[josevalim]

mutate_with is still lazy and therefore you can't transform it. If you need to use transform, then you need to do:

    new_column = Series.transform(df["column"], fn arg -> ... end)
    DF.put(df, "column", new_column)

which _is annoying_ since you are not supposed to use it. The correct way is to extend the Series API, which we will be very happy to!