Jurriaan Hage

Scripting the type inference process

To improve the quality of type error messages in functional programming languages,we propose four techniques which influence the behaviour of constraint-based type inference processes. These techniques take the form of externally supplied type inference directives, precluding the need to make any changes to the compiler. A second advantage is that the directives are automatically checked for soundness with respect to the underlying type system. We show how the techniques can be used to improve the type error messages reported for a combinator library. More specifically, how they can help to generate error messages which are conceptually closer to the domain for which the library was developed. The techniques have all been incorporated in the Helium compiler, which implements a large subset of Haskell.

Heuristics for type error discovery and recovery

Type error messages that are reported for incorrect functional programs can be difficult to understand. The reason for this is that most type inference algorithms proceed in a mechanical, syntax-directed way, and are unaware of inference techniques used by experts to explain type inconsistencies. We formulate type inference as a constraint problem, and analyze the collected constraints to improve the error messages (and, as a result, programming efficiency). A special data structure, the type graph, is used to detect global properties of a program, and furthermore enables us to uniformly describe a large collection of heuristics which embed expert knowledge in explaining type errors. Some of these also suggest corrections to the programmer. Our work has been fully implemented and is used in practical situations, showing that it scales up well. We include a number of statistics from actual use of the compiler showing us the frequency with which heuristics are used, and the kind and number of suggested corrections.

Type class directives

The goal of this paper is to improve the type error messages in the presence of Haskell 98 type classes, in particular for the non-expert user. As a language feature, type classes are very pervasive, and strongly influence what is reported and when, even in relatively simple programs. We propose four type class directives, and specialized type rules, to lend high-level support to compilers to improve the type error messages. Both have been implemented, and can be used to easily modify the behavior of the type inference process.

A method engineering based legacy to SOA migration method

Legacy systems are vitally important for the continuation of business in an enterprise as they support complex core business processes. However, legacy systems have several well-known disadvantages such as being inflexible and hard to maintain, so momentum is growing to evolve those systems into new technology environments. Recently, service-oriented architecture has emerged as a promising architectural style that enables existing legacy systems to expose their functionality as services, without making significant changes to the legacy systems themselves. A significant number of the legacy to service migration approaches address the technical perspective (i.e., supporting technology) to expose the legacy code as services. The other approaches focus on determining the feasibility of the migration that includes economical and technical feasibility, based on the characteristics of existing legacy system and the requirements of the target SOA system. In this paper, a legacy to SOA migration method that does not single out the migration feasibility and technical perspectives, but combines these two perspectives of migration, is proposed. Method engineering is used to develop the migration method by reusing method fragments from existing service-oriented development methods. Then, concept slicing is used to develop the service by extracting the relevant parts of the legacy code. The method is evaluated and enhanced by interviewing experts and further validated with two case studies. The method is found to be appropriate and effective in extracting services from legacy code with the aim of reusing these services in new configurations.

A structured legacy to SOA migration process and its evaluation in practice

Legacy to Service-Oriented Architecture migration approaches have been extensively researched over the last decade, primarily to reuse the valuable business logic that resides within legacy applications. Interestingly, most of the proposed approaches fail to cover the complete process from the technological, organizational and business perspectives. This paper presents a structured six-phase process that covers both migration planning and execution, and does so by considering the aforementioned perspectives. Furthermore, within each of the six phases of the process, we present a rationale to justify the need of each phase, current practices within each phase, and challenges that require further attention. The proposed structured process is then evaluated by (i) migrating features of two simple yet representative applications to SOA, and (ii) by mapping activities reported in literature. Based on our findings, we believe that the proposed structured process is successfully fitting to capture the essence of the activities that are performed within the legacy to SOA migration domain by combining various perspectives.

A generic usage analysis with subeffect qualifiers

Sharing analysis and uniqueness typing are static analyses that aim at determining which of a programs objects are to be used at most once. There are many commonalities between these two forms of usage analysis. We make their connection precise by developing an expressive generic analysis that can be instantiated to both sharing analysis and uniqueness typing. The resulting system, which combines parametric polymorphism with effect subsumption, is specified within the general framework of qualified types, so that readily available tools and techniques can be used for the development of implementations and metatheory.

Improving type error messages for generic Java

Since version 1.5, generics (parametric polymorphism) are part of the Java language. However, the combination of parametric polymorphism and inclusion polymorphism is complicated, particularly so for Generic Java. Indeed, the main Java compilers, Eclipse’s ejc and Sun’s javac, do not even accept the same set of programs. Moreover, experience with these compilers shows that the error messages provided by them leave more than a little to be desired.To alleviate the latter problem, we describe how to adapt the type inference process of Java to obtain better error diagnostics for generic method invocations. Although the extension by itself already helps to improve type error messages to some extent, another major advantage of the new type inference process is that it also paves the way for further heuristics can provide additional diagnostic information. The extension has been implemented into the JastAdd Extensible Java Compiler.

Strategies for Solving Constraints in Type and Effect Systems

Turning type and effect deduction systems into an algorithm is a tedious and error-prone job, and usually results in an implementation that leaves no room to modify the solving strategy, without actually changing it. We employ constraints to declaratively specify the rules of a type system. Starting from a constraint based formulation of a type system, we introduce special combinators in the type rules to specify in which order constraints may be solved. A solving strategy can then be chosen by giving a particular interpretation to these combinators, and the resulting list of constraints can be fed into a constraint solver; thus the gap between the declarative specification and the deterministic implementation is bridged. This design makes the solver simpler and easier to reuse. Our combinators have been used in the development of a real-life compiler.

Euler Graphs, Triangle-Free Graphs and Bipartite Graphs in Switching Classes

In the context of graph transformations we look at the operation of switching, which can be viewed as a method for realizing global transformations of graphs through local transformations of the vertices. A switching class is then a set of graphs obtainable from a given start graph by applying the switching operation.Continuing the line of research in Ehrenfeucht, Hage, Harju and Rozenberg we consider the problem of detecting three kinds of graphs in switching classes. For all three we find algorithms running in time polynomial in the number of vertices in the graphs, although switching classes contain exponentially many graphs.

How do professionals perceive legacy systems and software modernization

Existing research in legacy system modernization has traditionally focused on technical challenges, and takes the standpoint that legacy systems are obsolete, yet crucial for an organizations operation. Nonetheless, it remains unclear whether practitioners in the industry also share this perception. This paper describes the outcome of an exploratory study in which 26 industrial practitioners were interviewed on what makes a software system a legacy system, what the main drivers are that lead to the modernization of such systems, and what challenges are faced during the modernization process. The findings of the interviews have been validated by means of a survey with 198 respondents. The results show that practitioners value their legacy systems highly, the challenges they face are not just technical, but also include business and organizational aspects.