Need hints for little exercise : write the List.concat

Hi,


I am doing the Ocaml exercises from http://exercism.io/, I am at the 
part list-ops where you must rewrite some basics

List functions.


I have already done those ones (that was not easy I fell like I am retarded)


val length : 'a list -> int val reverse : 'a list -> 'a list

val map : f:('a -> 'b) -> 'a list -> 'b list

val filter : f:('a -> bool) -> 'a list -> 'a list

val fold : init:'acc -> f:('acc -> 'a -> 'acc) -> 'a list -> 'acc

val append : 'a list -> 'a list -> 'a list


But now I am stuck with this one:

val concat : 'a list list -> 'a list


I don't want the answer (I want to find it by myself) but just a hint 
that can help me.


For example I was thinking of taking recursively the sub-lists two by 
two and applying an "append"

function on them. Something like this :


let rec concat alist =

    let rec aux f s =

         match f with

         | [] -> []

         | hd :: tl -> hd :: (aux tl s) in

    match alist with

   | [] -> []

   | hd :: mid :: tl -> (aux hd mid) :: concat tl


This code doesn't work and I have the feeling that it is simpler than that.


Thanks