aucahuasi commented on a change in pull request #11159:
URL: https://github.com/apache/arrow/pull/11159#discussion_r712311252



##########
File path: cpp/src/arrow/compute/kernels/scalar_nested.cc
##########
@@ -80,6 +80,160 @@ const FunctionDoc list_value_length_doc{
      "Null values emit a null in the output."),
     {"lists"}};
 
+template <typename Type, typename IndexType>
+Status ListElementArray(KernelContext* ctx, const ExecBatch& batch, Datum* 
out) {
+  using ListArrayType = typename TypeTraits<Type>::ArrayType;
+  using IndexScalarType = typename TypeTraits<IndexType>::ScalarType;
+  const auto& index_scalar = batch[1].scalar_as<IndexScalarType>();
+  if (ARROW_PREDICT_FALSE(!index_scalar.is_valid)) {
+    return Status::Invalid("Index must not be null");
+  }
+  ListArrayType list_array(batch[0].array());
+  auto index = index_scalar.value;
+  if (ARROW_PREDICT_FALSE(index < 0)) {
+    return Status::Invalid("Index ", index,
+                           " is out of bounds: should be greater than or equal 
to 0");
+  }
+  std::unique_ptr<ArrayBuilder> builder;
+  RETURN_NOT_OK(MakeBuilder(ctx->memory_pool(), list_array.value_type(), 
&builder));
+  RETURN_NOT_OK(builder->Reserve(list_array.length()));
+  for (int i = 0; i < list_array.length(); ++i) {
+    if (list_array.IsNull(i)) {
+      RETURN_NOT_OK(builder->AppendNull());
+      continue;
+    }
+    std::shared_ptr<arrow::Array> value_array = list_array.value_slice(i);
+    auto len = value_array->length();
+    if (ARROW_PREDICT_FALSE(index >= static_cast<typename 
IndexType::c_type>(len))) {
+      return Status::Invalid("Index ", index, " is out of bounds: should be in 
[0, ", len,
+                             ")");
+    }
+    RETURN_NOT_OK(builder->AppendArraySlice(*value_array->data(), index, 1));
+  }
+  ARROW_ASSIGN_OR_RAISE(auto result, builder->Finish());
+  out->value = result->data();
+  return Status::OK();
+}
+
+template <typename InListScalarType, typename IndexType>
+Status ListElementScalar(KernelContext* /*ctx*/, const ExecBatch& batch, 
Datum* out) {
+  using IndexScalarType = typename TypeTraits<IndexType>::ScalarType;
+  const auto& index_scalar = batch[1].scalar_as<IndexScalarType>();
+  if (ARROW_PREDICT_FALSE(!index_scalar.is_valid)) {
+    return Status::Invalid("Index must not be null");
+  }
+  const auto& list_scalar = batch[0].scalar_as<InListScalarType>();
+  if (ARROW_PREDICT_FALSE(!list_scalar.is_valid)) {
+    out->value =
+        MakeNullScalar(checked_cast<const 
BaseListType&>(*batch[0].type()).value_type());
+    return Status::OK();
+  }
+  auto list = list_scalar.value;
+  auto index = index_scalar.value;
+  auto len = list->length();
+  if (ARROW_PREDICT_FALSE(index < 0 ||
+                          index >= static_cast<typename 
IndexType::c_type>(len))) {
+    return Status::Invalid("Index ", index, " is out of bounds: should be in 
[0, ", len,
+                           ")");
+  }
+  ARROW_ASSIGN_OR_RAISE(auto result, list->GetScalar(index));
+  out->value = result;
+  return Status::OK();
+}
+
+template <typename InListType, typename IndexType>
+void AddListElementArrayKernel(ScalarFunction* func) {
+  auto inputs = {InputType::Array(InListType::type_id),
+                 InputType::Scalar(IndexType::type_id)};
+  auto output = OutputType{ListValuesType};
+  auto sig =
+      KernelSignature::Make(std::move(inputs), std::move(output), 
/*is_varargs=*/false);
+  auto array_exec = ListElementArray<InListType, IndexType>;
+  ScalarKernel kernel{std::move(sig), std::move(array_exec)};
+  kernel.null_handling = NullHandling::COMPUTED_NO_PREALLOCATE;
+  kernel.mem_allocation = MemAllocation::NO_PREALLOCATE;
+  DCHECK_OK(func->AddKernel(std::move(kernel)));
+}
+
+template <typename InListScalarType, typename IndexType>
+void AddListElementScalarKernel(ScalarFunction* func) {
+  auto inputs = {InputType::Scalar(InListScalarType::TypeClass::type_id),
+                 InputType::Scalar(IndexType::type_id)};
+  auto output = OutputType{ListValuesType};
+  auto sig =
+      KernelSignature::Make(std::move(inputs), std::move(output), 
/*is_varargs=*/false);
+  auto scalar_exec = ListElementScalar<InListScalarType, IndexType>;
+  ScalarKernel kernel{std::move(sig), std::move(scalar_exec)};
+  kernel.null_handling = NullHandling::COMPUTED_NO_PREALLOCATE;
+  kernel.mem_allocation = MemAllocation::NO_PREALLOCATE;
+  DCHECK_OK(func->AddKernel(std::move(kernel)));
+}
+
+void AddListElementArrayKernels(ScalarFunction* func) {
+  AddListElementArrayKernel<ListType, UInt8Type>(func);
+  AddListElementArrayKernel<ListType, Int8Type>(func);
+  AddListElementArrayKernel<ListType, UInt16Type>(func);
+  AddListElementArrayKernel<ListType, Int16Type>(func);
+  AddListElementArrayKernel<ListType, UInt32Type>(func);
+  AddListElementArrayKernel<ListType, Int32Type>(func);
+  AddListElementArrayKernel<ListType, UInt64Type>(func);
+  AddListElementArrayKernel<ListType, Int64Type>(func);
+
+  AddListElementArrayKernel<LargeListType, UInt8Type>(func);
+  AddListElementArrayKernel<LargeListType, Int8Type>(func);
+  AddListElementArrayKernel<LargeListType, UInt16Type>(func);
+  AddListElementArrayKernel<LargeListType, Int16Type>(func);
+  AddListElementArrayKernel<LargeListType, UInt32Type>(func);
+  AddListElementArrayKernel<LargeListType, Int32Type>(func);
+  AddListElementArrayKernel<LargeListType, UInt64Type>(func);
+  AddListElementArrayKernel<LargeListType, Int64Type>(func);
+
+  AddListElementArrayKernel<FixedSizeListType, UInt8Type>(func);
+  AddListElementArrayKernel<FixedSizeListType, Int8Type>(func);
+  AddListElementArrayKernel<FixedSizeListType, UInt16Type>(func);
+  AddListElementArrayKernel<FixedSizeListType, Int16Type>(func);
+  AddListElementArrayKernel<FixedSizeListType, UInt32Type>(func);
+  AddListElementArrayKernel<FixedSizeListType, Int32Type>(func);
+  AddListElementArrayKernel<FixedSizeListType, UInt64Type>(func);
+  AddListElementArrayKernel<FixedSizeListType, Int64Type>(func);
+}
+
+void AddListElementScalarKernels(ScalarFunction* func) {
+  AddListElementScalarKernel<ListScalar, UInt8Type>(func);
+  AddListElementScalarKernel<ListScalar, Int8Type>(func);
+  AddListElementScalarKernel<ListScalar, UInt16Type>(func);
+  AddListElementScalarKernel<ListScalar, Int16Type>(func);
+  AddListElementScalarKernel<ListScalar, UInt32Type>(func);
+  AddListElementScalarKernel<ListScalar, Int32Type>(func);
+  AddListElementScalarKernel<ListScalar, UInt64Type>(func);
+  AddListElementScalarKernel<ListScalar, Int64Type>(func);
+
+  AddListElementScalarKernel<LargeListScalar, UInt8Type>(func);
+  AddListElementScalarKernel<LargeListScalar, Int8Type>(func);
+  AddListElementScalarKernel<LargeListScalar, UInt16Type>(func);
+  AddListElementScalarKernel<LargeListScalar, Int16Type>(func);
+  AddListElementScalarKernel<LargeListScalar, UInt32Type>(func);
+  AddListElementScalarKernel<LargeListScalar, Int32Type>(func);
+  AddListElementScalarKernel<LargeListScalar, UInt64Type>(func);
+  AddListElementScalarKernel<LargeListScalar, Int64Type>(func);
+
+  AddListElementScalarKernel<FixedSizeListScalar, UInt8Type>(func);
+  AddListElementScalarKernel<FixedSizeListScalar, Int8Type>(func);
+  AddListElementScalarKernel<FixedSizeListScalar, UInt16Type>(func);
+  AddListElementScalarKernel<FixedSizeListScalar, Int16Type>(func);
+  AddListElementScalarKernel<FixedSizeListScalar, UInt32Type>(func);
+  AddListElementScalarKernel<FixedSizeListScalar, Int32Type>(func);
+  AddListElementScalarKernel<FixedSizeListScalar, UInt64Type>(func);
+  AddListElementScalarKernel<FixedSizeListScalar, Int64Type>(func);

Review comment:
       @lidavidm Are you sure that compiles? It seems the kernel is invalid as 
generator template




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