ARROW-17639: [R] infer_type() fails for lists where the first element…

… is NULL (apache#14062)

This PR updates the internals of type inference on lists; instead of looking at the type of the first element, we instead iterate through all elements until we find a non-null element.

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>

r/src/type_infer.cpp

@@ -165,8 +165,13 @@ std::shared_ptr<arrow::DataType> InferArrowTypeFromVector<VECSXP>(SEXP x) {
         cpp11::stop(
             "Requires at least one element to infer the values' type of a list vector");
       }
-
-      ptype = VECTOR_ELT(x, 0);
+      // Iterate through the vector until we get a non-null result
+      for (R_xlen_t i = 0; i < XLENGTH(x); i++) {
+        ptype = VECTOR_ELT(x, i);
+        if (!Rf_isNull(ptype)) {
+          break;
+        }
+      }
     }
 
     return arrow::list(InferArrowType(ptype));
@@ -198,6 +203,8 @@ std::shared_ptr<arrow::DataType> InferArrowType(SEXP x) {
         return InferArrowTypeFromVector<STRSXP>(x);
       case VECSXP:
         return InferArrowTypeFromVector<VECSXP>(x);
+      case NILSXP:
+        return null();
       default:
         cpp11::stop("Cannot infer type from vector");
     }

r/tests/testthat/test-type.R

@@ -293,3 +293,19 @@ test_that("type() is deprecated", {
   )
   expect_equal(a_type, a$type)
 })
+
+test_that("infer_type() infers type for lists starting with NULL - ARROW-17639", {
+  null_start_list <- list(NULL, c(2, 3), c(4, 5))
+
+  expect_equal(
+    infer_type(null_start_list),
+    list_of(float64())
+  )
+
+  totally_null_list <- list(NULL, NULL, NULL)
+
+  expect_equal(
+    infer_type(totally_null_list),
+    list_of(null())
+  )
+})