[Rd] getting environment from "top" promise

[Romain Francois]

Hello, 

We have something very similar to your while loop in dplyr. 
https://github.com/hadley/dplyr/blob/02a609310184d003c2ae9e0c013bfa69fa4d257a/inst/include/tools/DataDots.h#L15

because we need to know exactly in which environment a promise is supposed to be evaluated, even though we might combine standard R evaluation with an alternative faster engine. this is the basis of what we called hybrid evaluation. 


For future work, I also have the while loop in the Promise class in Rcpp11, so that when you create a Promise in Rcpp11, its .environment() method gives you what you expect. 
https://github.com/romainfrancois/Rcpp11/blob/master/inst/include/Rcpp/Promise.h#L14 

So, this is something I find useful, although I’m not sure we are supposed to mess with promises. 

Romain

Le 11 févr. 2014 à 19:02, Michael Lawrence <lawrence.michael at gene.com> a écrit :

> Hi all,
> 
> It seems that there is a use case for obtaining the environment for the
> "top" promise. By "top", I mean following the promise chain up the call
> stack until hitting a non-promise.
> 
> S4 data containers often mimic the API of base R data structures. This
> means writing S4 methods for functions that quote their arguments, like
> with() and subset(). The methods package directly forwards any arguments
> not used for dispatch, so substitute(subset) is able to resolve the
> original quoted argument (this is not the case for naively written
> wrappers).  The problem then becomes figuring out the environment in which
> to evaluate the expression.
> 
> Consider:
> 
> setClass("A", representation(df = "data.frame"))
> 
> setMethod("subset", "A", function(x, subset) {
>  env <- parent.frame(2)
>  x at df <- x at df[eval(substitute(subset), x at df, env),,drop=FALSE]
>  x
> })
> 
> dropLowMpg <- function(x, cutoff=20) {
>  invisible(subset(x, mpg > cutoff))
> }
> 
> a <- new("A", df=mtcars)
> dropLowMpg(a)
> 
> The above works just fine, because we figured out that we need to evaluate
> in the grand-parent frame to avoid the frame of the generic call. But now
> let's assume A has a subclass B, and subset,B delegates to subset,A via
> callNextMethod(). The call stack is different, and our assumption is
> invalid.
> 
> setClass("B", representation(nrow="integer"), contains="A")
> setMethod("subset", "B", function(x, ...) {
>  ans <- callNextMethod()
>  ans at nrow <- nrow(ans at df)
>  ans
> })
> b <- new("B", df=mtcars)
> dropLowMpg(b)
> Error in eval(expr, envir, enclos) (from #3) : object 'cutoff' not found
> 
> We can fix this with a simple C function:
> SEXP top_prenv(SEXP nm, SEXP env)
> {
>  SEXP promise = findVar(nm, env);
>  while(TYPEOF(promise) == PROMSXP) {
>    env = PRENV(promise);
>    promise = PREXPR(promise);
>  }
>  return env;
> }
> 
> With R wrapper:
> top_prenv <- function(x) {
>  .Call2("top_prenv", substitute(x), parent.frame())
> }
> 
> Then this works (need to set subset,B again to reset cache):
> 
> setMethod("subset", "A", function(x, subset) {
>  env <- top_prenv(subset)
>  x at df <- x at df[eval(substitute(subset), x at df, env),,drop=FALSE]
>  x
> })
> setMethod("subset", "B", function(x, ...) {
>  ans <- callNextMethod()
>  ans at nrow <- nrow(ans at df)
>  ans
> })
> 
> b <- new("B", df=mtcars)
> dropLowMpg(b)
> 
> Would this be a useful addition to R? Is there a better way to solve this
> issue? We're using this successfully in the IRanges package now, but we'd
> like to avoid dealing with the internal details of R, and this is something
> that could be of general benefit.
> 
> Thanks,
> Michael
> 
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> 
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