Modular visitor components a practical solution to the expression families problem

The expression families problem can be defined as the problem of achieving reusability and composability across the components involved in a family of related datatypes and corresponding operations over those datatypes. Like the traditional expression problem, adding new components (either variants or operations) should be possible while preserving modular and static type-safety. Moreover, different combinations of components should have different type identities and the subtyping relationships between components should be preserved. By generalizing previous work exploring the connection between type-theoretic encodings of datatypes and visitors, we propose two solutions for this problem in Scala using modular visitor components. These components can be grouped into features that can be easily composed in a feature-oriented programming style to obtain customized datatypes and operations. © 2009 Springer Berlin Heidelberg.link_to_subscribed_fulltex

Functional programming with structured graphs

This paper presents a new functional programming model for graph structures called structured graphs. Structured graphs extend conventional algebraic datatypes with explicit definition and manipulation of cycles and/or sharing, and offer a practical and convenient way to program graphs in functional programming languages like Haskell. The representation of sharing and cycles (edges) employs recursive binders and uses an encoding inspired by parametric higher-order abstract syntax. Unlike traditional approaches based on mutable references or node/edge lists, well-formedness of the graph structure is ensured statically and reasoning can be done with standard functional programming techniques. Since the binding structure is generic, we can define many useful generic combinators for manipulating structured graphs. We give applications and show how to reason about structured graphs. © 2012 ACM.link_to_subscribed_fulltex

TypeCase: A design pattern for type-indexed functions

A type-indexed function is a function that is defined for each member of some family of types. Haskell's type class mechanism provides collections of open type-indexed functions, in which the indexing family can be extended by defining a new type class instance but the collection of functions is fixed. The purpose of this paper is to present TypeCase: a design pattern that allows the definition of closed type-indexed functions, in which the index family is fixed but the collection of functions is extensible. It is inspired by Cheney and Hinze's work on lightweight approaches to generic programming. We generalise their techniques as a design pattern. Furthermore, we show that type-indexed functions with type-indexed types, and consequently generic functions with generic types, can also be encoded in a lightweight manner, thereby overcoming one of the main limitations of the lightweight approaches. Copyright © 2005 ACM

Scala for generic programmers Comparing Haskell and Scala support for generic programming

Datatype-generic programming (DGP) involves parametrization of programs by the shape of data, in the form of type constructors such as list of. Most approaches to DGP are developed in pure functional programming languages such as Haskell. We argue that the functional object-oriented language Scala is in many ways a better choice. Not only does Scala provide equivalents of all the necessary functional programming features (such as parametric polymorphism, higher-order functions, higher-kinded type operations, and type-and constructor-classes), but it also provides the most useful features of object-oriented languages (such as subtyping, overriding, traditional single inheritance, and multiple inheritance in the form of traits). Common Haskell techniques for DGP can be conveniently replicated in Scala, whereas the extra expressivity provides some important additional benefits in terms of extensibility and reuse. We illustrate this by comparing two simple approaches in Haskell, pointing out their limitations and showing how equivalent approaches in Scala address some of these limitations. Finally, we present three case studies on how to implement in Scala real DGP approaches from the literature: Hinze's Generics for the Masses, Lmmel and Peyton Jones's Scrap your Boilerplate with Class, and Gibbons's Origami Programming. Copyright © 2010 Cambridge University Press

Scala for generic programmers

Datatype-generic programming involves parametrization by the shape of data, in the form of type constructors such as 'list of'. Most approaches to datatype-generic programming are developed in the lazy functional programming language Haskell. We argue that the functional object-oriented language Scala is in many ways a better setting. Not only does Scala provide equivalents of all the necessary functional programming features (such parametric polymorphism, higher-order functions, higher-kinded type operations, and type- and constructor-classes), but it also provides the most useful features of object-oriented languages (such as subtyping, overriding, traditional single inheritance, and multiple inheritance in the form of traits). We show how this combination of features benefits datatype-generic programming, using three different approaches as illustrations. Copyright © 2008 ACM.link_to_subscribed_fulltex

Functional programming with structured graphs

This paper presents a new functional programming model for graph structures called structured graphs. Structured graphs extend conventional algebraic datatypes with explicit definition and manipulation of cycles and/or sharing, and offer a practical and convenient way to program graphs in functional programming languages like Haskell. The representation of sharing and cycles (edges) employs recursive binders and uses an encoding inspired by para- metric higher-order abstract syntax. Unlike traditional approaches based on mutable references or node/edge lists, well-formedness of the graph structure is ensured statically and reasoning can be done with standard functional programming techniques. Since the binding structure is generic, we can define many useful generic combinators for manipulating structured graphs. We give applications and show how to reason about structured graphs. Copyright © 2012 ACM.link_to_subscribed_fulltex

Extensibility for the masses practical extensibility with object Algebras

This paper presents a new solution to the expression problem (EP) that works in OO languages with simple generics (including Java or C#). A key novelty of this solution is that advanced typing features, including F-bounded quantification, wildcards and variance annotations, are not needed. The solution is based on object algebras, which are an abstraction closely related to algebraic datatypes and Church encodings. Object algebras also have much in common with the traditional forms of the Visitor pattern, but without many of its drawbacks: they are extensible, remove the need for accept methods, and do not compromise encapsulation. We show applications of object algebras that go beyond toy examples usually presented in solutions for the expression problem. In the paper we develop an increasingly more complex set of features for a mini-imperative language, and we discuss a real-world application of object algebras in an implementation of remote batches. We believe that object algebras bring extensibility to the masses: object algebras work in mainstream OO languages, and they significantly reduce the conceptual overhead by using only features that are used by everyday programmers. © 2012 Springer-Verlag Berlin Heidelberg.link_to_subscribed_fulltex

Abstract syntax graphs for domain specific languages

This paper presents a representation for embedded domain specific languages (EDSLs) using abstract syntax graphs (ASGs). The purpose of this representation is to deal with the important problem of defining operations that require observing or preserving sharing and recursion in EDSLs in an expressive, yet easy-to-use way. In contrast to more conventional representations based on abstract syntax trees, ASGs represent sharing and recursion explicitly as binder constructs. We use a functional representation of ASGs based on structured graphs, where binders are encoded with parametric higher-order abstract syntax. We show how adapt to this representation to well-typed ASGs. This is especially useful for EDSLs, which often reuse the type system of the host language. We also show an alternative class-based encoding of (well-typed) ASGs that enables extensible and modular well-typed EDSLs while allowing the manipulation of sharing and recursion. Copyright © 2013 ACM.link_to_subscribed_fulltex

The essence of the ITERATOR pattern

The Iterator pattern gives a clean interface for element-by-element access to a collection, independent of the collection's shape. Imperative iterations using the pattern have two simultaneous aspects: mapping and accumulating. Various existing functional models of iteration capture one or other of these aspects, but not both simultaneously. We argue that C. McBride and R. Paterson's applicative functors (Applicative programming with effects, J. Funct. Program., 18 (1): 113, 2008), and in particular the corresponding traverse operator, do exactly this, and therefore capture the essence of the Iterator pattern. Moreover, they do so in a way that nicely supports modular programming. We present some axioms for traversal, discuss modularity concerns and illustrate with a simple example, the wordcount problem. © 2009 Copyright Cambridge University Press

"Scrap your boilerplate" reloaded

The paper "Scrap your boilerplate" (SYB) introduces a combinator library for generic programming that offers generic traversais and queries. Classically, support for generic programming consists of two essential ingredients: a way to write (type-)overloaded functions, and independently, a way to access the structure of data types. SYB seems to lack the second. As a consequence, it is difficult to compare with other approaches such as PolyP or Generic Haskell. In this paper we reveal the structural view that SYB builds upon. This allows us to define the combinators as generic functions in the classical sense. We explain the SYB approach in this changed setting from ground up, and use the understanding gained to relate it to other generic programming approaches. Furthermore, we show that the SYB view is applicable to a very large class of data types, including generalized algebraic data types. © Springer-Verlag Berlin Heidelberg 2006.link_to_subscribed_fulltex