This is the conversion mechanism for results returned from a server language interface.
By default, JSON is used to decode the string. Otherwise the result of the basic
decoding should be provided as argument object.
Should be called by the implementation of ServerEval for individual interface classes.
valueFromServer(string, key, get, evaluator, object)the string to be passed to JSON.
the key if a proxy was allowed, otherwise the empty string.
the logical controling whether a proxy or a converted value was wanted.
the evaluator object that issued the server language expression.
If JSON is not used, the call from the server language method should provide the
elementary conversion of the result (without using asRObject() and the string argument
should be omitted. If JSON is used, object should be computed by default from JSON.
the R object implied by the server result.
Chambers, John M. (2016) Extending R, Chapman & Hall/CRC. ( Chapter 13, discussing this package, is included in the package: ../doc/Chapter_XR.pdf.)