F# Data: XML Type Provider

This article demonstrates how to use the XML type provider to access XML documents in a statically typed way. We first look how the structure is inferred and then demonstrate the provider by parsing RSS feed.

The XML type provider provides a statically typed access to XML documents. It takes a sample document as an input (or document containing a root XML node with multiple child nodes that are used as samples). The generated type can then be used to read files with the same structure. If the loaded file does not match the structure of the sample, an runtime error may occur (but only when accessing e.g. non-existing element).

Introducing the provider

The type provider is located in the FSharp.Data.dll assembly. Assuming the assembly is located in the ../../bin directory, we can load it in F# Interactive as follows: (note we also need a reference to System.Xml.Linq, because the provider uses the XDocument type under the cover):

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#r "../../../bin/FSharp.Data.dll"
#r "System.Xml.Linq.dll"
open FSharp.Data

Inferring type from sample

The XmlProvider<...> takes one static parameter of type string. The parameter can be either a sample XML string or a sample file (relatively to the current folder or online accessible via http or https). It is not likely that this could lead to ambiguities.

The following sample generates a type that can read simple XML documents with a root node containing a two attributes:

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type Author = XmlProvider<"""<author name="Paul Feyerabend" born="1924" />""">
let sample = Author.Parse("""<author name="Karl Popper" born="1902" />""")

printfn "%s (%d)" sample.Name sample.Born

The type provider generates a type Author that has properties corresponding to the attributes of the root element of the XML document. The types of the properties are inferred based on the values in the sample document. In this case, the Name property has a type string and Born is int.

XML is quite flexible format, so we could represent the same document differently. Instead of using attributes, we could use nested nodes (<name> and <born> nested under <author>) that directly contain the values:

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type AuthorAlt = XmlProvider<"<author><name>Karl Popper</name><born>1902</born></author>">
let doc = "<author><name>Paul Feyerabend</name><born>1924</born></author>"
let sampleAlt = AuthorAlt.Parse(doc)

printfn "%s (%d)" sampleAlt.Name sampleAlt.Born

The generated type provides exactly the same API for reading documents following this convention (Note that you cannot use AuthorAlt to parse samples that use the first style - the implementation of the types differs, they just provide the same public API.)

The provider turns a node into a simply typed property only when the node contains just a primitive value and has no children or attributes.

Types for more complex structure

Now let's look at a number of examples that have more interesting structure. First of all, what if a node contains some value, but also has some attributes?

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type Detailed = XmlProvider<"""<author><name full="true">Karl Popper</name></author>""">
let info = Detailed.Parse("""<author><name full="false">Thomas Kuhn</name></author>""")

printfn "%s (full=%b)" info.Name.Value info.Name.Full

If the node cannot be represented as a simple type (like string) then the provider builds a new type with multiple properties. Here, it generates a property Full (based on the name of the attribute) and infers its type to be boolean. Then it adds a property with a (special) name Value that returns the content of the element.

Types for multiple simple elements

Another interesting case is when there are multiple nodes that contain just a primitive value. The following example shows what happens when the root node contains multiple <value> nodes (note that if we leave out the parameter to the Parse method, the same text used for the schema will be used as the runtime value)

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type Test = XmlProvider<"<root><value>1</value><value>3</value></root>">

Test.GetSample().Values
|> Seq.iter (printfn "%d")

The type provider generates a property Values that returns an array with the values - as the <value> nodes do not contain any attributes or children, they are turned into int values and so the Values property returns just int[]!

Processing philosophers

In this section we look at an example that demonstrates how the type provider works on a simple document that lists authors that write about a specific topic. The sample document data/Writers.xml looks as follows:

<authors topic="Philosophy of Science">
  <author name="Paul Feyerabend" born="1924" />
  <author name="Thomas Kuhn" />
</authors> 

At runtime, we use the generated type provider to parse the following string (which has the same structure as the sample document with the exception that one of the author nodes also contains died attribute):

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let authors = """
  <authors topic="Philosophy of Mathematics">
    <author name="Bertrand Russell" />
    <author name="Ludwig Wittgenstein" born="1889" />
    <author name="Alfred North Whitehead" died="1947" />
  </authors> """

When initializing the XmlProvider, we can pass it a file name or a web url. The Load and AsyncLoad methods allows reading the data from a file or from a web resource. The Parse method takes the data as a string, so we can now print the information as follows:

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type Authors = XmlProvider<"../data/Writers.xml">
let topic = Authors.Parse(authors)

printfn "%s" topic.Topic
for author in topic.Authors do
  printf " - %s" author.Name 
  author.Born |> Option.iter (printf " (%d)")
  printfn ""

The value topic has a property Topic (of type string) which returns the value of the attribute with the same name. It also has a property Authors that returns an array with all the authors. The Born property is missing for some authors, so it becomes option<int> and we need to print it using Option.iter.

The died attribute was not present in the sample used for the inference, so we cannot obtain it in a statically typed way (although it can still be obtained dynamically using author.XElement.Attribute(XName.Get("died"))).

Global inference mode

In the examples shown earlier, an element was never (recursively) contained in an element of the same name (for example <author> never contained another <author>). However, when we work with documents such as XHTML files, this can often be the case. Consider for example, the following sample (a simplified version of data/HtmlBody.xml):

<div id="root">
  <span>Main text</span>
  <div id="first">
    <div>Second text</div>
  </div>
</div>

Here, a <div> element can contain other <div> elements and it is quite clear that they should all have the same type - we want to be able to write a recursive function that processes <div> elements. To make this possible, you need to set an optional parameter Global to true:

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type Html = XmlProvider<"../data/HtmlBody.xml", Global=true>
let html = Html.GetSample()

When the Global parameter is true, the type provider unifies all elements of the same name. This means that all <div> elements have the same type (with a union of all attributes and all possible children nodes that appear in the sample document).

The type is located under a type Html, so we can write a printDiv function that takes Html.Div and acts as follows:

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/// Prints the content of a <div> element
let rec printDiv (div:Html.Div) =
  div.Spans |> Seq.iter (printfn "%s")
  div.Divs |> Seq.iter printDiv
  if div.Spans.Length = 0 && div.Divs.Length = 0 then
      div.Value |> Option.iter (printfn "%s")

// Print the root <div> element with all children  
printDiv html

The function first prints all text included as <span> (the element never has any attributes in our sample, so it is inferred as string), then it recursively prints the content of all <div> elements. If the element does not contain nested elements, then we print the Value (inner text).

Reading RSS feeds

To conclude this introduction with a more interesting example, let's look how to parse a RSS feed. As discussed earlier, we can use relative paths or web addresses when calling the type provider:

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type Rss = XmlProvider<"http://tomasp.net/blog/rss.aspx">

This code builds a type Rss that represents RSS feeds (with the features that are used on http://tomasp.net). The type Rss provides static methods Parse, Load and AsyncLoad to construct it - here, we just want to reuse the same uri of the schema, so we use the GetSample static method:

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let blog = Rss.GetSample()

Printing the title of the RSS feed together with a list of recent posts is now quite easy - you can simply type blog followed by . and see what the autocompletion offers. The code looks like this:

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// Title is a property returning string 
printfn "%s" blog.Channel.Title

// Get all item nodes and print title with link
for item in blog.Channel.Items do
  printfn " - %s (%s)" item.Title item.Link

Related articles

• API Reference: XmlProvider type provider
• API Reference: XElementExtensions module

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