chore: import upstream snapshot with attribution

2026-07-13 13:21:23 +08:00
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+---
+# These are optional elements. Feel free to remove any of them.
+status: proposed
+contact: markwallace-microsoft
+date: 2023-11-21
+deciders: SergeyMenshykh, markwallace, rbarreto, mabolan, stephentoub
+consulted: 
+informed: 
+---
+
+# 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.