A program for which prompt collection is practically impossible

promptness.fs

(*
The resizable array xs is required for the first half of the execution of
this "test" function but not the last half. Collecting xs as early as possible
would mean collecting it halfway through the execution of "test" but this
is practically impossible. Doing so in general is equivalent to solving the
halting problem.
*)

let next n =
  6364136223846793005UL*n + 1442695040888963407UL

let rec test (xs: ResizeArray<_>) n =
  xs.Add n
  let n = next n
  if n = 7331588424831733315UL then
    printfn "%d" xs.[n % uint64 xs.Count |> int]
  if n <> 10556961279111748739UL then
    test xs n

test (ResizeArray()) 3UL

Quick and dirty results using "time -v"

Toccata:
Command being timed: "./promptness"
User time (seconds): 0.21
Maximum resident set size (kbytes): 34584

FSharp
Command being timed: "./promptness.exe"
User time (seconds): 0.03
Maximum resident set size (kbytes): 40608