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  ANSI Common Lisp   14 Conses   14.2 Dictionary of Conses

14.2.23 endp Function

endp list    generalized-boolean

Arguments and Values:
list - a list, which might be a dotted list or a circular list.

generalized-boolean - a generalized boolean.

Returns true if list is the empty list. Returns false if list is a cons.

 (endp nil)  true
 (endp '(1 2))  false
 (endp (cddr '(1 2)))  true

Exceptional Situations:
Should signal an error of type type-error if list is not a list.

The purpose of endp is to test for the end of proper list. Since endp does not descend into a cons, it is well-defined to pass it a dotted list. However, if shorter "lists" are iteratively produced by calling cdr on such a dotted list and those "lists" are tested with endp, a situation that has undefined consequences will eventually result when the non-nil atom (which is not in fact a list) finally becomes the argument to endp. Since this is the usual way in which endp is used, it is conservative programming style and consistent with the intent of endp to treat endp as simply a function on proper lists which happens not to enforce an argument type of proper list except when the argument is atomic.

Allegro CL Implementation Details:

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