Functional programming languages have banned assignment because of its undesirable properties. The reward of this rigorous decision is that functional programming languages are side-effect free. There is another side to the coin: because assignment plays a crucial role in Input/Output (I/O), functional languages have a hard time dealing with I/O. Functional programming languages have therefore often been stigmatised as inferior to imperative programming languages because they cannot deal with I/O very well. In this paper, we show that I/O can be incorporated in a functional programming language without loss of any of the generally accepted advantages of functional programming languages. This discussion is supported by an extensive account of the I/O system offered by the lazy, purely functional programming language Clean. Two aspects that are paramount in its I/O system make the approach novel with respect to other approaches. These aspects are the technique of explicit multiple environment passing, and the Event I/O framework to program Graphical User I/O in a highly structured and high-level way. Clean file I/O is as powerful and flexible as it is in common imperative languages (one can read, write, and seek directly in a file). Clean Event I/O provides programmers with a high-level framework to specify complex Graphical User I/O. It has been used to write applications such as a window-based text editor, an object based drawing program, a relational database, and a spreadsheet program. These graphical interactive programs are completely machine independent, but still obey the look-and-feel of the concrete window environment being used. The specifications are completely functional and make extensive use of uniqueness typing, higher-order functions, and algebraic data types. Efficient implementations are present on the Macintosh, Sun (X Windows under Open Look) and PC (OS/2).

Abstract. Software testing is a labor-intensive and hence expensive, yet heavily used technique to control quality. In this paper we introduce Gast, a fully automatic test-tool. Properties from first order logic can be expressed in the system, Gast automatically generates appropriate test-data, evaluates the property for these values, and analyzes the test-results. In this way it becomes easier and cheaper to test software components. The distinguishing property of our system is that the test-data are generated in a systematic and generic way using generic programming techniques. This implies that there is no need for the user to indicate how data should be generated. Moreover, duplicated tests are avoided and for finite domains Gast is able to proof a property by testing it for all possible values. As an important side-effect, it also encourages stating formal properties of the software.

Task-Oriented Programming (TOP) is a novel programming paradigm for the construction of distributed systems where users work together on the internet. When multiple users collaborate, they need to interact with each other frequently. TOP supports the definition of tasks that react to the progress made by others. With TOP, complex multiuser interactions can be programmed in a declarative style just by defining the tasks that have to be accomplished, thus eliminating the need to worry about the implementation detail that commonly frustrates the development of applications for this domain. TOP builds on four core concepts: tasks that represent computations or work to do which have an observable value that may change over time, data sharing enabling tasks to observe each other while the work is in progress, generic type driven generation of user interaction, and special combinators for sequential and parallel task composition. The semantics of these core concepts is defined in this paper. As an example we present the iTask3 framework, which embeds TOP in the functional programming language Clean.

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