How can I measure the time taken to execute a function call in ATS?

Say I wrote a function foo and I would like to find out the time taken to execute foo(1000). Is there already a package available for doing this?

If seconds-resolution is OK, you can #include "libats/libc/DATS/time.dats" and then use time() and difftime():

implement main0() = {
  val before = time()
  val _ = foo(1000)
  val () = println!(difftime(time(), before))
}

If you'd like finer resolution you can, under Linux, use libc's gettimeofday():

extern fun reset_timer(): void = "ext#reset_timer"
extern fun elapsed_time(): double = "ext#elapsed_time"
%{
#include <sys/time.h>
#include <time.h>
struct timeval timer_timeval;
void reset_timer() { gettimeofday(&timer_timeval, NULL); }
double elapsed_time() {
  struct timeval now;
  gettimeofday(&now, NULL);
  int secs = now.tv_sec - timer_timeval.tv_sec;
  double ms = (now.tv_usec - timer_timeval.tv_usec) / ((double)1000000);
  return(secs + ms);
}
%}

/* OCaml-style helper to ignore the value of foo(1000) */
fn {a:t@ype} ignore(x: a): void = ()

implement main0() =
  begin
    reset_timer();
    ignore(foo(1000));
    println!("foo(1000) took: ", elapsed_time(), "s");
  end

Output (where foo(1000) takes about 3000.3 milliseconds):

foo(1000) took: 3.000296s