Emacs Lisp variables scoping

Question

Let's consider the following functions:

    (defun test (x)
      "X."
      (request "http://example.com"
        :parser 'json-read
        :complete (cl-function
                   (lambda (&key response &allow-other-keys)
                     (message "%s" x)))))

    (defun test2 (x)
      "X."
      (funcall (cl-function (lambda (z) (message "%s" z))) x))

Calling (test2 3) Works fine and produces the desired message. Calling (test 3), however, fails with the following error:

error in process sentinel: let*: Symbol’s value as variable is void: x
error in process sentinel: Symbol’s value as variable is void: x

My guess is that request is a macro doing something weird to variable scoping. Unfortunately, the documentation does not mention anything like this. Is there a way to overcome that?

sds · Accepted Answer · 2022-10-28 19:55:47Z

4

request is probably an ordinary function.

The problem is that the lambda you pass to it is not called immediately but is saved and called later:

(defun make-function (x)
  (lambda () (message "received %s" x)))
=> make-function
(defconst f (make-function 3))
=> f
(funcall f)
=> Lisp error: (void-variable x)

x that make-function bound no longer exists.

This is because by default Emacs uses dynamic binding and you need lexical binding to get the behavior you want.

If you add

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

to the first line of your file, it will be compiled with lexical binding and the code above will produce the message received 3 instead of an error.

edited Oct 28, 2022 at 19:55

answered Oct 28, 2022 at 18:34

sds

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3 Comments

Sventimir Over a year ago

When I add this to the top of the file, I get symbols's value as variable is void: -*- when I try to evaluate it with eval-buffer.

sds Over a year ago

@Sventimir: (1) I expected your file to begin with comments ;-) (2) please compile and load the file (C-u C-x C-f)

Sventimir Over a year ago

Okay, when the line is commented out, it works. Thanks a lot!

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