release code error but not in debug or on source - r

Question

My code runs fine but fails in a package
I boiled it down to
wtf<-function(r)
{
require(raster)
stopifnot(class(r) == "RasterLayer")
return(as.matrix(r))
}
When sourced, everything works fine. When the function is part of a package, it fails. It nicely runs in debug mode though, step by step.
library(mypackage)
r <- raster(ncol=6, nrow=6)
r[] <- runif(ncell(r),0,1)
extent(r) <- matrix(c(0, 0, 6, 6), nrow=2)
wtf(r)
# Error in as.vector(data) :
# no method for coercing this S4 class to a vector
# Traceback
# 5 as.vector(data)
# 4 array(x, c(length(x), 1L), if (!is.null(names(x))) list(names(x),
# NULL) else NULL)
# 3 as.matrix.default(r)
# 2 as.matrix(r) at terrain.R#7
# 1 wtf(s)
I'm a bit puzzeled as to why this happens and to how proceed.
The build went fine, the check went clean, so what is going on?
What would be the next question to ask and explore in order to solve the problem?
R version 3.1.1 (2014-07-10)
Platform: x86_64-apple-darwin10.8.0 (64-bit)
locale:
[1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] raster_2.3-0 spdep_0.5-77 Matrix_1.1-4 minerva_1.4.1 gdata_2.13.3 rgdal_0.9-1 sp_1.0-15
loaded via a namespace (and not attached):
[1] boot_1.3-11 coda_0.16-1 deldir_0.1-5 grid_3.1.1 gtools_3.4.1 lattice_0.20-29 LearnBayes_2.15 MASS_7.3-33 nlme_3.1-118 parallel_3.1.1 splines_3.1.1 tools_3.1.1

Answer 1

The traceback shows that the default as.matrix is used, rather than the raster variant. I believe this problem goes away if you add this line to your Namespace file:
import(raster)
Or when you are explicit about which as.matrix you want:
wtf <- function(r) {
stopifnot(inherits(r, "RasterLayer"))
raster::as.matrix(r)
}
Rather than 'manually' testing for class membership, you might consider a more formal (S4) approach:
if (!isGeneric("wtf")) {
setGeneric("wtf", function(x, ...)
standardGeneric("wtf"))
}
setMethod("wtf", signature(x='RasterLayer'),
function(x, ...) {
raster::as.matrix(x)
}
)