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

[Tassilo Horn <tsdh@gnu.org>]

Andreas Röhler <andreas.roehler@easy-emacs.de> writes:

> ;;; -*- 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.

You have to byte-compile.  Then you get for the lexical version:

--8<---------------cut here---------------start------------->8---
byte code for my-function:
  doc:   ...
  args: 257
0	constant  nil
1	constant  my-map
2	constant  make-byte-code
3	constant  257
4	constant  "\211\300>?\205\n.\301\302!\207"
5	constant  vconcat
6	constant  vector
7	stack-ref 6
9	call	  1
10	constant  [error "invalid mapping"]
11	call	  2
12	constant  3
13	constant  "\n\n(fn X)"
14	call	  5
15	stack-ref 3
16	call	  2
17	return	  
--8<---------------cut here---------------end--------------->8---

That gives:

  (my-function '(4 5)) => Lisp error: (error "invalid mapping")

For the dynamic version:

--8<---------------cut here---------------start------------->8---
byte code for my-function:
  args: (input)
0	constant  nil
1	varbind	  l
2	constant  my-map
3	constant  <compiled-function>
      args: (x)
    0	    varref    x
    1	    varref    l
    2	    memq      
    3	    not	      
    4	    goto-if-nil-else-pop 1
    7	    constant  error
    8	    constant  "invalid mapping"
    9	    call      1
    10:1    return    

4	varref	  input
5	call	  2
6	unbind	  1
7	return	  
--8<---------------cut here---------------end--------------->8---

Note the varref l.  When the lambda is called in the dynamic version, l
will refer to the l argument from my-map.

That gives:

  (my-function '(4 5)) => (nil nil)

Bye,
Tassilo