Error running timer: (void-variable message) in Emacs init.el

Question

Why do I get Error running timer: (void-variable message) in the function below in my `init.el - Emacs?

(defun cypher/cowsayx-sclock (in-minutes message)
  (interactive "nSet the time from now - min.: \nsWhat: ")
  (run-at-time (* in-minutes 60)
               nil
               (lambda ()
                 (message "%S" message)
                 (shell-command (format "xcowsay %s" (shell-quote-argument
                                                      message))))))

Answer 1

You need to turn on lexical-binding, for that message occurrence in the lambda not to be treated as a free variable. It's a lexical variable local to function cypher/cowsayx-sclock, but within the lambda it's free.

Otherwise, you need to instead substitute the value of variable message in the lambda expression, and use that as a list. Here's a backquoted expression that gives you that list with the message value substituted.

`(lambda ()
   (message "%S" ',message)
   (shell-command (format "xcowsay %s" (shell-quote-argument ',message)))

But this is less performant than using lexical-binding, which produces a closure for the lambda, encapsulating the value of message.

See the Elisp manual, node Using Lexical Binding.

You can, for example, just put this at the end of a comment line as the first line of your file:

  -*- lexical-binding:t -*-

For example, if your code is in file foo.el then this could be its first line:

;;; foo.el --- Code that does foo things.   -*- lexical-binding:t -*-