Partition By in Clojure

On every professional software engineering job I’ve had, data transformation always played a role. Most recently, at Spantree we needed to get a massive corpus of synonyms into the file format readable by Elasticsearch.

We received a 640MB text file, with over 5.4 million lines of the form


While a Groovy or Python solution was possible, this was definitely a problem that would require processing things line by line.

Instead of relying on keeping track of state line by line, this seemed like a great opportunity to try out Clojure. The crux of the application boils down to two functions.

First, the processLine function. This takes in a given string, and returns a map with the given ID and word.

(defn processLine
  (let [splitLine (str/split line #"\|")]
    {:id (nth splitLine 0)
     :word (nth splitLine 3)}))

While the syntax is different, this is fairly similar to what would have been used in Groovy or Python.

After we read in a line, we need to group the lines based on their ID. This is when Clojure began to really show improvement in brevity and style. The second function, partition-by, allows us to create sequences of groups based on the result of a given function. In this case, the given function is simply getting the ID of a line.

(defn groupSynonymsById
  (partition-by #(:id %) allSynonyms))

With some basic wrappers like importAllSynonyms to load the file and map processLine, and writeOutGroupedSynonyms to create the formatting we want, the entire main method becomes

(writeOutGroupedSynonyms (groupSynonymsById (importAllSynonyms)))

Nothing done was exclusive to Clojure, but the combination of built in functions and succinct syntax allowed us to produce a script with less than 15 lines of actual code. This will allow us to come back and understand the code with little effort, making any changes quick and painless.