How to map error in Combine

Issue #506

When a function expects AnyPublisher<[Book], Error> but in mock, we have Just

func getBooks() -> AnyPublisher<[Book], Error> {
    return Just([
        Book(id: "1", name: "Book 1"),
        Book(id: "2", name: "Book 2"),
    ])
    .eraseToAnyPublisher()
}

There will be a mismatch, hence compile error

Cannot convert return expression of type ‘AnyPublisher<[Book], Just

.Failure>’ (aka ‘AnyPublisher<Array, Never>’) to return type ‘AnyPublisher<[Book], Error>’

The reason is because Just produces Never, not Error. The workaround is to introduce Error

enum AppError: Error {
    case impossible
}

func getBooks() -> AnyPublisher<[Book], Error> {
    return Just([
        Book(id: "1", name: "Book 1"),
        Book(id: "2", name: "Book 2"),
    ])
    .mapError({ _ in  AppError.impossible })
    .eraseToAnyPublisher()
}

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Updated at 2020-11-07 20:30:01