Here's a much smaller and less ambitious interpreter in Python as a hackable taste of the language: https://github.com/darius/parson/blob/master/eg_fp.py -- or rather another dialect, since everyone who implements FP makes up their own concrete syntax. Mine reads left to right instead of right to left, and leaves out the noisy compose operator (it's implicit by juxtaposition), making it read kind of like Forth or Joy.

I also wrote an optimizing compiler to Scheme from this dialect back around 1990, but it's lost in the sands of time.