Half-assed ID3 attempt.

gistfile1.sml

fun name (n, v) = n;
fun vals (n, v) = v;

fun get([], a) = 0
  | get((b, v) :: tail, a) = if a = b then v else get(tail, a);

fun ig(a, S, C) =
    (* TODO :( *)
    C(S, name a);

fun pickBest(A, S, C) =
    foldl(fn ((n1, ig1), (n2, ig2)) => if ig1 >= ig2
                                      then (n1, ig1)
                                      else (n2, ig2))
         (hd A, 0)
         (map (fn a => (a, ig(a, S, C)))
              A);

fun filterS([], _, _) = []
  | filterS(s :: S, v, a) =
    if get(s a) = v
    then s :: filterS(S, v, a)
    else filterS(S, v, a);

fun removeA([], _) = []
  | removeA((b, v) :: A, a) =
    if b = a
    then removeA(A, a)
    else (b, v) :: removeA(A, a);

datatype Tree = Leaf of int | Branch of string * (int * Tree) list;

fun id3(S, C, []) = Leaf (C(S, "all"))
  | id3(S, C, A) = let val (a, _) = pickBest (A, S, C)
                   in Branch (name a,
                              map (fn v =>
                                      (v, id3(filterS(S, v, name a), C, removeA(A, name a))))
                                  (vals a))
                   end;