation. It contains a FILL-POINTER so that we
;; can easily call VECTOR-PUSH and VECTOR-POP to
;; add/remove elements.
(permutation (make-array (length items) :fill-pointer 0)))
;; Define a local recursive function named POPULATE, which
;; accepts a COUNT argument. The count starts at SIZE and
;; decreases at each recursive invocation, allowing the
;; function to know when it should end.
(labels ((populate (count)
(if (plusp count)
;; Loop over ITEMS by index
(dotimes (item-index size)
(let ((item (svref items item-index)))
;; We found an ITEM which is not yet
;; present in PERMUTATION.
(unless (eq item empty)
;; Push that element
(vector-push item permutation)
;; Replace current value in ITEMS by EMPTY
(setf (svref items item-index) empty)
;; POPULATE will recursively populate
;; the remaining elements in
;; PERMUTATION and call CALLBACK. Once
;; it is done, it will return here.
(populate (1- count))
;; There are other items to process in
;; current loop. Reset the state to how
;; it was before calling POPULATE.
;; Replace the EMPTY value by the
;; original ITEM at current index.
(setf (svref items item-index) item)
;; Remove ITEM from PERMUTATION.
(vector-pop permutation))))
;; We filled PERMUTATION with SIZE elements.
;; Call CALLBACK with PERMUTATION. Note: the
;; callback function is always given the same
;; vector, but its content changes over
;; time. The value passed to CALLBACK is thus
;; valid only during the time we are
;; executing CALLBACK. If the caller needs to
;; keep a copy of the current permutation, it
;; should COPY-LIST the value.
(funcall callback permutation))))
;; Initiate recursive function with current SIZE.
(populate size)))))
该函数接受一个列表和一个回调，这是一个接受一个参数的函数，即当前排列。请注意，此参数仅在调用的dynamic extent 期间有效，因为一旦调用返回，将修改传递给回调的相同数据结构。
如上所述，您可以调用任何函数，特别是闭包，它引用词法环境中的其他变量。在这里，匿名 lambda 增加了 count 变量，它允许计算排列的数量，而无需将它们存储在列表中并获取列表的大小：
(time
(let ((count 0))
(permutations% '(a b c d e f g h i j k) (lambda (p) (incf count)))
count))
=> 39916800
Evaluation took:
6.455 seconds of real time
6.438200 seconds of total run time (6.437584 user, 0.000616 system)
99.74% CPU
17,506,444,509 processor cycles
0 bytes consed
在上面的报告中，0 bytes consed代表了大约分配的内存数量（不包括堆栈分配）。
您还可以提供更安全的函数版本，在将每个排列发送到回调函数之前复制它。
(defun permutations (list callback)
(permutations% list (lambda (permutation)
(funcall callback (coerce permutation 'list)))))
另见
另请参阅the answer from Will Ness，它设法使用列表处理剩余元素的集合，从而避免过滤 EMPTY 元素的需要。