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bug#40693: 28.0.50; json-encode-alist changes alist
From: |
Basil L. Contovounesios |
Subject: |
bug#40693: 28.0.50; json-encode-alist changes alist |
Date: |
Sat, 23 May 2020 17:13:57 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) |
João Távora <joaotavora@gmail.com> writes:
> "Basil L. Contovounesios" <contovob@tcd.ie> writes:
>
>>> Am I missing something or doesn't this work like you want?
>>
>> There's a problem with the conditional require at byte-compile time. If
>> the version of Emacs doing the byte-compilation has native JSON support,
>> then it will generate invalid byte-code (and complain) for the case
>> where there is no native JSON support. In other words, there will be a
>> bug when an Emacs without native JSON loads a file that was
>> byte-compiled in an Emacs with native JSON.
>
> This doesn't give any any warning:
>
> (defvar foo-42 42)
>
> (provide 'foo)
> ;; foo.el ends here
>
> and a bar.el
>
> (eval-and-compile
> (defvar bar-have-native-42 t) ;; or nil
>
> (unless bar-have-native-42
> (require 'foo)))
>
> (defalias 'forty-two
> (if (eval-when-compile bar-have-native-42)
> (lambda () (message "%s" 42.0))
> (lambda () (message "%s" foo-42))))
> ;; bar.el ends here
>
> And it seems to work in both cases:
>
> ~/tmp ❯❯❯ emacs -Q --batch -L . -f batch-byte-compile bar.el
> ~/tmp ❯❯❯ emacs -Q --batch -L . -l bar.elc -f forty-two
> 42.0
> # now change that t to nil
> ~/tmp ❯❯❯ emacs -Q --batch -L . -f batch-byte-compile bar.el
> ~/tmp ❯❯❯ emacs -Q --batch -L . -l bar.elc -f forty-two
> 42
>
> But indeed in the recipe I gave you, I had forgotten the second
> eval-when-compile. If you take that away, it warns again.
This is not a representative example for the following reasons:
0. bar.el does not use lexical-binding.
1. The second lambda in forty-two does not let-bind foo-42.
2. If you byte-compile bar.el with bar-have-native-42 set to t, and then
load bar.elc in an Emacs that has bar-have-native-42 set to nil, then
42.0 gets printed, which is wrong. This is due to the incorrect
usage of eval-when-compile: we want the check to happen at runtime as
well.
Try this instead:
;;; foo.el --- foo -*- lexical-binding: t -*-
(eval-and-compile
(unless (fboundp 'json-parse-buffer)
(require 'json)))
(defalias 'foo
(if (eval-when-compile (fboundp 'json-parse-buffer))
(lambda ()
(json-parse-buffer :object-type 'plist
:null-object nil
:false-object :json-false))
(lambda ()
(let ((json-object-type 'plist))
(json-read)))))
;;; foo.el ends here
> No idea how to check if byte-code is "valid" or not: I just check the
> warnings. Can you tell me?
0. emacs -Q -batch -f batch-byte-compile foo.el
1. emacs -Q
2. (fset 'json-parse-buffer nil) C-j
3. M-x load-file RET foo.elc RET
4. (disassemble 'foo) C-j
This prints:
byte code for foo:
args: nil
0 constant plist
1 constant json-read
2 call 0
3 return
whereas I'd expect:
byte code for foo:
args: nil
0 constant plist
1 varbind json-object-type
2 constant json-read
3 call 0
4 unbind 1
5 return
>> Thanks. I've therefore gone with the original patch[1], as it exhibits
>> correct behaviour and is the least intrusive, but you should obviously
>> feel free to tweak it as you prefer.
>
> I think I'll let it be, the reason I'm not a huge fan is the foward
> declarations of individual functions and variables, but it's not so bad.
I think the declarations make the intention explicit to both the reader
and the byte-compiler in a simple way, without wrestling the
eval-*-compile machinery or allowing for subtle bugs like the ones
above.
Thanks,
--
Basil
- bug#40693: 28.0.50; json-encode-alist changes alist, Basil L. Contovounesios, 2020/05/17
- bug#40693: 28.0.50; json-encode-alist changes alist, Eli Zaretskii, 2020/05/18
- bug#40693: 28.0.50; json-encode-alist changes alist, Dmitry Gutov, 2020/05/18
- bug#40693: 28.0.50; json-encode-alist changes alist, João Távora, 2020/05/18
- bug#40693: 28.0.50; json-encode-alist changes alist, Basil L. Contovounesios, 2020/05/21
- bug#40693: 28.0.50; json-encode-alist changes alist, João Távora, 2020/05/21
- bug#40693: 28.0.50; json-encode-alist changes alist, Basil L. Contovounesios, 2020/05/22
- bug#40693: 28.0.50; json-encode-alist changes alist, João Távora, 2020/05/22
- bug#40693: 28.0.50; json-encode-alist changes alist,
Basil L. Contovounesios <=
- bug#40693: 28.0.50; json-encode-alist changes alist, João Távora, 2020/05/23
- bug#40693: 28.0.50; json-encode-alist changes alist, Basil L. Contovounesios, 2020/05/23
- bug#40693: 28.0.50; json-encode-alist changes alist, João Távora, 2020/05/23