Support thunks for Union.Types definitions

In schemas, it is possible to define the Union type before the
definitions of the object types that are part of the union. This
change adds a "UnionTypesThunk" option when creating a new Union,
similar to the existing FieldsThunk and InterfacesThunk.

This more closely matches the interface in graphql-js:
  https://github.com/graphql/graphql-js/blob/47bd8c8897c72d3efc17ecb1599a95cee6bac5e8/src/type/definition.ts#L1307

It is a first step in closing out #624

definition.go

@@ -796,15 +796,19 @@ type Union struct {
 	PrivateDescription string `json:"description"`
 	ResolveType        ResolveTypeFn
 
-	typeConfig    UnionConfig
-	types         []*Object
-	possibleTypes map[string]bool
+	typeConfig      UnionConfig
+	initalizedTypes bool
+	types           []*Object
+	possibleTypes   map[string]bool
 
 	err error
 }
+
+type UnionTypesThunk func() []*Object
+
 type UnionConfig struct {
-	Name        string    `json:"name"`
-	Types       []*Object `json:"types"`
+	Name        string      `json:"name"`
+	Types       interface{} `json:"types"`
 	ResolveType ResolveTypeFn
 	Description string `json:"description"`
 }
@@ -822,48 +826,80 @@ func NewUnion(config UnionConfig) *Union {
 	objectType.PrivateDescription = config.Description
 	objectType.ResolveType = config.ResolveType
 
-	if objectType.err = invariantf(
-		len(config.Types) > 0,
-		`Must provide Array of types for Union %v.`, config.Name,
-	); objectType.err != nil {
-		return objectType
+	objectType.typeConfig = config
+
+	return objectType
+}
+
+func (ut *Union) Types() []*Object {
+	if ut.initalizedTypes {
+		return ut.types
+	}
+
+	var unionTypes []*Object
+	switch utype := ut.typeConfig.Types.(type) {
+	case UnionTypesThunk:
+		unionTypes = utype()
+	case []*Object:
+		unionTypes = utype
+	case nil:
+	default:
+		ut.err = fmt.Errorf("Unknown Union.Types type: %T", ut.typeConfig.Types)
+		ut.initalizedTypes = true
+		return nil
+	}
+
+	ut.types, ut.err = defineUnionTypes(ut, unionTypes)
+	ut.initalizedTypes = true
+	return ut.types
+}
+
+func defineUnionTypes(objectType *Union, unionTypes []*Object) ([]*Object, error) {
+	definedUnionTypes := []*Object{}
+
+	if err := invariantf(
+		len(unionTypes) > 0,
+		`Must provide Array of types for Union %v.`, objectType.Name(),
+	); err != nil {
+		return definedUnionTypes, err
 	}
-	for _, ttype := range config.Types {
-		if objectType.err = invariantf(
+
+	for _, ttype := range unionTypes {
+		if err := invariantf(
 			ttype != nil,
 			`%v may only contain Object types, it cannot contain: %v.`, objectType, ttype,
-		); objectType.err != nil {
-			return objectType
+		); err != nil {
+			return definedUnionTypes, err
 		}
 		if objectType.ResolveType == nil {
-			if objectType.err = invariantf(
+			if err := invariantf(
 				ttype.IsTypeOf != nil,
 				`Union Type %v does not provide a "resolveType" function `+
 					`and possible Type %v does not provide a "isTypeOf" `+
 					`function. There is no way to resolve this possible type `+
 					`during execution.`, objectType, ttype,
-			); objectType.err != nil {
-				return objectType
+			); err != nil {
+				return definedUnionTypes, err
 			}
 		}
+		definedUnionTypes = append(definedUnionTypes, ttype)
 	}
-	objectType.types = config.Types
-	objectType.typeConfig = config
 
-	return objectType
-}
-func (ut *Union) Types() []*Object {
-	return ut.types
+	return definedUnionTypes, nil
 }
+
 func (ut *Union) String() string {
 	return ut.PrivateName
 }
+
 func (ut *Union) Name() string {
 	return ut.PrivateName
 }
+
 func (ut *Union) Description() string {
 	return ut.PrivateDescription
 }
+
 func (ut *Union) Error() error {
 	return ut.err
 }

definition_test.go

@@ -519,6 +519,7 @@ func TestTypeSystem_DefinitionExample_ProhibitsNilTypeInUnions(t *testing.T) {
 		Name:  "BadUnion",
 		Types: []*graphql.Object{nil},
 	})
+	ttype.Types()
 	expected := `BadUnion may only contain Object types, it cannot contain: <nil>.`
 	if ttype.Error().Error() != expected {
 		t.Fatalf(`expected %v , got: %v`, expected, ttype.Error())
@@ -666,3 +667,42 @@ func TestTypeSystem_DefinitionExample_CanAddInputObjectField(t *testing.T) {
 		t.Fatal("Unexpected result, inputObject should have a field named 'newValue'")
 	}
 }
+
+func TestTypeSystem_DefinitionExample_IncludesUnionTypesThunk(t *testing.T) {
+	someObject := graphql.NewObject(graphql.ObjectConfig{
+		Name: "SomeObject",
+		Fields: graphql.Fields{
+			"f": &graphql.Field{
+				Type: graphql.Int,
+			},
+		},
+	})
+
+	someOtherObject := graphql.NewObject(graphql.ObjectConfig{
+		Name: "SomeOtherObject",
+		Fields: graphql.Fields{
+			"g": &graphql.Field{
+				Type: graphql.Int,
+			},
+		},
+	})
+
+	someUnion := graphql.NewUnion(graphql.UnionConfig{
+		Name: "SomeUnion",
+		Types: (graphql.UnionTypesThunk)(func() []*graphql.Object {
+			return []*graphql.Object{someObject, someOtherObject}
+		}),
+		ResolveType: func(p graphql.ResolveTypeParams) *graphql.Object {
+			return nil
+		},
+	})
+
+	unionTypes := someUnion.Types()
+
+	if someUnion.Error() != nil {
+		t.Fatalf("unexpected error, got: %v", someUnion.Error().Error())
+	}
+	if len(unionTypes) != 2 {
+		t.Fatalf("Unexpected result, someUnion should have two unionTypes, has %d", len(unionTypes))
+	}
+}