Passing a symbol as an argument to a function

Question

I'm trying to understand why this doesn't work, and how to make it work:

(defvar test/var-1 nil)
(defvar test/var-2 nil)

(defun make-setter (var-to-set)
  (lambda (val-to-set)
    (setq var-to-set val-to-set)))

(defun do-the-thing ()
  (let ((the-val 5))
    (funcall (make-setter test/var-2) the-val)))

(do-the-thing) ;; After this function is called, I'd expect 
               ;; test/var-2 to have the value 5, but it's nil.

I assume the problem is that the call to setq is setting the value of the symbol var-to-set, overwriting the previous value of test/var-2 with 5. How do I pass in the name of the variable to set and and set the value of that variable?

Answer 1

First of all, closures won't work in Emacs Lisp without setting lexical-binding to t for the file. Dynamic scoping (which is used by default) makes it impossible to create closures.

To turn on lexical scoping, put this as the first line of the file with your code:

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

If you want to set the value of an arbitrary symbol, use set instead of setq, which quotes the name of the symbol you want to set.

When passing a symbol as an argument, make sure you quote it by putting an apostrophe in front of it.