CS1 Supervision 1.sml

fun normalizeDate (d1::d2::nil) = (if d1 < 5 then d1 + 10 else d1) * 10 + d2
  | normalizeDate _ = raise Match;
 
exception DateOutOfBounds
fun denormalizeDate date_number =
    if
        date_number < 50 orelse date_number > 149
    then
        raise DateOutOfBounds
    else let
        val decades = (date_number div 10)
        val years   = (date_number mod 10)
    in
        [decades mod 10, years]
    end;
 
fun compareDates date1 date2 = normalizeDate date1 > normalizeDate date2;
 
fun subtractYears num_years date = denormalizeDate ((normalizeDate date) - num_years);