On Jan 25, 2008 10:32 AM, Elf <address@hidden> wrote:
on that note, something that does work in the general case:
(define-macro (define-if-undefined name . val)
`(condition-case ,name
(() (define ,name ,@val))))
Thanks, Elf, I missed the "unless already exists" requirement.
(define-if-undefined) is reminiscent of DEFVAR in Common Lisp.
On a tangent, this reminds me of a macro I use. I wanted to introduce
formal hook-points in the main program, and let the run-script inject
hook-procedures into the main code if desired. The macro
(define-macro (hook-point hook-name)
(let ((hook (gensym "hook")))
`(and-let* ((,hook (condition-case ,hook-name (() #f))))
(,hook))))
is used in the main program like this:
(use ...)
(hook-point pre-init)
(initialize)
(hook-point post-init)
(main-loop)
(hook-point pre-shutdown)
(shutdown)
if the runscript defines a (pre-init) thunk, it's called at the right
moment; if not, no error is thrown, and the program continues. And I
like the way the main program reads; it's clear where the optional
behaviour might occur.
Graham