About equality in Emacs

About equality in Emacs

[Xue Fuqiao]

In the trunk version of (info "(cl) Equality Predicates"):

   Also note that the Common Lisp functions `member' and `assoc' use
`eql' to compare elements, whereas Emacs Lisp follows the MacLisp
tradition and uses `equal' for these two functions.  In Emacs, use
`memq' (or `cl-member') and `assq' (or `cl-assoc') to get functions
which use `eql' for comparisons.

I'm confused with the last sentence.  Don't `memq' and `assq' compare
objects with `eq'?  Why does the manual say that they "get functions
which use `eql' for comparisons"?

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

Re: About equality in Emacs

[Pascal J. Bourguignon]

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

> In the trunk version of (info "(cl) Equality Predicates"):
>
>    Also note that the Common Lisp functions `member' and `assoc' use
> `eql' to compare elements, whereas Emacs Lisp follows the MacLisp
> tradition and uses `equal' for these two functions.  In Emacs, use
> `memq' (or `cl-member') and `assq' (or `cl-assoc') to get functions
> which use `eql' for comparisons.
>
> I'm confused with the last sentence.  Don't `memq' and `assq' compare
> objects with `eq'?  

Perhaps.  But in Common Lisp, it would be EQL, since (eq 1 1) or (eq #\a
#\a) can return NIL. (member 1 '(1 2 3) :test (function eq)) is not
conforming in Common Lisp.

In emacs lisp, the difference between eq and eql is for floating point
numbers, where (eq 1.0 1.0) --> nil  while  (eql 1.0 1.0) --> t

In emacs-version "24.2.1":
  (memq 1 '(1 2))       --> (1 2)
  (memq 1.0 '(1.0 2.0)) --> nil 
memq uses eq, so it's no good on floating point numbers.


> Why does the manual say that they "get functions
> which use `eql' for comparisons"?

The sentence is: "In Emacs, use `memq' (or `cl-member') and `assq' (or
`cl-assoc') to get functions which use `eql' for comparisons."

The original sentence was: "In Emacs, use `member*' and `assoc*' to get
functions which use `eql' for comparisons."

The original meant that if you want to compare with eql, then instead of
using member or assoc, you would use member* and assoc*.

The new version indeed is hard to understand.  Does it mean that memq
and assq now use eql?  I'd be surprised if it did.

    New emacs:                     Old emacs:
    memq assq         ?  eq        memq assq
    member assoc         equal     member assoc
    cl-member cl-assoc   eql       member* assoc*


-- 
__Pascal Bourguignon__                     http://www.informatimago.com/
A bad day in () is better than a good day in {}.

Re: About equality in Emacs

[Michael Heerdegen]

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

> In the trunk version of (info "(cl) Equality Predicates"):
>
>    Also note that the Common Lisp functions `member' and `assoc' use
> `eql' to compare elements, whereas Emacs Lisp follows the MacLisp
> tradition and uses `equal' for these two functions.  In Emacs, use
> `memq' (or `cl-member') and `assq' (or `cl-assoc') to get functions
> which use `eql' for comparisons.
>
> I'm confused with the last sentence.  Don't `memq' and `assq' compare
> objects with `eq'?  Why does the manual say that they "get functions
> which use `eql' for comparisons"?

Doesn't make much sense to me as well.  I filed a bug report; it's now
bug#13620.  I think this sentence was just damaged when it was updated.


Regards,

Michael.

Re: About equality in Emacs

[Xue Fuqiao]

On Sun, 03 Feb 2013 12:00:44 -0500
Pascal J. Bourguignon wrote:

> Perhaps.  But in Common Lisp, it would be EQL, since (eq 1 1) or (eq #\a
> #\a) can return NIL. (member 1 '(1 2 3) :test (function eq)) is not
> conforming in Common Lisp.

Thanks for your reply, it is a bug and it will be fixed in GNU Emacs 24.2.93.  BTW, What does `#\a' mean?

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

RE: About equality in Emacs

[Drew Adams]

> What does `#\a' mean?

That's Common Lisp character syntax, in this case for the character `a' (?a in
Emacs Lisp).

Re: About equality in Emacs

[Michael Heerdegen]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> > I'm confused with the last sentence.  Don't `memq' and `assq' compare
> > objects with `eq'?  Why does the manual say that they "get functions
> > which use `eql' for comparisons"?
>
> Doesn't make much sense to me as well.  I filed a bug report; it's now
> bug#13620.  I think this sentence was just damaged when it was updated.

Has already been fixed by Glenn Morris.  This is the new version of the
paragraph:

,----------------------------------------------------------------------
|    Also note that the Common Lisp functions `member' and `assoc' use
| `eql' to compare elements, whereas Emacs Lisp follows the MacLisp
| tradition and uses `equal' for these two functions.  The functions
| `cl-member' and `cl-assoc' use `eql', as in Common Lisp.  The standard
| Emacs Lisp functions `memq' and `assq' use `eq', so you can use these
| if you do not care about the difference between `eq' and `eql'.
| 
`----------------------------------------------------------------------

Thanks Xue for finding this.


Regards,

Michael.