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I encountered an issue with the following code, provided by NickD as an answer to a previous question (link to the question):

(defun my-add-string-to-list (LIST-VAR STRING)
  (add-to-list LIST-VAR STRING t)
  (buffer-string)) ;; just to show that the function does not return a list

(defun my-failing-test ()
  (interactive)
  (let ((MYLIST '()))
    (my-add-string-to-list 'MYLIST "foo")
    (my-add-string-to-list 'MYLIST "bar")
    (message (format "%S" MYLIST))))

When I evaluate these functions in Emacs using eval-defun (C-M-x) in the *scratch* buffer and then run M-x my-failing-test, I encounter the following error (this happens also if I start Emacs with emacs -Q):

my-add-string-to-list: Symbol’s value as variable is void: MYLIST

Observations:

  1. If I evaluate the *scratch* buffer with eval-buffer, the my-failing-test function works as expected.

  2. If I save the same code (as suggested by NickD) in a file, for example, /tmp/fool.el, and then evaluate the functions with eval-defun, the my-failing-test function works as expected.

Additional information:

  • The modeline in the *scratch* buffer reads: (Lisp Interaction ElDoc).
  • The modeline in the fool.el buffer reads: (Elisp/d ElDoc).

Why is this happening? Until now, I've always used the scratch buffer to test function prototypes, and I believed that was its purpose. Am I mistaken, or is this some sort of bug?

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8

Your Emacs version (29, most likely) has silently changed the value of lexical-binding from nil to t in the *scratch* buffer.

When you use C-M-x to evaluate only those functions that variable isn't taken into account (it is nil by default in Emacs generally, still).

When you use M-x eval-buffer that variable's value in the buffer (t) is taken into account.

When lexical-binding is nil, variable MYLIST is automatically a special variable (i.e., dynamically bound). When lexical-binding is nil, variable MYLIST is by default lexically bound, unless you use (defvar MYLIST).

In your first test, there's no dynamic binding for MYLIST because lexical-binding is t in the buffer. In your second test it has a dynamic binding - the let binds with a dynamic binding because lexical-binding is nil.

The doc for add-to-list tells you this about argument LIST-VAR:

LIST-VAR should not refer to a lexical variable.

add-to-list looks for a dynamic binding for LIST-VAR.

Until now, I've always used the scratch buffer to test function prototypes, and I believed that was its purpose.

Yes, and you've counted on it using dynamic binding by default, as we all have. The mode-line indication of Lisp Interaction mode wasn't updated to use the suffix /l or /d to indicate lexical or dynamic binding. The mode-line indication of Elisp mode was updated to provide this info.

The comment text at the start of *scratch* should tell you that the buffer-local value of lexical-binding is t, where it tells you this less-important information:

;; This buffer is for text that is not saved, and for Lisp evaluation.
;; To create a file, visit it with C-x C-f and enter text in its buffer.

Maybe M-x report-emacs-bug to request such an enhancement?

This is a real gotcha, especially for users used to the old behavior in *scratch*. Changing the basic behavior silently is a no-no. (Yes, you can read about the change with C-h n (NEWS), but a clear indication in *scratch* itself would help.

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