T6: An Introduction to Type-Level and Generic Programming in Haskell

Abstract

Many Haskell functions can be defined for a large class of datatypes in a systematic way. Examples include structural equality and comparisons, all kind of (de)serialization functions (plain text, JSON, binary, etc.), traversal and access functions such as lenses and their various variants, functions for querying and updating databases and many more. For some built-in type classes, Haskell offers the "deriving" construct, but GHC also has a "generics" feature that allows users to define their own derivable classes, without having to resort to Template Haskell.

In this tutorial, we will introduce "generics-sop", a library that supports such datatype-generic programming by means of a number of powerful, higher-order combinators that can be reused and composed in several ways. This approach makes use of several type-system extensions available for GHC, such as GADTs (in particular, for heterogeneous lists), data kinds, kind polymorphism, constraint kinds, and rank-n polymorphism. We will introduce these concepts as needed and use "generics-sop" as motivation and example of how they can be put to good use. We will then also use "generics-sop" to define several example generic functions.

Tutorial objectives

Target audience

Intermediate and advanced Haskellers with solid knowledge of the core language features, and who want to learn more about type-level and/or datatype-generic programming. No prior knowledge of type system extensions such as GADTs or data kinds is assumed.

Participants need to bring a laptop with the GHC compiler and the generics-sop package (available on Hackage) installed.

Andres Löh

I am a Haskell consultant and co-owner of Well-Typed LLP, living in Regensburg, Germany. My first exposure to functional programming was during my undergraduate studies in Mathematics and Computer Science at the University of Konstanz. Inspired by that, I did a PhD on datatype-generic programming in Haskell at Utrecht University, which I completed in 2004. After being a postdoc and lecturer in Tallinn, Freiburg, Bonn, and Utrecht, I joined Well-Typed in 2010.

I am very interested in applying functional programming to real-world problems, and in particular in designing a programming language and its type system such that it facilitates code reuse and code evolution. To me, datatype-generic programming is an important technique to achieve these goals. Other research interests of mine include domain-specific languages, dependent types, parallel and concurrent programming, and version control.

At Well-Typed, I'm working on various projects for companies that use Haskell, as well as on open-source Haskell projects. I enjoy finding many opportunities to apply Computer Science research (other people's as well as mine) in practice. I am also responsible for designing and developing our Haskell training courses.