Add VARCHAR support to vector_set_value_at

- Add DUCKDB_TYPE_VARCHAR case using duckdb_vector_assign_string_element_len()
- Update type validation to allow :varchar in return types
- Add test_scalar_function_varchar_return_type test
- Update error test to use TIMESTAMP (now unsupported)

Author: suketa

ext/duckdb/scalar_function.c

@@ -243,6 +243,14 @@ static void vector_set_value_at(duckdb_vector vector, duckdb_logical_type elemen
         case DUCKDB_TYPE_DOUBLE:
             ((double *)vector_data)[index] = NUM2DBL(value);
             break;
+        case DUCKDB_TYPE_VARCHAR: {
+            // VARCHAR requires special API, not direct array assignment
+            VALUE str = rb_obj_as_string(value);
+            const char *str_ptr = StringValuePtr(str);
+            idx_t str_len = RSTRING_LEN(str);
+            duckdb_vector_assign_string_element_len(vector, index, str_ptr, str_len);
+            break;
+        }
         default:
             rb_raise(rb_eArgError, "Unsupported return type for scalar function");
             break;

lib/duckdb/scalar_function.rb

@@ -4,7 +4,7 @@ module DuckDB
   # DuckDB::ScalarFunction encapsulates DuckDB's scalar function
   class ScalarFunction
     # Sets the return type for the scalar function.
-    # Currently supports BOOLEAN, INTEGER, BIGINT, FLOAT, and DOUBLE types.
+    # Currently supports BOOLEAN, INTEGER, BIGINT, FLOAT, DOUBLE, and VARCHAR types.
     #
     # @param logical_type [DuckDB::LogicalType] the return type
     # @return [DuckDB::ScalarFunction] self
@@ -13,8 +13,8 @@ def return_type=(logical_type)
       raise DuckDB::Error, 'logical_type must be a DuckDB::LogicalType' unless logical_type.is_a?(DuckDB::LogicalType)
 
       # Check if the type is supported
-      unless %i[boolean integer bigint float double].include?(logical_type.type)
-        raise DuckDB::Error, 'Only BOOLEAN, INTEGER, BIGINT, FLOAT, and DOUBLE return types are currently supported'
+      unless %i[boolean integer bigint float double varchar].include?(logical_type.type)
+        raise DuckDB::Error, 'Only BOOLEAN, INTEGER, BIGINT, FLOAT, DOUBLE, and VARCHAR return types are currently supported'
       end
 
       _set_return_type(logical_type)

test/duckdb_test/scalar_function_test.rb

@@ -37,13 +37,13 @@ def test_return_type_setter
 
     def test_return_type_setter_raises_error_for_unsupported_type
       sf = DuckDB::ScalarFunction.new
-      varchar_type = DuckDB::LogicalType.new(17) # DUCKDB_TYPE_VARCHAR
+      timestamp_type = DuckDB::LogicalType.new(13) # DUCKDB_TYPE_TIMESTAMP (unsupported)
 
       error = assert_raises(DuckDB::Error) do
-        sf.return_type = varchar_type
+        sf.return_type = timestamp_type
       end
 
-      assert_match(/only.*INTEGER.*supported/i, error.message)
+      assert_match(/only.*supported/i, error.message)
     end
 
     def test_set_function
@@ -214,5 +214,22 @@ def test_scalar_function_float_return_type # rubocop:disable Metrics/MethodLengt
 
       assert_in_delta 3.0, result.first.first, 0.0001
     end
+
+    def test_scalar_function_varchar_return_type # rubocop:disable Metrics/MethodLength
+      @con.execute('SET threads=1')
+      @con.execute('CREATE TABLE test_table (name VARCHAR)')
+      @con.execute("INSERT INTO test_table VALUES ('Alice'), ('Bob')")
+
+      sf = DuckDB::ScalarFunction.new
+      sf.name = 'add_greeting'
+      sf.add_parameter(DuckDB::LogicalType.new(17)) # VARCHAR (type ID 17)
+      sf.return_type = DuckDB::LogicalType.new(17) # VARCHAR
+      sf.set_function { |name| "Hello, #{name}!" }
+
+      @con.register_scalar_function(sf)
+      result = @con.execute('SELECT add_greeting(name) FROM test_table ORDER BY name')
+
+      assert_equal [['Hello, Alice!'], ['Hello, Bob!']], result.to_a
+    end
   end
 end