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Re: Example use of macro to minimize and generalize the code
From: |
Jean Louis |
Subject: |
Re: Example use of macro to minimize and generalize the code |
Date: |
Tue, 1 Jun 2021 21:12:50 +0300 |
User-agent: |
Mutt/2.0.7+183 (3d24855) (2021-05-28) |
* Yuri Khan <yuri.v.khan@gmail.com> [2021-06-01 20:58]:
> On Wed, 2 Jun 2021 at 00:27, Jean Louis <bugs@gnu.support> wrote:
>
> > I have removed `id' argument and now I have:
> >
> > (macroexpand '(when-tabulated-id "people" (ignore)))
> > ⇒ (if id (progn (ignore))
> > (if (or (eq "people" 'any) (string-equal "people" rcd-current-table))
> > (let ((id (tabulated-list-get-id)))
> > (if id (progn (ignore)) (message "Did not get ID")))
> > (message "This function is for table `%s' only" "people")))
> >
> > If I would use ,d it would expand into number or nil, it seem it would end
> > up same.
> >
> > Now I don't have compiler warnings, and functions work fine.
> >
> > Let me know if I am making errors in this.
>
> Yes you are. Your macro expands to a form that refers to a symbol
> named “id” from outer scope. Next, there are several possibilities:
>
> * The symbol is not defined, in which case the result of expansion
> will signal (void-variable id):
>
> (when-tabulated-id "people" (ignore))
> ⇒ Debugger entered--Lisp error: (void-variable id)
Macro is exclusively used in functions as this:
(defun my-fun (&optional id)
""
(interactive)
(when-tabulated-id "hyobjects"
))
> * The user of your macro has to define a variable named “id” globally
> or buffer-locally. This is bad because the name “id” is not
> namespaced. In your own package, any variable names you define should
> start with a prefix unique to your package.
Do you mean all, or just global variables to have prefix?
> (setq id t)
> (when-tabulated-id "people" (ignore))
> ⇒ nil
I understand, just macro is used exclusively in functions as above. Is
that alright?
> * The user has to use your macro in a context where “id” is bound as a
> local variable or a function argument. If that’s your intent, you
> should document it in the docstring of the macro, but it’s fragile
> because nobody expects forms to behave this way.
>
> (let ((id t))
> (when-tabulated-id "people" (ignore)))
> ⇒ nil
>
> (defun foo (id)
> (when-tabulated-id "people" (ignore)))
> (foo t)
> ⇒ nil
(◍•ᴗ•◍) that can be, but that is exactly how I expected it to
behave. As the ID is either obtained programmatically which makes the
macro work or from (tabulated-list-get-id)
> * Lastly, you might have intended this macro to be used only by you.
Yes, and I see difficulties in future if somebody else wish to use it.
> In that case, it should be namespaced to your package with a double
> hyphen after the prefix. Anyway, three month later, you forget that
> the macro depends on “id” and you stare at your (let ((id t)) …) or
> (defun foo (id) …) which doesn’t obviously use “id” and remove the
> binding. Suddenly everything breaks.
Breaking I can also understand.
Macro or not macro my goal is to minimize programming, as 400+
functions are asking for (tabulated-list-get-id) and the ID can be in
different tables.
Maybe it can be solved without any macro.
--
Jean
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