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bug#25929: 25.2; map-delete doesn't delete permanently 1st alist elt


From: Michael Heerdegen
Subject: bug#25929: 25.2; map-delete doesn't delete permanently 1st alist elt
Date: Sat, 04 Mar 2017 01:04:18 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

Stefan Monnier <address@hidden> writes:

> >     (macroexp-let2 nil key
> >       `(progn
> >          (map--dispatch ,map
> >            :list (setf (alist-get ,key ,map nil t) nil)
> >            :hash-table (remhash ,key ,map)
> >            :array (and (>= ,key 0)
> >                        (<= ,key (seq-length ,map))
> >                        (aset ,map ,key nil)))
> >          ,map)))
>
> Note that this will make it pretty much impossible to use
> cl-generic dispatch.

It will also break cases where MAP is an expression that is not a
symbol:

#+begin_src emacs-lisp
(let ((thing (cons 'tag (list (cons 0 1) (cons 2 3)))))
  (map-delete (cdr thing) 0))
==> nil

(map-delete (list (cons 0 1) (cons 2 3)) 0)
==> if: Symbol's function definition is void: \(setf\ list\)
#+end_src

> A better option might be to provide a map-remove which works
> functionally (i.e. doesn't modify its argument by side-effects), and
> then change map-delete to signal an error when we ask to delete the
> first element of the list (and to stop returning the "new map" since
> it just works imperatively instead).

Why not make it just like `delete': return the map with the entry
removed, the original map might be altered - that's it.

If you want to assign the result back to a symbol/place, use setf or
callf.  Any unnecessary magic just blows up the semantics.

With other words, I would fix this issue by altering the docstring
("return the result" instead of "return MAP") - or am I missing
something?


Michael.





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