remoteData.ts

// USAGE EXAMPLE

render() {
    const { currentUserState, nearEnv } = this.state;

    return currentUserState.match({
        notAsked: () => <HomePage />,
        loading: () => <HomePage />,
        failure: (err) => <>Error occurred</>,
        success: (user) => <AppRouter currentUser={user} nearEnv={nearEnv} />,
    });
}



// DEFINITION

type RemoteDataInner<T> =
    { type: 'NOT_ASKED' }
    | { type: 'LOADING' }
    | { type: 'FAILURE', error: Error }
    | { type: 'SUCCESS', data: T }

interface MatchHandlers<T, R> {
    notAsked: () => R,
    loading: () => R,
    success: (data: T) => R,
    failure: (error: Error) => R,
}

class RemoteData<T> {
    inner: RemoteDataInner<T> = { type: 'NOT_ASKED' };

    constructor(inner: RemoteDataInner<T>) {
        this.inner = inner;
    }

    public static newNotAsked<T>(): RemoteData<T> {
        const inner: RemoteDataInner<T> = { type: 'NOT_ASKED' };
        return new RemoteData<T>(inner);
    }

    static newLoading<T>(): RemoteData<T> {
        const inner: RemoteDataInner<T> = { type: 'LOADING' };
        return new RemoteData<T>(inner);
    }

    static newFailure<T>(error: Error): RemoteData<T> {
        const inner: RemoteDataInner<T> = { type: 'FAILURE', error };
        return new RemoteData<T>(inner);
    }

    static newSuccess<T>(data: T): RemoteData<T> {
        const inner: RemoteDataInner<T> = { type: 'SUCCESS', data };
        return new RemoteData<T>(inner);
    }

    match<R>(handlers: MatchHandlers<T, R>): R {
        switch (this.inner.type) {
            case 'NOT_ASKED': {
                return handlers.notAsked();
            }
            case 'LOADING': {
                return handlers.loading();
            }
            case 'FAILURE': {
                return handlers.failure(this.inner.error);
            }
            case 'SUCCESS': {
                return handlers.success(this.inner.data);
            }
            default:
                throw new Error("match() is supposed to be exhaustive");
        }
    }
}