Publisher vs AnyPublisher in Combine

Question

What is the role of AnyPublisher in Combine, and why in many examples, including inWWDC Combine In practice, 27:40 they return AnyPublisher, using .eraseToAnyPublisher, and not just return a Publisher?

The Apple Documents says

Use AnyPublisher to wrap a publisher whose type has details you don’t want to expose to subscribers or other publishers.

But can anyone give an example of where it can be useful?

Answer 1

Publisher is a Protocol with associated types, while AnyPublisher is a struct.

Try casting to Publisher and you get an error

let x = Just(1) as Publisher

Protocol 'Publisher' can only be used as a generic constraint because it has Self or associated type requirements

This despite the fact that Just is a Publisher.

The Publisher type can't be utilized in the same way as AnyPublisher to achieve type erasure.

Where you could use Publisher is when you define a function that has generics as part of the definition.

Most common reason to use AnyPublisher:

Return an instance of a Publisher from a function.

Most common reason to use Publisher:

Create a protocol extension to create a custom Combine operator. For example:

extension Publisher {
  public func compactMapEach<T, U>(_ transform: @escaping (T) -> U?)
    -> Publishers.Map<Self, [U]>
    where Output == [T]
  {
    return map { $0.compactMap(transform) }
  }
}