Book Image

Haskell Design Patterns

By : Tikhon Jelvis, Ryan Lemmer
Book Image

Haskell Design Patterns

By: Tikhon Jelvis, Ryan Lemmer

Overview of this book

Design patterns and idioms can widen our perspective by showing us where to look, what to look at, and ultimately how to see what we are looking at. At their best, patterns are a shorthand method of communicating better ways to code (writing less, more maintainable, and more efficient code) This book starts with Haskell 98 and through the lens of patterns and idioms investigates the key advances and programming styles that together make "modern Haskell". Your journey begins with the three pillars of Haskell. Then you'll experience the problem with Lazy I/O, together with a solution. You'll also trace the hierarchy formed by Functor, Applicative, Arrow, and Monad. Next you'll explore how Fold and Map are generalized by Foldable and Traversable, which in turn is unified in a broader context by functional Lenses. You'll delve more deeply into the Type system, which will prepare you for an overview of Generic programming. In conclusion you go to the edge of Haskell by investigating the Kind system and how this relates to Dependently-typed programming
Table of Contents (14 chapters)

Abstracting datatypes


In this section, we will describe a series of patterns related to data abstraction. We start with existentially quantified types then progress to phantom types and end with GADTs. We'll see that these patterns fall within a spectrum of generality and power.

Universal quantification

Let's explore existential quantification from the perspective of its opposite, universal quantification.

All parametrically polymorphic functions, from Rank 1 to higher rank functions, are universally quantified. Similarly, parametrically polymorphic data­types are universally quantified. If the Haskell syntax for universally quantified functions and data­types were consistent, then we would have had to use the forall keyword in data­type signatures to indicate polymorphism, for example:

  data Maybe' a =           Nothing' | Just' a
  -- conceptually (but not practically!) the same as 
  data Maybe' a = forall a. Nothing' | Just' a

As another example, consider the universally quantified type...