[Rcpp-devel] Checking for a promise

[Dominick Samperi]

On Sat, Dec 25, 2010 at 4:08 PM, Gabor Grothendieck <ggrothendieck at gmail.com
> wrote:

> On Sat, Dec 25, 2010 at 3:58 PM, Dominick Samperi <djsamperi at gmail.com>
> wrote:
> > Using .Call appears to force the promise:
> >
> > msg="old"
> > delayedAssign("x",msg)
> > msg="new"
> > .Call('sexpType',x) # promise triggered here, returns 16
> > msg="even newer" # will not change already fired promise
> > .Call('sexpType',x) # returns 16
> > y = x
> > y # "new" (not "even newer")
> >
> > Here's sexpType:
> >
> > RcppExport SEXP sexpType(SEXP x_) {
> >     return Rcpp::wrap(TYPEOF(x_));
> > }
> >
> > The type returned is 16 here (STRSXP). If numbers were
> > assigned to msg instead 14 would be returned (REALSXP).
> >
>
> Note that the first attempt I posted tried to get around that by
> passing the environment and the name of the variable in the
> environment so that the object would not itself be passed yet it did
> not work either.  Here is a slight variation:
>

Yes, I tired your variant (just passing the environment) but it
still doesn't work. Here is the explanation: Rcpp currently
implements e[] (i.e., operator [] for Environment) via get(),
and when the variable is a promise it evaluates, effectively
forcing the promise. There is a Rcpp::Promise class, but it is
probably work in progress because it is not exposed.


> > library(Rcpp)
> > library(inline)
> > f <- cxxfunction(signature(env="environment", nm = "character"),
> +        body=' Environment e(env);
> +           std::string s = as<std::string>(nm);
> +           return wrap(TYPEOF (e[s])); ',
> +        plugin="Rcpp")
> >
> > # create promise
> > delayedAssign("prom", 3)
> >
> > # want it to return 5 but it returns 14
> > f(.GlobalEnv, "prom")
> [1] 14
>
>
>
> --
> Statistics & Software Consulting
> GKX Group, GKX Associates Inc.
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> email: ggrothendieck at gmail.com
>
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