Re: explicitly defining table length + _ARG proposal

[Quae Quack <quae@...>]

On 22 June 2010 14:31, David Manura <dm.lua@math2.org> wrote:
> To followup [1], can someone remind me what is the rationale for Lua
> 5.1 tables not storing an explicit table length, which thereby
> prevents trailing nils from being regularly counted in the table
> length?  I'm not suggesting storing length in a regular table key,
> like the Lua 5.0 "n" field which affects things like pairs iteration,
> but rather storing it in some hidden location accessible only via the
> # operator.
>
> My second comment is that Patrick mentioned something that made me
> think why not support the following:
>
>  function f(...)
>    for i=1,#_ARG do print(_ARG[i]) end
>    g(...)  -- or g(unpack(_ARG)) or maybe g(_ARG[*])
>  end
>
> where _ARG is a specially defined variable lexical to the current
> function with `...` params and containing the values in `...`.  It is
> "specially defined" in the manner of self and _ENV.  It is therefore
> not too unlike the `arg` variable in Lua 5.0 except for a couple
> differences.  Formally, _ARG is a userdata or table with __index and
> __len operators that serve the purpose of `select` and with the __len
> operator including trailing nils in its count.  _ARG may also be
> mutable, allowing things like `_ARG[i] = _ARG[i] or 0`.  Finally,
> there is an important performance optimization: if the function does
> not do things like store `_ARG` in another variable, pass it to
> another function, add keys to it, or use it as an upvalue, then `_ARG`
> can be optimized away by the compiler and doesn't actually need to be
> constructed in memory: #_ARG and _ARGV[i] merely render as opcodes in
> the VM.
>
> [1] http://lua-users.org/lists/lua-l/2010-05/msg00587.html
>

Why not just have #... defined to get the number of varargs?
sure, it might screw up some code that assumes that #... == #(...);
but #... has never looked good to me in the first place.
Maybe this is a step in the correct direction to stop varargs being
such a second class citizen/