Book Image

Haskell Design Patterns

By : Ryan Lemmer
Book Image

Haskell Design Patterns

By: 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)


A Lens provides access to a particular part of a data structure.

Lenses express a high-level pattern for composition and in that sense belong firmly in Chapter 3, Patterns for Composition. However, the concept of Lens is also deeply entwined with Foldable and Traversable, and so we describe it in this chapter instead.

Lenses relate to the getter and setter functions, which also describe access to parts of data structures. To find our way to the Lens abstraction (as per Edward Kmett's Lens library), we'll start by writing a getter and setter to access the root node of a tree.

Deriving Lens

Let's return to our Tree type from earlier:

data Tree a = Node a (Tree a) (Tree a)
            | Leaf a
deriving Show

   = Node 2 (Leaf 3)
            (Node 5 (Leaf 7)
            (Leaf 11))

   = Node [1 ,1 ] (Leaf [2,1 ])
                  (Node [3,2] (Leaf [5,2])
                  (Leaf [7,4]))

   = Node (1 ,1 ) (Leaf (2,1 ))
                  (Node (3,2) (Leaf (5,2))...