What is Balagan

Balagan is a tiny Clojure library for data structure transformation inspired by Enlive.

Supported Clojure Versions

Balagan targets Clojure 1.4+, tested against 3 Clojure versions x 2 JDKs on travis-ci.org, and is released under the Eclipse Public License.

Documentation and Examples

Balagan builds on ideas from Enlive. It may help to get familiar with them first.

Let’s say you’re working on a Data Access layer for the application. You have a user entry represented as hash:

(def user
  {:name "Alex"
   :birth-year 1990
   :nickname "ifesdjeen"})

Transformation

Now, we can start transforming users the way we want: add, remove fields based on certain conditions.

(update user
           []                  (add-field :cool-dude true) ;; adds a field :cool-dude with value true
           (new-path [:age])   #(- 2014 (:birth-year %))   ;; explicit adding of a new field, calculated from the existing data
           (new-path [:posts]) #(fetch-posts (:name %))    ;; fetching some related data from the DB
           [:posts :*]         #(update-posts %))       ;; apply some transformations to all the fetched posts, if there are any

Queries

Queries are very similar to how you’d query your data with filter in Clojure:

(let [data {:a {:b [{:c 1} {:c 2} {:c 3}]
                :d [{:c 5} {:c 6} {:c 7}]}}]
  (select data  [:* :* even? :c]))
;; => (1 3 5 7)

Results are returned in the order they’ve been seen in your data structure, however you should be aware of the fact that iterating over the hash in Clojure doesn’t guarantee you order.

Path Queries

Path queries are most useful when you’d like to fire a function against some part of your data (be it processing, database initialization or anything else.

You can also run predicate queries based on your map, for example if you want to configure your database servers from rather big and complex config:

(def conf {:db
           {:redis {:cache  [{:host "host01" :port 1234} {:host "host02" :port 1234}]
                    :pubsub [{:host "host01" :port 1234} {:host "host02" :port 1234}]}}
           :cassandra [{:host "host01"} {:host "host02"}]})

(with-paths conf
        [:db :redis :cache]  configure-redis-cache
        [:db :redis :pubsub] configure-redis-pubsub
        [:db :cassandra]     configure-cassandra)

In this example, configure-redis-cache funciton will receive two arguments: value and path:

(defn configure-redis-cache
  [value path]
  (println "Value: " value)
  (println "Path: " path))

;; => Value: [{:host host01, :port 1234} {:host host02, :port 1234}]
;; => Path: [:db :redis :cache]

You can also do wildcard-matching with :*, for example:

(b/select {:a {:b {:c 1} :d {:c 2}}}
          [:a :* :c] (fn [val path]
                       (if (= path [:a :b :c])
                         (is (= val 1))
                         (is (= val 2)))))

Learn more in the documentation section.

Community

To subscribe for announcements of releases, important changes and so on, please follow @ClojureWerkz on Twitter.

License

Distributed under the Eclipse Public License, the same as Clojure.

The source is available on GitHub. We also use GitHub to track issues.

About the Author

The ClojureWerkz Team.