Comments on: Shadowing a CL function definition

By: Pascal Bourguignon

Pascal Bourguignon — Tue, 01 Jul 2008 18:41:08 +0000

Indeed, there is nothing easier in Common Lisp than shadowing, be it a function or anything else named by a symbol (since CL:SHADOW applies to symbols).

(defpackage “TEST-APPOINTMENT”
(:use “CL”)
(:shadow “GET-UNIVERSAL-TIME”))
(in-package TEST-APPOINTMENT”)
(defun get-universal-time () (random 1e12))
(load “appointment.lisp”)

By: Tomas Zellerin

Tomas Zellerin — Tue, 01 Jul 2008 14:19:36 +0000

Why you do not use shadow for shadowing – function with same name in used package, calls COMMON-LISP counterpart, or – when testing – something other?

By: Richard Kreuter

Richard Kreuter — Tue, 01 Jul 2008 14:12:30 +0000

Strictly speaking, when the standard says that the consequences of an action are undefined, the implementation is permitted to do anything at all when a program tries it: silently ignore the attempt, ring the system bell, crash, blow up the universe, etc.; nor does the implementation have to do the same things for each attempt, or document what it’s to do, etc. So a conforming program can’t programmatically detect the consequences of actions whose consequences the standard and the implementation don’t define.

By: Leslie

Leslie — Tue, 01 Jul 2008 13:15:06 +0000

That’s good to know, thanks.

So reliance on this specific behaviour — if used at all — should be preceded by a check for it.

By: foo

foo — Tue, 01 Jul 2008 13:13:44 +0000

Well, ANSI Common Lisp does NOT allow to do that: changing definitions of the CL package. Many implementations will throw an error, because the package “CL” is locked.

Plus in compiled code the Common Lisp function might be inlined, so a redefinition would not have an effect. The Common Lisp file compiler can for example assume that within a file the definitions don’t change. It can also assume that the functions of the Common Lisp package don’t change.

So, what you describe may work in some implementation and may be useful there, in general it is not portable Common Lisp.

By: Zach Beane

Zach Beane — Tue, 01 Jul 2008 13:08:54 +0000

This is undefined behavior per 11.1.2.1.2.