Fix from_list/2 of list of structs when first is empty (#849)

* Fix `from_list/2` of list of structs when fist is empty

Closes #847

* Update test/explorer/series/list_test.exs

lib/explorer/polars_backend/shared.ex

@@ -126,7 +126,7 @@ defmodule Explorer.PolarsBackend.Shared do
     Native.s_from_list_of_series(name, series)
   end
 
-  def from_list(list, {:struct, fields} = _dtype, name) when is_list(list) do
+  def from_list(list, {:struct, fields} = dtype, name) when is_list(list) do
     series =
       for {column, values} <- Table.to_columns(list) do
         column = to_string(column)
@@ -135,6 +135,14 @@ defmodule Explorer.PolarsBackend.Shared do
       end
 
     Native.s_from_list_of_series_as_structs(name, series)
+    |> then(fn polars_series ->
+      if Native.s_dtype(polars_series) != dtype do
+        {:ok, casted} = Native.s_cast(polars_series, dtype)
+        casted
+      else
+        polars_series
+      end
+    end)
   end
 
   def from_list(list, dtype, name) when is_list(list) do

test/explorer/series/list_test.exs

@@ -168,6 +168,22 @@ defmodule Explorer.Series.ListTest do
                    "the value \"z\" does not match the inferred dtype {:s, 64}",
                    fn -> Series.from_list([[[[[1, 2], ["z", "b"]]]]]) end
     end
+
+    test "list of structs" do
+      series =
+        Series.from_list([[%{"a" => 42}], []], dtype: {:list, {:struct, %{"a" => :integer}}})
+
+      assert Series.dtype(series) == {:list, {:struct, %{"a" => {:s, 64}}}}
+      assert Series.to_list(series) == [[%{"a" => 42}], []]
+    end
+
+    test "list of structs with first empty" do
+      series =
+        Series.from_list([[], [%{"a" => 42}], []], dtype: {:list, {:struct, %{"a" => :integer}}})
+
+      assert Series.dtype(series) == {:list, {:struct, %{"a" => {:s, 64}}}}
+      assert Series.to_list(series) == [[], [%{"a" => 42}], []]
+    end
   end
 
   describe "cast/2" do