Reusable pure computation
A derive declares reusable pure computation. It has typed parameters and a typed result, and can be called where its dependencies are available. A helper does not create authority: record access, caller context, and expression evaluation restrictions still apply through its dependency graph. In a mutation, guards and effects execute in source order; the complete effect model belongs to scenarios and effects.
Nullability must be handled before using a nullable value as non-null. Finite enum match is exhaustive; if remains useful for intentional fallbacks. A derived value cannot turn opaque identifiers into readable records. See the exact expression and builtin catalog.