list_function.sml

fun sum_list (xs : int list) = 
    if null xs
    then 0
    else hd xs + sum_list(tl xs)

fun countdown (x : int) = 
    if x=0
    then []
    else x :: countdown(x-1)

fun append (xs : int list, ys : int list) = 
    if null xs
    then ys
    else (hd xs) :: (append((tl xs), ys))
(* (int list) * (int list) -> int list *)

(* function over pairs of lists *)

fun sum_pair_list (xs : (int * int) list) =
    if null xs
    then 0
    else #1 (hd xs) + #2 (hd xs) + sum_pair_list(tl xs)

    
fun firsts (xs : (int * int) list) =
    if null xs
    then []
    else (#1 (hd xs)) :: firsts(tl xs)

fun seconds (xs : (int * int) list) =
    if null xs
    then []
    else (#2 (hd xs)) :: seconds(tl xs)

fun sum_pair_list2 (xs : (int * int) list) =
    sum_list(firsts xs) + sum_list (seconds xs)