Categories as Types by Jeremy Kun.
From the post:
In this post we’ll get a quick look at two ways to define a category as a type in ML. The first way will be completely trivial: we’ll just write it as a tuple of functions. The second will involve the terribly-named “functor” expression in ML, which allows one to give a bit more structure on data types.
Jeremy gets us closer to application of category theory to programming with an introduction to types.