type_array_access env ctx (array: Types.Expression.t) index : Prog.Expression.t =
  let array_typed = type_expression env ctx array in
  let array_typ = array_typed.typ in
  let idx_typed = type_expression env ctx index in
  let element_typ = (assert_array (Types.Expression.tags array) array_typ).typ in
  assert_numeric (Types.Expression.tags index) idx_typed.typ |> ignore;
  { expr = ArrayAccess { array = array_typed; index = idx_typed; tags=Types.Expression.tags array };
    typ = element_typ;
    dir = array_typed.dir;
    tags = Types.Expression.tags array}

let assert_array = make_assert "array"
  begin function
  | Array array_typ -> Some array_typ
  | _ -> None
  end

type_assignment env ctx lhs rhs =
  let open Expression in 
  let open Prog.Statement in
  let expr_ctx = ExprContext.of_stmt_context ctx in
  let lhs_typed = type_expression env expr_ctx lhs in
  if not @@ is_lvalue env lhs_typed
  then raise_s [%message "Must be an lvalue"
                   ~lhs:(lhs:Types.Expression.t)]
  else
    let rhs_typed = cast_expression env expr_ctx lhs_typed.typ rhs in
    ignore (assert_same_type env (tags lhs) (tags rhs)
              lhs_typed.typ rhs_typed.typ);
    { stmt = Assignment { lhs = lhs_typed; rhs = rhs_typed; tags = Info.merge (tags lhs) (tags rhs) };
      typ = StmType.Unit },
    env