datatype student = Firstyear of string |
                   Sophomore of string |
                   Junior of string |
                   Senior of string;

fun studentToString (Firstyear x) = "First year " ^ x
  | studentToString (Sophomore x) = "Sophomore " ^ x
  | studentToString (Junior x)    = "Junior " ^ x
  | studentToString (Senior x)    = "Senior " ^ x;

fun isGraduatingLong (Firstyear _) = false
  | isGraduatingLong (Sophomore _) = false
  | isGraduatingLong (Junior _)    = false
  | isGraduatingLong (Senior _)    = true;

fun isGraduating (Senior _) = true
  | isGraduating _          = false;

fun funFacts x =
    if isGraduating x then
        (studentToString x) ^ " is graduating!"
    else
        (studentToString x) ^ " has more college fun coming!"

fun isOlder (Firstyear x) _             = false
  | isOlder (Sophomore x) (Firstyear y) = true
  | isOlder (Sophomore x) _             = false
  | isOlder (Junior x)    (Firstyear y) = true
  | isOlder (Junior x)    (Sophomore y) = true
  | isOlder (Junior x)    _             = false
  | isOlder (Senior x)    (Senior y)    = false
  | isOlder (Senior x)    _             = true;

fun compareAge x y =
    if isOlder x y then
        GREATER
    else if isOlder y x then
        LESS
    else
        EQUAL;