Machine Learning in Elixir: Mutate not working, missing pin ^ (Iris example, Chapter 1)

cols = ~w(sepal_width sepal_length petal_length petal_width)
normalised_iris =
  DF.mutate(
    iris,
    for col <- across(^cols) do
      {col.name, (col - mean(col)) / variance(col)}
    end
  )