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Haskell Design Patterns

Haskell Design Patterns

By : Lemmer
4.1 (9)
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Haskell Design Patterns

Haskell Design Patterns

4.1 (9)
By: 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 (9 chapters)
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Preface
Preface
What this book covers
What you need for this book
Who this book is for
Conventions
Reader feedback
Customer support
Free Chapter
1
1. Functional Patterns – the Building Blocks
1. Functional Patterns – the Building Blocks
Higher-order functions
Currying functions
Recursion
Types, pattern matching, and polymorphism
Lazy evaluation
Monads
Summary
2
2. Patterns for I/O
2. Patterns for I/O
I/O as a first class citizen
Imperative I/O
Lazy I/O
The problems with lazy I/O
Resource management with bracket
Iteratee I/O
Summary
3
3. Patterns of Composition
3. Patterns of Composition
Functor
Applicative functor
Monad
Monad transformers
Arrows
Summary
4
4. Patterns of Folding and Traversing
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4. Patterns of Folding and Traversing
Folding over lists
Summary
5
5. Patterns of Type Abstraction
5. Patterns of Type Abstraction
Abstracting function types: RankNTypes
Abstracting datatypes
Abstracting type-classes
Summary
6
6. Patterns of Generic Programming
6. Patterns of Generic Programming
Patterns of generic programming
The sum of products style
Origami programming
Scrap your boilerplate
Summary
7
7. Patterns of Kind Abstraction
7. Patterns of Kind Abstraction
Higher-order kinds
Higher-kinded polymorphism
Associated type synonyms
Type (synonym) families
Kind polymorphism
Type promotion
Type-level programming
Dependently-typed programming
Summary
Epilogue
8
Index
Index

Folding over lists

Recall the following from Chapter 1, Functional Patterns – the Building Blocks:

  sumLazy [] = 0
  sumLazy (x:xs) = x + sumLazy xs

Lazy recursion is captured by foldr:

  sumLazy' = foldr (+) 0

On the other hand, foldl captures tail recursion, for example consider a strict recursive sum:

  sumStrict acc [] = acc
  sumStrict acc (x:xs) = sumStrict (acc + x) xs

This is captured by the iterative process foldl:

sumStrict' = foldl (+)

The foldl function is tail recursive and strict in the accumulator. The foldr function is non-tail recursive and lazy. In fact, foldl is a special case of foldr (https://wiki.haskell.org/Foldl_as_foldr).

Folding with monadic functions

Let's write a tail-recursive sum that does some IO at each accumulation step:

  doSumStrict :: (Show a, Num a) => a -> [a] -> IO a

  doSumStrict acc [] = return acc
  doSumStrict acc (x:xs) = do 
    putStrLn $ " + " ++ (show x) ++ " = " ++ (show acc') 
    doSumStrict acc' xs
    where acc' = acc + x

  main = doSumStrict...

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