[ddragon]

I think the answer is fundamentally: giving the compiler a lot of information at compile time so it can optimize the generated code for the runtime, be it explicitly by type annotations or implicitly by type inference. If the compiler is sure a variable will be a 32 bits int, it can translate to the same instructions that any language that is not dynamic could.

What Julia language does is give programmers powerful tools to encode types, behaviors and even code manipulation/generation tools that can be fully resolved in compile time, while still having a fully dynamic runtime.