Carp is a small programming language designed to work well for interactive and performance sensitive use cases like games, sound synthesis and visualizations.
Carp is very tightly integrated with it's compiler which itself is written in a dynamic version of Carp (the dynamic version is implemented in C). To work on a Carp program you run ```carp``` which puts you in the compiler REPL. Everything you want to do with your program can be controlled from here.
This results in the compiler analyzing the code form for 'fib' and compiling it to some very fast binary code, loading this back into the REPL so that it can be called conveniently from there.