[GitHub] [arrow] rtpsw commented on a diff in pull request #13397: ARROW-16444: [R] Implement user-defined scalar functions in R bindings

[GitBox <gi...@apache.org>]

rtpsw commented on code in PR #13397:
URL: https://github.com/apache/arrow/pull/13397#discussion_r902546712


##########
r/src/compute.cpp:
##########
@@ -574,3 +576,90 @@ SEXP compute__CallFunction(std::string func_name, cpp11::list args, cpp11::list
 std::vector<std::string> compute__GetFunctionNames() {
   return arrow::compute::GetFunctionRegistry()->GetFunctionNames();
 }
+
+class RScalarUDFCallable : public arrow::compute::ArrayKernelExec {
+ public:
+  RScalarUDFCallable(const std::shared_ptr<arrow::Schema>& input_types,
+                     const std::shared_ptr<arrow::DataType>& output_type, cpp11::sexp fun)
+      : input_types_(input_types), output_type_(output_type), fun_(fun) {}
+
+  arrow::Status operator()(arrow::compute::KernelContext* context,
+                           const arrow::compute::ExecSpan& span,
+                           arrow::compute::ExecResult* result) {
+    std::vector<std::shared_ptr<arrow::Array>> array_args;
+    for (int64_t i = 0; i < span.num_values(); i++) {
+      const arrow::compute::ExecValue& v = span[i];
+      if (v.is_array()) {
+        array_args.push_back(v.array.ToArray());
+      } else if (v.is_scalar()) {
+        auto array = ValueOrStop(arrow::MakeArrayFromScalar(*v.scalar, span.length));
+        array_args.push_back(array);
+      }
+    }
+
+    auto batch = arrow::RecordBatch::Make(input_types_, span.length, array_args);
+
+    auto fun_result = SafeCallIntoR<std::shared_ptr<arrow::Array>>([&]() {
+      cpp11::sexp batch_sexp = cpp11::to_r6<arrow::RecordBatch>(batch);
+      cpp11::sexp batch_length_sexp = cpp11::as_sexp(span.length);
+
+      cpp11::writable::list udf_context = {batch_length_sexp};
+      udf_context.names() = {"batch_length"};
+
+      cpp11::sexp fun_result_sexp = fun_(udf_context, batch_sexp);
+      if (!Rf_inherits(fun_result_sexp, "Array")) {
+        cpp11::stop("arrow_scalar_function must return an Array");
+      }
+
+      return cpp11::as_cpp<std::shared_ptr<arrow::Array>>(fun_result_sexp);
+    });
+
+    if (!fun_result.ok()) {
+      return fun_result.status();
+    }
+
+    result->value.emplace<std::shared_ptr<arrow::ArrayData>>(
+        fun_result.ValueUnsafe()->data());
+    return arrow::Status::OK();
+  }
+
+ private:
+  std::shared_ptr<arrow::Schema> input_types_;
+  std::shared_ptr<arrow::DataType> output_type_;
+  cpp11::function fun_;
+};
+
+// [[arrow::export]]
+void RegisterScalarUDF(std::string name, cpp11::sexp fun) {
+  const arrow::compute::FunctionDoc dummy_function_doc{
+      "A user-defined R function", "returns something", {"..."}};
+
+  auto func = std::make_shared<arrow::compute::ScalarFunction>(
+      name, arrow::compute::Arity::VarArgs(), dummy_function_doc);

Review Comment:
   > I did see that the function docs are a required argument when creating a function and I saw the example in the Python UDFs that creates the documentation object. I think that the function documentation might be helpful for internal functions but for R and Python UDFs I think it unlikely that the documentation will ever be used. Until the feature is requested I'm inclined to leave the dummy function documentation instead of requiring a user to create it.
   
   A similar deferral conclusion was [reached here](https://github.com/ibis-project/ibis/pull/4029), though see [this](https://github.com/ibis-project/ibis/pull/4029#issuecomment-1145282945) and [this post](https://issues.apache.org/jira/browse/ARROW-16823?focusedCommentId=17554982&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-17554982) for where things may develop.



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