genericUnion.ts

type AsyncData<Data> = Initial | Retrieving | Success<Data> | Error;

type Initial = {
    kind: 'Initial'
};
type Retrieving = {
    kind: 'Retrieving'
};
type Success<Data> = {
    kind: 'Success',
    data: Data[]
};
type Error = {
    kind: 'Error',
    message: string
};

// ----

type User = {
    login: string,
    avatarUrl: string
};

type PageState = AsyncData<User>;