Emacs Lisp variables scoping-CodePudding

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?

CodePudding user response：

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.