Example of lua coroutines being used as iterators.

coroutine-iterator.lua

function permgen (a, n)
  if n == 0 then
    coroutine.yield(a)
  else
    for i=1,n do

      -- put i-th element as the last one
      a[n], a[i] = a[i], a[n]

      -- generate all permutations of the other elements
      permgen(a, n - 1)

      -- restore i-th element
      a[n], a[i] = a[i], a[n]

    end
  end
end

-- just to print the table properly
function printResult(a)
  for i,v in ipairs(a) do
    io.write(v, " ")
  end
  io.write("\n")
end

function perm (a)
  local co = coroutine.create(function () permgen(a, #a) end)
  local iterator = function ()
    local code, res = coroutine.resume(co)
    return res
  end
  return iterator
end

for p in perm({"a", "b", "c"}) do
  printResult(p)
end