this paper. Pleasingly, the arrow interface turned out to be applicable to other kinds of non-monadic library also, for example the fudgets library for graphical user interfaces [CH93], and a new library for programming active web pages. These applications will be described in sections 6 and 9. While arrows are a little less convenient to use than monads, they have significantly wider applicability. They can therefore be used to bring the benefits of monad-like programming to a much wider class of applications. 2 Background: Library Design Using Monads

Although detractors of functional programming sometimes claim that functional programming is too difficult or counterintuitive for most programmers to understand and use, evidence to the contrary can be found by looking at the popularity of spreadsheets. The spreadsheet paradigm, a first-order subset of the functional programming paradigm, has found wide acceptance among both programmers and end users. Still, there are many limitations with most spreadsheet systems.

The categorical notion of monad, used by Moggi to structure denotational descriptions, has proved to be a powerful tool for structuring combinator libraries. Moreover, the monadic programming style provides a convenient syntax for many kinds of computation, so that each library defines a new sublanguage. Recently, several workers have proposed a generalization of monads, called variously “arrows ” or Freyd-categories. The extra generality promises to increase the power, expressiveness and efficiency of the embedded approach, but does not mesh as well with the native abstraction and application. Definitions are typically given in a point-free style, which is useful for proving general properties, but can be awkward for programming specific instances. In this paper we define a simple extension to the functional language Haskell that makes these new notions of computation more convenient to use. Our language is similar to the monadic style, and has similar reasoning properties. Moreover, it is extensible, in the sense that new combining forms can be defined as expressions in the host language. 1.

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/0 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 stem 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 type...

Knit is a new component definition and linking language for systems code. Knit helps make C code more understandable and reusable by third parties, helps eliminate much of the performance overhead of componentization, detects subtle errors in component composition that cannot be caught with normal component type systems, and provides a foundation for developing future analyses over C-based components, such as cross-component optimization. The language is especially designed for use with component kits, where standard linking tools provide inadequate support for component configuration. In particular, we developed Knit for use with the OSKit, a large collection of components for building low-level systems. However, Knit is not OSKit-specific, and we have implemented parts of the Click modular router in terms of Knit components to illustrate the expressiveness and flexibility of our language. This paper provides an overview of the Knit language and its applications.

. We show how the features of modern integrated functional logic programming languages can be exploited to implement graphical user interfaces (GUIs) in a high-level declarative style. For this purpose, we have developed a GUI library in Curry, a multi-paradigm language amalgamating functional, logic, and concurrent programming principles. The functional features of Curry are exploited to define the graphical structure of an interface and to implement new graphical abstractions, and the logic features of Curry are used to specify the logical dependencies of an interface. Moreover, the concurrent and distributed features of Curry support the easy implementation of GUIs to distributed systems. 1 Introduction The implementation of graphical user interfaces for application programs is a non-trivial task which is usually supported by specific libraries. Although it is clear that any serious programming language must have a library for implementing GUIs, there are many different a...

Programmers confront a minefield when they design interactive Web programs. Web interactions take place via Web browsers. With browsers, consumers can whimsically navigate among the various stages of a dialog and can thus confuse the most sophisticated corporate Web sites. In turn, Web services can fault in frustrating and inexplicable ways. The quickening transition from Web scripts to Web services lends these problems immediacy.

. We describe a process extension to a lazy functional programming system, intended for applications with graphical user interfaces (GUIs). In the extended language, dynamically-created processes communicate by asynchronous message passing. We illustrate the use of the language, including as an extended example a simple board game in which squares are implemented as concurrent processes. We also describe a window manager, itself implemented in the extended functional language. Keywords: functional language, processes, concurrency, window manager, Gofer. 1 Introduction and Motivation Most of the time, elements of a graphical user interface (GUI) operate independently. For example, a menu doesn't interact with the rest of the program until the user selects an option. The user can highlight options, open up further menus or move the menu around the screen, all without doing anything that should concern any other element of the program. Popular languages such as C do not readily lend the...

In this paper we describe the implementation of several graphical programming paradigms (Model View Controller, Fudgets, and Functional Animations) using the GUI library TkGofer. This library relies on a combination of monads and multiple-parameter type classes to provide an abstract, type safe interface to Tcl/Tk. We show how choosing the right abstractions makes the given implementations surprisingly concise and easy to understand.

Many object-oriented languages used in practice descend from Algol. With this motivation, we study the theoretical issues underlying such languages via the theory of Algollike languages. It is shown that the basic framework of this theory extends cleanly and elegantly to the concepts of objects and classes. An important idea that comes to light is that classes are abstract data types, whose theory corresponds to that of existential types. Equational and Hoare-like reasoning methods, and relational parametricity provide powerful formal tools for reasoning about Algol-like object-oriented programs. 1