Re: new "empty" value/type in Lua?

[Dirk Laurie <dirk.laurie@...>]

2013/6/28 Tim Hill <drtimhill@gmail.com>:
> I'm going to get shot for suggesting this… (hides under bed)
>
> I've been writing a lot of Lua code recently, some of it quite generic in nature. One thing I frequently come up against is the need to handle data polymorphically in tables. And an effect of this is to hit against the overloaded use of "nil" in Lua.
>
> It seems to me that "nil" is overloaded; it means both a noun: "no value" and a verb: "delete this table entry". Now, most of the time, the delete is hidden since reading a non-existent table entry returns nil. So having an explicitly stored nil versus having no value stored at all is equivalent (all this I love, btw … 100 points to Lua).
>
> But it all rather breaks down with arrays (sequences). I cannot have sparse arrays, which in Lua terms means I cannot have "nil" in an array element. So, when writing polymorphic code, how do I handle cases (which, trust me, i DO have to handle, with "no value" in an array element? There are, of course, a whole bunch of work-arounds: Use a sentinel value; use an empty table (assuming empty tables aren't value data, and ignoring that each one is unique); use a magic userdata (assuming I have access to C code to make one); etc etc.
>
> But, let's face it; they ARE all workarounds. I think there IS a case to be made for being able to store a value in a Lua array that is uniquely NOT any other Lua value. Essentially, I think that value is "empty"; a valid Lua value that is simply not the same as any other Lua value (including nil).
>
> Basically, empty would act like nil in most regards, except:
> -- Storing empty in a table element actually stores the value, rather than deletes it.
> -- When used in a boolean context, the empty value evaluates to true (unlike nil).
> -- Has a type() of "empty"
>
> Other arguments in favor of this:
> -- Judging by the mail list chatter, a lot of Lua beginners struggle with the problem of nil in sequences.
>
> -- Better impedance matching to SQL NULL values; since empty can bridge to a SQL NULL when a Lua array is used to hold a SQL row.
>
> Thoughts anyone?
> --Tim
>
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