Haskell Cheat Sheet

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

-- let expression
result = let x = 10
             y = 20
         in x + y

-- where clause
bmi weight height = category
  where
    category
      | index < 18.5 = "underweight"
      | index < 25.0 = "normal"
      | otherwise = "overweight"
    index = weight / height ^ 2

-- Single line comment
{- Multi-line
   comment -}

add :: Int -> Int -> Int
add x y = x + y

greeting :: String -> String
greeting name = "Hello, " ++ name

x :: Int        -- Fixed-precision integer
y :: Integer    -- Arbitrary-precision
z :: Double     -- Floating point
b :: Bool       -- True / False
c :: Char       -- Single character
s :: String     -- [Char]

safeDivide :: Int -> Int -> Maybe Int
safeDivide _ 0 = Nothing
safeDivide a b = Just (a `div` b)

validate :: String -> Either String Int
validate s = case reads s of
  [(n, "")] -> Right n
  _         -> Left "Invalid number"

data Color = Red | Green | Blue
data Shape = Circle Double | Rectangle Double Double

area :: Shape -> Double
area (Circle r) = pi * r * r
area (Rectangle w h) = w * h

data Person = Person
  { firstName :: String
  , lastName  :: String
  , age       :: Int
  } deriving (Show, Eq)

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

head' :: [a] -> a
head' (x:_) = x
head' []    = error "empty list"

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

-- (.) composes functions
shout :: String -> String
shout = map toUpper . filter isAlpha

-- ($) avoids parentheses
result = sum $ map (*2) $ filter even [1..10]

double = map (\x -> x * 2) [1..5]
filtered = filter (>3) [1,2,3,4,5]
result = foldr (+) 0 [1..100]  -- sum

head [1,2,3]       -- 1
tail [1,2,3]       -- [2,3]
length [1,2,3]     -- 3
[1,2] ++ [3,4]     -- [1,2,3,4]
1 : [2,3]          -- [1,2,3]
reverse [1,2,3]    -- [3,2,1]

[x * 2 | x <- [1..10], even x]
-- [4,8,12,16,20]

pairs = [(x,y) | x <- [1..3], y <- [1..3], x /= y]

map (+1) [1,2,3]         -- [2,3,4]
filter even [1..10]      -- [2,4,6,8,10]
foldl (+) 0 [1..5]       -- 15
zip [1,2] ["a","b"]      -- [(1,"a"),(2,"b")]
take 3 [1..]             -- [1,2,3]

main :: IO ()
main = do
  putStrLn "What's your name?"
  name <- getLine
  putStrLn ("Hello, " ++ name)

lookup' :: String -> [(String, Int)] -> Maybe Int
lookup' key pairs = do
  value <- lookup key pairs
  guard (value > 0)
  return (value * 2)

-- Without do notation
main = getLine >>= \name -> putStrLn ("Hello, " ++ name)

-- Chain operations
result = Just 5 >>= \x -> Just (x + 1) >>= \y -> Just (y * 2)