"Symbol's value as variable is void: file-name" when using let

Question

I want to create an Emacs lisp function which

1. composes a path of a diary file which has the format <some directory>/t<timestamp>.org and
2. determines whether or not such file exists.

I wrote this:

(defun open-diary ()
  (interactive)
  ;; TODO: Find out whether or not file-name exists
  (let ((file-name (format-time-string "/some-dir/diary-files/t%Y%m%d.org"))
        (diary-file-exists (file-exists-p file-name)))
    (message diary-file-exists)))

When I run M-x open-diary I get the error Symbol's value as variable is void: file-name.

How can I fix it, i. e. make sure that (file-exists-p file-name) sees the file-name variable defined above?

Answer 1

First, lispers read code by indentation, not by counting parens, so your code was hard to read (I fixed that).

Second, let binds variables in parallel, while let* does it sequentially (see Local Variables).

Thus you need to use the latter:

(defun open-diary ()
  (interactive)
  ;; TODO: Find out whether or not file-name exists
  (let* ((file-name (format-time-string "/some-dir/diary-files/t%Y%m%d.org"))
         (diary-file-exists (file-exists-p file-name)))
    (message "%s exists? %s" file-name diary-file-exists)))