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I've seen ;;; -*- lexical-binding: t; -*- at the top of a file and (setq lexical-binding t) as how to turn on lexical binding for elisp code, but is there a way to tell Emacs to simply always do lexical binding, i.e., in your init file? From what I've seen, my guess would be it's a buffer-only option. Also, how can lexical binding be turned on in an org file? Does each babel emacs lisp code block have to specify (setq lexical-binding t)? BTW, putting ;;; -*- lexical-binding: t; -*- is not dealt with well on an export to HTML, etc. Using version 26.1.

4

Elisp

AFAIK the only two reliable ways for using lexical binding in Elisp files are:

  1. Setting the buffer-local variable lexical-binding as file-local variable at the beginning of the Elisp file with
    ;;; -*- lexical-binding: t; -*-

  2. Using eval with non-nil LEXICAL argument. (LEXICAL can be t or the lexical environment to be used)

Motivation for that behavior is that a global setting would potentially break your emacs configuration. Lexical binding in Elisp is a relatively new feature and older packages can rely on dynamic binding. See the difference in the behavior of lex-p below for both variants of binding.

Org-mode

I don't know where you got (setq lexical-binding t) from. That just does not work as the following Org source block shows:

#+BEGIN_SRC emacs-lisp
(setq lexical-binding t)
(defun lex-p ()
  "Return t if lexical binding is in effect."
  (let (lex
    _lex-p)
    (let ((lex t))
      (setq _lex-p
        (lambda ()
          lex)))
    (funcall _lex-p)))

(if (lex-p) "Yes" "No")
#+END_SRC

#+RESULTS:
: No

Nevertheless there is the header argument :lexical for Org source block that just does what you want:

#+BEGIN_SRC emacs-lisp :lexical t
(defun lex-p ()
  "Return t if lexical binding is in effect."
  (let (lex
    _lex-p)
    (let ((lex t))
      (setq _lex-p
        (lambda ()
          lex)))
    (funcall _lex-p)))

(if (lex-p) "Yes" "No")
#+END_SRC

#+RESULTS:
: Yes

You can set that header argument file global by the header-args property:

#+PROPERTY: header-args:emacs-lisp :lexical t

#+BEGIN_SRC emacs-lisp
(defun lex-p ()
  "Return t if lexical binding is in effect."
  (let (lex
    _lex-p)
    (let ((lex t))
      (setq _lex-p
        (lambda ()
          lex)))
    (funcall _lex-p)))

(if (lex-p) "Yes" "No")
#+END_SRC

#+RESULTS:
: Yes

#+BEGIN_SRC emacs-lisp
(if (lex-p) "Foo" "Bar")
#+END_SRC

#+RESULTS:
: Foo
  • Thanks, this makes sense. Still, in the second code block I see #+BEGIN_SRC emacs-lisp :lexical t but the next line is (setq lexical-binding t). You say above the second is ineffective? What was your thinking there? – 147pm Mar 29 at 3:37
  • @147pm Thanks for the note. The presence of (setq lexical-binding t) in the second block was a copy-paste error. I have corrected it. – Tobias Mar 29 at 3:47
  • (setq lexical-binding t) is needed to use lexical-binding in *scratch*. – npostavs Mar 29 at 4:16
  • @npostavs if I put the code from the first code block into *scratch*, change the (if (lex-p) "Yes" "No") into (message (if (lex-p) "Yes" "No")) and evaluate the buffer with M-x eval-buffer I get the message "No". On the other hand if I put the stuff from the second source block into the scratch buffer wrap the last form into message and wrap everything by a (eval '(progn ...) t) I get a "Yes". – Tobias Mar 29 at 4:52
  • @npostavs Even more important: If I put the stuff from second source block into *scratch* with the last form wrapped in (message ...), insert ;; -*- lexical-binding: t -*- at the beginning of the buffer, and run M-x eval-buffer I get a "Yes". – Tobias Mar 29 at 5:03

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