(* --- list functions --- *)

(* examples from the text *)

fun repeatEach([]) = []
  | repeatEach(a::rest) = a::a::repeatEach(rest);

fun sum([]) = 0
  | sum(a::rest) = a + sum(rest);

fun switchPair([]) = []
  | switchPair((a,b)::rest) = (b, a)::switchPair(rest);



(* 2.2.4 -- in class*)
fun count([]) = 0
    | count(a::rest) = 1 + count(rest);


(* 2.2.5 -- in class *)
fun findIth([], i) = (print("item not found"); ~1)
  | findIth(a::rest, 0) =a
  | findIth(a::rest, i) = findIth(rest, i - 1);


(* 2.2.6 -- in class *)
fun reverse([]) =[]
  | reverse(a::rest) = reverse(rest)@[a];


(* Ex 2.2.10 -- in class *)
fun listify([]) =[]
  | listify(a::rest) = [a] ::listify(rest);




(* Ex 2.2.12 -- in class *)
fun splitList([]) = ([] , [] )
 | splitList((a,b)::rest) = 
    let val (aRest, bRest) =splitList(rest);
    in
        (a::aRest, b::bRest)
   end;

(* sample use of splitlist: *)

splitList([(1,2), (3,4), (5,6)]);

(* results in ([1,3,5],[2,4,6])  *)


(* problems below marked "homework" assigned 1/28, due 1/30 *)

(* Ex 2.2.13 -- homework  *)
fun tuplify([], []) = []
  | tuplify(a::rest, []) =  ?? 
  | tuplify([], b::rest) =  ??
  | tuplify(a::aRest, b::bRest) =  ??


(* Ex 2.2.14 -- in class *)
fun sumPairs([]) = ??
  | sumPairs((a,b)::rest) = ??


(* Ex 2.2.15  -- homework *)
fun sumPairList([]) = (0, 0)
  | sumPairList((a,b)::rest) =
    let val (c, d) = sumPairList(rest)
    in
        ??
    end;

(* Extra hint on sumPairList: use splitList (above) as a pattern to follow. 
    Sample us of sumPairList: *)
sumPairList([(1,2), (3,4), (5,6)]);

(* Result should be (9, 12), that is, (1+3+5, 2+4+6) *)