[GitHub] [arrow] nealrichardson commented on a change in pull request #7514: ARROW-6235: [R] Implement conversion from arrow::BinaryArray to R character vector

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nealrichardson commented on a change in pull request #7514:
URL: https://github.com/apache/arrow/pull/7514#discussion_r443733083



##########
File path: r/src/array_from_vector.cpp
##########
@@ -1067,12 +1110,22 @@ std::shared_ptr<arrow::DataType> InferArrowTypeFromVector<VECSXP>(SEXP x) {
   if (Rf_inherits(x, "data.frame")) {
     return InferArrowTypeFromDataFrame(x);
   } else {
-    if (XLENGTH(x) == 0) {
-      Rcpp::stop(
-          "Requires at least one element to infer the values' type of a list vector");
-    }
+    SEXP ptype = Rf_getAttrib(x, symbols::ptype);
+    if (ptype == R_NilValue) {

Review comment:
       is this the same as `Rf_isNull()` (which I think we use more generally)? 

##########
File path: r/src/array_from_vector.cpp
##########
@@ -1067,12 +1110,22 @@ std::shared_ptr<arrow::DataType> InferArrowTypeFromVector<VECSXP>(SEXP x) {
   if (Rf_inherits(x, "data.frame")) {
     return InferArrowTypeFromDataFrame(x);
   } else {
-    if (XLENGTH(x) == 0) {
-      Rcpp::stop(
-          "Requires at least one element to infer the values' type of a list vector");
-    }
+    SEXP ptype = Rf_getAttrib(x, symbols::ptype);
+    if (ptype == R_NilValue) {
+      if (XLENGTH(x) == 0) {
+        Rcpp::stop(
+            "Requires at least one element to infer the values' type of a list vector");
+      }
 
-    return arrow::list(InferArrowType(VECTOR_ELT(x, 0)));
+      return arrow::list(InferArrowType(VECTOR_ELT(x, 0)));
+    } else {
+      // special case list(raw()) -> BinaryArray
+      if (TYPEOF(ptype) == RAWSXP) {
+        return arrow::binary();
+      }
+
+      return arrow::list(InferArrowType(ptype));

Review comment:
       If I read this correctly, you're introducing general support for `vctrs::list_of ptype` here? IIUC then it would make sense for the ListArray to R vector conversion to set that class and attribute too then, right? (If so, feel free to add that here or defer to a followup)

##########
File path: r/src/array_to_vector.cpp
##########
@@ -693,6 +741,9 @@ std::shared_ptr<Converter> Converter::Make(const std::shared_ptr<DataType>& type
     case Type::BOOL:
       return std::make_shared<arrow::r::Converter_Boolean>(std::move(arrays));
 
+    case Type::BINARY:

Review comment:
       See also https://jira.apache.org/jira/browse/ARROW-6543




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