Can I simplify this recursive concat function using List.fold_left?

I have created a working solution for concat, but I feel that I can reduce this using List.fold_lift.

Here is my current code:

let rec concat (lists : 'a list list) : 'a list =    match lists with    | [] -> []    | hd :: tl -> hd @ concat tl ;;

Here is what I have tried:

let concat (lists : 'a list list) : 'a list =    List.fold_left @ lists ;;

This gives me the error: This expression has type 'a list list but an expression was expected of type'a list

I think this is because the return value of list.fold_left gives us a list, but we are feeding it a list of lists so it then returns a list of lists again. How can I get around this without matching?

I was also playing around with List.map but with no luck so far:

let concat (lists : 'a list list) : 'a list =    List.map (fun x -> List.fold_left @ x) lists ;;