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bug#24969: 26.0.50; number-at-point

From: Andreas Röhler
Subject: bug#24969: 26.0.50; number-at-point
Date: Tue, 22 Nov 2016 09:45:01 +0100
User-agent: Mozilla/5.0 (X11; Linux i686; rv:45.0) Gecko/20100101 Icedove/45.4.0

On 22.11.2016 00:07, Drew Adams wrote:
In Emacs 25.1 (emacs -Q), `number-at-point' at either
the `-' or the `1' returns nil, for me.  And I do not
see why it should return a number.

`number-at-point' is defined using `form-at-point' with
THING `sexp' and predicate `numberp'.  The sexp picked
up at point is `foo-1', and that fails `numberp'.
 From the time when i opened Bug#24605, the
implementation, in master branch, of `number-at-point'
was different: it changed in commit 786ab4a5 (Bug#8634).
My patch was driven by this implementation.  I didn't notice
that `number-at-point' behaves different in emacs-25.
It behaves the same in Emacs 25.1 as previously.  If something
broke this after Emacs 25.1 then it should be reverted.

What am I missing?  Why should this rightfully return
a number?  I'm guessing that you are all using a more
recent version of `number-at-point' than what is in
Emacs 25.1 (?).  But to me the Emacs 25.1 behavior I
see (i.e., returning nil) is correct.

Did someone change the meaning of `number-at-point'
so that it now picks up a numeral that is not isolated?
If so, why would that be considered proper behavior?
At the very least it is not backward-compatible behavior.
That's right.  Commit above breaks backward-compatibility.
If it returns a number for point on a numeral in the middle
of a symbol name etc. then it breaks not only backward
compatibility - it breaks the very notion of a number at
point.  A number at point should be a number as delimited
and distinguished in the current mode.

The longstanding definition uses Lisp `read', so it distinguishes
a _Lisp_ number.  It uses what Lisp uses to delimit a numeral.

A better implementation of `number-at-point' than what has
always existed would do this:

1. Get (thing-at-point 'sexp)
2. If it is not a string, return nil.
3. Else match it against a regexp that tests for a numeral
    in the current mode/context.  Or use another such test
    other than regexp matching.  If there the mode/context
    defines numeric syntax then perhaps use a function that
    tests that way.
4. For Lisp, the result must coincide with the longstanding
    behavior, one way or another.

Unless `number-at-point' is extended in such a way, it should
simply be restored to what it has always been.  It's behavior
in Lisp should in any case be to return a Lisp number.

Why? A lisp-number is useful for further computations, for adding them for example.
It's not useful when editing the numbers text at point.

As (thing-at-point 'sexp) internally will get the buffer-substring with the number's literal, why not make is accessible for edits?

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