Type inference issues when organizing resolvers in separate files

[lorefnon]

Hello, thanks for maintaining garph as an open source project. This is perhaps related to #78 but I am creating as a new issue because it does seem like there is an issue with types somewhere.

My goal is to restructure a project that uses garph such that resolvers are grouped into separate modules. This has proven to be surprisingly difficult because I keep running into type errors that defy intuition.

I have a minimal reproducer here that illustrates the problem.

The api-schema.ts has the type definitons for graphql. And the resolvers are located here.

The crux is that I have a Resolvers type that I infer from the schema definiton:

export type Resolvers = InferResolvers<{ Query: typeof QueryT }, { context: YogaInitialContext }>

and then use its members when defining specific resolvers, for example:

export const locations: Resolvers["Query"]["locations"] = async (_parent, { page }) => {
  return {
    locations: [],
    page: {
      current: page.num,
      size: page.size
    }
  }
}

The above type checks without issues.

However when I try to compose these resolvers into a Query/Mutation type, I get obscure type errors that are hard to fathom:

export const Query: Resolvers["Query"] = {
  locations,
}

❯ pnpm run ts:check

> [email protected] ts:check /home/lorefnon/Workspace/garph-reproducer-1702804756397
> tsc --noEmit

server/resolvers/query.ts:4:14 - error TS2322: Type '{ locations: ((parent: {}, args: { page: { __typename?: "PageInput" | undefined; num: number; size: number;
 }; }, context: YogaInitialContext, info: GraphQLResolveInfo) => any) | { ...; } | { ...; } | { ...; } | undefined; }' is not assignable to type '{ locations?:
((parent: {}, args: { page: { __typename?: "PageInput" | undefined; num: number; size: number; }; }, context: YogaInitialContext, info: GraphQLResolveInfo) => a
ny) | undefined; } | ({ ...; } & { ...; })'.
  Type '{ locations: ((parent: {}, args: { page: { __typename?: "PageInput" | undefined; num: number; size: number; }; }, context: YogaInitialContext, info: Gra
phQLResolveInfo) => any) | { ...; } | { ...; } | { ...; } | undefined; }' is not assignable to type '{ locations?: { resolve: (parent: {}, args: { page: { __typ
ename?: "PageInput" | undefined; num: number; size: number; }; }, context: YogaInitialContext, info: GraphQLResolveInfo) => any; } | { ...; } | { ...; } | undef
ined; } & { ...; }'.
    Type '{ locations: ((parent: {}, args: { page: { __typename?: "PageInput" | undefined; num: number; size: number; }; }, context: YogaInitialContext, info: G
raphQLResolveInfo) => any) | { ...; } | { ...; } | { ...; } | undefined; }' is not assignable to type '{ locations?: { resolve: (parent: {}, args: { page: { __t
ypename?: "PageInput" | undefined; num: number; size: number; }; }, context: YogaInitialContext, info: GraphQLResolveInfo) => any; } | { ...; } | { ...; } | und
efined; }'.
      Types of property 'locations' are incompatible.
        Type '((parent: {}, args: { page: { __typename?: "PageInput" | undefined; num: number; size: number; }; }, context: YogaInitialContext, info: GraphQLRes
olveInfo) => any) | { ...; } | { ...; } | { ...; } | undefined' is not assignable to type '{ resolve: (parent: {}, args: { page: { __typename?: "PageInput" | un
defined; num: number; size: number; }; }, context: YogaInitialContext, info: GraphQLResolveInfo) => any; } | { ...; } | { ...; } | undefined'.
          Type '(parent: {}, args: { page: { __typename?: "PageInput" | undefined; num: number; size: number; }; }, context: YogaInitialContext, info: GraphQLRe
solveInfo) => any' is not assignable to type '{ resolve: (parent: {}, args: { page: { __typename?: "PageInput" | undefined; num: number; size: number; }; }, con
text: YogaInitialContext, info: GraphQLResolveInfo) => any; } | { ...; } | { ...; } | undefined'.

4 export const Query: Resolvers["Query"] = {
               ~~~~~


Found 1 error in server/resolvers/query.ts:4

 ELIFECYCLE  Command failed with exit code 2.

It is not clear why an object of type { locations: Resolvers["Query"]["locations"] } is not assignable to Resolvers["Query"] when Query doesn't have any other fields.

However, if I inline the locations resolver within the Query resolver here, it typechecks fine.

[mishushakov]

Do you still have the issue? Why was it closed?

[lorefnon]

Hi @mishushakov - I sorted it out - was an error at my end.

While logging the issue, I hadn't realized that Resolvers["Query"] was a union of multiple types. So Resolvers["Query"]["locations"] would end up being union of the type of that field from different members and if I wrap that into an object it would no longer be assignable to Resolvers["Query"].

So the pattern I adopted was always using Resolvers["SomeType"] when splitting, which works great.

// One file:

const QueryLocationsResolvers: Resolvers["Query"] = { locations: (...) => { ... } }

// Another file 
const QueryPricesResolvers: Resolvers["Query"] = { prices: (...) => { ... } }

// Finally while merging: 
const QueryResolvers: Resolvers["Query"] = {
    ...QueryLocationsResolvers,
    ...QueryPricesResolvers,
}

export const schema = buildSchema({
  g,
  resolvers: {
    Query: QueryResolvers,
    // ...
  },
})

Thanks for taking a look though.