how can i show a stack trace in r?

If you want to see the call stack without an error being thrown, you could try something like this:

show_stack <- function() {
  cat("#----- Stack containing call to show_stack -----#\n\n")
  x <- sys.calls()
  lapply(head(x, -1), function(x) {print(x); cat("\n")})
  cat("#-----------------------------------------------#\n\n")
}

Which you just insert in whichever function you want to trace:

foo <- function() bar()
bar <- function() baz()
baz <- function() show_stack()

Resulting in:

foo()
#> #----- Stack containing call to show_stack -----#
#> 
#> foo()
#> 
#> function() bar()
#> 
#> function() baz()
#> 
#> #-----------------------------------------------#

Or, for a more real-world example:

my_mean <- function(x) {
  show_stack()
  sum(x)/length(x)
}

Which may be run multiple times in a call:

tapply(iris$Sepal.Length, iris$Species, my_mean)
#> #----- Stack containing call to show_stack -----#
#> 
#> tapply(iris$Sepal.Length, iris$Species, my_mean)
#> 
#> lapply(X = ans[index], FUN = FUN, ...)
#> 
#> FUN(X[[i]], ...)
#> 
#> #-----------------------------------------------#
#> 
#> #----- Stack containing call to show_stack -----#
#> 
#> tapply(iris$Sepal.Length, iris$Species, my_mean)
#> 
#> lapply(X = ans[index], FUN = FUN, ...)
#> 
#> FUN(X[[i]], ...)
#> 
#> #-----------------------------------------------#
#> 
#> #----- Stack containing call to show_stack -----#
#> 
#> tapply(iris$Sepal.Length, iris$Species, my_mean)
#> 
#> lapply(X = ans[index], FUN = FUN, ...)
#> 
#> FUN(X[[i]], ...)
#> 
#> #-----------------------------------------------#
#> 
#>     setosa versicolor  virginica 
#>      5.006      5.936      6.588