Advice is a mechanism, widely used in aspect-oriented languages, that allows one program component to augment or modify the behavior of other components. Advice is useful for modularizing concerns, including logging, error handling, and some optimizations, that would otherwise require code to be scattered throughout a system. When advice and other components are composed together they become tightly coupled, sharing both control and data flows. However this creates important problems: modular reasoning about a component becomes very difficult; and two tightly coupled components may interfere with the control and data flows of each other.
This paper presents EffectiveAdvice, a disciplined model of advice, inspired by Aldrich's Open Modules, that has full support for effects in both base components and advice. With EffectiveAdvice, equivalence of advice, as well as base components, can be checked by equational reasoning. The paper describes an implementation of EffectiveAdvice as a Haskell library and shows how to use it to solve well-known programming problems. Advice is modeled by mixin inheritance and effects are modeled by monads. Interference patterns previously identified in the literature are expressed as combinators. Parametricity, together with the combinators, is used to prove two harmless advice theorems. The result is an effective model of advice that supports effects in both advice and base components, and allows these effects to be separated with strong non-interference guarantees, or merged as needed.