Abstract: Experience shows that users of the Hat viewing tools find it hard to keep orientation and navigate to a point of interest in the trace. Hence this paper describes a new viewing tool where navigation through the trace is based on the program source. The tool combines ideas from algorithmic debugging, traditional stepping debuggers and dynamic program slicing. 1

The world outside of academia lacks large scale real examples of applications written in functional programming languages. This technical report considers the development of one such application, a Geometric Modelling System. The core of the report describes the application in some depth. The report concludes with an appraisal of the results and considers the appropriateness of the functional programming style. 1 Introduction The advantages claimed of the functional programming style over the conventional imperative style are now well rehearsed [Jon84] [Hug89] [Hud89]. Even so functional languages have only flourished in the laboratories of academia. This fact is currently being addressed by the FLARE project. "The effectiveness of functional programming has been amply demonstrated on a laboratory scale and it is now appropriate to scale up the experiments with a view to encouraging the wider community of applications developers to embrace this technology." --- FLARE project proposal ...

Introduction This chapter initially considers pure lazy functional languages: their philosophy, advantages and disadvantages. We then examine how to develop efficient lazy functional programs. One way to achieve efficiency is to introduce impurities. In the final section the two schools of lazy functional programming, pure and impure, are assessed. The assessment centres around two partial implementations of the Hopcroft Tarjan graph planarity algorithm. Profiling tools are used to make an experimental comparison and optimisation of each program. 4.1 Lazy Functional Programming In his book [42] Reade suggests that the user of a traditional imperative language is required to do the following: 1. describe the result to be computed; 2. impose an order on the steps required in the computation; 3. create and destroy, as required, any data structures used by the computation. 74 The first item is concerned with the extensional prope