Re: Proposal: Make creating large arrays faster and "allow" nils in arrays

[DarkWiiPlayer <darkwiiplayer@...>]

This was actually my first thought when I read the proposal: There
should be a `__setlen` metamethod to allow resizing of custom types.
Probably also a `rawsetlen` method that can be called within the
metamethod of a table.

On 17/07/2020 17:53, Mason Bogue wrote:
> > What happens if the __len metamethod is overridden?
>
> Good question! The best idea I have is to treat this like __index and
> add a __setlen metamethod. Length assignment on any non-table value
> (userdata eg) would then be a type error unless there is a __setlen
> metamethod defined for that value. Of course the __setlen metamethod
> then calls for a rawsetlen() function which only works on tables.
>
> You can't currently implement this behavior in pure Lua — you need to
> change the internal representation of the table to get the speedup.
> What I wanted was to set the length of the array segment. If there is
> a __len metamethod, the programmer must ensure their code makes sense,
> but it's true in that case `#t = 5` does not guarantee the truth of
> `#t == 5`. 
>
> It seems like this should mean that if the table length is set
> downwards, all entries between the old (raw) len and new len are
> deleted. (Hence we may also want `rawgetlen()`, although that is
> already a question raised by __len). 
>
> On Thursday, July 16, 2020, Sean Conner <sean@conman.org
> <mailto:sean@conman.org>> wrote:
>
>     It was thus said that the Great Mason Bogue once stated:
>     > We are all generally aware of the issues with the length operator on
>     > tables containing nil. There was an idea to fix this in 5.4 by
>     > creating a special t[x] = undef syntax, which was considered too
>     ugly.
>     > Here I propose an alternative:
>     >
>     > #t = 5 -- sets the length of the array portion of t to 5
>
>       What happens if the __len metamethod is overridden?
>
>       -spc
>

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