F is written in K, another small language. In fact, the entire implementation is in one file: http://www.nsl.com/k/f/f.k.

Example program (defines factorial and computes fac(6), from http://www.nsl.com/k/f/f/fac.f):

 f

    
[[1=][][dup!pred!fac!*]cond!]fac


  

The main site, "no stinking loops", is full of links including to other languages (scroll down) and resources on K. Although many are broken 🙁.