Re: Empty? No. Array? No. Has? Yes.

[Eike Decker <zet23t@...>]

2013/7/3 Coda Highland <chighland@gmail.com>:
> On Wed, Jul 3, 2013 at 12:39 PM, Eike Decker <zet23t@googlemail.com> wrote:
>>
>> Am 03.07.2013 20:46 schrieb "Roberto Ierusalimschy"
>> <roberto@inf.puc-rio.br>:
>>>
>>> If we add a new value in Lua (call it null, empty, nothing, whatever)
>>> to represent null in JSON, the problem with JSON is solved, because
>>> we do not change JSON. This change is trivial to do, but only solves
>>> the JSON problem.
>>>
>>> If we add a new value in Lua (call it null, empty, nothing, whatever)
>>> to represent null in *Lua*, the problem is not solved. Now Lua has a new
>>> value, and therefore we need yet another value to represent this new
>>> Lua value. Adding new values will not solve the problem in Lua. Period.
>>>
>>> -- Roberto
>>>
>>
>> Just an unfinished premature loud thought: what if nil is a type with
>> different states similar to Boolean that can be true or false? Maybe this is
>> a very stupid idea but I want to elaborate on it a bit...
>>
>> Setup:
>> The states could be nil(defined)/nil(undefined). Both values are very
>> special:
>> nil(defined) == nil(undefined) -- yes, this results in "true"
>> a,b = nil -- a is nil(defined), b is nil(undefined)
>>
>> Actually, both nil states are indistinguishable on a syntax level - the
>> distinguishing can only happen through a special function that can tell the
>> difference, nothing else can. This somewhat similar to the SQL concept where
>> null is checked by doing "is null" and not "= null", which is false (if I
>> recall correctly).
>>
>> Consequences:
>> I think the result of this logic could be disastrous.... But maybe this
>> crazy logic still makes sense:
>>
>>   a,b = nil
>>   function f(a,b,c)
>>     -- if called this way: f(a,b), a is nil(defined), b and c are
>> nil(undefined)
>>     -- if called this way: f(a,b or nil), a and b are nil(defined), c is
>> nil(undefined)
>>   end
>>
>> Interesting is also the case for tables: if a value is nil(defined), it
>> continues to be present in the table and the slot is NOT removed. Thus:
>>
>>   function delete() end -- returns nil(undefined)
>>   t = { a=1, b=nil, c=delete()}
>>   t.a = delete()
>>   -- status: t.b is nil(defined), t.a and t.c are nil(undefined) and thus do
>> not exist
>>
>> Consequently, popping an array element works by doing "t[#t] = delete()" or
>> "_,t[#t] = nil"  - which is interestingly quite close to my last suggestion
>> of storing nil values in tables. Also interesting is, that arrays remain
>> arrays as long defined nil values are present and forming a sequence. Also,
>> it can be distinguished between functions that return a value or no value -
>> or if the number of returned values is invalid (somehow, this is where
>> debugging becomes interesting).
>>
>>
>> The concept is maybe very wacky, but somehow, it's intriguing me ... What I
>> strongly dislike is, that it's difficult to explain to beginners and also
>> hard to understand maybe. On the contrary, the current nil concept and the
>> deletion of array elements is also lacking.
>>
>> From my point of view, this would be almost completely compatible with
>> existing code, except that table element deletion needs to be revisited (we
>> had this discussion in the other thread).
>>
>> Well, as crazy as this idea might be, I hope have given you something
>> interesting/fun to think on :)
>>
>>
>> Cheers,
>>
>> Eike
>>
>
>
> It's a brilliant idea, but is "nil" an alias for "nil(defined)" or
> "nil(undefined)"? It'd have to be an alias for "nil(undefined)" to
> avoid breaking "t[#t] = nil" and similar idioms.
>
> /s/ Adam

If you write "nil", you specify it, so it's nil(defined). The
undefined nil value is only created when it originated from a lack of
specification.
Actually, deletion could work this way:

t[#t] = jfdosaifjosaifjos

... as long as the environment table / upvalue lookup for that value
is not defined ;).
So you could also do this:

local del
t[#t] = undefined
t[#t] = del

... whatever you use, as long as your source is an undefined nil
value, the table value gets deleted. Creating an undefined nil value
is simple - just don't initialize the local variable and you have it
:)

Eike