Re: NaN in Lua 5.3 Reference Manual

[Dirk Laurie <dirk.laurie@...>]

2017-06-23 12:41 GMT+02:00 John Hind <john.hind@zen.co.uk>:

> I notice that Lua 5.3 Reference Manual has been extensively edited over the
> last few point releases, but one aspect that is still not very well
> described is NaN. There are three references to it, all tangential. Two
> state that it is not a valid table key while the third describes its
> treatment by comparison operators. The thing itself is never properly
> introduced.

NaN is an IEEE-754 subtlety and no more worthy of elaboration in
the Lua manual than a discussion of why n*(1/n)==1 might for some
values of n be false. The properties mentioned in the manual are
precisely the ways in which it impinges on essential Lua issues.

It does fall within the scope of an auxiliary glossary [1] for careful readers,
and there I wrote the following:

NaN
---

Acronym for "Not A Number". An exceptional value of type [number] (also
written 'NAN' and 'nan'), which arises as the result of an undefinable
arithmetic [operation]. NaN is not a Lua keyword, but appears in
output (in some implementations, even the nonsensical '-nan' may
be printed). If you need the notation NaN, assign such
a value to the name.

NaN is not a legal [key] in a table, but counts as [true](truth).

    NaN = 0/0
    NaN + 1        -- NaN. All arithmetic operations involving NaN have
                   -- result NaN.
    NaN <= NaN     -- false. All comparison operations involving NaN have
                   -- result false except the following.
    NaN ~= NaN     -- true. The only Lua value with this property.

[1] https://rawgit.com/dlaurie/lua-notes/master/glossary.html