Directive proposal for opting out of null bubbling

Author: benjie

spec/Section 3 -- Type System.md

@@ -2018,7 +2018,8 @@ by a validator, executor, or client tool such as a code generator.
 
 :: A _built-in directive_ is any directive defined within this specification.
 
-GraphQL implementations should provide the `@skip` and `@include` directives.
+GraphQL implementations should provide the `@skip`, `@include` and
+`@nullOnError` directives.
 
 GraphQL implementations that support the type system definition language must
 provide the `@deprecated` directive if representing deprecated portions of the
@@ -2240,3 +2241,96 @@ to the relevant IETF specification.
 ```graphql example
 scalar UUID @specifiedBy(url: "https://tools.ietf.org/html/rfc4122")
 ```
+
+### @nullOnError
+
+```graphql
+directive @nullOnError on QUERY | MUTATION | SUBSCRIPTION
+```
+
+The `@nullOnError` _built-in directive_ may be provided on query, mutation and
+subscription operations, and disables the error propagation behavior described
+in [Handling Field Errors](#sec-Handling-Field-Errors) by treating all Non-Null
+types as if they were instead Null-Only-On-Error types.
+
+Note: This is useful for clients that still wish to receive sibling fields when
+an error on a Non-Null value occurs. Effectively, `@nullOnError` enables the
+client to opt in to handling errors locally; for example, a client might use
+this to limit the scope of null propagation to a fragment rather than the entire
+field, or to update a normalized store even when an error occurs.
+
+Consider the following schema:
+
+```graphql
+type Query {
+  me: Viewer
+}
+
+type Viewer {
+  username: String!
+  bestFriend: Viewer!
+}
+```
+
+If the `bestFriend` field were to return `null`, then the following operation:
+
+```graphql example
+query myQuery {
+  me {
+    username
+    bestFriend {
+      username
+    }
+  }
+}
+```
+
+Would return a result such as:
+
+```json example
+{
+  "errors": [
+    {
+      "message": "Value cannot be null",
+      "locations": [{ "line": 4, "column": 5 }],
+      "path": ["me", "bestFriend"]
+    }
+  ],
+  "data": {
+    "me": null
+  }
+}
+```
+
+However, if we apply the `@nullOnError` directive to our operation:
+
+```graphql example
+query myQuery @nullOnError {
+  me {
+    username
+    bestFriend {
+      username
+    }
+  }
+}
+```
+
+The result would contain identical errors, but the "me" field will be populated:
+
+```json example
+{
+  "errors": [
+    {
+      "message": "Value cannot be null",
+      "locations": [{ "line": 4, "column": 5 }],
+      "path": ["me", "bestFriend"]
+    }
+  ],
+  "data": {
+    "me": {
+      "username": "billy",
+      "bestFriend": null
+    }
+  }
+}
+```