Subject: bug#34708: alist-get has unclear documentation

From: Eric Abrahamsen <[email-masked]>
Date: Sat, 02 Mar 2019 11:06:39 -0800

Michael Heerdegen <[email-masked]> writes:

> "Miguel V. S. Frasson" <[email-masked]> writes:
>
>> I can't imagine how to *set* anything with alist-get. It seams to me
>> that it just use the value of ALIST for look up, so talk about
>> generalized variables is meaningless to me here.
>
> You use it like this: say variable V is bound to an alist, then you can
> do (setf (alist-get key V) value).  After that, (alist-get key V) will
> evaluate to VALUE, so you have "set" that place.  In the general case, V
> can also be a generalized variable, e.g. (car SOMETHING-ELSE).
>
> To replace the word "this" with something better is not so easy.  We
> could write "The name of this function can be used to build expressions
> that can be used as a generalized variable", but I doubt it will make
> things clearer for somebody not familiar with the concept of generalized
> variables.  Using this function name to build place expressions is not
> different from using other function names that allow to be used for
> generalized variables.

One other phrase you often see here is "setf-able place". I don't know
if that's formally acceptable in docstrings, but it would be much more
comprehensible to say "this form is a setf-able place", and would give
the key hint (setf) as well. It's true it's pretty weird to refer to a
function call as a variable.

> I would rather go with an example, which I think is justified because
> using this function name in place expressions is the canonical way to
> modify alists and people need to use it (there is no `alist-put') no
> matter if they are familiar with generalized variables.

Most definitely, this needs examples. I also agree that the REMOVE usage
needs an example -- I made it work eventually, but it took a fair bit of
experimentation.

Eric