Re: Multiple return value in __index metamethod

[Rici Lake <lua@...>]

On 10-Jul-07, at 12:31 PM, Lavergne Thomas wrote:

> The program I currently write with Lua and C use a lot of objects with
> an __index metamethod. And today I've wanted to extend one of these
> objects and return more information from one of theses __index. The
> frist thing that come to my mind was to use the lua ability to return
> multiple values from functions. This have the advantage of keeping
> compatibility (with checking for some special case) and nearly no
> overhead for the existing code.
> But in metamethods (except for __call) the number of return values is
> adjusted to 1.
>
> I easily understand why in case of metamethod like __add or like but is
> there any reason to do this for __index ?


__index is triggered by the use of an index operation in Lua, including
t[3], t.foo, t:meth()

Like the arithmetic operators, these operations are adjusted to
a single value; in the compiled VM code, they compile to a single
operation. Allowing multiple returns would be a complication to
the VM design, and would likely be considerable overhead for a
fairly rare use case.

> I think the documentation is false for this. In lua code for __index 
> the
> return value is :
>   return h(table, key)
> and for __call :
>   return h(func, ...)
> So the same call. I've assumed that in both case, all the result were
> returned.

The number of return values consumed is determined by the caller,
not the function called. So the description is technically correct.

> In my case I have different possibility do handle this. I can make my
> __index methods return a table of these values, but this have the
> ovrhead to create a lot of very small tables destroyed almost imediatly
> and so trigger the gc a lot more often, and I must modify all the
> existing code.

Why not just use classic accessor functions? That is:
  obj:values()
instead of
  obj.values

It seems easier than the other possibilities, and has the advantage
of working :)