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bug#39557: 27.0.60; Elisp manual, doc about bignums

From: Paul Eggert
Subject: bug#39557: 27.0.60; Elisp manual, doc about bignums
Date: Mon, 17 Feb 2020 15:52:56 -0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1

On 2/17/20 3:19 PM, Drew Adams wrote:

this doc should probably also mention that the numerical value of
a marker is an integer
It already says "Many of the functions described in this chapter accept
markers for arguments in place of numbers.... When the argument value
is a marker, its position value is used and its buffer is ignored."

Not really the same thing.  Nothing there says that
a marker position is an integer (fixnum or bignum),
and not some other kind of number.

That's clear from context. This section is titled "Integer Basics" and it's all about integers.

Personally I'd even trim some text from the above quote, as it's not basic to integers. I would rather not make that text longer or trickier.

if you compare an integer
against an integer numeral then you had better use
`eql', unless you know that both are fixnums

No, it suffices if *either* is a fixnum. For example, (eq 0 FOO) tests
FOO is the integer zero, and works regardless of whether FOO is a bignum.

I see.  Then please say that.

I'd rather not. Again, this section is "Integer Basics" and the reference manual should not bog itself down various possible ways to use integers in programs (there are too many ways).

If we're going to talk about "older" code then
we should specify older than what.

I originally wrote "older than Emacs 27" but trimmed it as being nonessential before installing the patch. It's not a big deal either way.

I don't
think there should be any need to talk about
older code or say "should now".

This bug report assumed that Emacs is basically like Common Lisp in this area. However, Emacs is not there yet (though we've made progress), and it's better if the documentation reflects that fact rather than pretending there's no difference from Common Lisp.

Any code -
old or new - that uses `eq' to compare
integers needs to know that at least one of
the operands is a fixnum.

It's sometimes OK to use eq even when both arguments are bignums. It depends on the circumstances.

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