Series.cast/2 with type error when used with across/1 and summarise/2

[viniciussbs]

Hey! I guess we have another typespec issue.

This is working:

grouped_df
|> DF.summarise(
  for col <- across(~r/^group_\d+_price$/) do
    {col.name, Series.cast(mean(col), :integer)}
  end
)

But Dialyzer is complaining about Explorer.Series.cast/2:

The function call will not succeed.

Explorer.Series.cast(:infinity | :nan | :neg_infinity | nil | float(), :integer)

will never return since the success typing is:
(%Explorer.Series{:dtype => _, _ => _}, any()) :: any()

and the contract is
(series :: t(), dtype :: dtype() | dtype_alias()) :: t()

[philss]

@viniciussbs That's because mean may return a lazy series in the context of a Query. If you want to send a PR, please take a look the specs from the size/1 function (inside the Series module) - we want to do the same: @spec mean(series :: Series.t()) :: float() | non_finite() | nil | lazy_t().