Iñigo Serna <inigoserna@gmail.com> writes:
> Hi,
>
> Pascal J. Bourguignon writes:
>>> ; I can't use a variable inside the success lambda function of a web
>>> request call.
>>> ; As this is difficult to explain I'll show some things I've tried.
>>
>> Add:
>> (setf lexical-binding t)
>> before fn1, and:
>> ;; -*- mode:emacs-lis;lexical-binding:t -*-
>> as the first line of all your .el files.
>
> Thanks a lot Pascal, that makes fn1 `work'!
> And to learn more myself, do you have any hint why `fn3' or `fn4' don't work
> and how to make
> it work?
>
> Thanks,
> Iñigo Serna
>
Use pp macroexpand to see what your macro calls expand to:
(pp (macroexpand '(my-request2 "https://api.stackexchange.com/2.1/questions";
#'(lambda (data &optional d)
(let* ((item (elt (assoc-default 'items data) 0))
(title (assoc-default 'title item))
(tags (assoc-default 'tags item)))
(message "PARAMS: %S" d)
(message "=> %s %S" title tags)
(message "LAST: %S" (nth 2 d))))
'("OOOO" "AAAA" c))))
(request "https://api.stackexchange.com/2.1/questions"; :params
'((order . "desc")
(sort . "activity")
(site . "stackoverflow"))
:parser 'json-read
:success
(function*
(lambda
(&key data &allow-other-keys)
(funcall
#'(lambda
(data &optional d)
(let*
((item
(elt
(assoc-default 'items data)
0))
(title
(assoc-default 'title item))
(tags
(assoc-default 'tags item)))
(message "PARAMS: %S" d)
(message "=> %s %S" title tags)
(message "LAST: %S"
(nth 2 d))))
data
'("OOOO" "AAAA" c)))))
>From this, you can see that the parameter c of fn3 is never used
(you just pass the symbol c to your callback).
Similarly, for fn4:
(pp (macroexpand '(my-request2 "https://api.stackexchange.com/2.1/questions";
#'(lambda (data &optional d)
(let* ((item (elt (assoc-default 'items data) 0))
(title (assoc-default 'title item))
(tags (assoc-default 'tags item)))
(message "PARAMS: %S" d)
(message "=> %s %S" title tags)
(message "LAST: %S" (nth 2 d))))
(list "OOOO" "AAAA" c))))
(request "https://api.stackexchange.com/2.1/questions"; :params
'((order . "desc")
(sort . "activity")
(site . "stackoverflow"))
:parser 'json-read
:success
(function*
(lambda
(&key data &allow-other-keys)
(funcall
#'(lambda
(data &optional d)
(let*
((item
(elt
(assoc-default 'items data)
0))
(title
(assoc-default 'title item))
(tags
(assoc-default 'tags item)))
(message "PARAMS: %S" d)
(message "=> %s %S" title tags)
(message "LAST: %S"
(nth 2 d))))
data
(list "OOOO" "AAAA" c)))))
You can see that your macro expands to a call to request with as
argument an anonymous function that refers a free variable named c.
However, without lexical-binding, variables are dynamic.
lexical = space = where
dynamic = time = when
Therefore WHEN this anonymous function is called, it may happen that no
variable named c exist anymore. Or worse, than another variable named c
THEN exist that is not the one that existed when your function fn4 was
executing!
In the context of Common Lisp, here is an explaination of
lexical/dynamic and global/local
https://groups.google.com/forum/#!msg/comp.lang.lisp/4VyopdWcFI4/1sDQU-3H8VgJ
Google also about the FUNARG problem.
--
__Pascal Bourguignon__ http://www.informatimago.com/
âThe 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.â -- Carl Bass CEO Autodesk
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