common-lisp 照应宏
示例
照应宏是一种引入变量（通常为IT）的宏，该变量捕获用户提供的表单的结果。一个常见的示例是AnaphoricIf，它与regular一样IF，但也定义了变量IT以引用测试表单的结果。
(defmacro aif (test-form then-form &optional else-form)
`(let ((it ,test-form))
(if it ,then-form ,else-form)))
(defun test (property plist)
(aif (getf plist property)
(format t "The value of ~s is ~a.~%" property it)
(format t "~s wasn't in ~s!~%" property plist)))
(test :a '(:a 10 :b 20 :c 30))
; The value of :A is 10.
(test :d '(:a 10 :b 20 :c 30))
; :D wasn't in (:A 10 :B 20 :C 30)!
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