RFC: Define custom scalars in terms of built-in scalars.

This proposes an additive change which allows custom scalars to be defined in terms of the built-in scalars. The motivation is for client-side code generators to understand how to map between the GraphQL type system and a native type system. As an example, a `URL` custom type may be defined in terms of the built-in scalar `String`. It could define additional serialization and parsing logic, however client tools can know to treat `URL` values as `String`. Presently, we do this by defining these mappings manually on the client, which is difficult to scale, or by giving up and making no assumptions of how the custom types serialize.

Another real use case of giving client tools this information is GraphiQL: this change will allow GraphiQL to show more useful errors when a literal of an incorrect kind is provided to a custom scalar. Currently GraphiQL simply accepts all values.

To accomplish this, this proposes adding the following:

* A new property when defining `GraphQLScalarType` (`ofType`) which asserts that only built-in scalar types are provided.

* A second type coercion to guarantee to a client that the serialized values match the `ofType`.

* Delegating the `parseLiteral` and `parseValue` functions to those in `ofType` (this enables downstream validation / GraphiQL features)

* Exposing `ofType` in the introspection system, and consuming that introspection in `buildClientSchema`.

* Adding optional syntax to the SDL, and consuming that in `buildASTSchema` and `extendSchema` as well as in `schemaPrinter`.

* Adding a case to `findBreakingChanges` which looks for a scalar's ofType changing.

Author: leebyron

src/language/__tests__/schema-kitchen-sink.graphql

@@ -65,6 +65,8 @@ extend union Feed @onUnion
 
 scalar CustomScalar
 
+scalar StringEncodedCustomScalar = String
+
 scalar AnnotatedScalar @onScalar
 
 extend scalar CustomScalar @onScalar

src/language/__tests__/schema-parser-test.js

@@ -712,6 +712,7 @@ type Hello {
         {
           kind: 'ScalarTypeDefinition',
           name: nameNode('Hello', { start: 7, end: 12 }),
+          type: null,
           directives: [],
           loc: { start: 0, end: 12 },
         },

src/language/__tests__/schema-printer-test.js

@@ -111,6 +111,10 @@ describe('Printer', () => {
 
       scalar AnnotatedScalar @onScalar
 
+      scalar StringEncodedCustomScalar = String
+
+      scalar AnnotatedScalar @onScalar
+
       extend scalar CustomScalar @onScalar
 
       enum Site {

src/language/ast.js

@@ -448,6 +448,7 @@ export type ScalarTypeDefinitionNode = {
   +loc?: Location,
   +description?: StringValueNode,
   +name: NameNode,
+  +type?: NamedTypeNode;
   +directives?: $ReadOnlyArray<DirectiveNode>,
 };
 

src/language/parser.js

@@ -890,18 +890,23 @@ function parseOperationTypeDefinition(
 }
 
 /**
- * ScalarTypeDefinition : Description? scalar Name Directives[Const]?
+ * ScalarTypeDefinition :
+ *   - Description? scalar Name ScalarOfType? Directives[Const]?
+ *
+ * ScalarOfType : = NamedType
  */
 function parseScalarTypeDefinition(lexer: Lexer<*>): ScalarTypeDefinitionNode {
   const start = lexer.token;
   const description = parseDescription(lexer);
   expectKeyword(lexer, 'scalar');
   const name = parseName(lexer);
+  const type = skip(lexer, TokenKind.EQUALS) ? parseNamedType(lexer) : null;
   const directives = parseDirectives(lexer, true);
   return {
     kind: SCALAR_TYPE_DEFINITION,
     description,
     name,
+    type,
     directives,
     loc: loc(lexer, start),
   };

src/language/printer.js

@@ -111,7 +111,7 @@ const printDocASTReducer = {
   OperationTypeDefinition: ({ operation, type }) => operation + ': ' + type,
 
   ScalarTypeDefinition: addDescription(({ name, directives }) =>
-    join(['scalar', name, join(directives, ' ')], ' '),
+    join(['scalar', name, wrap(' = ', type), join(directives, ' ')], ' '),
   ),
 
   ObjectTypeDefinition: addDescription(

src/language/visitor.js

@@ -101,7 +101,7 @@ export const QueryDocumentKeys = {
   SchemaDefinition: ['directives', 'operationTypes'],
   OperationTypeDefinition: ['type'],
 
-  ScalarTypeDefinition: ['description', 'name', 'directives'],
+  ScalarTypeDefinition: ['description', 'name', 'type', 'directives'],
   ObjectTypeDefinition: [
     'description',
     'name',

src/type/__tests__/introspection-test.js

@@ -1307,8 +1307,8 @@ describe('Introspection', () => {
             'An enum describing what kind of type a given `__Type` is.',
           enumValues: [
             {
-              description: 'Indicates this type is a scalar.',
-              name: 'SCALAR',
+              description: 'Indicates this type is a scalar. `ofType` is a valid field.',
+              name: 'SCALAR'
             },
             {
               description:

src/type/definition.js

@@ -449,16 +449,30 @@ function resolveThunk<+T>(thunk: Thunk<T>): T {
 export class GraphQLScalarType {
   name: string;
   description: ?string;
+  ofType: ?GraphQLScalarType;
   astNode: ?ScalarTypeDefinitionNode;
 
   _scalarConfig: GraphQLScalarTypeConfig<*, *>;
 
   constructor(config: GraphQLScalarTypeConfig<*, *>): void {
     this.name = config.name;
     this.description = config.description;
+    this.ofType = config.ofType || null;
     this.astNode = config.astNode;
     this._scalarConfig = config;
     invariant(typeof config.name === 'string', 'Must provide name.');
+    if (this.ofType) {
+      const ofTypeName = this.ofType.name;
+      invariant(
+        ofTypeName === 'String' ||
+        ofTypeName === 'Int' ||
+        ofTypeName === 'Float' ||
+        ofTypeName === 'Boolean' ||
+        ofTypeName === 'ID',
+        `${this.name} may only be described in terms of a built-in scalar ` +
+        `type. However ${ofTypeName} is not a built-in scalar type.`
+      );
+    }
     invariant(
       typeof config.serialize === 'function',
       `${this.name} must provide "serialize" function. If this custom Scalar ` +
@@ -478,12 +492,13 @@ export class GraphQLScalarType {
   // Serializes an internal value to include in a response.
   serialize(value: mixed): mixed {
     const serializer = this._scalarConfig.serialize;
-    return serializer(value);
+    const serialized = serializer(value);
+    return this.ofType ? this.ofType.serialize(serialized) : serialized;
   }
 
   // Parses an externally provided value to use as an input.
   parseValue(value: mixed): mixed {
-    const parser = this._scalarConfig.parseValue;
+    const parser = this._scalarConfig.parseValue || (this.ofType && this.ofType.parseValue);
     if (isInvalid(value)) {
       return undefined;
     }
@@ -492,7 +507,7 @@ export class GraphQLScalarType {
 
   // Parses an externally provided literal value to use as an input.
   parseLiteral(valueNode: ValueNode, variables: ?ObjMap<mixed>): mixed {
-    const parser = this._scalarConfig.parseLiteral;
+    const parser = this._scalarConfig.parseLiteral || (this.ofType && this.ofType.parseLiteral);
     return parser
       ? parser(valueNode, variables)
       : valueFromASTUntyped(valueNode, variables);
@@ -513,6 +528,7 @@ GraphQLScalarType.prototype.toJSON = GraphQLScalarType.prototype.inspect =
 export type GraphQLScalarTypeConfig<TInternal, TExternal> = {
   name: string,
   description?: ?string,
+  ofType?: ?GraphQLScalarType,
   astNode?: ?ScalarTypeDefinitionNode,
   serialize: (value: mixed) => ?TExternal,
   parseValue?: (value: mixed) => ?TInternal,

src/type/introspection.js

@@ -381,7 +381,7 @@ export const __TypeKind = new GraphQLEnumType({
   values: {
     SCALAR: {
       value: TypeKind.SCALAR,
-      description: 'Indicates this type is a scalar.',
+      description: 'Indicates this type is a scalar. `ofType` is a valid field.',
     },
     OBJECT: {
       value: TypeKind.OBJECT,