[Rcpp-devel] Checking for a promise

Le 25/12/10 02:10, Gabor Grothendieck a ?crit :

How do I fix this so it works? ?The code is intended to return TRUE or
FALSE according to whether the variable with name nm in environment
env is a promise or not. ?In the example code below it returns FALSE
but I would like it to return TRUE. ?Is the promise being forced
somewhere along the way? ?Some other problem?

library(Rcpp)
f<- cxxfunction(signature(env="environment", nm = "character"),
? ? ? ? body=' Environment e(env);
? ? ? ? ? ?std::string s = as<std::string>(nm);
? ? ? ? ? ?return wrap(TYPEOF (e[s]) == PROMSXP); ',
? ? ? ? plugin="Rcpp")

# create promise
delayedAssign("prom", 3)

# want it to return TRUE but it returns FALSE
f(.GlobalEnv, "prom")

Hello,

Sorry for jumping late in this thread, xmas oblige.

Environment::operator[] eventually calls Environment::get() which is
implemented (in Environment.cpp) as follows:

? ?SEXP Environment::get( const std::string& name) const {
? ? ? ?SEXP res = Rf_findVarInFrame( m_sexp, Rf_install(name.c_str()) ?) ;

? ? ? ?if( res == R_UnboundValue ) return R_NilValue ;

? ? ? ?/* We need to evaluate if it is a promise */
? ? ? ?if( TYPEOF(res) == PROMSXP){
? ? ? ? ? ? ? ?res = Rf_eval( res, m_sexp ) ;
? ? ? ?}
? ? ? ?return res ;
? ?}

So the promise is explicitely forced.


We could augment the interface of Environment so that it adds something that
would not force the promise. Perhaps adding ', bool force = false' to get
and find.

We could also add a ctor to Rcpp::Promise that would grab a promise from an
environment, something like:

Promise::Promise( const Environment& env, std::string& name){
? ? ? ?SEXP res = Rf_findVarInFrame( m_sexp, Rf_install(name.c_str()) ?) ;
? ? ? ?if( TYPEOF(res) != PROMSXP){ be unhappy ; }
? ? ? ?setSEXP( res ) ;
}

Promises is a beast I rarely use, so the interface is still in a "to be
improved" state. Suggestions are much welcome, for example pseudo code
chunks of the sort of things one would do with them.