Here are some resources about the free monad in scala . At some point I need to write summaries for each entry :)
Blog posts
Free Monads I had bookmarked this sometime ago and seems to be very to the point without a lot of theotitical analysis
Free Monad - Cats Documentation by Pascal Voitot
herding cats - Free monads by Eugene Yokota
Deriving the Free Monad by Noel Welsh
Free Monads Are Simple by Noel Welsh
Free Monads, Part One by Timothy Perrett
On Free Monads by Pere Villega
Free Monads using FreeK by Pere Villega
Freek, a freaky simple Free to combine your DSL seamlessly by Pascal Voitot
Haskell GADTs in Scala by Sean Seefried
Alternatives to GADTs in Scala by Paul Chiusano
Invasion of the Monad Transformers
Video talks
The Interpreter Pattern Revisited by Rúnar Bjarnason
Composable application architecture with reasonably priced monads by Rúnar Bjarnason - slides code
A Year living Freely by Chris Myers
Run free with the monads: Free Monads for fun and profit by Ken Scambler
Move Over Free Monads: Make Way for Free Applicatives! by John De Goes
Free Monads and Free Applicatives by Markus Hauck
Why The Free Monad isn’t Free by Kelley Robinson
Examples
MonsantoCo/stoop A Scala DSL for interfacing with CouchDB.
tpolecat/doobie principled database access for scala In Programs as Values Rob Norris explains how to create an algebra with free monads for safe dB access.
Workshops
underscoreio/scalax15-interpreters Slides and companion material for the Scala Interpreters workshop from Scala Exchange 2015.
Misc
jdegoes/FreeMonad.scala A pedagogical free(er) monad in 23 lines of Scala