eraseToAnyPublisher

Combine Mid-level Swift 5.1+ iOS 13+ Reviewed 31 Oct 2026 By Alok Singh

Pick the fix

Why does func searchResults() -> AnyPublisher<[Item], Never> end with .eraseToAnyPublisher()?

OptionsTo hide the long nested operator type so callers depend only on Output and FailureTo make the publisher run on the main threadTo make the pipeline fasterTo keep the subscription alive

Hint 1 of 1

Look at the type a chain of operators produces.

Choose an answer, then open Show answer below to check it.

Show answer

Answer: A To hide the long nested operator type so callers depend only on Output and Failure

Correct. The implementation can change without breaking callers.

Why not the others?

B To make the publisher run on the main thread. Use receive(on:) for that.
C To make the pipeline faster. Erasure adds a small cost; it doesn't speed anything up.
D To keep the subscription alive. That is what AnyCancellable does.

30-second answer

Each operator wraps the previous publisher in a new generic type, so a pipeline's real type is long and leaks implementation details. eraseToAnyPublisher() wraps it in AnyPublisher<Output, Failure>, so an API exposes only what it emits and can change its internals freely. Opaque some Publisher also hides the type, but returns one fixed concrete type.

Practise saying it · 30 s. No recording, just a timer

Explain type erasure in Combine.

Say your answer out loud, then check which of these points you covered:

Nested operator types
AnyPublisher at boundaries
some Publisher alternative

Deep dive

Erase at module or API boundaries, not between every operator.

Use some Publisher<Output, Failure> (Swift 5.7 primary associated types) when one concrete type is returned.

Erasing has a tiny runtime cost.

Subjects are often exposed through an erased publisher so callers can't send.

Common mistakes

Exposing a Subject publicly.
Erasing after every operator.

Senior take & follow-ups

Seniors use erasure to design APIs, and know the opaque alternative.

Follow-up questions

Think of your answer first, then open the hint.

Why expose a subject as AnyPublisher?
Show hint
Callers can subscribe but not send.
When is some Publisher better?
Show hint
When the function always returns the same concrete pipeline.

Suggest a fix if something here is wrong or out of date.