Tested each sample.

gem/ex6.lua

@@ -0,0 +1,14 @@
+function source.chain(src, f)
+  return function()
+    if not src then
+      return nil
+    end
+    local chunk, err = src()
+    if not chunk then
+      src = nil
+      return f(nil)
+    else
+      return f(chunk)
+    end
+  end
+end