new primitive for iterating partially over an array

array.lua

@@ -26,3 +26,21 @@ function array.any(arr, f)
   end
   return false
 end
+
+-- like ipairs, but support an index to start iterating from
+-- idx is inclusive; the first call to the iterator will return idx
+function array.each(arr, idx)
+  if idx then
+    if type(idx) ~= 'number' then
+      error("argument #2 to 'array.each' must be an integer if provided")
+    end
+    assert(idx >= 1, ("can't start iterating over an array from %d; the starting index needs to be at least 1"):format(idx))
+  end
+  idx = idx and idx-1 or 0
+  local iter = ipairs(arr)
+  return function()
+    local elem
+    idx, elem = iter(arr, idx)
+    return idx, elem
+  end
+end