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Re: [Emacs-diffs] trunk r116995: cl-lib defstruct introspection

From: Stefan Monnier
Subject: Re: [Emacs-diffs] trunk r116995: cl-lib defstruct introspection
Date: Mon, 21 Apr 2014 18:03:18 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4.50 (gnu/linux)

> That's new. Using the whole ChaneLog message has been a recommendation,
> but never a requirement.

For `elpa', that's true, but for `emacs' it's always been a requirement,
on the premise that this should/will allow us to drop the ChangeLog
files at some point.

> Now there's one more step on the commit path, and a useless one at
> that: the changelog entry is available in the change itself and in the
> message to the mailing list.

C-x v v can copy the message from ChangeLog for you (and set Author:
and Fixes: at the same time), so it's not so bad.

The way to fix this, is to make ChangeLog unneeded.  First step on this
path is to provide some way to make `C-x 4 a' usable without ChangeLog.

>>> +The @code{cl-defstruct} package also provides a few structure
>>> +introspection functions.
>> I'm curious: when/where did you bump against a need for that?
> I have a few private macros that lexically bind structure slots,

Which part makes it impossible/impractical to use standard accessors for

> and this information is also needed for some interface-generation work
> I'm thinking of doing.

Not sure what "interface-generation" means, but it sounds interesting.

>>> address@hidden cl-struct-set-slot-value struct-type slot-name inst value
>> We don't need this, since we can always use setf instead.
> So? We have both (setf (aref ...) ...) and (aset ...).

That's only because (setf (aref ...) ...) needs to macroexpand to something.
[ It's one of the differences between Common-Lisp and Elisp.  ]

In your case, (setf (cl-struct-slot-value ...) ...) can macroexpand to
something without needing cl-struct-set-slot-value.  Actually, in order
for (incf (cl-struct-slot-value ...)) not to compute the offset twice,
(setf (cl-struct-slot-value ...) ...) will end up expanding to something
else than a call to cl-struct-set-slot-value.

> That test was there in cl-check-type. The test doesn't make sense to me
> either. We should drop it in both places if we drop it in cl-the.

Great, let's drop it then.  Thanks.

>>> +(cl-define-compiler-macro cl-struct-slot-value
>> Please use (declare (compiler-macro ..)).
> Why? In both cases, the compiler macro is written out-of-line and in
> both cases, we just stick the compiler macro on the symbol's plist.

Because that's the style we use in Elisp.
Note that (declare (compiler-macro ..)) can provide the compiler-macro
"inline" or "out-of-line".

>> I guess this goes back to the earlier question about when/where the use
>> for this functionality came up.
> Unless we're using this functionality in generated code where, while the
> slot is constant, it's more convenient to use that slot's name than to
> try to determine the accessor name.

Ah, so it's for code generated based on cl-struct-slot-info?
Right, that makes sense.


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