Generics

examples/production-app/graphql/Connection.ts

@@ -0,0 +1,27 @@
+/** @gqlType */
+export type Connection<T> = {
+  /** @gqlField */
+  edges: Edge<T>[];
+  /** @gqlField */
+  pageInfo: PageInfo;
+};
+
+/** @gqlType */
+export type Edge<T> = {
+  /** @gqlField */
+  node: T;
+  /** @gqlField */
+  cursor: string;
+};
+
+/** @gqlType */
+export type PageInfo = {
+  /** @gqlField */
+  startCursor: string | null;
+  /** @gqlField */
+  endCursor: string | null;
+  /** @gqlField */
+  hasNextPage: boolean;
+  /** @gqlField */
+  hasPreviousPage: boolean;
+};

examples/production-app/models/LikeConnection.ts

@@ -3,7 +3,7 @@ import * as DB from "../Database";
 import { Ctx } from "../ViewerContext";
 import { Query, Subscription } from "../graphql/Roots";
 import { Like } from "./Like";
-import { PageInfo } from "./PageInfo";
+import { PageInfo } from "../graphql/Connection";
 import { connectionFromArray } from "graphql-relay";
 import { Post } from "./Post";
 import { PubSub } from "../PubSub";

examples/production-app/models/PostConnection.ts

@@ -3,32 +3,16 @@ import * as DB from "../Database";
 import { Ctx } from "../ViewerContext";
 import { Query } from "../graphql/Roots";
 import { Post } from "./Post";
-import { PageInfo } from "./PageInfo";
+import { Connection } from "../graphql/Connection";
 import { connectionFromArray } from "graphql-relay";
 
-/** @gqlType */
-export type PostConnection = {
-  /** @gqlField */
-  edges: PostEdge[];
-  /** @gqlField */
-  pageInfo: PageInfo;
-};
-
 /**
  * Convenience field to get the nodes from a connection.
  * @gqlField */
-export function nodes(userConnection: PostConnection): Post[] {
+export function nodes(userConnection: Connection<Post>): Post[] {
   return userConnection.edges.map((edge) => edge.node);
 }
 
-/** @gqlType */
-type PostEdge = {
-  /** @gqlField */
-  node: Post;
-  /** @gqlField */
-  cursor: string;
-};
-
 // --- Root Fields ---
 
 /**
@@ -43,7 +27,7 @@ export async function posts(
     before?: string | null;
   },
   ctx: Ctx,
-): Promise<PostConnection> {
+): Promise<Connection<Post>> {
   const rows = await DB.selectPosts(ctx.vc);
   const posts = rows.map((row) => new Post(row));
   return connectionFromArray(posts, args);

examples/production-app/models/User.ts

@@ -4,8 +4,8 @@ import { Ctx } from "../ViewerContext";
 import { GraphQLNode } from "../graphql/Node";
 import { Model } from "./Model";
 import { Post } from "./Post";
-import { PostConnection } from "./PostConnection";
 import { Mutation } from "../graphql/Roots";
+import { Connection } from "../graphql/Connection";
 
 /** @gqlType */
 export class User extends Model<DB.UserRow> implements GraphQLNode {
@@ -21,7 +21,7 @@ export class User extends Model<DB.UserRow> implements GraphQLNode {
   /**
    * All posts written by this user. Note that there is no guarantee of order.
    * @gqlField */
-  async posts(_: unknown, ctx: Ctx): Promise<PostConnection> {
+  async posts(_: unknown, ctx: Ctx): Promise<Connection<Post>> {
     const rows = await DB.selectPostsWhereAuthor(ctx.vc, this.row.id);
     const posts = rows.map((row) => new Post(row));
     return connectionFromArray(posts, {});

examples/production-app/models/UserConnection.ts

@@ -3,32 +3,16 @@ import * as DB from "../Database";
 import { Ctx } from "../ViewerContext";
 import { Query } from "../graphql/Roots";
 import { User } from "./User";
-import { PageInfo } from "./PageInfo";
+import { Connection } from "../graphql/Connection";
 import { connectionFromArray } from "graphql-relay";
 
-/** @gqlType */
-export type UserConnection = {
-  /** @gqlField */
-  edges: UserEdge[];
-  /** @gqlField */
-  pageInfo: PageInfo;
-};
-
 /**
  * Convenience field to get the nodes from a connection.
  * @gqlField */
-export function nodes(userConnection: UserConnection): User[] {
+export function nodes(userConnection: Connection<User>): User[] {
   return userConnection.edges.map((edge) => edge.node);
 }
 
-/** @gqlType */
-type UserEdge = {
-  /** @gqlField */
-  node: User;
-  /** @gqlField */
-  cursor: string;
-};
-
 // --- Root Fields ---
 
 /**
@@ -43,7 +27,7 @@ export async function users(
     before?: string | null;
   },
   ctx: Ctx,
-): Promise<UserConnection> {
+): Promise<Connection<User>> {
   const rows = await DB.selectUsers(ctx.vc);
   const users = rows.map((row) => new User(row));
   return connectionFromArray(users, args);

src/Errors.ts

@@ -412,3 +412,28 @@ export function missingSpecifiedByUrl() {
 export function specifiedByOnWrongNode() {
   return `Unexpected \`@${SPECIFIED_BY_TAG}\` tag on non-scalar declaration. \`@${SPECIFIED_BY_TAG}\` can only be used on custom scalar declarations. Are you missing a \`@${SCALAR_TAG}\` tag?`;
 }
+
+export function missingGenericType(
+  templateName: string,
+  paramName: string,
+): string {
+  return `Missing type argument for generic GraphQL type. Expected \`${templateName}\` to be passed a GraphQL type argument for type parameter \`${paramName}\`.`;
+}
+
+export function nonGraphQLGenericType(
+  templateName: string,
+  paramName: string,
+): string {
+  return `Expected \`${templateName}\` to be passed a GraphQL type argument for type parameter \`${paramName}\`.`;
+}
+
+export function genericTypeUsedAsUnionMember(): string {
+  return `Unexpected generic type used sa union member. Generic type may not currently be used as members of a union. Grats requires that all union members define a \`__typename\` field typed as a string literal matching the type's name. Since generic types are synthesized into multiple types with different names, Grats cannot ensure they have a correct \`__typename\` property and thus cannot be used as members of a union.`;
+}
+export function genericTypeImplementsInterface(): string {
+  return `Unexpected \`implements\` on generic \`${TYPE_TAG}\`. Generic types may not currently declare themselves as implementing interfaces. Grats requires that all types which implement an interface define a \`__typename\` field typed as a string literal matching the type's name. Since generic types are synthesized into multiple types with different names, Grats cannot ensure they have a correct \`__typename\` property and thus declare themselves as interface implementors.`;
+}
+
+export function concreteTypeMissingTypename(implementor: string): string {
+  return `Missing \`__typename\` on \`${implementor}\`. The type \`${implementor}\` is used in a union or interface, so it must have a \`__typename\` field.`;
+}