Re: High-level vs low-level overrides / Multi-level overrides

["Soni L." <fakedme@...>]

On 01/08/16 04:48 PM, Nagaev Boris wrote:
> On Mon, Aug 1, 2016 at 8:50 PM, Soni L. <fakedme@gmail.com> wrote:
> > Lua 5.2 introduced __ipairs. It's a high-level ipairs() override, as it
> > overrides ipairs() itself.
> >
> > Lua 5.3 removed __ipairs and instead added the low-level override __index.
> > It overrides the primitive (low-level) operations used by ipairs(), instead
> > of ipairs() itself.
> >
> > Why can't we have both? I.e. try __ipairs first, if that fails use __index,
> > etc.
> >
> > We could also get a __next metamethod and a rawnext() function. pairs()
> > would then first try __pairs, then try __next.
> >
> > --
> > Disclaimer: these emails may be made public at any given time, with or
> > without reason. If you don't agree with this, DO NOT REPLY.
> What is the motivation behind this proposal? I think the whole idea is
> not ideal. It adds to complexity of the language.

stdlib.

> PS. Given Lua had __pairs, ipairs (and __ipairs) would be unneeded. To
> get current ipairs, one would set __pairs to current ipairs and invoke
> pairs.

Yeah we don't need ipairs we could just do

local i = 1
local v = t[i]
while v ~= nil do
  i = i + 1
  v = t[i]
end

or

local function ipairs_aux(t, i)
  i = i + 1
  local v = t[i]
  if v then return i, v end
  return nil
end

function ipairs(t)
  return ipairs_aux, t, 1
end

However, we do have ipairs, so it'd be nice to have multi-level 
overrides where it makes sense.

__next and __index is consistent.
__pairs and __ipairs is consistent.
__pairs and __index is not consistent.

Having the former 2 gives us the best of both worlds.

--
Disclaimer: these emails may be made public at any given time, with or without reason. If you don't agree with this, DO NOT REPLY.