Warnings about malformed functions when using lexical-let

Question

I have the following function:

(defun filter-repl-show-after (line-num)
  "Show lines after LINE-NUM."
  (filter-repl--foreach-line
   (lexical-let ((line-num line-num))
     (lambda ()
       (when (> (line-number-at-pos) line-num)
         (filter-repl--show-line))))))

(defun filter-repl--foreach-line (f)
  "Apply F to each line in the buffer."
  (save-excursion
    (goto-char (point-min))
    (while (not (eobp))
        (funcall f)
        (forward-line))))

The elisp linter is warning me that ‘(line-num line-num)’ is a malformed function. The problem seems to be due to the lexical letter, because I don't get the warning with

(defun filter-repl-show-after-tweaked (line-num)
  "Show lines after LINE-NUM."
  (filter-repl--foreach-line
     (lambda ()
       (when (> (line-number-at-pos) line-num)
         (filter-repl--show-line)))))

Is this warning a false positive and I avoid it if so?

Regarding checkdoc demanding uppercase names in docstring

Repro M-x checkdoc on

(defun hello (f)
  "Say hello to 'f'."
)

Results in the followign message:

Argument ‘f’ should appear (as F) in the doc string (C-h,f,e,n,p,q)

Answer 1

The elisp linter is warning me that ‘(line-num line-num)’ is a malformed function.

The elisp linter is saying that the list (line-num line-num) doesn't make sense as a function object. The reason it thinks it should be a function object is probably that it doesn't know that lexical-let is a macro, and also because it's the first item in the unquoted ((line-num line-num)) list.

After initially copying your first code block into my *scratch* buffer, I got the same "malformed function" warning (plus a warning stating that "the function" lexical-let doesn't appear to be defined).

If you add (require 'cl) above the code, both warnings disappear. However, an additional warning appears, about the "cl" library being deprecated.

From at least Emacs 28 forward, lexical-let is only found in the (deprecated) "cl" library. Based on the commentary to "cl-lib", it may have at one point been included in that (non-deprecated) library, but it isn't anymore. So you might want to look for an alternative to using the lexical-let macro.