Two Guys Arguing

CSS Selectors, Java Interop and Scraping

Posted in clojure by youngnh on 11.03.10

Building a DOM

Parsing HTML can be tricky, most of my naive attempts to parse real-world pages produced a lot of stack traces. The Validator.nu HTML parser has so far cleared those low hurdles. It’s implemented in Java and it has a maven artifact, which makes it easy to include in a leiningen project, so it’s my current weapon of choice.

:dependencies [[org.clojure/clojure "1.2.0"]
           [org.clojure/clojure-contrib "1.2.0"]
       [nu.validator.htmlparser/htmlparser "1.2.1"]]

It’s easy to get a DOM from a webpage using Validator.nu (api docs here), feed HtmlDocumentBuilder an InputSource which you feed a java.io.Reader, which is easily created via the reader fn from clojure.java.io:

(defn build-document [file-name]
  (.parse (HtmlDocumentBuilder.) (InputSource. (reader file-name))))

Converting the DOM to a seq

Clojure comes with a few very nice tree walking facilities. We can’t use them until we convert a dom with nodes of type, well, Node and branches of NodeList into seqs that Clojure is more adept at manipulating.

NodeList has two methods on it, getLength() and item(int index). One approach is to close over an index binding and recursively create the seq:

(defn nodelist-seq [node-list]
  (letfn [(internal [i]
            (lazy-seq
          (when (< i (.getLength node-list))
            (cons (.item node-list i) (internal (inc i))))))]
  (internal 0)))

Another is to keep the current index in an atom, and implement Iterator with it, which Clojure can make into a seq for you:

(defn nodelist-seq [node-list]
  (iterator-seq
   (let [i (atom 0)]
     (reify Iterator
       (hasNext [_]
         (< @i (.getLength node-list)))
       (next [_]
         (try
       (.item node-list @i)]
           (finally
        (swap! i inc)))))))

Where I’m using try/finally as a replacement for Common Lisp’s prog1.

With that in place, it’s not hard to turn a DOM into a nested seq, which either zippers found in clojure.zip or Stuart Sierra’s clojure.walk should be able to navigate for you quite adeptly.

Selectors

I’d like to be able to select a node by:

  • id: #statTable1
  • tag name: table
  • class attribute: .class

And I’d like selectors to work from any node I give it. This way I can write a selector that will work at multiple places in a tree, making them more reusable. Being able to turn a DOM into a seq suggests that filtering it on a predicate would be a quick way to write the above selectors, here are supporting functions to inspect the nodes themselves:

(defn element-tagname [elt]
  (when (= Node/ELEMENT_NODE (.getNodeType elt))
    (.getNodeName elt)))

(defn get-attribute [elt attr]
  (.?. elt getAttributes (getNamedItem attr) getValue))

(defn hasclass? [elt class]
  (when-let [class-attr (get-attribute elt "class")]
    (some #(= class %) (split class-attr #" "))))

The .?. method in get-attribute is remarkably useful. It’s analogous to the .. operator in clojure.core for chaining method invocations on objects. As not all Node objects have attributes on them, and not all attributes have the one we’re looking for, in both cases, a null value is returned by the method invoked. Trying to invoke any other method returns an NPE. .?. does the grunt-work of handling that and short-circuiting to return nil.

The Document object has two methods on it that are just too good to pass up, though. getElementById and getElementsByTagName might give better performance than scanning the entire tree, so if we’re selecting from the root, then we’d like to use them. Multimethods solve our dilemma nicely.

(defn doc-or-node [node & _]
  (if (instance? Document node)
    Document
    Node)))

(defmulti id-sel doc-or-node)

(defmulti element-sel doc-or-node)

(defmethod id-sel Document [document id]
  (.getElementById document (.substring id 1)))

(defmethod id-sel Node [node id]
  (filter #(= (.substring id 1) (get-attribute % "id")) (dom-seq node)))

(defmethod element-sel Document [document elt-name]
  (.getElementsByTagname document elt-name))

(defmethod element-sel Node [node elt-name]
  (filter #(= elt-name (element-tagname %)) (dom-seq node)))

Uniformity

Finally, if each selector takes a single Node and returns a list of Nodes, then I’ll note that you can “chain” selectors together with mapcat.

(->> (element-sel document "body")
     (mapcat #(element-sel % "table"))
     (mapcat #(element-sel % "tr"))
     (mapcat #(class-sel % ".odd")))

With this property, we’d need to make sure that Document version of id-sel above wraps it’s single Node in a list. This sort of chaining ability, of taking a bunch of things, and applying them in sequence to get a single thing throws up the use reduce flags in my head. My first attempt nearly works out of the gate:

(defn $ [node & selectors]
  (reduce mapcat node selectors))

The problems with it being that mapcat takes it’s function argument first, while we’re passing our selector functions in second, and that mapcat takes a list, not a single item. Here’s how I fixed it:

(defn flip [f]
  (fn [& args]
    (apply f (reverse args))))

(defn $ [node & selectors]
  (reduce (flip mapcat) [node] selectors))

So now we have a new selector that composes the behavior of a bunch of selectors.

The ‘M’ Word

By now, you may have realized that this approach is the same as that suddenly ubiquitous and hip mathematical notion, the List monad. I won’t expound any further on the point, you’re either interested in monads or you’re not. I’m of the mind that they’re a remarkably useful construct, but a bit obtuse when approached from the narrow description of only their mathematical properties.

You can find a larger working example expanding upon all the code in this post on my github

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