sml - What's meant by "isSome oldest(tl dates)" here; why "oldest(tl dates)" has a option type?

Question

fun oldest (dates: (int*int*int) list) =
    if null dates
    then NONE
    else let
    val ans = oldest(tl dates)
    in
    if isSome ans andalso is_older(valOf ans, hd dates)
    then ans
    else SOME (hd dates)
    end

a function compares all dates(like 2018,2,28) in the list, and get the oldest date

question from:https://stackoverflow.com/questions/65873724/whats-meant-by-issome-oldesttl-dates-here-why-oldesttl-dates-has-a-opt

Answer 1

Waitting for answers