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steve donovan wrote:
for i,v in t do

In this context, 'for' expects t to be a callable thing. Normally this is a function, as the first parameter of a tuple:

   for k, v in ipairs(table) do
=> for k, v in fn, table, 0 do

'for' then calls fn repeatedly to fetch the values, passing in the rest of the tuple (which gets modified during the iteration).

The problem is, the rest of the tuple is *optional*. It's perfectly possible to use an iterator function without them.

  i = 0
  function iterator()
    i = i + 1
    return i

  for a in iterator do

And, of course, tables can be overridden using the __call metatable entry to look like functions.

The upshot of this is, if you add an __iter metatable entry to allow iteration on tables, you also add a nasty ambiguity as to what happens if the table has both a __call *and* an __iter. That's not to say it can't be done... just that ambiguities like this are best avoided if you wish to retain your sanity.

(And implementing iteration using __call is tricky because you don't get told when the iteration starts or stops. Note that my iterator function above uses *global* state, not a per-invocation state.)

David Given