"truly-the" 函数是什么？

Question

在 SBCL 2.0.1 中，(macroexpand '(dolist (x '(1 2 3)) (princ x))) returns:

(BLOCK NIL
  (LET ((#:N-LIST385 '(1 2 3)))
    (TAGBODY
     #:START386
      (UNLESS (ENDP #:N-LIST385)
        (LET ((X (TRULY-THE (MEMBER 3 2 1) (CAR #:N-LIST385))))
          (SETQ #:N-LIST385 (CDR #:N-LIST385))
          (TAGBODY (PRINC X)))
        (GO #:START386))))
  NIL)
T

什么是TRULY-THE？它似乎是非标准的，因为我在 Common Lisp HyperSpec 中找不到它。

TRULY-THE和THE有什么区别？

Answer 1

来自 SBCL manual:

Special Operator: truly-the [sb-ext] value-type form

Specifies that the values returned by form conform to the value-type, and causes the compiler to trust this information unconditionally.

Consequences are undefined if any result is not of the declared type -- typical symptoms including memory corruptions. Use with great care.

换句话说，the 运算符在某些情况下（例如具有高级别调试）编译为测试以检查指定类型的正确性，而 truly-the 指示编译器从不执行此检查。

"truly-the" 函数是什么？

What is the "truly-the" function?

common-lisp

sbcl