How to add external variable to fragment?

I have read many topics and pages about this issue and while it seems this should work I am probably missing something.

I have this query

      from(v in Visit,
        right_join:
          day in fragment(
            "generate_series(CURRENT_DATE - INTERVAL '? day', CURRENT_DATE, '1 day')",
            30
          ),
        on: day == fragment("date(?)", v.inserted_at),
        group_by: day,
        select: %{
          day: fragment("date(?)", day),
          count: count(v.id)
        }
      )
      |> Repo.all()

Which returns

[
  %{count: 0, day: ~D[2019-09-03]},
  %{count: 0, day: ~D[2019-09-26]},
  %{count: 3, day: ~D[2019-09-15]},
  %{count: 0, day: ~D[2019-09-11]},
     ...
]

I want that 30 to be a variable so I change the query to

      num_of_days = 30

      from(v in Visit,
        right_join:
          day in fragment(
            "generate_series(CURRENT_DATE - INTERVAL '? day', CURRENT_DATE, '1 day')",
            ^num_of_days
          ),
        on: day == fragment("date(?)", v.inserted_at),
        group_by: day,
        select: %{
          day: fragment("date(?)", day),
          count: count(v.id)
        }
      )
      |> Repo.all()

This fails with

[debug] QUERY ERROR source="visits" db=9.5ms queue=1.4ms
SELECT date(f1), count(v0."id") FROM "visits" AS v0 RIGHT OUTER JOIN generate_series(CURRENT_DATE - INTERVAL '$1 day', CURRENT_DATE, '1 day') AS f1 ON f1 = date(v0."inserted_at") GROUP BY f1 [30]
** (ArgumentError) parameters must be of length 0 for query %Postgrex.Query{cache: :reference, columns: ["date", "count"], name: "ecto_1187", param_formats: [], param_oids: [], param_types: [], ref: #Reference<0.3361914078.3108241410.64835>, result_formats: [:binary, :binary], result_oids: [1082, 20], result_types: [Postgrex.Extensions.Date, Postgrex.Extensions.Int8], statement: "SELECT date(f1), count(v0.\"id\") FROM \"visits\" AS v0 RIGHT OUTER JOIN generate_series(CURRENT_DATE - INTERVAL '$1 day', CURRENT_DATE, '1 day') AS f1 ON f1 = date(v0.\"inserted_at\") GROUP BY f1", types: {Postgrex.DefaultTypes, #Reference<0.3361914078.3108372482.49372>}}
    (postgrex) lib/postgrex/query.ex:80: DBConnection.Query.Postgrex.Query.encode/3
    (db_connection) lib/db_connection.ex:1148: DBConnection.encode/5
    (db_connection) lib/db_connection.ex:1246: DBConnection.run_prepare_execute/5
    (db_connection) lib/db_connection.ex:1342: DBConnection.run/6
    (db_connection) lib/db_connection.ex:540: DBConnection.parsed_prepare_execute/5
    (db_connection) lib/db_connection.ex:533: DBConnection.prepare_execute/4
    (ecto_sql) lib/ecto/adapters/sql.ex:562: Ecto.Adapters.SQL.execute!/4
    (ecto_sql) lib/ecto/adapters/sql.ex:554: Ecto.Adapters.SQL.execute/5
    (ecto) lib/ecto/repo/queryable.ex:153: Ecto.Repo.Queryable.execute/4
    (ecto) lib/ecto/repo/queryable.ex:18: Ecto.Repo.Queryable.all/3

So the variable is not interpolated

I also tried the macro approach

  def count_by_date_with(identifier, num_of_days) do

    num_of_days = 30

    from(v in Visit,
      right_join: day in days(num_of_days),
      on: day == fragment("date(?)", v.inserted_at),
      group_by: day,
      select: %{
        day: fragment("date(?)", day),
        count: count(v.id)
      }
    )
    |> Repo.all()
  end

  defmacro days(num_of_days) do
    quote do
      fragment(
        "generate_series(CURRENT_DATE - INTERVAL '? day', CURRENT_DATE, '1 day')",
        unquote(num_of_days)
      )
    end
  end

But this doesn’t even compile

iex(25)> recompile
Compiling 1 file (.ex)

== Compilation error in file lib/tinyclone/visit.ex ==
** (Ecto.Query.CompileError) malformed join `days(num_of_days)` in query expression
    (ecto) lib/ecto/query.ex:755: Ecto.Query.from/5
    (ecto) expanding macro: Ecto.Query.from/2
    (tinyclone) lib/tinyclone/visit.ex:43: TinyClone.Visit.count_by_date_with/2
    (elixir) expanding macro: Kernel.|>/2
    (tinyclone) lib/tinyclone/visit.ex:52: TinyClone.Visit.count_by_date_with/2
    (elixir) lib/kernel/parallel_compiler.ex:229: anonymous fn/4 in Kernel.ParallelCompiler.spawn_workers/7
** (exit) shutdown: 1
    (mix) lib/mix/tasks/compile.all.ex:59: Mix.Tasks.Compile.All.do_compile/4
    (mix) lib/mix/tasks/compile.all.ex:24: anonymous fn/1 in Mix.Tasks.Compile.All.run/1
    (mix) lib/mix/tasks/compile.all.ex:40: Mix.Tasks.Compile.All.with_logger_app/1
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/tasks/compile.ex:96: Mix.Tasks.Compile.run/1
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (iex) lib/iex/helpers.ex:104: IEx.Helpers.recompile/1

How can I interpolate that variable in the fragment?