Convert a matrix into a list by flattening it ocaml

I want to convert a matrix(type matrix = float list list) into a list(so basically I’m writing a function to concatenate a list of lists, but I’m not allowed to use List.append, List.flatten, etc.).
So for example, into_list [[1.; 2.]; [3.; 4.] = [1.; 2.; 3.; 4.]. I wrote my helper function to append two lists together:

let rec append_list (a: float list) (b: float list): float list =
  match a with
  | [] -> b
  | h :: t -> h :: append_list t b

right now, my into_list function looks like this:

let rec into_list (m : matrix): float list =
  match m with
  | [] -> []
  | h1 :: h2 :: t -> append_list h1 h2 :: into_list t

I pretty sure | h1 :: h2 :: t -> append_list h1 h2 :: into_list t is wrong but I have no idea how to fix it, right now utop gives me the error message says "Unbound type constructor matrix".

>Solution :

It would appear matrix is simply:

type matrix = float list list

As such:

append_list h1 h2 :: into_list t

Would yield float list list because the type of :: is 'a -> ‘a list -> ‘a list`.

You’re looking for something like a fold, that continually appends the lists into a final float list.

let rec into_list m = List.fold_left append_list [] m

Of course, it doesn’t seem like you’re allowed to use List.fold_left. Replicate that functionality (shown many times here) and you’re good to go.