Haskell Cheat Sheet

Haskell syntax, pure functions, algebraic data types, and lazy evaluation for statically typed functional programming.

2 PagesAdvancedApr 2, 2026

Basic Syntax

Bindings, control flow, and IO.

haskell

main :: IO ()main = do  let age = 30      name = "Ada"      pi' = 3.14159  if age >= 18    then putStrLn (name ++ " is an adult")    else putStrLn (name ++ " is a minor")  mapM_ (\i -> putStrLn ("Count: " ++ show i)) [0..4]

Functions & Pattern Matching

Defining functions by matching on structure.

haskell

-- function with type signaturesquare :: Int -> Intsquare x = x * x-- pattern matching on listsdescribe :: [Int] -> Stringdescribe []     = "empty"describe [x]    = "one element: " ++ show xdescribe (x:xs) = "head " ++ show x ++ ", rest has " ++ show (length xs)factorial :: Int -> Intfactorial 0 = 1factorial n = n * factorial (n - 1)

Types & Type Classes

Algebraic data types and ad-hoc polymorphism.

haskell

data Shape = Circle Double | Rectangle Double Doublearea :: Shape -> Doublearea (Circle r) = pi * r * rarea (Rectangle w h) = w * hclass Describable a where  describe :: a -> Stringinstance Describable Shape where  describe (Circle r) = "Circle with radius " ++ show r  describe (Rectangle w h) = "Rectangle " ++ show w ++ "x" ++ show h

Core Syntax & Types

Common Haskell notation.

::- reads as 'has type', used for type signatures
->- function type/arrow, e.g. Int -> Int
data- defines a new algebraic data type
Maybe a- Just value or Nothing, avoids null
Either a b- Left error or Right success value
($)- low-precedence function application operator, avoids parens
(.)- function composition operator
IO a- type wrapping actions that perform side effects

Pro Tip

Haskell is lazily evaluated by default — use `seq` or strict data types (via `!` bang patterns) when accumulating large sums to avoid building up unevaluated thunks that blow the stack.

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