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This is my first foray into emacs macros, so likely silly mistakes ahead: I'm trying to mimic the definition of pop (see the manual here), but I want to remove an element of alist indexed by key, returning the value corresponding to key from alist while altering alist in place. Here's my attempt:

  (defun alist-remove (alist key)
    (seq-filter
     (lambda (pair) (not (eq key (car pair))))
     alist))

  (defmacro alist-pop (alist key)
    `(prog1
         (cdr (assq ,key ,alist))
       (setq alist (alist-remove ,alist ,key))))

The function alist-remove works just fine. The macro alist-pop returns the value of the key correctly, but doesn't modify the value of alist. What am I missing?

1 Answer 1

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-       (setq alist (alist-remove ,alist ,key))))
+       (setq ,alist (alist-remove ,alist ,key))))

Get into the habit of using M-x pp-macroexpand-last-sexp.

Used following (alist-pop a 'b) it will show you:

(prog1
    (cdr
     (assq 'b a))
  (setq alist
    (alist-remove a 'b)))

See also pp-macroexpand-expression as well as the functions described in C-hig (elisp)Expansion

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  • One little comma. Thank you. Apr 11, 2023 at 1:02
  • If the answer has successfully resolved the problem, please accept it.
    – NickD
    Apr 11, 2023 at 1:14

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