Copying a matrix into another matrix ocaml

I am trying to copy a the values of the first matrix to the second element by element. I don't understand why "<-" doesn't work for me when I want to set a new value. Here's the code:

let y=[|[|"true";"true";"false";"false"|];
        [|"true";"false";"true";"false"|];
        [|"false";"false";"true";"true"|];
        [|"false";"true";"false";"true"|]|]
;;

let fill_array = 
  let l=Array.length x in
  for i=0 to l-1 do
    for j=0 to l-1 do
      x.(i).(j)<-(y(i).(j))
    done;
  done;
  x;;

I get the following-Error: This expression has type string array array This is not a function; it cannot be applied. Ideally this would be a bool array but I wanted to try string first.

You are missing a dot. In

x.(i).(j)<- y(i).(j)

y(i) is a function application whereas you wanted y.(i)

x.(i).(j)<- y.(i).(j)

Note that for an element-by-element copy, another option would be to use higher-order functions rather than low-level loops:

let copy x y =
  Array.iteri (fun i xi ->
    Array.iteri (fun j x -> y.(i).(j) <- x) xi
  ) x