pub fn day(s: String) -> Result(Int, Snag)

pub fn days(l: List(String)) -> Result(List(Int), Snag)

pub fn int(s: String) -> Result(Int, Snag)

pub fn valid_int(s: String, is_valid: fn(Int) -> Bool, invalid_msg: String) -> Result(
  Int,
  Snag,
)