Alan Mackenzie <
address@hidden> schrieb am Di., 14. Apr. 2015 um 00:03 Uhr:
Hello, Emacs.
I recently tried out M-x auto-insert in a file.el, and it caused a
testing macro to fail. The critical point was the default setting of
lexical-binding to t in the first line.
Reduced to its essentials, my problem is a file looking like this:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; lexical-bug.el --- lexical bug -*- lexical-binding: t; -*-
(eval-when-compile
(defmacro test-ptr (x)
`(let* ((ptr (copy-tree ,x))
(form '(setcar ptr 'a))
(result (eval form)))
(message "result is %s, ptr is %s" result ptr))))
(test-ptr '(x y z))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
When I byte-compile it, I expect on loading it to get the result message
in the message area. This works fine without the lexical binding. With
the LB, instead I get an error about `ptr' being unbound - clearly, in
the form `(eval form)', i.e. `(eval `(setcar ptr 'a))', there is no `ptr'
in `eval''s stack frame.
Obviously, this bit of the code needs dynamic binding. So I should be
able to bind `lexical-binding' to nil at some strategic place to achieve
this. I'm not quite sure where this place is, but nowhere seems to
work. For example, if I change the file to the following:
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; lexical-bug.el --- lexical bug -*- lexical-binding: t; -*-
(setq lexical-binding nil) ; 1
(eval-when-compile
(let (lexical-binding) ; 2
(defmacro test-ptr (x)
(let (lexical-binding ; 3
)
`(let* (lexical-binding ; 4
(ptr (copy-tree ,x))
(form '(setcar ptr 'a))
(result (eval form)))
(message "result is %s, ptr is %s" result ptr))))))
(eval-when-compile (setq lexical-binding nil)) ; 5
(setq lexical-binding nil) ; 6
(test-ptr '(x y z))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
, I still get an error with `ptr' being unbound.
What is going on here? Why is the byte compiler continuing to use
lexical binding despite the variable being set to nil in six different
ways?
IIUC only the local variable is relevant. At least that's the case for loading, and it would make sense for byte compilation as well. Byte compilation transforms the lexical binding into a stack access and gets rid of the 'ptr' symbol entirely.
What do I have to do to get `ptr' in this context a special variable
(whilst still having it lexically bound in other code)?
The correct way is to avoid eval and use a closure instead:
(let* ((ptr (copy-tree ,x))
(form (lambda () (setcar ptr 'a)))
(result (funcall form)))
(message "result is %s, ptr is %s" result ptr))
Not only will this work with lexical binding, the byte compiler is able to provide better error messages (e.g. if you try to call an undefined function in the form) and enable additional optimizations.