Emacs lisp. One function to has no access to variables in another function

Question

Code in emacs lisp:

 


(defun test-sub-fun-var()
  (message "SUB test-sub-fun-var: my-local-var: %s" my-local-var)
  )

(defun test-main()
  (message "\nSTART test-main")
  (let
      (
       (my-local-var)
       )
    (setq my-local-var "MY LOCAL VARIABLE VALUE")
    (message "MAIN my-local-var: %s" my-local-var)
    (test-sub-fun-var)
    )
  (message "FINISH test-main")
  )

(test-main)

When run function "test-main" we have result:

START test-main

MAIN my-local-var: MY LOCAL VARIABLE VALUE

SUB test-sub-fun-var: my-local-var: MY LOCAL VARIABLE VALUE

FINISH test-main

As you can see, the function "test-sub-fun-var" has access to variable "my-local-var" that declare in function "test-main". But I need function "test-sub-fun-var" to has NO access to variable "my-local-var". How I can do this? Thanks.

Answer 1

You don't say what version of Emacs you're using. Before 24.1 there wasn't really a way to do what you want, you just had to be careful. But, quoting from the latest version of the Info manual (26.0.50.1):

11.9.3 Lexical Binding

Lexical binding was introduced to Emacs, as an optional feature, in version 24.1. [...] A lexically-bound variable has “lexical scope”, meaning that any reference to the variable must be located textually within the binding construct.

You enable lexical binding on a per-file basis, with the file-local variable lexical-binding. So,

;; -*- lexical-binding:t -*-

(defun test-sub-fun-var()
  (message "SUB test-sub-fun-var: my-local-var: %s" my-local-var))
(defun test-main()
   (message "\nSTART test-main")
   (let ((my-local-var))
       (setq my-local-var "MY LOCAL VARIABLE VALUE")
       (message "MAIN my-local-var: %s" my-local-var)
       (test-sub-fun-var))
   (message "FINISH test-main"))

(test-main)

Now your output is:

START test-main
MAIN my-local-var: MY LOCAL VARIABLE VALUE
test-sub-fun-var: Symbol’s value as variable is void: my-local-var