16167152
Functional Programming with Overloading and Higher-Order Polymorphism
The Hindley/Milner type system has been widely adopted as a basis for statically typed functional languages. One of the main reasons for this is that it provides an elegant compromise between flexibility, allowing a single value to be used in different ways, and practicality, freeing the programmer from the need to supply explicit type information. Focusing on practical applications rather than implementation or theoretical details, these notes examine a range of extensions that provide more flexible type systems while retaining many of the properties that have made the original Hindley/Milner system so popular. The topics discussed, some old, but most quite recent, include higher-order polymorphism and type and constructor class overloading. Particular emphasis is placed on the use of these features to promote modularity and reusability.
15051631
Type Classes with Functional Dependencies
Type classes in Haskell allow programmers to define functions that can be used on a set of different types, with a potentially different implementation in each case. For example, type classes are used to support equality and numeric types, and for monadic programming. A commonly requested extension to support 'multiple parameters' allows a more general interpretation of classes as relations on types, and has many potentially useful applications. Unfortunately, many of these examples do not work well in practice, leading to ambiguities and inaccuracies in inferred types and delaying the detection of type errors. 
 
This paper illustrates the kind of problems that can occur with multiple parameter type classes, and explains how they can be resolved by allowing programmers to specify explicit dependencies between the parameters. A particular novelty of this paper is the application of ideas from the theory of relational databases to the design of type systems.
1424753
Monad transformers and modular interpreters
We show how a set of building blocks can be used to construct programming language interpreters, and present implementations of such building blocks capable of supporting many commonly known features, including simple expressions, three different function call mechanisms (call-by-name, call-by-value and lazy evaluation), references and assignment, nondeterminism, first-class continuations, and program tracing.
The underlying mechanism of our system is monad transformers, a simple form of abstraction for introducing a wide range of computational behaviors, such as state, I/O, continuations, and exceptions.
Our work is significant in the following respects. First, we have succeeded in designing a fully modular interpreter based on monad transformers that incudes features missing from Steele's, Espinosa's, and Wadler's earlier efforts. Second, we have found new ways to lift monad operations through monad transformers, in particular difficult cases not achieved in Moggi's original work. Third, we have demonstrated that interactions between features are reflected in liftings and that semantics can be changed by reordering monad transformers. Finally, we have implemented our interpreter in Gofer, whose constructor classes provide just the added power over Haskell's type classes to allow precise and convenient expression of our ideas. This implementation includes a method for constructing extensible unions and a form of subtyping that is interesting in its own right.
14632058
First-class polymorphism with type inference
Languages like ML and Haskell encourage the view of values as first-class entities that can be passed as arguments or results of functions, or stored as components of data structures. The same languages offer parametric polymorphism, which allows the use of values that behave uniformly over a range of different types. But the combination of these features is not supported-- polymorphic values are not first-class. This restriction is sometimes attributed to the dependence of such languages on type inference, in contrast to more expressive, explicitly typed languages, like System F, that do support first-class polymorphism.This paper uses relationships between types and logic to develop a type system, FCP, that supports first-class polymorphism, type inference, and also first-class abstract datatypes. The immediate result is a more expressive language, but there are also long term implications for language design.
2346657
ML Typing, Explicit Polymorphism and Qualified Types
The ML type system was originally introduced as a means of identifying a class of terms in a simple untyped language, often referred to as core-ML, whose evaluation could be guaranteed not to “go wrong”. In subsequent work, the terms of core-ML have also been viewed as a ‘convenient shorthand’ for programs in typed languages. Notable examples include studies of ML polymorphism and investigations of overloading, motivated by the use of type classes in Haskell.
6608650
Dictionary-free overloading by partial evaluation
One of the most novel features in the functional programming language Haskell is the system oftype classes used to support a combination of overloading and polymorphism. Current implementations of type class overloading are based on the use ofdictionary values, passed as extra parameters to overloaded functions. Unfortunately, this can have a significant effect on run-time performance, for example, by reducing the effectiveness of important program analyses and optimizations.This paper describes how a simple partial evaluator can be used to avoid the need for dictionary values at run-time by generating specialized versions of overloaded functions. This eliminates the run-time costs of overloading. Furthermore, and somewhat surprisingly given the presence of multiple versions of some functions, for all of the examples that we have tried so far, specialization actually leads to a reduction in the size of compiled programs.
6816342
SkipNet: A Scalable Overlay Network with Practical Locality Properties
Scalable overlay networks such as Chord, CAN, Pastry, and Tapestry have recently emerged as flexible infrastructure for building large peer-to-peer systems. In practice, such systems have two disadvantages: They provide no control over where data is stored and no guarantee that routing paths remain within an administrative domain whenever possible. SkipNet is a scalable overlay network that provides controlled data placement and guaranteed routing locality by organizing data primarily by string names. SkipNet allows for both fine-grained and coarse-grained control over data placement: Content can be placed either on a pre-determined node or distributed uniformly across the nodes of a hierarchical naming sub-tree. An additional useful consequence of SkipNet's locality properties is that partition failures, in which an entire organization disconnects from the rest of the system, can result in two disjoint, but well-connected overlay networks.
39264003
HyperAT: HCI and Web Authoring
We review HCI problems with hypertext, and for authoring World Wide Web documents in particular. We suggest that a framework is required to understand the usability issues, and that these issues cannot be seen as psychological or computing: they are multi-disciplinary. We discuss HyperAT, a protorype authoring tool, being implemented to test these ideas.
22331464
Vector-City Vector Distance Transform
This paper will examine the current chamfer and vector distance transforms for encoding objects as distance fields. A new vector distance transform is introduced which uses the city-block chamfer distance transform as a basis. Detailed error analysis using real CT data is presented, demonstrating the improved accuracy of the new approach over existing methods. The production of a subvoxel accurate distance field is also demonstrated by employing an improved classification. Distance fields are shown for skull and chess piece datasets.
5463102
Improving photo searching interfaces for small-screen mobile computers
In this paper, we conduct a thorough investigation of how people search their photo collections for events (a set of photographs relating to a particular well defined event), singles (individual photographs) and properties (a set of photographs with a common theme) on PDAs. We describe a prototype system that allows us to expose many issues that must be considered when designing photo searching interfaces. We discuss each of these issues and make recommendations where applicable. Our major observation is that several different methods are used to locate photographs. In light of this, we conclude by discussing how photo searching interfaces might embody or support such an approach.
39410616
Theme issue on interactive mobile information access
This paper introduces this issue on interactive mobile information access. Papers are presented issues relating to a diversity of user groups and environments, with a central theme of accessing information whilst on the move.