It's compiling a subset of Scheme that can be statically typed. It's not for full-blown live-hackable Scheme programs. You'd use Crunch or PreScheme to implement things that you can't implement in Scheme, like a garbage collector. I don't know if Crunch has this feature, but with PreScheme you can run your PreScheme programs at the Scheme REPL so you can have your comfy REPL-driven dev environment and then create a static executable when you're ready.