[Ecls-list] symbol-macro usage

[Alexander Gavrilov]

Hi,

Could you also comment on the troubles that happen over here?

http://dwim.hu/darcsweb/darcsweb.cgi?r=HEAD%20hu.dwim.def;a=headblob;f=/integration/common.lisp

Namely (and in case the above link is temporarily
unavailable), the original non-ecl code is as follows:

#-ecl
(eval-when (:compile-toplevel :load-toplevel :execute)
  (unintern 'common-lisp:in-package :hu.dwim.common)
  (shadow "IN-PACKAGE" :hu.dwim.common))

#-ecl
(eval-when (:compile-toplevel :load-toplevel :execute)
  ;; this export is not inside the above eval-when for a reason.
  (export 'hu.dwim.common::in-package :hu.dwim.common)
  (assert (not (eq 'hu.dwim.common::in-package 'common-lisp:in-package))))

#-ecl
(def macro hu.dwim.common::in-package (package-name)
   ...)

When one tries to use it with ECL, the compiling part seems
to work as expected, and the symbol is recorded in the fasl
as hu.dwim.common::in-package. However, when it is loaded,
the references are resolved before any code is executed,
so the assertion fails - and if it is removed, the macro
definition overwrites the original common-lisp macro with
disastrous consequences.

Alexander
 
> The error is very subtle. You have to go here
>    http://www.lispworks.com/documentation/HyperSpec/Body/03_bcaa.htm
> 
> define-symbol-macro does not have compile-time side effects. That means that
> when the compiler reaches the form (defvar a m) it does not know that M is a
> macro! This causes a load time error because the code is designed to find
> out the value of the variable "M", not to use "tstst".