Advent of Code 2020 in Nix

day1.nix

#!/usr/bin/env -S nix-instantiate --eval --strict
# real  0m4.143s
with (builtins.getFlake "nixos").lib;
let
  tails = l: if l == [] then [] else [ l ] ++ tails (tail l); 
  sum = foldl' (x: y: x + y) 0;
  product = foldl' (x: y: x * y) 1;
  lines = s: splitString "\n" (removeSuffix "\n" s); 

  nums = map toInt (lines (builtins.readFile ./input1));
  solve = nums: n: let
    go = nums: n: xs: if n == 0 then
      optional (sum xs == 2020) xs
    else
      concatMap (ys:
        go (tail ys) (n - 1) ([ (head ys) ] ++ xs) 
      ) (tails nums);
  in product (head (go nums n []));
in
  map (solve nums) [ 2 3 ]