Why does this straightforward use of a closure fail?

Question

Suppose you save the following code to some file test.el and then do load-file test.el

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

(let ((closure-vbl 0))
  (defun tst ()
    (eval 'closure-vbl)))

(tst)

You get the error tst: Symbol's value as variable is void: closure-vbl. (Replacing (eval 'closure-vbl) with (symbol-function 'closure-vbl) gives the same error.)

Why is this? I don't understand what is wrong with the above code? Am I miunderstanding some property of emacs closures? Or are they simply not developed enough to deal with non-trivial situations.

Answer 1

There's a few pitfalls here:

• Using eval with lexical-binding will fail unless you pass it an alist environment argument holding the bindings in question (see F1 f eval)
• Looking up symbol slots only behaves correctly with dynamic scoping, if you attempt it despite this, you'll get the global value (see F1 f symbol-value)

FWIW, this is what I'd call the straight-forward use:

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

(let ((closure-variable 0))
  (defun test ()
    closure-variable))

(test)