Implement Series.from_list of null type

[lkarthee]

Fix for #806

Support Series.from_list([nil, nil])

Even though tests pass - DataFrame.concat_rows may fail and Series.concat will fail if one of dtypes is :null (irrespective of this fix) - need to address it later. Series concat nulls has no test cases

sn = Series.from_list([nil, nil, nil], dtype: :null)
s2 = Series.from_list([4.0, 5.0, 6.4])
Series.concat([s2, sn]) # will fail - it is valid in polars

[lkarthee]

Switched {:f, 64} to :null when a list of nils is passed to Series.from_list/1

[josevalim]

I think we can instead change this line to:

Suggested change

	type || preferable_type || {:f, 64}
	type || preferable_type || :null

And then we can most likely skip other changes to this file?

[josevalim]

To be precise, skip the changes to type/2 and new_type_matches?. :)

[lkarthee]

3 existing test cases fail :

s = Series.from_list([])
p = Series.cumulative_product(s)
**  (ArgumentError) Explorer.Series.cumulative_product/2 not implemented for dtype :null.
p = Explorer.Series.product(s)
**  (ArgumentError) Explorer.Series.product/1 not implemented for dtype :null
p = Series.less(s, nil)
**  cannot invoke Explorer.Series.less/2 with mismatched dtypes: {:f, 64} and nil

2 new test cases fail:

s = Series.from_list([1, 2, 3], dtype: :null)
** (ArgumentError) the value 1 does not match the inferred series dtype :null
s =
     Series.from_list([
       %{a: nil, b: nil},
       %{a: 3, b: nil},
       %{a: 5, b: nil}
     ])
** (CaseClauseError) no case clause matching: nil

There is assumption that empty array is {:f, 64}

[josevalim]

What about a list of lists of nils?

Series.from_list([[nil, nil], [nil, nil, nil]])

[lkarthee]

test/explorer/series/list_test.exs has below test case:

test "list of list of nulls" do
       series = Series.from_list([[nil, nil], [nil]])
       assert series.dtype == {:list, :null}
       assert series[0] == [nil, nil]
       assert Series.to_list(series) == [[nil, nil], [nil]]
     end

[josevalim]

Ah, fantastic!

[josevalim]

💚 💙 💜 💛 ❤️

I will investigate the failures then, thank you!