selection types for graphql queries

index.ts

type Scalar = string | number | boolean | undefined | null

type ScalarsOf<T> = {[Key in keyof T]: T[Key] extends Scalar ? Key : never}[keyof T]
type ObjectsOf<T> = {[Key in keyof T]: T[Key] extends Scalar ? never : Key}[keyof T]

type Simplify<T> = T extends Function ? T : {[K in keyof T]: Simplify<T[K]>};
type ValuableKeys<T> = {[Key in keyof T]: T[Key] extends never ? never : Key}[keyof T]
type Nevertheless<T> = T extends {} ? {[K in ValuableKeys<T>]: Nevertheless<T[K]>} : T

type CompleteSelector<T> = {[Key in ScalarsOf<T>]: true} & {
    [Key in ObjectsOf<T>]: T[Key] extends (infer U)[] ? CompleteSelector<U> : CompleteSelector<T[Key]>
}
type Selector<S, T> = CompleteSelector<T> extends S ? S & {__resultType: SelectionResult<S, T>} : never // throw away invalid selectors
type Selected<S, T> = T extends Scalar 
    ? T 
    : T extends (infer U)[] 
        ? SelectionResult<S, U>[] 
        : SelectionResult<S, T>
type SelectionResult<S, T> = CompleteSelector<T> extends S ? {
    [Key in keyof S]: Key extends keyof T 
        ? Selected<S[Key], T[Key]> 
        : never // throw away impossible keys
} : never // throw away invalid selectors

const s = <T>() => <S>(s: S): Selector<S, T> => s as any
type Infer<S extends {__resultType: any}> = Simplify<S['__resultType']>


type User = {
id: string
org: Organization
}

type Organization = {
id: string
leader: User
second: User
members: User[]
}

type Query = {
getUser: User
}

const q = s<Query>()

const result = q({
    getUser: {
        id: true,
        org: {
            leader: {
                id: true
            },
            second: {
                id: true
            },
            members: {
                id: true
            }
        }
    }
})

type Result = Infer<typeof result>