Re: Real-life examples of lexical binding in Emacs Lisp

[Andreas Röhler]

Am 17.06.2015 um 16:42 schrieb Stefan Monnier:
> > Without lexical binding, to try to get some independence from other
> > functions, you would have to PREFIX ALL THE LOCAL VARIABLES WITH
> > FUNCTION SPECIFIC PREFIXES!
> Actually, not really.  The Elisp convention to only use prefixes for
> global variables is 99% sufficient.  In your example, the problem is
> that `some-function' modifies the (presumably global) variable
> `scratch-buffer' and that this variable does not have an
> appropriate prefix.
>
> The only case where non-prefixed local variables cause problem is when
> you introduce higher-order functions, as in:
>
>     (defun my-map (f l)
>       (if l (cons (funcall f (car l)) (my-map f (cdr l)))))
>
>     (defun my-function (input)
>       (let ((l ...))
>         (my-map (lambda (x) (unless (memq x l) (error "invalid mapping")))
>                 input)))
>
> Suddenly the `l' used inside the lambda will not refer to the `l' nearby
> but to the `l' argument of `my-map'.  For this reason, some higher-order
> functions used to use weird argument names to try and avoid such
> name capture.  Nowadays they can use lexical scoping and hence choose
> their variable names sanely and without fear.

Checked this with

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

(defun my-map (f l)
  (if l (cons (funcall f (car l)) (my-map f (cdr l)))))

(defun my-function (input)
  (let ((l ()))
    (my-map (lambda (x) (unless (memq x l) (error "invalid mapping")))
        input)))

(my-function '(4 5))

Got identic results also without lexical-binding seen here. l is taken 
from inside lambda in both modes.