[Jtsummers]

This is also true of most lisps, and, IME, any language with a good REPL. You can start with snippets which become the bodies of loops and functions, which grow into the full program with (as he points out) various tests and examples constructed along the way.

It's a useful way for approaching a sort of (in my use) hybrid of top-down/bottom-up programming. The things you see on the REPL growing into functions or structs or classes is the bottom-up part, but still with some specification (in this case KWIC indexing) guiding the ultimate interface.