Re: Why is implicit and explicit 'nil' treated differently?
[
Date Prev][
Date Next][
Thread Prev][
Thread Next]
[
Date Index]
[
Thread Index]
- Subject: Re: Why is implicit and explicit 'nil' treated differently?
- From: Jan Behrens <jbe-lua-l@...>
- Date: 2015年4月19日 12:41:17 +0200
On 2015年4月18日 23:21:47 -0500
Andrew Starks <andrew.starks@trms.com> wrote:
> On Saturday, April 18, 2015, Andrew Starks <andrew.starks@trms.com>
> wrote:
>
> >
> > > [...]
> >
> > There is one other case where nil/none appear. It almost never
> > matters, but it thwarted me from being too creative with tables:
> > __index is always adjust to one return value. So:
> >
> > __index = function(t, i, x)
> > --x is always absent (empty/nil), btw. This would almost
> > certainly never matter, but it's true. I'm not sure how I know
> > this, actually.it wouldn't happen from trying `t[1, 2]`, because
> > that's an error. Anyway... end
> >
> > f = function()
> > end
> >
> > print (select("#", __index()), select("#", f()))
> > -->1 0
> >
> > So you can never signal the difference between nothing and `nil`
> > from your index metamethod. Again, not complaining, I'm just sayin'.
> >
> >
> Blech the above is wrong, unless you take it to mean that __index is
> being invoked as a metamethod on a table, not just a function hanging
> out in space, as the variable named __index. Sorry for the noise.
Indeed:
function __index(self, key)
return "a", "b"
end
t = setmetatable({}, {__index=__index})
print(select("#", __index()), select("#", t.x))
--> 2 1
print(__index())
--> a b
-- multiple values passed to the print function, because
-- __index() is last argument
print(t.x)
--> a
-- because t.x is expected to be a _single_ value, the list
-- of arguments is truncated
Similarly, "none" would be converted to "nil" if the __index function
returns no value *and* is invoked as a metamethod:
function __index(self, key)
return
end
t = setmetatable({}, {__index=__index})
print(select("#", __index()), select("#", t.x))
--> 0 1
print(__index())
-->
-- nil is not shown, because __index() may pass a variable
-- number of arguments (here: zero) to the print function
-- (__index() is the last entry in the argument list for
-- print)
print(t.x)
--> nil
-- because t.x is expected to be a _single_ value, nil is
-- printed
print((__index()))
--> nil
-- also here (__index()) in parenthesis must be a single
-- value, hence nil is printed
Regards,
Jan
- References:
- Varargs efficiency, Soni L.
- Re: Varargs efficiency, Sean Conner
- Re: Varargs efficiency, Soni L.
- Re: Varargs efficiency, Tim Hill
- Re: Varargs efficiency, Soni L.
- Re: Varargs efficiency, Hisham
- Why is implicit and explicit 'nil' treated differently?, tonyp
- Re: Why is implicit and explicit 'nil' treated differently?, Jan Behrens
- Re: Why is implicit and explicit 'nil' treated differently?, Andrew Starks
- Re: Why is implicit and explicit 'nil' treated differently?, Andrew Starks