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

[Philipp Janda <siffiejoe@...>]

Am 03.07.2013 18:58 schröbte Andrew Starks:
> I replied to a different thread (too much splitting). To keep things
> together, I will cross-post:
>
> I'd suggest not having `nothing`.

I agree. Otherwise we would need a concept for the absence of this 
`nothing` value. `nothing` only makes sense in the context of a 
collection of values (like Lua's argument/return value lists).

We need some other way to detect `nothing` like `select('#',...)`, or 
`table.has()/defined()/exists()`, or raising an error when trying to 
access it.

>          a = function() end
>          b = function() return end
>          c = function() return nil end
>
> print(select('#', a()), select('#', b()), select('#', c())
> --> 0, 0, 1

function
> exists(...)
>    return not select('#', ...) == 0
> end
> local foo
> print(exists(foo))
> -->true
> print(exists(baz))
> --false --currently true

Perhaps we could make every table access a vararg expression of zero or 
one value (with the usual argument adjustments) ...

So:
    select( '#', baz, baz, baz )  --> 2
    select( '#', baz, baz, baz, foo ) --> 4
    { 1, baz, 2, baz }        --> { 1, nil, 2 }
    local x, y = baz, 3  --> x = nil, y = 3
    t = { a = nil }
    select( '#', t.a )   --> 1
    select( '#', t.b )   --> 0

> print(exists(a()))
> --false --currently true

This already prints `false`. The list of return values can hold nothing, 
or one or more values.

> print(exists(b()))
> --false --currently true

Same here, already prints `false`.

> print(exists(c()))
> --true
>
> This is not to suggest an implementation. It is only to illustrate behavior.
>
> -Andrew

Philipp