Extensible Markup Language (XML) is an encoding of plain text to provide machine-readable annotations on a document. The standard is specified by W3C (http://www.w3.org/TR/2008/REC-xml-20081126/).
In this recipe, we will parse an XML document representing an e-mail conversation and extract all the dates.
We will first set up an XML file called input.xml with the following values, representing an e-mail thread between Databender and Princess on December 18, 2014 as follows:
$ cat input.xml
<thread>
<email>
<to>Databender</to>
<from>Princess</from>
<date>Thu Dec 18 15:03:23 EST 2014</date>
<subject>Joke</subject>
<body>Why did you divide sin by tan?</body>
</email>
<email>
<to>Princess</to>
<from>Databender</from>
<date>Fri Dec 19 3:12:00 EST 2014</date>
<subject>RE: Joke</subject>
<body>Just cos.</body>
</email>
</thread>Using Cabal, install the HXT library which we use for manipulating XML documents:
$ cabal install hxt
We only need one import, which will be for parsing XML, using the following line of code:
import Text.XML.HXT.Core
Define and implement
mainand specify the XML location. For this recipe, the file is retrieved frominput.xml. Refer to the following code:main :: IO () main = do input <- readFile "input.xml"Apply the
readStringfunction to the input and extract all the date documents. We filter items with a specific name using thehasName :: String -> a XmlTree XmlTreefunction. Also, we extract the text using thegetText :: a XmlTree Stringfunction, as shown in the following code snippet:dates <- runX $ readString [withValidate no] input //> hasName "date" //> getTextWe can now use the list of extracted dates as follows:
print dates
By running the code, we print the following output:
$ runhaskell Main.hs ["Thu Dec 18 15:03:23 EST 2014", "Fri Dec 19 3:12:00 EST 2014"]
The library function, runX, takes in an
Arrow. Think of an Arrow as a more powerful version of a Monad. Arrows allow for stateful global XML processing. Specifically, the runX function in this recipe takes in IOSArrow XmlTree String and returns an IO action of the String type. We generate this IOSArrow object using the readString function, which performs a series of operations to the XML data.
For a deep insight into the XML document, //> should be used whereas /> only looks at the current level. We use the //> function to look up the date attributes and display all the associated text.
As defined in the documentation, the hasName function tests whether a node has a specific name, and the getText function selects the text of a text node. Some other functions include the following:
All the Arrow functions can be found on the Text.XML.HXT.Arrow.XmlArrow documentation at http://hackage.haskell.org/package/hxt/docs/Text-XML-HXT-Arrow-XmlArrow.html.