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)

Dependently-typed programming


Dependently-typed programming refers to type-level programming where prior data types determine the types of subsequent values.

By doing type-checking computations, the dependently-typed programming style allows for more nuanced type definitions. For instance, instead of defining a type for "list of numbers", we might go further with the dependently-typed "list of numbers of size n" or "list of distinct strings".

For example, it is easy to implement sprintf in an untyped way, but this function is notoriously difficult to implement in a type-safe language because the return type depends on the value of the format string:

  sprintf "%s"  :: String -> String
  sprintf "%d"  :: Int -> String

To do this in Haskell, we require dependently-typed programming. Let's explore a simplified example (refer to Fun with Types, Kiselyov et al, for more information). First, we create an embedded language for format strings:

  data L      -- literals e.g. "hello"
  data V val...