Oh, those wonderful four words every Java programmer knows so well . Defining a constant, never to be changed and accessible from everywhere. A powerful tool, if used right and with care! But how can I do such a thing in R? Is there a common pratice to do this?

Well, if you don’t know, you must ask and you shall be answered: https://stat.ethz.ch/pipermail/r-help/2010-July/245480.html. Duncan Murdoch replied:

If they are constants, you won’t need to use <<- or assign() to change them:  just define them at top level in one of the source files for your package.

Simpler than I thought, and more elegant.

I use this method for setting package wide names for the operations, connections method names, character encoding, as well as namespace attributes so far. All of these are used to switch between different cases at some point in the code, so their correctness is important which makes the single point of change very useful.

See the class Constants.R, excerpt below:

sosService <- "SOS"
sosNamespacePrefix <- "sos"
sosGetCapabilitiesName <- "GetCapabilities"
sosDescribeSensorName <- "DescribeSensor"

# [...]

.sosConnectionMethodGet <- "GET"
.sosConnectionMethodPost <- "POST"

These constants are then used for example in the main request function and encoding functions:

if(sos@method == .sosConnectionMethodGet) {
.kvpEncoding = .encode(request, verbose)

# [...]

}

# [...]

sosEncodeRequestXMLGetObservation_1.0.0 <- function(obj) {
.xmlDoc <- xmlNode(name = sosGetObservationName,
namespace = sosNamespacePrefix,
namespaceDefinitions = c(.sosNamespaceDefinitionsAll,
.sosNamespaceDefinitionsGetObs),
attrs=c(.xsiSchemaLocationAttribute, service = obj@service,
version = obj@version))

# required and optional are mixed as schema requires a particular order:
.offering <- xmlNode(name = "offering", namespace = sosNamespacePrefix,
obj@offering)
.xmlDoc <- addChildren(node = .xmlDoc, .offering)
#  [...]

The “namespace” attribute should not be highlighted in the code, I’m sorry about that (because of a lack of alternatives I’m using C++ syntax highlighting).