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I like this. In fact, I'd say that replacing the __pairs and __ipairs metamethods with an __iter metamethod (that is used by the generic "for" when encountering non-functions, with the default case of next()) would be "cleaner" (as you're now not mixing up standard library functions like pairs with language features like metatables).
On Sat, 22 May 2010 11:48:11 -0700, Jonathan Castello <twisolar@gmail.com> wrote: 


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