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Re: Real-life examples of lexical binding in Emacs Lisp
From: |
Pascal J. Bourguignon |
Subject: |
Re: Real-life examples of lexical binding in Emacs Lisp |
Date: |
Fri, 29 May 2015 14:28:13 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux) |
Marcin Borkowski <mbork@mbork.pl> writes:
> Hi all,
>
> I googled a bit, and could not find /real-world/ examples of using
> lexical binding and its advantages /in Emacs Lisp/. I understand that
> it's a nice thing to be able to create closures, and that lexical
> binding is in general faster than dynamic binding (which is a bonus in
> itself), but could anyone show me a real /text editing/ problem that
> lexical binding solves, like something that is easier done with
> l.b. than with d.b.? (Examples of general-purpose programming problems
> made easier with l.b. are more or less obvious/easy to find, but Emacs
> is a text editor, after all, and this is its primary area.)
Lexical binding matters for two things:
- it allows the creation of closures.
- it prevents the clobbering of variables.
Closures:
A typical example, is visible in the thread "~`symbol-function' to
get code as list even when byte-compiled?":
;;;; -*- mode:emacs-lisp;lexical-binding:t;coding:utf-8 -*-
(defun add-one-shot-meat (hook fun)
(let ((name (gensym)))
(setf (symbol-function name)
(lambda ()
(remove-hook hook name)
(funcall fun)))
(add-hook hook name)))
Without lexical binding, fun and hook would be dynamic, and
therefore their bindings would disappear when add-one-shot-meat
returns. Therefore they would be undefined variable when the
function is called, or worse, they may be bound at that time by some
other function to something different.
Compare:
(setf lexical-binding t)
(defun e (f)
(let ((v 42))
(funcall f)))
(let ((v 33))
(e (lambda () v)))
--> 33
(setf lexical-binding nil)
(defun e (f)
(let ((v 42))
(funcall f)))
(let ((v 33))
(e (lambda () v)))
--> 42
Clobering variables:
For example, if we have a package such as:
(setf lexical-binding nil)
(defun d () v)
(defun e (f)
(let ((v 42))
(funcall f)))
and we used it with a function in another package such as:
(defun h ()
(let ((v 33))
(d)))
we obtain:
(e (function h))
--> 33
instead of the expected 42.
Hence the workaround of prefixing all variables by the package name,
but this is often insufficient (because package names being often
generic, a different package may name its internal variables
similarly) and not always applied, notably for internal variables.
To be 100% safe without lexical binding, you would have to prefix
ALL your variables with very long package and function name
prefixes.
--
__Pascal Bourguignon__ http://www.informatimago.com/
“The factory of the future will have only two employees, a man and a
dog. The man will be there to feed the dog. The dog will be there to
keep the man from touching the equipment.” -- Carl Bass CEO Autodesk
Re: Real-life examples of lexical binding in Emacs Lisp,
Pascal J. Bourguignon <=
Re: Real-life examples of lexical binding in Emacs Lisp, Emanuel Berg, 2015/05/29
Re: Real-life examples of lexical binding in Emacs Lisp, Rusi, 2015/05/30
Re: Real-life examples of lexical binding in Emacs Lisp, Emanuel Berg, 2015/05/30