default.nix

with import <nixpkgs/lib>;

let

  printf' =
    result: # String
    format: # [ Char ]
    builtins.trace "printf' \"${result}\" \"${builtins.toJSON format}\"" (
    if length format == 0 then result else
      if head format == "%" then
        let
          type = head (tail format);
          rest = tail (tail format);
        in
        if type == "s" then
          string: printf' (result + string) rest
        else if type == "d" then
          number: printf' (result + toString number) rest
        else if type == "%" then
          printf' (result + "%") rest
        else throw "Invalid format string"

      else
        printf' (result + head format) (tail format));

  printf = format: printf' "" (stringToCharacters format);


in 

  printf "%%%ste%dst" "hi" 10