Publisher vs AnyPublisher in Combine

Publisher is a protocol and AnyPublisher is a concrete implementation of Publisher.

AnyPublisher is a type-erased struct that conforms the Publisher protocol. Type erasure allows to hide details about the publisher that may not want to expose to subscribers — or downstream publishers.

Note: AnyPublisher does not have a send(_:) operator, so new values cannot be added to that publisher.

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) }
  }
}