common-lisp - Common Lisp 中 &environment 的作用是什么？

Question

我对常见的 lisp 中的 &environment 参数感到困惑。特别是它有什么用，为什么它是一个参数，而不是一个特殊的变量？

编辑:也很高兴看到 &environment 的具体示例。将在代码中使用。

Answer 1

文档

Macro Lambda Lists :

&environment is followed by a single variable that is bound to an environment representing the lexical environment in which the macro call is to be interpreted. This environment should be used with macro-function, get-setf-expansion, compiler-macro-function, and macroexpand (for example) in computing the expansion of the macro, to ensure that any lexical bindings or definitions established in the compilation environment are taken into account.

解释

定义宏(本地或全局)的运算符必须在评估或编译之前定义代码如何扩展，因此可能需要扩展现有宏，其扩展可能取决于环境 - 因此宏定义需要环境。

不是特殊变量

特殊变量更危险，因为用户可能会重新绑定(bind)它们并且因为它们更难在多线程代码中正确处理。

例子

遍布 places.lisp :

(defmacro psetf (&whole whole-form
                 &rest args &environment env)
  (labels ((recurse (args)
             (multiple-value-bind (temps subforms stores setterform getterform)
                 (get-setf-expansion (car args) env)
               (declare (ignore getterform))
               (when (atom (cdr args))
                 (error-of-type 'source-program-error
                   :form whole-form
                   :detail whole-form
                   (TEXT "~S called with an odd number of arguments: ~S")
                   'psetf whole-form))
               (wrap-let* (mapcar #'list temps subforms)
                 `(MULTIPLE-VALUE-BIND ,stores ,(second args)
                    ,@(when (cddr args) (list (recurse (cddr args))))
                    ,@(devalue-form setterform))))))
    (when args `(,@(recurse args) NIL))))

来自 iolib/src/new-cl/definitions.lisp :

(defmacro defconstant (name value &optional documentation
                       &environment env)
  (destructuring-bind (name &key (test ''eql))
      (alexandria:ensure-list name)
    (macroexpand-1
     `(alexandria:define-constant ,name ,value
        :test ,test
        ,@(when documentation `(:documentation ,documentation)))
     env)))