It's indeed very interesting!

What I meant is that it might be simpler to just have pure functions operating on immutable data structures, which you could then compose in whichever you want. I.e. you could have a function collapse-vertices that takes some geometry and produces some new geometry (without altering the initial one!), which you could then compose with some other pure function, like produce-random-vertices, and so on, much like a Unix pipe.

A free monad is just a formalization of the command/interpreter pattern in a purely functional context (i.e. most useful in languages like Haskell or Clojure).