Variables in pattern matching vector

let box_value llval =
  let value_t = match type_by_name the_module "value_t" with
      Some t -> t
    | None -> raise (Error "Could not look up value_t")
  in
  let value_ptr = build_alloca value_t "value" builder in
  let idx n = [| const_int i32_type 0; const_int i32_type n |] in
  let dst = match type_of llval with
      ty when ty = i64_type ->
      build_in_bounds_gep value_ptr (idx 0) "boxptr" builder
    | ty when ty = i1_type ->
       build_in_bounds_gep value_ptr (idx 1) "boxptr" builder
    | ty when ty = vector_type m n ->
       build_in_bounds_gep value_ptr (idx 3) "boxptr" builder
    | _ -> raise (Error "Don't know how to box type") in
  ignore (build_store llval dst builder); value_ptr