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Re: Contributing (setf (assoc ...))
From: |
Stefan Monnier |
Subject: |
Re: Contributing (setf (assoc ...)) |
Date: |
Wed, 18 Nov 2009 09:23:03 -0500 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.1.50 (gnu/linux) |
> (defsetf assoc (key place) (value)
> (let ((s1 (gensym)))
> `(let ((,s1 (assoc ,key ,place)))
> (if ,s1 (setf (second ,s1) ,value)
> (push (list ,key ,value) ,place)))))
This code has 2 problems:
1- association lists have elements of the form (KEY . VAL) rather than
(KEY VAL), so rather than (list ,key ,value) it should use
(cons ,key ,value) and rather than (second ,s1) it should use
(cdr ,s1).
2- it's unsatisfactory iin that it's asymmetric w.r.t assoc; because
assoc returns not just the VAL associated to a KEY but the whole
(KEY . VAL).
Usually (setf <foo> <bar>) should imply that a subsequent evaluation
of <foo> should return the value of <bar>, but here this can't be the
case. IOW `assoc' is inherently incompatible with setf.
Problem 1 is trivial, obviously. Problem 2 is more philosophical than
anything, but it makes the macro unsatifactory.
WDPT?
Stefan
- Re: Contributing (setf (assoc ...)),
Stefan Monnier <=