Developing Web Applications with Haskell and Yesod

Appendix F. xml-conduit

Many developers cringe at the thought of dealing with XML files. XML has the reputation of having a complicated data model, with obfuscated libraries and huge layers of complexity sitting between you and your goal. I’d like to posit that a lot of that pain is actually a language and library issue, not inherent to XML.

Once again, Haskell’s type system allows us to easily break down the problem to its most basic form. The xml-types package neatly deconstructs the XML data model (both a streaming and DOM-based approach) into some simple ADTs. Haskell’s standard immutable data structures make it easier to apply transforms to documents, and a simple set of functions makes parsing and rendering a breeze.

We’re going to be covering the xml-conduit package. Under the surface, this package uses a lot of the approaches Yesod in general utilizes for high performance: blaze-builder, text, conduit, and attoparsec. But from a user perspective, it provides everything from the simplest APIs (readFile/writeFile) through full control of XML event streams.

In addition to xml-conduit, there are a few related packages that come into play, like xml-hamlet and xml2html. We’ll cover both how to use all these packages, and when they should be used.

Synopsis

Example F-1. Input XML file

<document title="My Title">
    <para>This is a paragraph. It has <em>emphasized</em> and <strong>strong</strong> words.</para>
    <image href="myimage.png"/>
</document>

Example F-2. Haskell code

{-# LANGUAGE QuasiQuotes ...