validate_schema: unify check of root types

Replicates graphql/graphql-js@839f244

Author: Cito

src/graphql/type/validate.py

@@ -103,33 +103,22 @@ def report_error(
 
     def validate_root_types(self) -> None:
         schema = self.schema
-        query_type = schema.query_type
-        if not query_type:
+        if not schema.query_type:
             self.report_error("Query root type must be provided.", schema.ast_node)
-        elif not is_object_type(query_type):
-            self.report_error(
-                f"Query root type must be Object type, it cannot be {query_type}.",
-                get_operation_type_node(schema, OperationType.QUERY)
-                or query_type.ast_node,
-            )
-
-        mutation_type = schema.mutation_type
-        if mutation_type and not is_object_type(mutation_type):
-            self.report_error(
-                "Mutation root type must be Object type if provided,"
-                f" it cannot be {mutation_type}.",
-                get_operation_type_node(schema, OperationType.MUTATION)
-                or mutation_type.ast_node,
-            )
-
-        subscription_type = schema.subscription_type
-        if subscription_type and not is_object_type(subscription_type):
-            self.report_error(
-                "Subscription root type must be Object type if provided,"
-                f" it cannot be {subscription_type}.",
-                get_operation_type_node(schema, OperationType.SUBSCRIPTION)
-                or subscription_type.ast_node,
-            )
+        for operation_type in OperationType:
+            root_type = schema.get_root_type(operation_type)
+            if root_type and not is_object_type(root_type):
+                operation_type_str = operation_type.value.capitalize()
+                root_type_str = inspect(root_type)
+                if_provided_str = (
+                    "" if operation_type == operation_type.QUERY else " if provided"
+                )
+                self.report_error(
+                    f"{operation_type_str} root type must be Object type"
+                    f"{if_provided_str}, it cannot be {root_type_str}.",
+                    get_operation_type_node(schema, operation_type)
+                    or root_type.ast_node,
+                )
 
     def validate_directives(self) -> None:
         directives = self.schema.directives