xquery: Evaluate an XQuery expression
In omegahat/RXQuery: Bi-directional interface to an XQuery engine

Description Usage Arguments Value Author(s) References See Also Examples

This function provides an interface to the XQuery engine Zorba and allows the caller to evaluate an XQuery expression(s). This interface also allows the caller to provide R functions that act as XQuery functions which can be invoked within the XQuery expressions. In other words, the R functions become available as regular XQuery expressions.

xquery(query, ..., funcs = list(...),
        addQuery = sapply(funcs, inherits, "XQueryContextFunction"),
         prefix = character(),  ns = character(),
          baseURI = paste("file://", getwd(), "/", sep = ""), 
           ctxt = NULL, module = createModule(, ctxt),
            .deterministic = rep(TRUE, length(funcs)),
             resultSize = 100, expandFactor = 2,
            registerModule = length(funcs) > 0)

`query`	character string giving the XQuery expression/code.
`...`	a collection of R function objects and/or names which specifies R functions that are exported to XQuery and which can be used within XQuery expressions.
`funcs`	a named list of R function names and/or objects that will be registered with and made available to the XPath engine so that they can be called with the XPath query.
`addQuery`	No longer used! See registerFunctions. This now corresponds to a non-deterministic function. a logical vector with as many elements as `funcs` which indicates whether the function needs to be passed a reference to the XQuery context object. A function can use this to perform computations within the XQuery engine.
`prefix`	a character vector which is used as content to place at the start of the XPath expression provided in `query`. This is intended to allow the separation of function declarations and definitions that are used in multiple XPath queries and are not specific to a particular query.
`ns`	a named character vector that is used to declare namespaces in the prolog of the query.
`baseURI`	the URI used to resolve relative URLs.
`ctxt`	the static context in which to evaluate this query.
`module`	if there are any functions to be registered with Zorba as external functions (`funcs`), we use this ExternalModule object to house and register them.
`.deterministic`	passed on to `registerFunctions`
`resultSize`	a guess as to how many elements (zorba::Item objects) will be returned from the query. This is used to allocate the R object to hold the results. The closer this is to the actual number the more efficient R will be. However, the package will do the correct thing regardless of the initial guess.
`expandFactor`	a number greater than 1 that is used to grow the R object used to collect the results from the query when we need more space.
`registerModule`	a logical that controls whether to register the R external function module with the Zorba context. When we specify R functions to use in the query, this is automatically done. However, one can force the registration of the module without specifying any functions to explicitly register. In that case, when the functions are needed, we look in the R search path (including the module's environment) and find the functions directly. We currently assume all functions found in this way are "deterministic" (wrt the Zorba compiler). In the future, we will look at the class of the object.

An R object that is the result of the XQuery expression.

Duncan Temple Lang

XQuery http://www.w3.org/TR/xquery/

Zorba http://www.zorba-xquery.com/

compileXQuery

xquery( "1 + 2")


  # invoke XQuery functions. This checks the conversion.
  # Check the class of these objects are POSIXt, POSIXct.

 xquery( "fn:current-dateTime()")
 xquery( "fn:current-date()")
 xquery( "fn:current-time()")

  #

query = 'declare namespace R = "urn:R" ; 
         declare function R:nchar($x) external; 
         declare function R:rnorm($n) external ; 
         R:nchar("a string with 27 characters") + R:rnorm(1)'


     # call with two R functions.
 i = xquery( query, rnorm, "nchar")


     # Specify the functions as a list with explicit names.
 i = xquery(query, funcs = list(rnorm = rnorm, nchar = "nchar"))