Writing implementations of some of the core functions of a language can be a great way to really get to understand the language.
Included here are implementations of reduce, count, filter, map, and pmap. The implementations are only models of the core functions, and are not intended to provide the same efficiency benefits that the core functions offer.
There are both unit and property-based tests to demonstrate that the functions behave as desired. The unit tests can be run with lein spec, and the property-based tests with lein test.
For further details, see Understanding Core Clojure Functions and Property-Based Testing in Clojure.