ocaml error

let uniq x =
    let (acc : 'a list) = [] in
    let h (a : 'a list) (l : 'a list) = match l with
        |[] -> a
        |hd::[] ->
            (if index a hd = -1 then
                a
            else
                a::hd
            )
        |hd::tl ->
            (if index a hd = -1 then
                a
            else
                h a::hd tl
            )
    in
    h acc x