First 1000 lines of Clojure

April 16, 2014

This is a short summary after having written my first 1000 lines of Clojure.

It all started when I was assigned the task of importing some stuff into our stuff and nobody cared to specify which language to write the importer in.

First I thought I’d write it in Scala, but when push came to shove, I couldn’t be bothered learning a new syntax, so I figured Clojure would be a good fit. I’ve always been intrigued by lisp, so a lisp running on the JVM should be a good fit.

#The importer Basically I was given a huge XMl-file and some content files, and I wrote a thingy which transformed this XML-file into a nice tree-structure, which I then fed to a rest-client responsible for creating content by interacting with our rest-api.

#Lessons learned I’ve been working on and off on this importer for the last four months, with breaks from it in up to a month in length. I was scared about coming back to the code after such long time, but that has not been a problem at all, even though

(defn parents-of-files [folder path]
  (let [children (:children folder)
        docs (filter :document children)
        sub-nodes (remove :document children)
        new-path (conj path (:name folder))
        rest (flatten (map #(parents-of-files (:folder %1) new-path ) sub-nodes))]
    (if (seq docs)
      (cons {:path new-path :documents docs} rest)

(defn flatten-children [folder]
  (let [flat-tree (parents-of-files folder [])]
    (flatten (map #(add-path-to-docs (:documents %) (:path %)) flat-tree))))

Still makes me a bit scared. Having said that, given the shortish functions and their descriptive names, it’s quite easy to follow the code.

##Persistent data structures Working with persistent data structures was difficult before I understood that Clojure has some nice functions to make it much easier. In the beginning, when I wanted to change baz to qux in

(def h {:foo {:bar {:baz "qix"}}})

I ended up with code like

(defn update-baz [h val]
  (let [foo (:foo h)
	bar (:bar foo)]
    (assoc h :foo (assoc foo :bar (assoc bar :baz val)))))

which took me 10 minutes to write. Then you find assoc-in, and the whole thing is done in one line:

(assoc-in h [:foo :bar :baz] "qux")

If you want to capitalize the value instead of just replacing it, you use update-in

(update-in h [:foo :bar :baz] clojure.string/capitalize)

##The hard bits The hardest bit in this project was due to the fact that the XML was actually a dump of a javax.swing.DefaultMutableTreeNode. This meant that to start the transformation of this XML, I needed to use the java-beans that were serialized. This meant that in order to test my functions, I first had to get hold of the Java-objects, which could only be found by deserializing the stuff. So, even though the Java-interop worked as a charm, it was a hassle to get test-data. Lesson learned from this is to get stuff over to lists and maps as soon as possible, and the bean function is your friend here.

##The REPL Working in the repl is sooo nice. You start out with an idea of what you want to make, then you polish it in the REPL. As far as I can see TDD becomes useless, Jay Fields has more on this.

A downside to working in the REPL (through CIDER in emacs) is that I constantly forget to save my buffers. This leads to my unsureness about the state of the file I’m currently editing on. Don’t really know how to handle that.

#The big bonus Was actually something I discovered today, as the project is getting finished. Running the import in the REPL means I can interact with the import after it is done

Take this function, which takes a document and creates it on the server:

(defmethod create-document false [document]
    (let [node-id (rest/create-document (remove-audit-props document))]
      (rest/set-properties (assoc (get-audit-props document) :nodeId node-id))
      (swap! document-count inc)
    (catch Exception e
      (swap! errors conj document)
      (error e "Could not create document "   document))))

As you can see, whenever it fails, it adds the document to the errors atom. This lives on after the import is done, and I can inspect the errors to see how many errors have occured, and I can even manually retry the documents that failed. I would never have thought about making that possible had I written this importer in Java.

#In ending I do find it kind of sad that most developers seem to be frightened by Lisps in general, since it is a really nice language-family to work with.

I find that wrapping my head around some of the stuff I need to do is hard, but that actually makes me think harder about the problem at hand, and in the end seems to leed to better solutions.