Optics in the abstract

This repository contains materials to accompany the Well-Typed workshop at ZuriHac 2021 on Optics.

• Workshop will be streamed on YouTube at 14:00 CEST (13:00 BST, 08:00 ET) on Saturday 19th June

• Discussion in the #optics channel on the ZuriHac Discord

• Optics on Hackage

• You may wish to install optics-0.4 so you can experiment in GHCi (any of GHC 8.2 or later should work). The exercises/ directory contains a .cabal package that can be loaded with cabal repl.

Abstract

Haskell programming often involves manipulating data structures. Optics are a compositional abstraction representing "notions of substructure" as first-class values. For example, lenses capture the notion of one data structure containing another, and there are more general optics for read-only, write-only, optional or repeated substructures. Judicious use of optics can lead to succinct, expressive code, yet optics also have a reputation of being impenetrable.

In this workshop, we will explore optics-the-concept in general and the optics library in particular. This library provides interfaces for defining, combining and using various kinds of optics. Crucially, these interfaces are abstract, so they can be understood without reference to the underlying implementation.

We will also discuss how optics can help is in addressing the limitations of Haskell's record system, in conjunction with language extensions such as DuplicateRecordFields, OverloadedLabels and the forthcoming RecordDotSyntax and NoFieldSelectors.

Finally, we will use optics as a case study to reflect on library design in Haskell. How can we build libraries that capture powerful concepts and yet remain easy to use by Haskellers with a wide range of skill levels?

This workshop will be suitable for Haskell programmers with a knowledge of basic Haskell concepts (polymorphic functions, data types, type classes). It does not require familiarity with advanced type system features such as higher-rank types or type families.

Further reading

• Optics Haddocks

• Profunctor Optics: Modular Data Accessors by Matthew Pickering, Jeremy Gibbons, Nicolas Wu (The Art, Science, and Engineering of Programming, 2017).

• Applicative programming with effects by Conor McBride and Ross Patterson (JFP, 2008). The paper that introduced Applicative and Traversable.

• The essence of the iterator pattern by Jeremy Gibbons and Bruno C. d. S. Oliveira (JFP, 2009). Relates traversals to iterators, and discusses the laws for Traversable.

• Free Monoids in Haskell by Dan Doel. Explains why Fold s a is more general than s -> [a] once infinite structures get involved.

• Lens over tea by Artyom Kazak. A blog post series exploring the implementation of lens.

• Glassery by Oleg Grenrus. A detailed reference to (profunctor) optics.

• A reddit comment from Edward Kmett on the history of ideas in lens.

• Optics by Example by Chris Penner. A book giving a detailed introduction to lens.