[GitHub] [arrow] pitrou commented on a diff in pull request #36310: GH-36309: [C++] Add ability to cast between scalars of list-like types

["pitrou (via GitHub)" <gi...@apache.org>]

pitrou commented on code in PR #36310:
URL: https://github.com/apache/arrow/pull/36310#discussion_r1243605216


##########
cpp/src/arrow/scalar.cc:
##########
@@ -1127,6 +1127,28 @@ Status CastImpl(const StructScalar& from, StringScalar* to) {
   return Status::OK();
 }
 
+// casts between variable-length and fixed-length list types
+template <typename ToScalar>
+enable_if_t<is_list_like_type<typename ToScalar::TypeClass>::value &&
+                ToScalar::TypeClass::type_id != Type::MAP,
+            Status>
+CastImpl(const BaseListScalar& from, ToScalar* to) {
+  DCHECK(from.type->id() == Type::LIST || from.type->id() == Type::LARGE_LIST ||
+         from.type->id() == Type::FIXED_SIZE_LIST);

Review Comment:
   Careful: `MapScalar` inherits from `BaseListScalar`. Are you sure it can't use this overload?



##########
cpp/src/arrow/scalar_test.cc:
##########
@@ -391,6 +390,8 @@ class TestRealScalar : public ::testing::Test {
     ASSERT_TRUE(list_nan.ApproxEquals(list_other_nan, options));
   }
 
+  void TestListOf() { TestListOf<ListScalar>(list(type_)); }

Review Comment:
   And can we add a cast test between map and non-map-list-like somewhere?



##########
cpp/src/arrow/scalar_test.cc:
##########
@@ -1135,6 +1136,25 @@ class TestListScalar : public ::testing::Test {
     ASSERT_NE(a0->hash(), b0->hash());
   }
 
+  void TestCast(ScalarType& scalar, const std::shared_ptr<DataType>& to_type) {

Review Comment:
   Can we avoid non-const refs?
   ```suggestion
     void TestCast(const ScalarType& scalar, const std::shared_ptr<DataType>& to_type) {
   ```



##########
cpp/src/arrow/scalar_test.cc:
##########
@@ -391,6 +390,8 @@ class TestRealScalar : public ::testing::Test {
     ASSERT_TRUE(list_nan.ApproxEquals(list_other_nan, options));
   }
 
+  void TestListOf() { TestListOf<ListScalar>(list(type_)); }

Review Comment:
   Is it me, or is `TestListOf` never called with types other than plain list?



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