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

[Scott Fial <scott@...>]

Just another array idea to clutter the discussion...

Provide a syntax to specify an explicit array-length for a table t, e.g.:

  #t = 100

Once an explicit array-length has been set for t, let it take on the 
following new behaviors:

  1. type(t) shall return "array"

  2. #t shall return the array-length rather than lua's internally
     computed length (although the internal length is still
     maintained)

  3. For any integer index i > array-length, the assignment t[i] = v
     shall have the side-effect of setting array-length to i (even
     when v is nil).

  4. ipairs(t) and unpack(t) shall iterate over the entire range 1..#t

  5. Setting the array-length to nil shall remove the explicit
     length and restore the table to normal behavior.


EXAMPLE SESSION

Supposing that the parser is modified to allow the # operator in an 
l-value context as a syntax for setting an explicit length for a table 
(I have no idea if this is feasible), here's an example interactive session:

  > t = {1, 2, 3}
  > print(type(t), #t)
  table   3

  > #t = 5        -- set explicit array-length
  > print(type(t), #t)
  array   5

  > for i, v in ipairs(t) do print(i, v) end
  1   1
  2   2
  3   3
  4   nil
  5   nil

  > #t = 2        -- array-length explicitly decreased
  > print(unpack(t))
  1   2

  > t[6] = 6      -- array-length implicitly increased
  > t[7] = nil    -- and again
  > print(unpack(t))
  1   2   3   nil   nil   6   nil

  > #t = nil      -- restore normal behavior
  > print(type(t), #t)
  table   3


Scott