microscheme

That's true; on the ATMega32u4 it's the 2.5kb of RAM that kills you. The 16MHz clock speed is plenty fast for most microcontroller applications. That's why Microscheme is just a subset of Scheme; there are no continuations or runtime eval in order to save memory. It's true that there are no structs, but it wouldn't be much of a stretch to add them. There's a discussion of writing a macroexpander in another Scheme runtime like Guile which can compile macros to microscheme output. (This is a bit like how ClojureScript lacks runtime eval but allows you to write compile-time macros in Clojure.) Writing a define-struct macro that compiles to vector-ref at runtime would be pretty trivial once you have a macroexpander working. https://github.com/ryansuchocki/microscheme/issues/8 If you're interested in hacking on a small Scheme implementation

I doubt PICBIT (or Microscheme for that matter) could run on the PIC16 family, with it's 25 bytes of RAM. Wouldn't be much of stretch to port Microscheme to PIC18s though (with 512+ bytes of RAM), the code generator is pretty straight-forward: https://github.com/ryansuchocki/microscheme/blob/master/src/...