OCaml recursive function int list -> int -> (int list * int list) -

studying midterm , looking through old exam questions. 1 doesn't have solution posted , stumping me:

partition: int list -> int -> (int list * int list) divides first argument 2 lists, 1 containing elements less second argument, , other elements greater or equal second argument. partition [5;2;10;4] 4 = ([2], [5;10;4])

oh, , i'm supposed able find solution without using auxiliary function

here far i've gotten:

let rec partition l n = match l      | [] -> ([], []) (* must return type *)     | x :: xs -> if x < n (* append x first list, continue recursing *)                     else (* append x second list, continue recursing *) 

normally, i'd use aux function parameter store pair of lists i'm building, can't done here. i'm bit stuck

you should use let in construction match return value of recursive call:

let rec partition l n = match l      | [] -> ([], [])     | x :: xs -> let a, b = partition xs n in                     if x < n (x::a), b                     else          a, (x::b);;