Haskell



Haskell :: Functional Programming with Types

Haskell is a functional programming language. If you have programmed before and would like to see a little bit of how Haskell works and is different from other programming languages, you can get an overview.

Haskell is unique for two reasons. The first is that it is purely functional. If you have a function and you call it twice in two different places with the same arguments then it will return exactly the same value both times. Secondly, Haskell provides a very modern type system which incorporates features like typeclasses and generalized algebraic data types. (We don't expect you to know what these terms mean for now, but we do hope they'll be rolling smoothly off your tongue by the time you are done with this book.)

So, why do Haskellers like the language then? Is it just because we like using things that are unique and modern? No, Haskell programmers enjoy their work because dealing with just pure functions makes it so much easier to understand our programs and prove them correct. Moreover, having an advanced type system helps us catch our mistakes, both the silly and profound ones.

In this book we aim to introduce you both to the Haskell language, from the very basics to its most advanced features, and to computer programming in general. Seasoned programmers, we urge you to be especially patient with this process. In all likelihood, the languages you are most familiar with differ greatly from Haskell, and habits from those languages might make it more difficult to understand how things work: simple, but different. So treat this as an adventure, a whole different world, seeing programming through the warped and mathematical mindset of a functional programmer.

Overview

The book is divided into three sections, namely, the Beginner's Track, the Advanced Track, and Practical Haskell. The last is designed to cover the more day-to-day issues, and to be readable with simply the knowledge of the Beginner's Track.

Beginner's Track

This section will introduce you to the very basics of the language and some of the more frequently used libraries. This will enable you to build simple programs and prepare you for the advanced track.

Most chapters contain exercises where you can train the newly learned features of Haskell. At the end of each chapter is a link to solutions so you can check if your solution is correct.

Haskell Basics Getting set up Variables and functions Truth values Type basics Lists and tuples Type basics II Next steps Building vocabulary Simple input and output

Elementary Haskell Recursion More about lists List processing Type declarations Pattern matching Control structures More on functions Higher order functions Using GHCi effectively

Intermediate Haskell Modules Indentation More on datatypes Other data structures Classes and types The Functor class

Monads Understanding monads Maybe - List do Notation IO - State Additive monads (MonadPlus) Monadic parser combinators Monad transformers Practical monads

Advanced Track

This section will introduce wider functional programming concepts such as different data structures and type theory. It will also cover more practical topics like concurrency.

Advanced Haskell Arrows Understanding arrows Continuation passing style (CPS) Zippers Value recursion (MonadFix) Applicative Functors Monoids Mutable objects Concurrency

Fun with Types Polymorphism basics Existentially quantified types Advanced type classes Phantom types Generalised algebraic data-types (GADT) Datatype algebra Type constructors & Kinds

Wider Theory Denotational semantics Equational reasoning Program derivation Category theory The Curry-Howard isomorphism fix and recursion

Haskell Performance Introduction Step by Step Examples Graph reduction Laziness Time and space profiling Strictness Algorithm complexity Data structures Parallelism

Haskell in Practice

This section covers the more day-to-day issues of working with Haskell, issues such as making use of the standard library, building graphical interfaces or working with databases. You should be able to jump directly to this section from the basic track.

Libraries Reference The Hierarchical Libraries Lists - Arrays - Maybe - Maps IO - Random Numbers

General Practices Building a standalone application Debugging Testing Packaging your software (Cabal) Using the Foreign Function Interface (FFI) Generic Programming : Scrap your boilerplate

Specialised Tasks Graphical user interfaces (GUI) Databases Web programming Working with XML Using Regular Expressions Parsing Mathematical Expressions Writing a Basic Type Checker

Appendices

Syntactic sugar

Answers to exercises

Authors and Acknowledgements

About the book

Notes for contributors

Style conventions

To do

Templates for the Haskell wikibook

Experimental Modules

List of topics

Alternate tutorials

• Haskell Meta-tutorial - the tutorial to find other tutorials
• Learn you a Haskell for great good - Tutorial aimed at people who have experience in imperative programming languages but haven't programmed in a functional language before.
• Write Yourself a Scheme in 48 Hours - (imported) An alternate approach to teaching Haskell (and perhaps Scheme), aimed at a more advanced audience (though not necessarily one that knows how to program!)
• Yet Another Haskell Tutorial - (imported) is aimed at beginners and takes a practical approach to things. It is now part of this wikibook and will eventually be mixed in to the main text.

Additional resources

• haskell.org - Resources for learning Haskell.
• The resource page for Haskell: the craft of functional programming