readXML: Read In an XML Document

Description

Return a function which reads in an XML document. The structure of the XML document is described with a specification.

Usage

readXML(spec, doc)

Arguments

spec

A named list of lists each containing two components. The constructed reader will map each list entry to the content or metadatum of the text document as specified by the named list entry. Valid names include content to access the document's content, and character strings which are mapped to metadata entries.

Each list entry must consist of two components: the first must be a string describing the type of the second argument, and the second is the specification entry. Valid combinations are:

type = "node", spec = "XPathExpression": The XPath expression spec extracts information from an XML node.
type = "attribute", spec = "XPathExpression": The XPath expression spec extracts information from an attribute of an XML node.
type = "function", spec = function(tree) …: The function spec is called, passing over a tree representation (as delivered by xmlInternalTreeParse from package XML) of the read in XML document as first argument.
type = "unevaluated", spec = "String": The character vector spec is returned without modification.

doc

An (empty) document of some subclass of TextDocument.

Value

A function with the following formals:

elem: a named list with the component content which must hold the document to be read in.
language: a string giving the language.
id: a character giving a unique identifier for the created text document.

The function returns doc augmented by the parsed information as described by spec out of the XML file in elem$content. The arguments language and id are used as fallback: language if no corresponding metadata entry is found in elem$content, and id if no corresponding metadata entry is found in elem$content and if elem$uri is null.

Details

Formally this function is a function generator, i.e., it returns a function (which reads in a text document) with a well-defined signature, but can access passed over arguments (e.g., the specification) via lexical scoping.

Examples

Run this code

# NOT RUN {
readGmane <-
readXML(spec = list(author = list("node", "/item/creator"),
                    content = list("node", "/item/description"),
                    datetimestamp = list("function", function(node)
                    strptime(sapply(XML::getNodeSet(node, "/item/date"), XML::xmlValue),
                             format = "%Y-%m-%dT%H:%M:%S",
                             tz = "GMT")),
                    description = list("unevaluated", ""),
                    heading = list("node", "/item/title"),
                    id = list("node", "/item/link"),
                    origin = list("unevaluated", "Gmane Mailing List Archive")),
                    doc = PlainTextDocument())
# }