Prefixing the c function from terra - r

Question

In R, I can prefix functions with the name of the package they belong to (e.g., dplyr::select). Nevertheless, I am having problems when doing this with c from the terra package. I can do it fine with base::c (should I want to):
base::c(1, 2, 3)
# [1] 1 2 3
However, I run into problems when running similar code for terra:
# Dummy SpatRaster
foo <- terra::rast(matrix(1:9, ncol=3))
# Works fine
c(foo, foo)
# Not so much
terra::c(foo, foo)
# Error: 'c' is not an exported object from 'namespace:terra'
I am confused how c is not an exported function of terra and yet I can access and use it just fine... so long as I don't use a prefix.
Q: Can someone explain why this is the case and how I can explicitly refer to c from terra?
PS ?terra::c gives a help page explaining how c combines SpatRasterobjects into a new SpatRaster object, which suggests to me that this function must've been implemented in the terra package.

Answer 1

This is because c is a "primitive" function --- these follow their own rules. A package cannot / need not import and export them
c
#function (...) .Primitive("c")
This is not the case, for, for example, nrow
nrow
#function (x)
#dim(x)[1L]
#<bytecode: 0x000000001662d228>
#<environment: namespace:base>
And terra creates a generic function from it, and exports it so that you can do terra::nrow().
Your question triggered me to look a bit more at this, and I noted that in the current version of terra many primitive functions do not work if you do not load the package with library(terra). For example, you get
foo <- terra::rast(matrix(1:9, ncol=3))
x <- c(foo, foo)
max(x)
#Error in x#ptr$summary(fun, na.rm, .terra_environment$options#ptr) :
# trying to get slot "ptr" from an object of a basic class ("NULL") with no slots
I just fixed that in the development version; and with that version the above returns
#class : SpatRaster
#dimensions : 3, 3, 1 (nrow, ncol, nlyr)
#resolution : 1, 1 (x, y)
#extent : 0, 3, 0, 3 (xmin, xmax, ymin, ymax)
#coord. ref. : +proj=longlat +datum=WGS84 +no_defs
#source : memory
#names : max
#min values : 1
#max values : 9