Mutual recursion and Tailcall() in R

I toyed with the experimental Tailcall() feature in R-devel. I can’t actually run this code when rendering the notebook, but the basic idea is that sys.nframe() shows the height of the stack when the recursion bottoms out and it is much smaller in the Tailcall() examples.

is_even <- function(n) {
  if (n == 0) list(TRUE, sys.nframe()) else Tailcall(is_odd, n - 1)
}
is_odd <- function(n) {
  if (n == 0) list(FALSE, sys.nframe()) else Tailcall(is_even, n - 1)
}
is_odd(30)
#> [[1]]
#> [1] FALSE
#> 
#> [[2]]
#> [1] 40

naive_is_even <- function(n) {
  if (n == 0) list(TRUE, sys.nframe()) else naive_is_odd(n - 1)
}
naive_is_odd <- function(n) {
  if (n == 0) list(FALSE, sys.nframe()) else naive_is_even(n - 1)
}
naive_is_odd(30)
#> [[1]]
#> [1] FALSE
#> 
#> [[2]]
#> [1] 70

(Note that the sys.nframe() values are inflated by the targets and knitr functions that are generate this notebook entry.)