Previous implementations of generic rewriting libraries have a number of limitations: they require the user to either adapt the datatype on which rewriting is applied, or the rewriting rules are specified as functions, which makes it hard or impossible to document, test, and analyse them. We describe a library that demonstrates how to overcome these limitations by defining rules in terms of datatypes, and show how to use a type-indexed datatype to automatically extend a datatype for syntax trees with a case for metavariables. We then show how rewrite rules can be implemented without any knowledge of how the datatype is extended with metavariables. We use Haskell, extended with associated type synonyms, to implement both type-indexed datatypes and generic functions. We analyse the performance of our library and compare it with other approaches to generic rewriting.

Previous implementations of generic rewriting libraries have a number of limitations: they require the user to either adapt the datatype on which rewriting is applied, or the rewriting rules are specified as functions, which makes it hard or impossible to document, test, and analyse them. We describe a library that demonstrates how to overcome these limitations by defining rules in terms of datatypes, and show how to use a type-indexed datatype to automatically extend a datatype for syntax trees with a case for metavariables. We then show how rewrite rules can be implemented without any knowledge of how the datatype is extended with metavariables. We use Haskell, extended with associated type synonyms, to implement both type-indexed datatypes and generic functions. We analyse the performance of our library and compare it with other approaches to generic rewriting.

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Workflow management systems (WFMS) are software systems that coordinate the tasks human workers and computers have to perform to achieve a certain goal based on a given workflow description. Due to changing circumstances, it happens often that some tasks in a running workflow need to be performed differently than originally planned and specified. Most commercial WFMSs cannot deal with the required run-time changes properly. These changes have to be specified at the level of the underlying Petri-Net based semantics. Moreover, the implicit external state has to be adapted to the new task as well. Such low-level updates can easily lead to wrong behaviour and other errors. This problem is known as the dynamic change bug. In the iTask WFMS, workflows are specified using a radically different approach: workflows are constructed in a compositional style, using pure functions and combinators as selfcontained building blocks. This paper introduces a change concept for the iTask system where self-contained tasks can be replaced by other self-contained tasks, thereby preventing dynamic change bugs. The static and dynamic typing system furthermore guarantees that these tasks have compatible types.

Static typing in functional programming languages such as Clean, Haskell, and ML is highly beneficial: it prevents erroneous be haviour at run time and provides opportunities for optimisations. However, dynamic typing is just as important as sometimes types are not known until run time. Examples are exchanging values be tween applications by deserialisation from disk, input provided by a user, or obtaining values via a network connection. Ideally, a static typing system works in close harmony with an orthogonal dynamic typing system; not discriminating between statically and dynamically typed values. In contrast to Haskell's minimal sup port for dynamic typing, Clean has an extensive dynamic typing; it adopted ML's support for monomorphism and parametric polymor phism and added the notion of type dependencies. Unfortunately, ad-hoc polymorphism has been left out of the equation over the years. While both ad-hoc polymorphism and dynamic typing have been studied in-depth earlier, their interaction in a statically typed functional language has not been studied before. In this paper we explore the design space of their interactions.