Semantic Kernel Functions are defined using Interface or Abstract Base Class

Context and Problem Statement

The Semantic Kernel must define an abstraction to represent a Function i.e. a method that can be called as part of an AI orchestration. Currently this abstraction is the ISKFunction interface. The goal of the ADR is decide if this is the best abstraction to use to meet the long term goals of Semantic Kernel.

Decision Drivers

• The abstraction must extensible so that new functionality can be added later.
• Changes to the abstraction must not result in breaking changes for consumers.
• It is not clear at this time if we need to allow consumers to provide their own SKFunction implementations. If we do we this may cause problems as we add new functionality to the Semantic Kernel e.g. what if we define a new hook type?

Considered Options

• ISKFunction interface
• SKFunction base class

ISKFunction Interface

• Good, because implementations can extend any arbitrary class
• Bad, because we can only change the default behavior of our implementations and customer implementations may become incompatible.
• Bad, because we cannot prevent customers for implementing this interface.
• Bad, because changes to the interface are breaking changes for consumers.

SKFunction Case Class

• Good, because the changes to the interface are not breaking changes for consumers.
• Good, because class constructor can be made internal so we can prevent extensions until we know there are valid use cases.
• Good, because we can change the default implementation easily in future.
• Bad, because implementations can only extend SKFunction.

Decision Outcome

Chosen option: "SKFunction base class", because we can provide some default implementation and we can restrict creation of new SKFunctions until we better understand those use cases.