let f n arr =
    Array.map (Array.make n)
    |> Array.fold_left Array.append [| |]

File "solution.ml", line 3, characters 7-41:
Error: This expression has type 'a array array -> 'a array
       but an expression was expected of type
         ('b array -> 'b array array) -> 'c
       Type 'a array array is not compatible with type
         'b array -> 'b array array