Suggestion: Deprecate "Attempt to index a nil value" error; instead, return nil or create table

[Anton Jordaan <anton.jordaan@...>]

Lua is memory efficient for huge but sparse multi-dimensional arrays, 
since nil values aren't stored in the tables.

However, reading and writing to such sparse multi-dimensional arrays can 
be quite a hassle.  If I want to access the value of
t[a][b][c][d][e], I first have to check whether t[a] is a table, t[a][b] 
is a table, t[a][b][c] is a table etc, otherwise Lua throws an error 
"Attempt to index a nil value".


For example, to read the value at t[a][b][c][d][e], I cannot simply use:
v = t[a][b][c][d][e]
Instead, the code must look something like:
v = t and t[a] and t[a][b] and t[a][b][c] and t[a][b][c][d] and 
t[a][b][c][d][e]


Or, to write a value to t[a][b][c][d][e], I cannot simply use:
t[a][b][c][d][e] = v

Instead, the code must look something like:

if not t then t = {[a] = {[b] = {[c] = {[d] = {[e] = v}}}}}
elseif not t[a] then t[a] = {[b] = {[c] = {[d] = {[e] = v}}}}
elseif not t[a][b] then t[a][b] = {[c] = {[d] = {[e] = v}}}
elseif not t[a][b][c] then t[a][b][c] = {[d] = {[e] = v}}
elseif not t[a][b][c][d] then t[a][b][c][d] = {[e] = v}
else t[a][b][c][d][e] = v
end

I suggest that it would be more useful -- and more consistent -- if the 
"Attempt to index a nil value" error is deprecated and, instead, the 
indexing of undefined variables does the following:

When reading: simply return nil.  Given that undefined variables and 
non-existent table entries both return nil, I think it would be more 
consistent if an attempt to index an undefined variable also simply 
returns nil.  (The Lua FAQ states that: "In many languages, trying to 
access a non-existent key in a list or dictionary causes an exception; 
in Lua the result is simply nil. This is unambiguous because nil cannot 
be usefully put into tables.")  If t is nil, then t[a] should also 
simply be nil, as should t[a][b][c][d][e].

When writing: automatically assign a single-entry table to each 
undefined variable, with the given index the sole entry.  For example, 
if t is an undefined variable, then t[a] = v would be syntactic sugar 
for t = {[a] = v}.  Similarly, if t[a][b] is already declared as a table 
but t[a][b][c] is nil, then t[a][b][c][d][e] = v would mean t[a][b][c] = 
{[d] = {[e] = v}}.


-- Anton