# binaryMerge :: (b -> b -> b) -> [b] -> b | null
merge:
list:

let
  # `binaryMerge start end` merges the elements at indices `index` of `list` such that `start <= index < end`
  # Type: Int -> Int -> b
  binaryMerge' = start: end:
    # assert start < end; # Invariant
    if end - start >= 2 then
      let
        # If there's at least 2 elements, split the range in two, recurse on each part and merge the result
        # The invariant is satisfied because each half will have at least 1 element
        firstHalf = binaryMerge' start (start + (end - start) / 2);
        secondHalf = binaryMerge' (start + (end - start) / 2) end;
      in merge firstHalf secondHalf
    else
      # Otherwise there will be exactly 1 element due to the invariant, in which case we just return it directly
      builtins.elemAt list start;
in
if list == [ ] then
  # Calling binaryMerge as below would not satisfy its invariant
  null
else
  binaryMerge' 0 (builtins.length list)

# Source: taken and modified from
# https://github.com/NixOS/nixpkgs/blob/3cae63521d93eb2fb0aba28218ca9b00150ffdc1/lib/attrsets.nix#L1488-L1507