“A well-written program in an abstract language like Ocaml or Lisp has the quality of an elegant mathematical proof: beautiful and concise, but you can’t change anything without breaking it. Most programs in C++ are more like an elaborate model train layout, supporting endless tinkering without actually stopping the train from going ‘round.

The ideal language would let you write functionality quickly and concisely like a high-level language, but let you tinker and optimize to your heart’s content. To support these two goals, it needs to read two separate sets of source code: a high-level part and an optional low-level part that guides compiler optimizations.