Haskell语言速查

函数与类型

-- 类型签名
add :: Int -> Int -> Int
add x y = x + y

-- 函数组合
transform :: [String] -> [String]
transform = filter (not . null) . map (map toUpper)

-- Lambda 表达式
double = \x -> x * 2
evens = filter (\x -> x `mod` 2 == 0) [1..10]

模式匹配

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

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

-- 守卫
classify :: Int -> String
classify n
  | n < 0    = "negative"
  | n == 0   = "zero"
  | n < 10   = "small"
  | otherwise = "large"

数据类型

-- 代数数据类型
data Shape = Circle Double
           | Rectangle Double Double
           | Triangle Double Double Double

area :: Shape -> Double
area (Circle r)        = pi * r * r
area (Rectangle w h)   = w * h
area (Triangle a b c)  = ...

-- 记录语法
data Person = Person { name :: String, age :: Int }
alice = Person { name = "Alice", age = 28 }
olderAlice = alice { age = 29 }  -- 更新

Maybe Monad

-- Maybe 用于安全计算
safeDiv :: Int -> Int -> Maybe Int
safeDiv _ 0 = Nothing
safeDiv x y = Just (x `div` y)

-- do 表示法
compute :: Int -> Int -> Maybe Int
compute x y = do
  a <- safeDiv x y
  b <- safeDiv a 2
  return (a + b)

-- fmap 和 >>=
fmap (*2) (Just 5)        -- Just 10
Just 5 >>= safeDiv 10     -- Just 2
Nothing >>= safeDiv 10    -- Nothing