Contributing (setf (assoc ...))

Contributing (setf (assoc ...))

[Daniel Pittman]

G'day.

I recently wanted to update as associative list, but apparently Emacs
doesn't presently have an implementation of (setf (assoc ...)), at least
in the snapshot I use.

I believe I have paperwork on file already with the FSF for Emacs, even
assuming that this is large enough to require such paperwork:

(defsetf assoc (key place) (value)
  (let ((s1 (gensym)))
    `(let ((,s1 (assoc ,key ,place)))
       (if ,s1 (setf (second ,s1) ,value)
         (push (list ,key ,value) ,place)))))

That implementation should work fine in all circumstances, I believe,
and passes my local testing.

Um, generally speaking there doesn't seem to be anything like a test
suite for Emacs Lisp code that I could locate.  Did I miss something, or
is it generally the practice to just get it right? ;)

Regards,
        Daniel

Re: Contributing (setf (assoc ...))

[Lennart Borgman]

On Sat, May 9, 2009 at 3:55 PM, Daniel Pittman <daniel@rimspace.net> wrote:
> Um, generally speaking there doesn't seem to be anything like a test
> suite for Emacs Lisp code that I could locate.  Did I miss something, or
> is it generally the practice to just get it right? ;)

There is a subdirectory emacs/test if you check out the sources. There
is however not very much there yet if I remember correctly.

On EmacsWiki you can find this page:

   http://www.emacswiki.org/emacs/UnitTesting

And I have a modified version of the ert test library in nXhtml. This
modified version adds some things I need for testing syntax
fontification.

Re: Contributing (setf (assoc ...))

[Stefan Monnier]

> (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 ...))

[David Kastrup]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> (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?

The above does not have the right setf semantics.  One would need to
write

(setf (cdr (assoc key ...)) value)

in order to set just the value of a key value pair.  If one does, it is
perfectly fine that the return value is just value.

Or one would have to use something like

(setf (assoc-default key ...) value)

-- 
David Kastrup