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)

Imperative I/O


Even though Haskell I/O code is purely functional, this does not prevent us from writing imperative style I/O code. Let's start by printing all the lines of the file:

import System.IO
import qualified Data.ByteString as B
import qualified Data.ByteString.Char8 as B8
import Data.Char (chr)

main = do
  h <- openFile "jabberwocky.txt" ReadMode
  loop h
  hClose h
  where
    loop h' = do   
     isEof <- hIsEOF h'
     if isEof
      then putStrLn "DONE..."
      else do
        line  <- hGetLine h'
        print $ words line
        loop h'

Instead of the hGetLine function, let's use a Data.ByteString.hGet function to read from the file in chunks of 8 bytes:

  chunk  <- B.hGet h' 8
  
  print . words $ show chunk
  –- vs
  –- line  <- hGetLine h'
  –- print $ words line

The splitting of a chunk into words is not meaningful anymore. We need to accumulate the chunks until we reach the end of a line and then capture the accumulated line and possible remainder:

data...