Here are some excerpts from my .emacs:
(setq lexical-binding t)
;; .emacs
(custom-set-variables
;; custom-set-variables was added by Custom.
;; If you edit it by hand, you could mess it up, so be careful.
;; Your init file should contain only one such instance.
;; If there is more than one, they won't work right.
'(diff-switches "-u")
'(tab-always-indent (quote complete)))
;...
(require 'dired)
;...
(dotimes (i 12) (define-key dired-mode-map (kbd (concat "<f" (number-to-string (1+ i)) ">"))
(lambda ()
(interactive)
(goto-char (point-min))
(forward-line (+ 4 i)))))
This should bind keys f1 to f12 in dired-mode to commands that jump to particular files in the list of files (ignoring . and ..). However, initially, after starting emacs, these keys don't work - I get an error message forward-line: Symbol's value as variable is void: i. However, when I go to the top line of my .emacs and press C-x C-e to evaluate that line, and then go to the last line quoted above and press C-x C-e to evaluate that dotimes expression, those function keys start working!
Why is that?
By the way, it also doesn't work if I evaluate the whole buffer.
It turns out that it is necessary to replace
(setq lexical-binding t)
with
;; -*- lexical-binding: t -*-
The manual hints suggestively at this, but does not actually say so outright.