# Free Category
[](http://github.com/coot)
[](https://circleci.com/gh/coot/free-category/tree/master)
This package contains efficient free categories. There are two presentations:
* using realtime queues (C. Okasaki 'Pure Functional Data Structures')
* using continuation passing style
Free arrows and free Kleisli categories are also included.
Free categories are useful to model state machines in a simple yet type safe
manner. For that purpose `Kleisli` categroies are a very useful target which
allows to include monadic computations. This packge contains a useful
generalisation of `Kliesli` categories captured by `EffCategory` class
(effectful categories), and a (free) transformer which lifts a category to
an effectful one.
## Some examples
* [LoginStateMachine](https://github.com/coot/free-category/blob/master/examples/src/LoginStateMachine.hs):
based on [State Machines All The Way
Down](https://www.youtube.com/watch?v=xq7ZuSRgCR4) by Edwin Bradly, 2017 You
can run it with `cabal new-run examples:login-state-machine`.
* Read more [here](https://coot.me/posts/finite-state-machines.html) on
a simple example of a finite state machine encoded using a free category
using a simple GADT.
* Another
[example](https://github.com/coot/free-algebras/blob/master/examples/src/Control/Category/Free.hs).
* [Blog post](https://coot.me/posts/kleisli-categories-and-free-monads.html) on Keleisli categories.