Re: Feature request: allow nil as table key

[Duncan Cross <duncan.cross@...>]

On Wed, Jun 10, 2009 at 6:16 PM, Jerome Vuarand<jerome.vuarand@gmail.com> wrote:
> Hi,
>
> Following some recent comments about the next version of Lua, I
> remembered a proposal I did on IRC (as doub) but forgot to post here.
>
> It would be nice to allow nil as a key in Lua tables. I don't see any
> syntactic or semantical reason to prevent it, and I'm sure any
> technical reason in the Lua sources can be overcome. It would have the
> same semantics as all other immutable values. The initial/default
> value of the nil key would be nil. It would then be read with "t[nil]"
> and changed with "t[nil] = value". It would be usable in a table
> constructor as "t = {[nil] = value}". Indexing with nil would trigger
> the __index and __newindex metamethods.
>
> The current behaviour could be emulated with a __newindex metamethod
> that raise an error when passed a nil second parameter.
>
> I think that modification would improve the overall consistency of
> Lua. It would save us from handling the current special case when the
> current behaviour is not needed.
>
> Please comment (and I'd love to have the authors opinion on the
> subject). I've never modified the Lua sources this extensively, but I
> could give it a try if a patch would help the feature being
> incorporated in the next version.
>

It is quite an important sentinel value for using next() directly
(which I do fairly often, certainly more often than I've ever felt the
need to store a value for the nil key in a table) - next(t, nil) could
no longer be used to fetch the first pair, and (next(t, nil) == nil)
would no longer indicate that t is an empty table.

-Duncan