Haskell Introduction

Haskell is a purely functional, lazily evaluated, statically typed programming language. Named after logician Haskell Curry, it is the gold standard for functional programming and has influenced languages like Rust, Elm, TypeScript, and Scala.

What makes Haskell different?

Most languages are imperative — you write a sequence of instructions that change state. Haskell is functional — you compose pure functions that transform data without side effects.

-- Imperative (Java/Python style): "do this, then this, then that"
result = []
for i in range(10):
    if i % 2 == 0:
        result.append(i * i)

-- Functional (Haskell style): "what I want"
result = map (^2) (filter even [0..9])

Key concepts

Where Haskell is used

• Finance — Standard Chartered, Barclays, Jane Street use Haskell for trading systems
• Tech — Facebook's spam filter, GitHub's semantic versioning library
• Compilers — GHC (Haskell's own compiler) is written in Haskell
• Data engineering — Pandoc (universal document converter) is Haskell

Hello World

main :: IO ()
main = putStrLn "Hello, Nepal!"

Save as Main.hs and compile:

ghc Main.hs -o hello
./hello

Or run directly:

runhaskell Main.hs

Basic Types and Functions

-- Integers
x :: Int
x = 42

-- Floats
pi :: Double
pi = 3.14159

-- Strings (actually [Char])
greeting :: String
greeting = "Hello, Nepal!"

-- Booleans
isActive :: Bool
isActive = True

-- Functions (type signature goes on the line before)
add :: Int -> Int -> Int
add a b = a + b

-- Functions are curried by default!
-- add 3 4 is actually (add 3) 4
add5 :: Int -> Int
add5 = add 5

Guards

grade :: Int -> String
grade score
    | score >= 90 = "A"
    | score >= 80 = "B"
    | score >= 70 = "C"
    | score >= 60 = "D"
    | otherwise   = "F"

What you will learn