The "called from C" case isn't the interesting one, even though it's what the grandparent mentioned. It's the "called from a scripting language" case. You can call Haskell or Ocaml or even Java code from within CPython or Matz Ruby. However, if you do this, you bring along an entirely different runtime system and memory layout. Usually this means that you face some very odd memory management bugs and lose any speed advantage you gain as soon as you have to marshal objects across the language boundary.