Re: Is it necessary to kill ipairs()?

[Jonathan Castello <twisolar@...>]

On Sat, May 22, 2010 at 11:33 AM, Joshua Jensen
<jjensen@workspacewhiz.com> wrote:
> I am still fond of 'for k,v in t do'.  I was very sad to see it go.  It was
> simple.  Lua is supposed to be simple, but ipairs/pairs complicated it.
>
> Josh
>

Could we perhaps return to "for k,v in t do", and have the generic-for
use the __pairs metamethod of its target as the iterator factory? If
no __pairs exists, default to next(). So this:

for k,v in t do
  print(k, v)
end

would normally act like you used pairs(). But if you set a metatable:

setmetatable(t, {
  __pairs = function(t)
    return function(t, i)
      i = i + 1
      local v = t[i]
      if v then
        return i, v
      end
    end, t, 0
  end,
})

then it iterates over the numeric indices in order. Further, you could
easily define a "false ipairs()" that returns a table with the __pairs
metamethod set:

function ipairs(t)
  local copy = {}
  for k,v in t do
    copy[k] = v
  end
  return setmetatable(copy, {
    __pairs = the_iter, -- same as above
  })
end

~Jonathan