This repository includes sample implementation of a partially type-erased publisher that retains significant properties from its upstream publisher.
Exposing type-erased publishers across APIs boundaries ensures that changing the underlying implementations won't affect existing code using such APIs. It is also useful to prevent callers from accessing certain methods from the wrapped types (see Subject example provided in AnyPublisher documentation).
But along with those undesired type details, type erasure -as implemented by AnyPublisher- also discards any intrinsic semantics carried by the concrete publisher type being wrapped; including properties that could help API consumers to reason about their code and play a role in its (compile-time) type safety.
As a side note, if you are not familiar with type-rich programming you can watch Bjarne Stroustrup talk on the topic (I found it via this SO answer).
The CombineSemantics library demonstrates how to model meaningful publisher properties into Swift protocols. Concrete publishers can declare their compliance to the semantics associated with a protocol by conforming to it.
The sample implementation focuses only on single value publishers. This property, modeled by the SingleValueSemantics protocol, defines a publisher that cannot:
- finish successfully without receiving a value
- fail if it has already received a value
- receive more than one value
In order to make conditional conformance to SingleValueSemantics more fine-grained, another protocol is declared, NonEmptySemantics that models non-empty publishers
The codebase is structured in the following folders:
Sources/CombineSemanticsTests/CombineSemanticsresourcesscripts
It contains the new publisher protocols:
NonEmptySemanticsSingleValueSemantics
And their corresponding type-erased concrete implementations:
AnyNonEmptyPublisherAnySingleValuePublisher.
Lastly, there are extensions for Combine's publishers to conditionally conform the aforementioned protocols.
It contains both the gyb template and the generated test case where partially type-erased publishers behavior is checked.
It contains the csv representation of the tests (it is required from the gyb template).
It contains the shell scripts used to download and execute gyb:
- use
scripts/download_gyb.shto download a local copy of gyb - use
scripts/process_gyb.sh Tests/CombineSemanticsTests/to regenerate the tests