A prototype for a mechanism for specifying the types of
parameters and the return value for an R function. This is
meta-information that can be used to generate stubs for servers
and various interfaces to these functions. Additionally, the
arguments in a call to a typed function can be validated using
the type specifications.
We allow types to be specified as either
i) by class name using either inheritance - is(x, className),
or strict instance of - class(x) %in% className, or
ii) a dynamic test given as an R expression which is evaluated
at run-time.
More precise information and interesting tests can be done via
ii), but it is harder to use this information as meta-data as
it requires more effort to interpret it and it is of course
run-time information. It is typically more meaningful.