Re: new "empty" value/type in Lua?

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

2013/7/1 Roberto Ierusalimschy <roberto@inf.puc-rio.br>:

> Much more serious, for me, is the incompatibility that t[#t] = nil will
> not "pop" elements form a list (and. of course, the extra complexity).

Exactly. Deletion by assigning nil is too ingrained in existing Lua
programs to be replaced now.

The need is not for nil to change its nature. The need is for table keys
to be classifiable as legal or illegal before they are used. The length
function would work on legal keys, not on keys in use. The proposed 'has'
function would simply distinguish between legal and illegal keys.

One possible mechanism could be a metatable entry '__has'. If this
is nil, we are back in the present situation, so no incompatibility.
If this is present, __newindex will not assign and __index will not
retrieve illegal keys.

If `__has` is a table, then 'has(tbl)' looks in it for the key. If it
is a function, 'has(tbl)' calls it and returns the result. If it is
a number, 'has' accepts all integer keys between 1 and `__has` and
# returns `__has`. To create a fixed-size, no-holes table that can
take 100 entries, all you need to do is

   tbl=setmetatable({},{__has=100})

By all means offer syntactic sugar `tbl=array(100)` for this.  The
array part would be allocated to take 100 entries. Nil would still
be used to identify currently unused keys, but the array would not be
shrunk even if less than half full.

The proposed 'delete' function does not seem to be as useful. Just
deleting the present value can be done by assigning nil, and permanently
changing the legality of a key should not be as easy as calling a
standard library function. It should require explicit reassigning of
`getmetatable(tbl).__has`.