Haskell (.hs)

Overview

Haskell is a purely functional programming language known for its mathematical elegance, type safety, and lazy evaluation. Named after mathematician Haskell Curry, it emphasizes immutability, higher-order functions, and advanced type systems. Haskell is widely used in academia, research, and industries requiring high reliability and correctness, such as finance and aerospace.

Technical Details

• File Extension: .hs
• MIME Type: text/x-haskell
• Category: Programming Language
• First Appeared: 1990
• Paradigm: Functional, lazy evaluation
• Platform: Cross-platform (GHC compiler)

Key Features

Pure Functional Programming

• No side effects in pure functions
• Immutable data structures
• Referential transparency
• Mathematical function composition

Advanced Type System

• Static type checking
• Type inference
• Algebraic data types
• Type classes and instances

Lazy Evaluation

• Expressions evaluated only when needed
• Infinite data structures support
• Memory efficient for large computations
• Call-by-need evaluation strategy

Syntax Example

-- Basic function definition
factorial :: Integer -> Integer
factorial 0 = 1
factorial n = n * factorial (n - 1)

-- List comprehension
squares :: [Integer]
squares = [x^2 | x <- [1..]]

-- Higher-order function
map' :: (a -> b) -> [a] -> [b]
map' _ [] = []
map' f (x:xs) = f x : map' f xs

-- Algebraic data type
data Tree a = Empty | Node a (Tree a) (Tree a)
    deriving (Show, Eq)

-- Pattern matching
treeSize :: Tree a -> Int
treeSize Empty = 0
treeSize (Node _ left right) = 1 + treeSize left + treeSize right

Language Constructs

Type Classes

class Eq a where
    (==) :: a -> a -> Bool
    (/=) :: a -> a -> Bool

instance Eq Bool where
    True == True = True
    False == False = True
    _ == _ = False

Monads

-- Maybe monad for handling null values
safeDivide :: Double -> Double -> Maybe Double
safeDivide _ 0 = Nothing
safeDivide x y = Just (x / y)

-- Do notation
calculateResult :: Double -> Double -> Double -> Maybe Double
calculateResult x y z = do
    result1 <- safeDivide x y
    result2 <- safeDivide result1 z
    return result2

List Operations

-- List manipulation
numbers = [1, 2, 3, 4, 5]
doubled = map (*2) numbers
evens = filter even numbers
sum' = foldr (+) 0 numbers

Development Tools

Compilers and Interpreters

• GHC: Glasgow Haskell Compiler
• GHCi: Interactive interpreter
• Hugs: Haskell interpreter
• UHC: Utrecht Haskell Compiler

Build Tools

• Cabal: Package system and build tool
• Stack: Build tool with curated package sets
• Nix: Functional package manager

IDEs and Editors

• VSCode with Haskell extensions
• IntelliJ IDEA with IntelliJ-Haskell
• Emacs with haskell-mode
• Vim with vim-haskell

Libraries and Frameworks

Core Libraries

• base: Fundamental definitions
• containers: Data structures
• text: Efficient Unicode text
• bytestring: Fast byte arrays

Web Development

• Yesod: Type-safe web framework
• Scotty: Lightweight web framework
• Servant: Type-level web DSL
• Warp: High-performance web server

Concurrency

• STM: Software Transactional Memory
• async: Asynchronous programming
• parallel: Parallel programming

Parsing

• Parsec: Parser combinator library
• Attoparsec: Fast parser combinator
• Megaparsec: Industrial-strength parser

Common Use Cases

Academic Research

• Programming language research
• Type theory implementation
• Mathematical modeling
• Algorithm prototyping

Financial Systems

• Quantitative analysis
• Risk modeling
• Trading systems
• Cryptocurrency platforms

Compilers and Interpreters

• Domain-specific languages
• Code analysis tools
• Theorem provers
• Language implementations

Data Analysis

• Statistical computing
• Machine learning research
• Data transformation pipelines
• Scientific computing

File Structure

Basic Module Structure

module MyModule 
    ( exportedFunction1
    , exportedFunction2
    , DataType(..)
    ) where

import Data.List (sort)
import qualified Data.Map as M

-- Type definitions
data DataType = Constructor1 | Constructor2

-- Function definitions
exportedFunction1 :: Int -> Int
exportedFunction1 x = x + 1

Common Pragmas

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE DeriveFunctor #-}

Package Management

Cabal Configuration

name: my-project
version: 0.1.0.0
synopsis: A sample Haskell project

library
    exposed-modules: MyModule
    build-depends: base >=4.7 && <5
    hs-source-dirs: src
    default-language: Haskell2010

Stack Configuration

resolver: lts-18.0
packages:
- .
extra-deps: []

Notable Applications

• Pandoc: Universal document converter
• Xmonad: Tiling window manager
• Cardano: Blockchain platform
• Facebook's Haxl: Data fetching library
• GitHub's Semantic: Code analysis tool

Learning Resources

• "Learn You a Haskell for Great Good!" by Miran Lipovača
• "Real World Haskell" by O'Sullivan, Goerzen, and Stewart
• "Haskell Programming from First Principles" by Allen & Moronuki
• School of Haskell online tutorials
• Official Haskell documentation

Haskell continues to influence programming language design and serves as a powerful tool for developers who prioritize correctness, expressiveness, and mathematical elegance in their code.