[Rcpp-devel] Evaluating a call in a particular environment

[Douglas Bates]

A short version of this question is "can I evaluate an Rcpp::Language
instance in an Rcpp::Environment instance"?  Initially I thought this
was a given but on looking closer at the unit tests I don't see how
exactly it would be done.

A second short version of this question is "could someone, probably
Romain, remind me what is done to activate the debugging code in Rcpp
or point me to the documentation for it?".

The long version follows:

In the lme4a package (only on R-forge and requiring the R-forge
version of the Matrix package) the nlmer function to fit nonlinear
mixed-effects models has a nonlinear model, expressed as a call, and
an environment in which to evaluate this call.  The operation of
evaluating the deviance of the model involves changing the contents of
some numeric vectors in this environment then evaluating the call.

The nlmerResp class is declared (in lme4a/src/mer.h) as
    class nlmerResp : public rwResp {
	Rcpp::Environment nlenv;
	Rcpp::Language nlmod;
	Rcpp::CharacterVector pnames;
    public:
	nlmerResp(Rcpp::S4 xp);
	void updateMu(Rcpp::NumericVector const &gamma);
    };

and initialized from an Rcpp::S4 object.  I have some debugging code
in the current version of the updateMu method to check that the values
in the environment are what I expect them to be
> nm1 <- lme4a:::nlmer2(circumference ~ SSlogis(age, Asym, xmid, scal) ~ 0 + Asym + xmid + scal + (0 + Asym|Tree), Orange, start = c(Asym = 200, xmid = 725, scal = 350))
> .Call("nlmerDeEval", nm1)
past initialization of nlmerDe object
gamma after offset: 0, 0, 0, 0, 0
u: 0, 0, 0, 0, 0
b: 0, 0, 0, 0, 0
gamma after reUpdate: 0, 0, 0, 0, 0
gamma after feUpdate: 200, 200, 200, 200, 200
gamma in resp::updateMu: 200, 200, 200, 200, 200
p = 0, parameter name = Asym
current: 200, 200, 200, 200, 200
updated: 200, 200, 200, 200, 200
p = 1, parameter name = xmid
current: 725, 725, 725, 725, 725
updated: 725, 725, 725, 725, 725
p = 2, parameter name = scal
current: 350, 350, 350, 350, 350
updated: 350, 350, 350, 350, 350

Before evaluation
Error in cpp_exception(message = "object 'xmid' not found", class =
"Rcpp::eval_error") :
  object 'xmid' not found

So I am a bit surprised at xmid not being found because I just
modified it.  The objects at the R level are

> nm1 at resp@nlmod
SSlogis(age, Asym, xmid, scal)
> class(nm1 at resp@nlmod)
[1] "call"
> ls.str(nm1 at resp@nlenv)
age :  num [1:35] 118 484 664 1004 1231 ...
Asym :  num [1:35] 200 200 200 200 200 200 200 200 200 200 ...
scal :  num [1:35] 350 350 350 350 350 350 350 350 350 350 ...
xmid :  num [1:35] 725 725 725 725 725 725 725 725 725 725 ...

and I can evaluate the call in R

> head(eval(nm1 at resp@nlmod, nm1 at resp@nlenv))
[1]  30.00804  66.87019  91.30771 137.87260 161.86760 172.79186

I am trying to evaluate the call in the C++ code as

   NumericVector rr = nlmod.eval(nlenv);

which, I assume, expands to

  NumericVector rr = nlmod.eval(SEXP(nlenv));

Is there some trick I am missing?