+One motivation for using a functional programming language is that
+it is more difficult (or even impossible) to make low-level mistakes,
+and it is easier to reason about programs. But even the most advanced
+functional programmers are not infallible; they misunderstand the
+properties of their own programs, or those of others, and so commit
+errors. We therefore aim to provide functional programmers with
+tools for testing and tracing programs. In broad terms, testing
+means first specifying what behaviour is acceptable in principle,
+then finding out whether behaviour in practice matches up to it across
+the input space. Tracing means first recording the internal details
+of a computation, then examining what is recorded to gain insight, to
+check hypotheses or to locate faults. We concentrate on QuickCheck,
+a tool for testing Haskell programs, and Hat, a tool for tracing them.
+Each tool is useful in its own right, but they are even more useful
+in combination: testing using QuickCheck can identify failing cases,
+tracing using Hat can reveal the causes of failure.
+
Postscript (103 KB)
+
+ +
+Hat is a programmer's tool for generating a trace of a computation of
+a Haskell 98 program and viewing such a trace in various different
+ways. Applications include program comprehension and debugging.
+A new version of Hat uses a stand-alone program transformation to
+produce self-tracing Haskell programs. The transformation is small
+and works with any Haskell 98 compiler that implements the standard
+foreign function interface. We present general techniques for
+building compiler independent tools similar to Hat based on program
+transformation. We also point out which features of Haskell 98 caused
+us particular grief.
+
Postscript (84 KB)
+
+ +
+We define a small step operational semantics for a core of Haskell. We modify
+this semantics to generate traces, specifically Augmented Redex Trails. This
+small and direct definition of Augmented Redex Trails shall improve our
+understanding of them and shall help to extend them systematically.
+
+Postscript (44 KB)
+
+
hunk ./docs/publications.html 100
-Freja, Hat and Hood - A Comparative Evaluation of Three Systems for Tracing and Debugging Lazy Functional Programs
+Freja, Hat and Hood - A Comparative Evaluation of Three Systems
+ for Tracing and Debugging Lazy Functional Programs
hunk ./docs/publications.html 116
-
hunk ./docs/publications.html 123
+Publications not directly about Hat, but produced as a spin-off
+result of the Hat project:
+
+The type systems of most typed functional programming languages
+are based on the Hindley-Milner type system. A practical problem
+with these type systems is that it is often hard to understand why a
+program is not type correct or a function does not have the intended
+type. We suggest that at the core of this problem is the difficulty
+of explaining why a given expression has a certain type. The type
+system is not defined compositionally. We propose to explain types
+using a variant of the Hindley-Milner type system that defines a
+compositional type explanation graph of principal typings. We describe
+how the programmer understands types by interactive navigation through
+the explanation graph. Furthermore, the explanation graph can be
+the foundation for algorithmic debugging of type errors, that is,
+semi-automatic localisation of the source of a type error without
+even having to understand the type inference steps. We implemented a
+prototype of a tool to explore the usefulness of the proposed methods.
+
+Postscript (139 KB)
+
+ +
+There are several Haskell libraries for converting tree structured data
+into indented text, but they all make use of some backtracking. Over
+twenty years ago Oppen published a more efficient imperative
+implementation of a pretty printer without backtracking. We show that
+the same efficiency is also obtainable without destructive updates
+by developing a similar but purely functional Haskell implementation
+with the same complexity bounds. At its heart lie two lazy double
+ended queues.
+
+Postscript (138 KB)
+
+ +