Hi Peter,

On Wed, Dec 30, 2009 at 7:18 AM, Peter Olsen pcolsen@gmail.com wrote:

I'm writing to ask how to redefine the "or" macro in ABCL. I'm working on a project and there's one package in which I need "or" to mean something else than its usual definition.  I've tried simply to redefine it, but it doesn't work.  I understand that the Common Lisp spec requires special provision for that to be done --- such as the package-lock construct in SBCL.

Well, actually, ABCL doesn't have package locks, so this should be easier in ABCL. Your approach was going in the right direction, however, you've hit a little snag which is in the way: OR is a special operator and a macro. The special operator makes the interpreter perform better because it prevents object allocations.

How can I do that in ABCL?  I want to be able to do this within the code, for example, with in a "let" so that I can bound its scope. Can anyone point me in the right direction?  My first --- failed --- experiment is below.

My comments below.

A way that doesn't work. slipstream:~ pcolsen$ abcl Armed Bear Common Lisp 0.17.0 Java 1.5.0_20 Apple Inc. Java HotSpot(TM) Client VM Low-level initialization completed in 1.476 seconds. Startup completed in 4.363 seconds. Type ":help" for a list of available commands. CL-USER(1): (or nil t) T CL-USER(2): (defmacro or (x) (+ 1 x))  ; My very own new "or" OR

Here, OR is still both a special operator and a macro. The special operator has precedence on evaluation. The actual macro function gets stored in the symbol-plist. If you execute

(setf (symbol-function 'or) (get 'or 'sys::macroexpand-macro))

before running the defmacro, you're done. However: note that if you trigger any part of the interpreter or compiler depending on the behaviour of OR, you're in trouble: it'll see the incorrect version of OR if it's within the scope of the specific block.

Another solution could be to rename the OR in your code to something else, but I guess you thought of that.

CL-USER(3): (or 3) ; Result should be 4 3 CL-USER(4): (or nil nil) ; My "or" should take only one argument. NIL CL-USER(5): (or nil t) T CL-USER(6):  ;;; Redefinition didn't work.

Hope that helps; it works on my system, at least.

Bye,

Erik.

On Wed, Dec 30, 2009 at 7:18 AM, Peter Olsen pcolsen@gmail.com wrote:

I'm writing to ask how to redefine the "or" macro in ABCL. I'm working on a project and there's one package in which I need "or" to mean something else than its usual definition.  I've tried simply to redefine it, but it doesn't work.  I understand that the Common Lisp spec requires special provision for that to be done --- such as the package-lock construct in SBCL. How can I do that in ABCL?  I want to be able to do this within the code, for example, with in a "let" so that I can bound its scope. Can anyone point me in the right direction?  My first --- failed --- experiment is below.

In addition to what Erik said, if you need you own OR to be effective only in your package, you can use your-package:or instead of common-lisp:or to name your macro, for example:

(defpackage :my-package (:use :common-lisp) (:shadow #:or) (:export ...))

then, start your source files with

(in-package :my-package)

and in one of them put

(defmacro or ...) ;the symbol is my-package:or, not common-lisp:or

Of course, this way your macro will only get used by your own code (and code that imports the symbol OR from your package); it won't affect other libraries or the system itself, so it's not equivalent to Erik's advice. This solution has the advantage of being portable Common Lisp.

hth, Alessio