Tested each sample.

gem/ex5.lua

@@ -0,0 +1,15 @@
+function source.empty(err)
+  return function()
+    return nil, err
+  end
+end
+
+function source.file(handle, io_err)
+  if handle then
+    return function()
+      local chunk = handle:read(20)
+      if not chunk then handle:close() end
+      return chunk
+    end
+  else return source.empty(io_err or "unable to open file") end
+end