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;; The ONCE-ONLY macro is hard to explain, hard to understand, hard to
;; write, hard to modify, and hard to live without once you figure out
;; how to use it. It's used in macros to guard against multiple
;; evaluation of arguments. My version is longer than most, but it
;; does some error checking and it gives gensym'd variables more
;; meaningful names than usual.
(in-package :cl-utilities)
(defun %check-once-only-names (names)
"Check that all of the NAMES are symbols. If not, raise an error."
;; This only raises an error for the first non-symbol argument
;; found. While this won't report multiple errors, it is probably
;; more convenient to only report one.
(let ((bad-name (find-if-not #'symbolp names)))
(when bad-name
(error "ONCE-ONLY expected a symbol but got ~S" bad-name))))
(defmacro once-only (names &body body)
;; Check the NAMES list for validity.
(%check-once-only-names names)
;; Do not touch this code unless you really know what you're doing.
(let ((gensyms (loop for name in names collect (gensym (string name)))))
`(let (,@(loop for g in gensyms
for name in names
collect `(,g (gensym ,(string name)))))
`(let (,,@(loop for g in gensyms for n in names
collect ``(,,g ,,n)))
,(let (,@(loop for n in names for g in gensyms
collect `(,n ,g)))
,@body)))))
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