Presented to the Melbourne Scala User Group on March 24th 2014, of which you can find the slides on SlideShare.
The Tank Game uses Free Monads to drive the AIs of the tanks. Each Tank holds a Free[TankMove, Unit] (aka AI[Unit]), which gets consumed one instruction at a time per game frame.
The "game" consists of a number of tanks with hard-wired AI scripts; there is no user input.
There are two interpreters, a Hard interpreter, and an Easy interpreter which shoots slightly to the right.
A simple example of an ADT/Functor that can generate a useful Free Monad, based on Runar Bjarnason's Dead Simple Dependency Injection talk.
This illustrates the steps to convert a conceptual DSL to a Free Monad script, and has an example of both a pure and effectful interpreter.
Simple example of a mutually tail-recursive algorithm that cannot be optimised by scalac, based on Runar's Stackless Scala paper. There is a naive example that will blow the stack, and a Trampoline translation of the same code that lifts the recursion onto the heap.
Minimal implementation of Free, from first principles. This was an effective exercise for me to learn the structure, and may be otherwise useful as a reference with less noise than scalaz.Free or Control.Monad.Free.