Re: lists with nil play nice for Lua 5.2

[Irayo <irayo.lt@...>]

Norman Ramsey wrote:
> Perhaps Lua 5.2 can address the policy disorder by
> providing a value 'table.hole' whose sole purpose is to be distinct
> from every other value (including nil) :-)
>   

This makes the most sense to me out of any of the other solutions.

Nil in tables is not "nothingness"--it means, technically, that the key 
is non-existent.  So, asking Lua if a specific table element is nil is, 
in essence, asking if that element is non-existent.  Setting a table 
element to nil specifically is just *telling* Lua that the element 
doesn't exist.  Lua doesn't handle non-existent elements because they 
*don't exist*.  So you can't expect to be able to tell Lua that an 
element doesn't exist and then have it know that you meant that the 
element existed, but was blank.

People use the value 'nil' in such a way as to say "nothing".  'blah = 
nothing'.  But if you're writing that with a nil, you're telling Lua 
that blah doesn't exist.  A "hole" or "blank" value, as Norman 
suggested, addresses this by being able to differentiate between

t[1] = nil;  -- a nonexistant element, and
t[2] = table.hole;  -- an element that exists, but has no properly 
defined value

#t = 2, because 'hole' is has a value.  Of course, I rather think we 
could come up with a more friendly name than "hole"--like maybe 
"blank".  I dunno.

Users can continue using nil if they want, and use all the work-arounds 
to have iterators work in sparse tables, which means it's fully 
backward-compatible.  Nothing changes except the addition of another 
element to the table library (as far as I know).  Other users can 
properly use new functions.

It may also be useful to define a function in such a way that  f(a) is 
"(a==nil) or (a==table.hole)", so that you can check whether a value is 
nil *or* a hole.

As always, I'm not a Lua expert and I don't know what most people in the 
community are looking for; this just seems to be a very simple and 
elegant solution to a long-lasting problem.  Feel free to get all up in 
my grill if you disagree--I want to see what others think.

--
Irayo