如何以 let 方式格式化嵌套的多值绑定？

Question

最近，我经常嵌套几个返回多个值的函数。然而，与let允许将这些调用优雅地写成一个大语句的 不同，我总是以大量缩进结束。

我的问题是：有几个多值函数，例如

(defun return-1-and-2 ()
  (values 1 2))

(defun return-3-and-4 ()
  (values 3 4))

是否有可能达到与

(multiple-value-bind (one two)
    (return-1-and-2)
  (multiple-value-bind (three four)
      (return-3-and-4)
    (list one two three four)))

但写得更简洁 - 方式let，即，类似

(multiple-value-bind (one two)
    (return-1-and-2)
  (multiple-value-bind (three four)
      (return-3-and-4)
    (list one two three four)))

?

Answer 1

图书馆中可能有类似的结构。

请注意，它更类似于let*，而不是let，因为范围是嵌套的。

一个人可以写一个宏。例如：

(defmacro multiple-value-let* ((&rest bindings) &body body)

  "Sets the scope for several ((var-0 ... var-n) form)
  binding clauses, using the multiple return values of the form."

  (if (null bindings)
      `(progn ,@body)
    (destructuring-bind (((&rest vars) form) &rest rest-bindings)
        bindings
      `(multiple-value-bind ,vars
           ,form
         (multiple-value-let* ,rest-bindings
           ,@body)))))

例子：

CL-USER 33 > (walker:walk-form
              '(multiple-value-let* (((one two)    (return-1-and-2))
                                     ((three four) (return-3-and-4)))
                 (list one two three four)))
(MULTIPLE-VALUE-BIND (ONE TWO)
    (RETURN-1-AND-2)
  (MULTIPLE-VALUE-BIND (THREE FOUR)
      (RETURN-3-AND-4)
    (PROGN (LIST ONE TWO THREE FOUR))))

Answer 2

我越来越喜欢这个库let-plus，它提供了一个let+具有此选项（以及其他选项）的宏：

\n\n

(let+ (((&values one two) (return-1-and-2))\n       ((&values three four) (return-3-and-4))\n       (foo (bar))                  ; other examples\n       (#(a b c) (some-vector)))    ;\n  #| body\xe2\x80\xa6 |#)\n

\n

Answer 3

在塞拉皮姆，mvlet*：

展开一系列嵌套的多值绑定表单。

  (mvlet* ((minutes seconds (truncate seconds 60))
           (hours minutes (truncate minutes 60))
           (days hours (truncate hours 24)))
    (declare ((integer 0 *) days hours minutes seconds))
    (fmt "~d day~:p, ~d hour~:p, ~d minute~:p, ~d second~:p"
         days hours minutes seconds))

https://github.com/ruricolist/serapeum/blob/master/REFERENCE.md#mvlet-rest-bindings-body-body