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Cratis

CHR0065: Read model fold method will not be discovered

Rule description

Section titled “Rule description”

Chronicle discovers a self reducer from public static methods named Reduce. An otherwise fold-shaped method named Reducer, reduce, ReduceOn, or another name is ignored. An instance Reduce method alone does not establish self-reducer discovery either. The warning names the method, explains which requirement it misses, and shows the expected signature.

The rule requires all of these signals:

  • The containing type is a concrete, non-generic class or record that does not implement IReducer or IReducerFor<T>.
  • The type carries Arc’s [ReadModel] attribute, including inherited metadata, or already has a valid self-reduction method or a static Reduce near miss recognized by Chronicle’s self-reducer policy.
  • The method is public and non-generic, with two or three ordinary parameters: a class or interface with event declaration metadata ([EventType] or [EventTypeGenerationFor<T>], including inherited metadata), exactly the containing model type (nullable or non-nullable), and optionally EventContext.
  • The result is exactly the containing model or Task of that model, including nullable results.
  • The name is not exactly Reduce, or the method is not static.

Ordinary helpers, factories, collection-returning queries, non-event arguments, and unrelated types are not reported. The rule requires visible event declaration metadata and respects attribute inheritance. Historical generations and types inheriting event markers count; event membership supplied only by registration or inferred from known derived events of an unmarked dispatch base type does not. A method already reported by CHR0058 as an invalid sibling Reduce declaration does not also receive CHR0065.

Severity

Section titled “Severity”

Warning

How to fix

Section titled “How to fix”

If the method is intended to fold events, rename it to Reduce and make it public static. Accept the nullable current model because the first event receives absent state. Use (event, current) or (event, current, EventContext) and return the model or Task of the model.

Review instance-state access before making a method static, and check callers and other overloads before renaming it. If the fold needs injected services, use a separate IReducerFor<T> instead. There is no automatic code fix.

Section titled “Related topics”

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