bug#30749: 26.0.91; (cl-)case is confused when a nil atom is used instead of a keylist

bug#30749: 26.0.91; (cl-)case is confused when a nil atom is used instead of a keylist

[Alex Bennée]

I got confused why my case statement was failing as I'd used the second
form assuming that having nil as an atom would work. I'm not sure why
the quoting fixed it but it's certainly a confusing inconsistency. The
following behaviour shows up on emacs -Q:

(require 'cl)

(case nil
  ((nil) "nil")
  ((t) "default"))
"nil"

(case nil
  (nil "nil")
  (t "default"))
"default"

(case nil
  ('nil "nil")
  ('t "default"))
"nil"

(case t
  ('nil "nil")
  ('t "default"))
"default"

I think this is probably fixable with just making the docstring clearer
as the results of handling the duel atom/list of nil might have
unexpected results.

In GNU Emacs 26.0.91 (build 16, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2018-03-06 built on zen
Repository revision: 0efe0bd233de20bfb5bd9d06b255fc8ecf04602b
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:	Ubuntu 16.04.3 LTS

--
Alex Bennée

bug#30749: 26.0.91; (cl-)case is confused when a nil atom is used instead of a keylist

[Noam Postavsky]

Alex Bennée <alex.bennee@linaro.org> writes:

> I got confused why my case statement was failing as I'd used the second
> form assuming that having nil as an atom would work. I'm not sure why
> the quoting fixed it but it's certainly a confusing inconsistency.

'nil is the same as (quote nil)

> I think this is probably fixable with just making the docstring clearer
> as the results of handling the duel atom/list of nil might have
> unexpected results.

So just add a "non-nil" perhaps?

--- i/lisp/emacs-lisp/cl-macs.el
+++ w/lisp/emacs-lisp/cl-macs.el
@@ -763,7 +763,7 @@ cl-case
   "Eval EXPR and choose among clauses on that value.
 Each clause looks like (KEYLIST BODY...).  EXPR is evaluated and compared
 against each key in each KEYLIST; the corresponding BODY is evaluated.
-If no clause succeeds, cl-case returns nil.  A single atom may be used in
+If no clause succeeds, cl-case returns nil.  A single non-nil atom may be used in
 place of a KEYLIST of one atom.  A KEYLIST of t or `otherwise' is
 allowed only in the final clause, and matches if no other keys match.
 Key values are compared by `eql'.

bug#30749: 26.0.91; (cl-)case is confused when a nil atom is used instead of a keylist

[Alex Bennée]

Noam Postavsky <npostavs@gmail.com> writes:

> Alex Bennée <alex.bennee@linaro.org> writes:
>
>> I got confused why my case statement was failing as I'd used the second
>> form assuming that having nil as an atom would work. I'm not sure why
>> the quoting fixed it but it's certainly a confusing inconsistency.
>
> 'nil is the same as (quote nil)

Ahh so ends up as a list containing nil?

>
>> I think this is probably fixable with just making the docstring clearer
>> as the results of handling the duel atom/list of nil might have
>> unexpected results.
>
> So just add a "non-nil" perhaps?
>
> --- i/lisp/emacs-lisp/cl-macs.el
> +++ w/lisp/emacs-lisp/cl-macs.el
> @@ -763,7 +763,7 @@ cl-case
>    "Eval EXPR and choose among clauses on that value.
>  Each clause looks like (KEYLIST BODY...).  EXPR is evaluated and compared
>  against each key in each KEYLIST; the corresponding BODY is evaluated.
> -If no clause succeeds, cl-case returns nil.  A single atom may be used in
> +If no clause succeeds, cl-case returns nil.  A single non-nil atom may be used in
>  place of a KEYLIST of one atom.  A KEYLIST of t or `otherwise' is
>  allowed only in the final clause, and matches if no other keys match.
>  Key values are compared by `eql'.

Works for me.

--
Alex Bennée

bug#30749: 26.0.91; (cl-)case is confused when a nil atom is used instead of a keylist

[Noam Postavsky]

tags 30749 fixed
close 30749 26.1
quit

Alex Bennée <alex.bennee@linaro.org> writes:

> Noam Postavsky <npostavs@gmail.com> writes:
>
>> 'nil is the same as (quote nil)
>
> Ahh so ends up as a list containing nil?

Yup.

>> -If no clause succeeds, cl-case returns nil.  A single atom may be used in
>> +If no clause succeeds, cl-case returns nil.  A single non-nil atom may be used in

> Works for me.

Pushed to emacs-26.

[1: e244fed5e0]: 2018-03-10 17:56:04 -0500
  Clarify that nil doesn't match itself as a cl-case clause (Bug#30749)
  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=e244fed5e041c706dd10c60bd893634902f04aaa