About setf extensions

About setf extensions

[Xue Fuqiao]

In (info "(cl) Setf Extensions"):

     Most of these have directly corresponding "set" functions, like
     `use-local-map' for `current-local-map', or `goto-char' for
     `point'.  A few, like `point-min', expand to longer sequences of
     code when they are used with `setf' (`(narrow-to-region x
     (point-max))' in this case).

I don't understand the `point-min' example.  What does the "x" mean
here?  I know it is the START argument of `narrow-to-region', but where
is it from?  And where will it be used?  Thanks.

-- 
Best regards, Xue Fuqiao.
http://www.emacswiki.org/emacs/XueFuqiao

Re: About setf extensions

[Michael Heerdegen]

Xue Fuqiao <xfq.free@gmail.com> writes:

> In (info "(cl) Setf Extensions"):
>
>      Most of these have directly corresponding "set" functions, like
>      `use-local-map' for `current-local-map', or `goto-char' for
>      `point'.  A few, like `point-min', expand to longer sequences of
>      code when they are used with `setf' (`(narrow-to-region x
>      (point-max))' in this case).
>
> I don't understand the `point-min' example.  What does the "x" mean
> here?  I know it is the START argument of `narrow-to-region', but where
> is it from?  And where will it be used?  Thanks.

I guess it's the value the generalized variable `(point-min)' should be
set to.  I.e.,

  (setf (point-min) x)

is translated to

  (narrow-to-region x (point-max))

This seems reasonable, because afterwards, (point-min) will give x.


Regards,

Michael.

Re: About setf extensions

[Stefan Monnier]

>      Most of these have directly corresponding "set" functions, like
>      `use-local-map' for `current-local-map', or `goto-char' for
>      `point'.  A few, like `point-min', expand to longer sequences of
>      code when they are used with `setf' (`(narrow-to-region x
>      (point-max))' in this case).

> I don't understand the `point-min' example.  What does the "x" mean
> here?

(setf (point-min) x) gets turned into (narrow-to-region x (point-max)).


        Stefan