# Computes the ackermann function by recursively importing derivations. Evaluate with
#
# nix-instantiate --eval --read-write-mode ./ack.nix -A result --arg m 3 --arg n 2
{ m, n }:
let
  pkgs = import <nixpkgs> {};
  recurse = m': n': ''
    (import ${toString __curPos.file} { m = ${toString m'}; n = ${toString n'}; }).result
  '';
  text =
    if m == 0 then toString (n + 1)
    else if n == 0 then recurse (m - 1) 1
    else recurse (m - 1) (recurse m (n - 1));
  drv = pkgs.writeText "ack-${toString m}-${toString n}" text;
in drv // {
  result = import drv;
}