Laws, laws, laws. It seems as though whenever we learn about a new abstraction in functional programming, we hear about its associated laws. Laws come up when we learn about type classes like Functors, Monoids, Monads, and more! Usually laws are mentioned and swiftly brushed past as we move on to examples and applications of whatever structure we're learning about. But not today.

In this talk, we'll learn about Functors and Monoids, paying close attention to their laws. Why should our abstractions have laws? We'll answer this question both by seeing powers we gain by having laws, and by seeing tragedies that can befall us without laws.

From CSV, to JSON, to YAML, DER, and the dreaded XML, many programmers are dealing with data formats all day. In statically-typed languages like Haskell, we can get a great benefit by imposing a rigid structure of types on the data we consume from these formats. We call this process of imposing structure on data "decoding".

Decoding libraries are available in many languages and for many formats. This talk explores the design decisions behind a new Haskell library, sv, which decodes CSV and similar formats such as PSV. sv addresses what we perceived as problems with other libraries. We will discuss the benefits and drawbacks of the interesting or noteworthy choices made in sv's philosophy and design, and make recommendations as to how these lessons could be applied to other formats.

Since their introduction, Applicative and its sidekick Alternative have become ubiquitous and beloved type classes in Haskell and similar languages such as Purescript. By contrast, their contravariant counterparts are relatively unknown. These contravariant structures are useful, and their time has come!

In this talk, we will learn about Contravariant, and its subclasses Divisible and Decidable. How do they work? How do they relate to the classes we know? Are they useful? For what? This talks aims to demystify these classes in an approachable way, by demonstrating their use with practical motivating examples.