Haskell Basic Notes

Platform

apt-get install haskell-Platform
apt-get install ghc-mod
atom plugins : language-haskell autocomplete-haskell ide-haskell haskell-ghc-mod

ghci

• :l: load file.
• :r: reload file.
• :cd.
• :edit: $EDITOR.
• :m: module.
• :q: quit.
• :?.
• :k: kind.
• :t: type function.
• :info: data/TypeClass.

ghc

runghc *.hs/*.lhs

Unique Mark

• :+: 复数符, 2 :+ 3 -> 2+3i.
• _: 泛匹配符, 表示不关心此部分具体内容.
• <-: 属于符号, 用于 ListRange 中.
• =>: 类型约束分隔符

Expression

if 语句也是表达式

doubleSmallNumber' x = (if x > 100 then x else x*2) + 1

Type

基本类型

ghci> :t 'a'
'a' :: Char

ghci> :t True
True :: Bool

ghci> :t "HELLO!"
"HELLO!" :: [Char]

ghci> :t (True, 'a')
(True, 'a') :: (Bool, Char)

Int

有限整数

Integer

无限整数(效率低)

Float

单精度浮点数

Double

双精度浮点数

Bool

True/False

Char

String

Ordering

LT,GT,EQ

Word

Data.Word - unsigned int

Rational

有理数类型,用于高精度数学运算

List

Operator

• ++.
• :.
• !!.
• >/</==.

Function

• head List 首元素.
• last List 尾元素.
• init List 除去尾元素的部分.
• tail List 除去首元素的部分.
• length List 长度.

Tips:

fromIntegral (length [1,2,3,4]) + 3.2

• null List BestPractice: 检查一个 List 是否为空

ghci> null [1,2,3]
False
ghci> null []
True

• reverse List 反转
• take num List 返回 List 前 num 个元素组成的 List

ghci> take 3 [5,4,3,2,1]
[5,4,3]
ghci> take 5 [1,2]
[1,2]
ghci> take 0 [6,6,6]
[]

• drop num List 删除 List 前 num 个元素

ghci> drop 3 [8,4,2,1,5,6]
[1,5,6]
ghci> drop 0 [1,2,3,4]
[1,2,3,4]
ghci> drop 100 [1,2,3,4]
[]

• maximum List 返回最大元素

• minimum List 返回最小元素

• sum List 返回 List 元素和

• product List 返回 List 元素积

• elem elem List 判断元素存在性

ghci> 4 `elem` [3,4,5,6]
True
ghci> 10 `elem` [3,4,5,6]
False

• cycle List 返回循环无限数组(Haskell 惰性特性)
• repeat Elem 返回循环无限数组(Haskell 惰性特性)
• replicate num Elem 返回循环无限数组

take 10 (cycle [1,2,3]) -> [1,2,3,1,2,3,1,2,3,1]
take 10 (repeat 5)      -> [5,5,5,5,5,5,5,5,5,5]
replicate 3 10          -> [10,10,10]

• takeWhile :: (a -> Bool) -> [a] -> [a] 遇到不符合限制条件的元素便停止遍历 List

ghci> sum (takeWhile (<10000) (filter odd (map (^2) [1..])))
166650

Range

三要素: , 与 ..

• 上限
• 下限
• 步长(仅可标明一次)

上下限: [1..20]
步长为2: [2,4..20]
步长为13无限List: [13,26..]
take 24 [13,26..]

List Comprehension

由类似集合定义的离散数学定义,来定义复杂的 List:

[expression | filter]

[expression | x <- Range, Predicate(断言/限制条件)]

• Range: ,分隔多个 Range(一般为 List)
• Predicate: ,分隔多个断言;每个断言均为 Boolean 表达式

ghci> [x*2 | x <- [1..10], x*2 >= 12]
[12,14,16,18,20]

ghci> [ x | x <- [50..100], x `mod` 7 == 3]
[52,59,66,73,80,87,94]


boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]

ghci> boomBangs [7..13]
["BOOM!","BOOM!","BANG!","BANG!"]

• 多个 Range

ghci> [ x*y | x <- [2,5,10], y <- [8,10,11]]
[16,20,22,40,50,55,80,100,110]

• 嵌套 Comprehension

ghci> let xxs = [[1,3,5,2,3,1,2,4,5],[1,2,3,4,5,6,7,8,9],[1,2,4,2,1,6,3,1,3,2,3,6]]

ghci> [ [ x | x <- xs, even x ] | xs <- xxs]
[[2,2,4],[2,4,6,8],[2,4,2,6,2,6]]

Tuple

内部差异性

• 同一 Tuple 里可存放不同 Type 的项

外部差异性

• 数目不同或某项不同的 Tuple 属于不同 Type
  • 不可置于同一 List 中
  • 不同长度的 Tuple 不可比较(比较符只可用于相同 Type)

Tuple Function

二元组

• fst/snd tuple: 返回首项/尾项.
• zip List1 List2: 对应项配对, 组成二元组 List.

ghci> zip [5,3,2,6,2,7,2,5,4,6,6] ["im","a","turtle"]
[(5,"im"),(3,"a"),(2,"turtle")]

ghci> zip [1..] ["apple", "orange", "cherry", "mango"]
[(1,"apple"),(2,"orange"),(3,"cherry"),(4,"mango")]

三元组

first :: (a, b, c) -> a
first (x, _, _) = x

second :: (a, b, c) -> b
second (_, y, _) = y

third :: (a, b, c) -> c
third (_, _, z) = z

泛型

运用 Type 变量 (只可为单字符), 实现泛型参数与多态函数.

借助 TypeClass 可轻松实现多态函数:

ghci> :t head
head :: [a] -> a

-- a 和 b 可为同类型
-- 第一个参数与返回值必须同类型
ghci> :t fst
fst :: (a, b) -> a

-- 所有参数必须同类型,且必须为Num成员
ghci> :t (*)
(*) :: (Num a) => a -> a -> a

函数类型

• 单个参数

removeNonUppercase :: [Char] -> [Char]
removeNonUppercase st = [ c | c <- st, c `elem` ['A'..'Z']]

• 多个参数

addThree :: Int -> Int -> Int -> Int
addThree x y z = x + y + z

Pattern Matching

当函数拥有多个函数体(模式)时,会从上至下进行匹配各模式,一旦匹配则只应用这一函数体.

As Pattern Matching

all@(pattern): all 为指向 pattern 整体的引用.

all@(x:y:xs) -- 其中all与(x:y:xs)等价

capital :: String -> String
capital "" = "Empty string, whoops!"
capital all@(x:xs) = "The first letter of " ++ all ++ " is " ++ [x]

List Pattern Matching

• x:xs
• x:y:z:xs.

head' :: [a] -> a
head' [] = error "Can't call head on an empty list, dummy!"
head' (x:_) = x

length' :: (Num b) => [a] -> b
length' [] = 0
length' (_:xs) = 1 + length' xs

sum' :: (Num a) => [a] -> a
sum' [] = 0
sum' (x:xs) = x + sum' xs

Tuple Pattern Matching

• (x, y)
• (x, y, z)

Guard Pattern Matching and Where Binding

子模式匹配: 运用布尔表达式实现判断, 应用对应函数体:

• 关键符号: | 与 where.
• | 分隔函数体.
• where:
  • 可见性: 定义只对本模式可见的 (私有) 名字与 (私有) 函数.
  • where 定义在最外层, 使得各模式共享 (私有) 名字与 (私有) 函数.
  • 名字定义时可使用模式匹配 where (head:_) = firstName.

bmiTell :: (RealFloat a) => a -> a -> String
bmiTell weight height
    | bmi <= skinny = "You're underweight, you emo, you!"
    | bmi <= normal = "You're supposedly normal. Pet, I bet you're ugly!"
    | bmi <= fat    = "You're fat! Lose some weight, fatty!"
    | otherwise     = "You're a whale, congratulations!"
    where bmi = weight / height ^ 2
          skinny = 18.5
          normal = 25.0
          fat = 30.0

Let Binding

类似 where, 绑定对象为表达式/函数:

let bindings
in  expressions

let sideArea = 2 * pi * r * h
    topArea = pi * r ^2
in  sideArea + 2 * topArea

• 可见性: in 作用域, 只对本 guard 可见.
• 可使用模式匹配.
• 可用于 List Range 中.

Case Pattern Matching

• 模式匹配是 case 表达式的特殊情况(语法糖:简化写法).
• 在函数中, 模式匹配只能用于参数定义中, case 表达式可用于其他地方 (let/where 绑定, 普通表达式, guard 语句).

case expression of pattern -> result
                   pattern -> result
                   pattern -> result
                   ...

describeList :: [a] -> String
describeList xs = "The list is " ++ case xs of [] -> "empty."
                                               [x] -> "a singleton list."
                                               xs -> "a longer list."

Pattern Matching Best Practice

• 代替 if-else/switch 语句
• 递归算法(将递归基础作为首模式,递归函数体作为尾模式)
• List Range 中亦可使用模式匹配

addVectors :: (Num a) => (a, a) -> (a, a) -> (a, a)
addVectors (x1, y1) (x2, y2) = (x1 + x2, y1 + y2)

Type Class

ghci> :t (==)
(==) :: (Eq a) => a -> a -> Bool

ghci> :t fromIntegral
fromIntegral :: (Integral a, Num b) => a -> b

=> 符号

=> 左部: 类约束(Class Constraint) => 右部: 函数类型(参数/返回值类型),其中参数类型同属 Class

Basic Type Class

ghci> :info typeClassName

Eq

• 功能: 成员类型可判断相等性
• 成员: 大部分基本类型(不包含函数类型)
• 方法: == 与 /= 函数

class Eq a where
    (==) :: a -> a -> Bool
    (/=) :: a -> a -> Bool
    x == y = not (x /= y)
    x /= y = not (x == y)

Ord

Ord 成员必为 Eq 成员: class (Eq a) => Ord a where

• 功能: 成员类型可排序
• 成员: 大部分基本类型(不包含函数类型)
• 方法:
  • < > <= >= 函数
  • compare 函数 (Ord a) => a -> a -> Ordering

Show

• 功能: 成员类型可用字符串表示
• 成员: 大部分基本类型(不包含函数类型)
• 方法: show 函数 (Show a) => a -> String

Tips: 结合 Read, 可用于字符串与数值之间的转化

Read

• 功能: 可以将字串转为 Read 某成员类型
• 成员: 大部分基本类型(不包含函数类型)
• 方法: read 函数 (Read a) => String -> a

Tips: 结合 Show, 可用于字符串与数值之间的转化

Enum

• 功能: 连续性(可枚举), 其成员类型可用于Range中
• 成员: () Bool Char Ordering Int Integer Float Double

[Thursday .. Sunday]

ghci> succ Monday
Tuesday
ghci> pred Saturday
Friday

Bounded

• 功能: 成员类型具有上下限
• 方法: minBound/maxBound 函数 (Bounded a) => a 无参多态常量/定义

ghci> minBound :: Day
Monday
ghci> maxBound :: Day
Sunday

Num

• 功能: 成员类型具有数字特征.
• 成员: 实数 整数 (Int/Integer/Float/Double).
• 方法: +/-/*/abs 函数.
• 实例: 所有数字都是多态常量/定义(可视为函数).

ghci> :t 20
20 :: (Num t) => t

Integral

• 功能: 成员类型具有数字特征
• 成员: 整型 - Int Integer

Floating

• 功能: 成员类型具有数字特征
• 成员: 浮点型 - Float Double

TypeClass	Method Feature
Functor	f a + (a -> b) -> f b
Applicative	f a + f (a -> b) -> f b
Monad	m a + (a -> m b) -> m b

Functor

• 成员: Maybe a, [], Either a, IO
  • 成员 kind 必须为 * -> *
  • f 一元类型构造符(type constructor)
• 必须遵守准则:
  • fmap id = id
  • fmap (f . g) F = fmap f (fmap g F)

ghci> :info Functor
class Functor (f :: * -> *) where
    fmap :: (a -> b) -> f a -> f b
    ($) :: a -> f b -> f a

instance Functor [] where
    fmap = map

instance Functor Maybe where
    fmap f (Just x) = Just (f x)
    fmap f Nothing = Nothing

instance Functor (Either a) where
    fmap f (Right x) = Right (f x)
    fmap f (Left x) = Left x

instance Functor IO where
    fmap f action = do
        result <- action
        return (f result)

Control Applicative

• 成员: f :: * -> * 一元类型构造符 (Type Constructor).
• <*>: 参数为 2 个 functor 实例, 其中一个包含一个函数.

(<$>) :: (Functor f) => (a -> b) -> f a -> f b
f <$> x = fmap f x

• 作用: 可以用单一一个函数操作多个 functor

class (Functor f) => Applicative f where
    pure :: a -> f a
    (<*>) :: f (a -> b) -> f a -> f b

Maybe

instance Applicative Maybe where
    pure = Just
    Nothing <*> _ = Nothing
    (Just f) <*> something = fmap f something

Collection []

instance Applicative [] where
    pure x = [x]
    fs <*> xs = [f x | f <- fs, x <- xs]

IO

instance Applicative IO where
    pure = return
    a <*> b = do
        f <- a
        x <- b
        return (f x)

ZipList

instance Applicative ZipList where
        pure x = ZipList (repeat x)
        ZipList fs <*> ZipList xs = ZipList (zipWith (\f x -> f x) fs xs)

Multi Functor

ghci> pure (+) <*> Just 3 <*> Just 5
Just 8
ghci> pure (+) <*> Just 3 <*> Nothing
Nothing
ghci> pure (+) <*> Nothing <*> Just 5
Nothing

ghci> (*) <$> [2,5,10] <*> [8,10,11]
[16,20,22,40,50,55,80,100,110]

myAction :: IO String
myAction = (++) <$> getLine <*> getLine

ghci> getZipList $ max <$> ZipList [1,2,3,4,5,3] <*> ZipList [5,3,1,2]
[5,3,3,4]

高度封装函数

liftA2, 对两个 applicative 运用二元函数:

liftA2 :: (Applicative f) => (a -> b -> c) -> f a -> f b -> f c
liftA2 f a b = f <$> a <*> b

ghci> liftA2 (:) (Just 3) (Just [4])
Just [3,4]
ghci> (:) <$> Just 3 <*> Just [4]
Just [3,4]

Control Monad

• 成员: 类型构造符(type constructor)

class Monad m where
    return :: a -> m a

{- bind -}(>>=) :: m a -> (a -> m b) -> m b

    (>>) :: m a -> m b -> m b
    x >> y = x >>= _ -> y

    fail :: String -> m a
    fail msg = error msg

• 特性: 允许返回值之间具有弹性交互

{- 当出现异常后,之后所有的值都变为Nothing -}
ghci> return (0,0) >>= landRight 2 >>= landLeft 2 >>= landRight 2
Just (2,4)
ghci> return (0,0) >>= landLeft 1 >>= landRight 4
  \ >>= landLeft (-1) >>= landRight (-2)
Nothing

Monad Laws:

• return 满足 Left identity: return x >>= f 等于 f x
• return 满足 right identity: m >>= return 等于 m
• Associativity: 结合律 (m >>= f) >>= g 等于 m >>= (\x -> f x >>= g)

ghci> return 3 >>= (\x -> Just (x+100000))
Just 100003
ghci> (\x -> Just (x+100000)) 3
Just 100003

ghci> Just "move on up" >>= (\x -> return x)
Just "move on up"
ghci> [1,2,3,4] >>= (\x -> return x)
[1,2,3,4]
ghci> putStrLn "Wah!" >>= (\x -> return x)
Wah!

{-Tips: 利用结合律合并两个 Monadic Function-}
(<=<) :: (Monad m) => (b -> m c) -> (a -> m b) -> (a -> m c)
f <=< g = (\x -> g x >>= f)
ghci> let f x = [x,-x]
ghci> let g x = [x*3,x*2]
ghci> let h = f <=< g
ghci> h 3
[9,-9,6,-6]

Maybe Monad

具有失败可能性的 context 封装,灵活处理异常(返回值为 Nothing)

实现

applyMaybe :: Maybe a -> (a -> Maybe b) -> Maybe b
applyMaybe Nothing f  = Nothing
applyMaybe (Just x) f = f x

instance Monad Maybe where
    return x = Just x
    Nothing >>= f = Nothing
    Just x >>= f  = f x
    fail _ = Nothing

do 表示法

• 在 do expression 中，每一行都是一个 monadic value
• 检查返回值，使用 <-

foo :: Maybe String
foo = do
    x <- Just 3
    y <- Just "!"
    Just (show x ++ y)

foo :: Maybe String
foo = Just 3   >>= (\x ->
      Just "!" >>= (\y ->
      Just (show x ++ y)))

routine :: Maybe Pole
routine = do
    start <- return (0,0)
    first <- landLeft 2 start
    Nothing
    second <- landRight 2 first
    landLeft 1 second

List Monad

• non-determinism(不确定性)

ghci> (*) <$> [1,2,3] <*> [10,100,1000]
[10,100,1000,20,200,2000,30,300,3000]

• 实现

instance Monad [] where
    return x = [x]
    xs >>= f = concat (map f xs)
    fail _ = []

• 返回值交互: 下例中 n 与 return (n, ch) 进行交互
  • list comprehension 与 do 表示法 均是 >>= 的语法糖
• list comprehension: <- 与 条件表达式
• do 表示法: <- 与 guard 函数

ghci> [1,2] >>= \n -> ['a','b'] >>= \ch -> return (n,ch)
[(1,'a'),(1,'b'),(2,'a'),(2,'b')]

{- do 表示法 -}
listOfTuples :: [(Int,Char)]
listOfTuples = do
    n <- [1,2]
    ch <- ['a','b']
    return (n,ch)

sevensOnly :: [Int]
sevensOnly = do
    x <- [1..50]
    guard ('7' `elem` show x)
    return x

{- list comprehension -}
ghci> [ (n,ch) | n <- [1,2], ch <- ['a','b'] ]
[(1,'a'),(1,'b'),(2,'a'),(2,'b')]

MonadPlus

使 Monad 具有 Monoid 的性质(二元封闭运算)

instance MonadPlus [] where
    mzero = []
    mplus = (++)

Monad Algorithms

马走日

• 计算出可移动位置

moveKnight :: KnightPos -> [KnightPos]
moveKnight (c,r) = do
    (c',r') <- [(c+2,r-1),(c+2,r+1),(c-2,r-1),(c-2,r+1)
                ,(c+1,r-2),(c+1,r+2),(c-1,r-2),(c-1,r+2)
                ]
    guard (c' `elem` [1..8] && r' `elem` [1..8])
    return (c',r')

• 利用 >>= 向后传递多个可交互的位置

in3 start = return start >>= moveKnight >>= moveKnight >>= moveKnight

in3 :: KnightPos -> [KnightPos]
in3 start = do
    first <- moveKnight start
    second <- moveKnight first
    moveKnight second

• 最后完成完整函数: 产生所有三步的可能位置，检查其中一个位置是否在里面

canReachIn3 :: KnightPos -> KnightPos -> Bool
canReachIn3 start end = end `elem` in3 start

Foldable

import qualified Data.Foldable as F

foldMap :: (Monoid m, Foldable t) => (a -> m) -> t a -> m

instance F.Foldable Tree where
    foldMap f Empty = mempty
    foldMap f (Node x l r) = F.foldMap f l `mappend`
                                f x           `mappend`
                                F.foldMap f r

Data Monoid

• 成员: 必须为具体类型 (不可是类型构造符 (Type Constructor)).
• 准则 (Monoid Law):
  • 结合律 a·(b·c) = (a·b)·c.
  • 无需满足 a mappend b == b mappend a.

class Monoid m where
    mempty :: m             -- identity
    mappend :: m -> m -> m
    mconcat :: [m] -> m
    mconcat = foldr mappend mempty

• 实例

instance Monoid [a] where
    mempty = []
    mappend = (++)

newtype Product a =  Product { getProduct :: a }
    deriving (Eq, Ord, Read, Show, Bounded)

instance Num a => Monoid (Product a) where
    mempty = Product 1
    Product x `mappend` Product y = Product (x * y)

ghci> getProduct $ Product 3 `mappend` Product 4 `mappend` Product 2
24

newtype Any = Any { getAny :: Bool }
    deriving (Eq, Ord, Read, Show, Bounded)

instance Monoid Any where
    mempty = Any False
    Any x `mappend` Any y = Any (x || y)

ghci> getAny . mconcat . map Any $ [False, False, False, True]
True

newtype All = All { getAll :: Bool }
    deriving (Eq, Ord, Read, Show, Bounded)

instance Monoid All where
    mempty = All True
    All x `mappend` All y = All (x && y)

ghci> getAll . mconcat . map All $ [True, True, False]
False

instance Monoid Ordering where
    mempty = EQ
    LT `mappend` _ = LT
    EQ `mappend` y = y
    GT `mappend` _ = GT

-- Tips:
-- mappend 在左边不等于 EQ 的情况下都会回传左边的值。相反地则回传右边的值
-- 可代替多个 if/else 语句
import Data.Monoid

lengthCompare :: String -> String -> Ordering
lengthCompare x y = (length x `compare` length y) `mappend`
                    (vowels x `compare` vowels y) `mappend`
                    (x `compare` y)
    where vowels = length . filter (`elem` "aeiou")

instance Monoid a => Monoid (Maybe a) where
    mempty = Nothing
    Nothing `mappend` m = m
    m `mappend` Nothing = m
    Just m1 `mappend` Just m2 = Just (m1 `mappend` m2)

instance Monoid (First a) where
    mempty = First Nothing
    First (Just x) `mappend` _ = First (Just x)
    First Nothing `mappend` x = x

ghci> getFirst $ First (Just 'a') `mappend` First Nothing
Just 'a'

Custom Type Class

• 创建新类: 可以只有声明没有实现

class ClassName where
    defining code

• 创建已有类的实例: 必须实现所有已声明函数
  • 作用等同于 deriving(自由度更大)
  • 可以重写函数,去除默认函数处理,达到特定目的

- 先创建新类型
data TrafficLight = Red | Yellow | Green

instance Eq TrafficLight where
    Red == Red = True
    Green == Green = True
    Yellow == Yellow = True
    _ == _ = False

instance Show TrafficLight where
    show Red = "Red light"
    show Yellow = "Yellow light"
    show Green = "Green light"

• 创建新类和实现实例时,使用 class constraint
  • 可达到类似于继承的效果
  • 可达到限制类型的效果

class (Eq a) => Num a where
    ...

instance (Eq m) => Eq (Maybe m) where
    Just x == Just y = x == y
    Nothing == Nothing = True
    _ == _ = False

data type

data SelfDefinedTypeName =
  ValueConstructorName ValueType .. | .. deriving (TypeClass, ..)

• data 范例

data Point = Point Float Float deriving (Show)
data Shape = Circle Point Float | Rectangle Point Point deriving (Show)

• 导出 data

module Shapes
( Point(..)
, Shape(..)
) where

• 后构造器 > 前构造器

e.g True > False

data Bool = False | True deriving (Ord)

函数特性

data type 也是函数,若省略参数亦会造成 Curry 化.

e.g map fx list

ghci> map (Circle 10 20) [4,5,6,6]
[Circle 10.0 20.0 4.0,
Circle 10.0 20.0 5.0,
Circle 10.0 20.0 6.0,
Circle 10.0 20.0 6.0
]

• Value Constructor:使用ValueConstructorName ValueType ..可构造出一个该类型的定义/名字

ghci > Circle 10 20 30
Circle 10 20 30

记录语法(Record Syntax)

• 定义

data Person = Person { firstName :: String
                     , lastName :: String
                     , age :: Int
                     , height :: Float
                     , phoneNumber :: String
                     , flavor :: String
                     } deriving (Show)

• 使用

ghci> Car {company="Ford", model="Mustang", year=1967}
Car {company = "Ford", model = "Mustang", year = 1967}

类型参数(Type Parameters)

提高代码的复用性

data Car a b c = Car { company :: a
                       , model :: b
                       , year :: c
                        } deriving (Show)

tellCar :: (Show a) => Car String String a -> String
tellCar (Car {company = c, model = m, year = y}) =
  "This " ++ c ++ " " ++ m ++ " was made in " ++ show y

Maybe value constructor

data Maybe a = Nothing | Just a

• Just 可实现转化:

Just :: a -> Maybe a

Deriving(派生)

data Day = Monday | Tuesday | Wednesday | Thursday | Friday | Saturday | Sunday
           deriving (Eq, Ord, Show, Read, Bounded, Enum)

ghci> Wednesday
Wednesday
ghci> show Wednesday
"Wednesday"
ghci> read "Saturday" :: Day
Saturday

ghci> Saturday == Sunday
False
ghci> Saturday == Saturday
True
ghci> Saturday > Friday
True
ghci> Monday `compare` Wednesday
LT

ghci> minBound :: Day
Monday
ghci> maxBound :: Day
Sunday

ghci> succ Monday
Tuesday
ghci> pred Saturday
Friday
ghci> [Thursday .. Sunday]
[Thursday,Friday,Saturday,Sunday]

ghci> [minBound .. maxBound] :: [Day]
[Monday,Tuesday,Wednesday,Thursday,Friday,Saturday,Sunday]

type 定义

为 data 声明别名 - typedef

type String = [Char]
type PhoneNumber = String
type Name = String
type PhoneBook = [(Name,PhoneNumber)]

• type 类型参数: 匹配 data 类型参数

type AssocList k v = [(k,v)]

type IntMap v = Map.Map Int v
type IntMap = Map.Map Int

类型别名,只可以在 Haskell 的类型部分中使用:

• 定义新类型
• 类型声明
• 类型注释(::)
• 禁止: 定义名字/定义 AssocList [(1,2),(4,5),(7,9)]

高级数据结构

栈

type Stack = [Int]

pop :: Stack -> (Int,Stack)
pop (x:xs) = (x,xs)

push :: Int -> Stack -> ((),Stack)
push a xs = ((),a:xs)

链表

data List a = Empty | Cons a (List a) deriving (Show, Read, Eq, Ord)
data List a = Empty
  | Cons { listHead :: a, listTail :: List a} deriving (Show, Read, Eq, Ord)

二叉树

data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show, Read, Eq)

singleton :: a -> Tree a
singleton x = Node x EmptyTree EmptyTree

treeInsert :: (Ord a) => a -> Tree a -> Tree a
treeInsert x EmptyTree = singleton x
treeInsert x (Node a left right)
      | x == a = Node x left right
      | x < a  = Node a (treeInsert x left) right
      | x > a  = Node a left (treeInsert x right)

treeElem :: (Ord a) => a -> Tree a -> Bool
treeElem x EmptyTree = False
treeElem x (Node a left right)
    | x == a = True
    | x < a  = treeElem x left
    | x > a  = treeElem x right

ghci> let nums = [8,6,4,1,7,3,5]
ghci> let numsTree = foldr treeInsert EmptyTree nums

函数

递归函数

• 边界条件
• 递归基础
• 递归函数体

List 函数

• 边界条件: 空 List
• 递归函数体: x:xs 取出首元素进行一般操作,对尾部进行递归操作.

maximum' :: (Ord a) => [a] -> a
maximum' [] = error "maximum of empty list"
maximum' [x] = x
{-
maximum' (x:xs)
    | x > maxTail = x
    | otherwise = maxTail
    where maxTail = maximum' xs
-}
maximum' (x:xs) = max x (maximum' xs)

replicate' :: (Num i, Ord i) => i -> a -> [a]
replicate' n x
    | n <= 0    = []
    | otherwise = x:replicate' (n-1) x

take' :: (Num i, Ord i) => i -> [a] -> [a]
take' n _
    | n <= 0   = []
take' _ []     = []
take' n (x:xs) = x : take' (n-1) xs

reverse' :: [a] -> [a]
reverse' [] = []
reverse' (x:xs) = reverse' xs ++ [x]

repeat' :: a -> [a]
repeat' x = x:repeat' x

zip' :: [a] -> [b] -> [(a,b)]
zip' _ [] = []
zip' [] _ = []
zip' (x:xs) (y:ys) = (x,y):zip' xs ys

elem' :: (Eq a) => a -> [a] -> Bool
elem' a [] = False
elem' a (x:xs)
    | a == x    = True
    | otherwise = a `elem'` xs

Awesome Quick Sort

quicksort :: (Ord a) => [a] -> [a]
quicksort [] = []
quicksort (x:xs) =
  let smallerSorted = quicksort [a | a <- xs, a <= x]
      biggerSorted = quicksort [a | a <- xs, a > x]
  in smallerSorted ++ [x] ++ biggerSorted

高阶函数

Curry 化

当传入不全参数时,会改变函数的类型,返回值从单类型变成函数类型.

• 当传入不全参数时:
• compare 的类型变为 (Ord a) => a -> (a -> Ordering)

compareWithHundred :: (Num a, Ord a) => a -> Ordering
compareWithHundred x = compare 100 x

• 通过给二元中缀函数传递唯一参数:
• 中缀函数类型由 a -> a -> a 转为 a -> a

divideByTen :: (Floating a) => a -> a
divideByTen = (/10)

ghci> :t (/10)
(/10) :: (Fractional a) => a -> a

• 包装函数:
  • 传入一个二元函数作为参数,便可实现 zipWithFunc
  • 若在定义时便传入一个函数参数,便可实现 Curry 化

zipWith' :: (a -> b -> c) -> [a] -> [b] -> [c]
zipWith' _ [] _ = []
zipWith' _ _ [] = []
zipWith' f (x:xs) (y:ys) = f x y : zipWith' f xs ys


ghci> zipWith' (+) [4,2,5,6] [2,6,2,3]
[6,8,7,9]
ghci> zipWith' max [6,3,2,1] [7,3,1,5]
[7,3,2,5]
ghci> zipWith' (++) ["foo "，"bar "，"baz "] ["fighters"，"hoppers"，"aldrin"]
["foo fighters","bar hoppers","baz aldrin"]
ghci> zipWith' (*) (replicate 5 2) [1..]
[2,4,6,8,10]
ghci> zipWith' (zipWith' (*)) [[1,2,3],[3,5,6],[2,3,4]] [[3,2,2],[3,4,5],[5,4,3]]
[[3,4,6],[9,20,30],[10,12,12]]

• 通过 Curry 化,还可省略参数

foo a = bar b a ->
foo = bar b

map 函数

映射函数 - List Comprehension 的函数化

map :: (a -> b) -> [a] -> [b]
map _ [] = []
map f (x:xs) = f x : map f xs

• 如果 map 传入的函数参数的类型为 a -> a -> a,则返回的 List 类型为[a -> a](f x 传参不完全,造成了 Curry 化).

ghci> let listOfFun = map (*) [0..]
ghci> (listOfFun !! 4) 5
20

ghci> map (+3) [1,5,3,1,6]
[4,8,6,4,9]
ghci> map (++ "!") ["BIFF"，"BANG"，"POW"]
["BIFF!","BANG!","POW!"]
ghci> map (replicate 3) [3..6]
[[3,3,3],[4,4,4],[5,5,5],[6,6,6]]
ghci> map (map (^2)) [[1,2],[3,4,5,6],[7,8]]
[[1,4],[9,16,25,36],[49,64]]
ghci> map fst [(1,2),(3,5),(6,3),(2,6),(2,5)]
[1,3,6,2,2]

filter 函数

过滤函数 - Comprehension 的函数化

filter :: (a -> Bool) -> [a] -> [a]
filter _ [] = []
filter p (x:xs)
    | p x       = x : filter p xs
    | otherwise = filter p xs

ghci> filter (>3) [1,5,3,2,1,6,4,3,2,1]
[5,6,4]
ghci> filter (==3) [1,2,3,4,5]
[3]
ghci> filter even [1..10]
[2,4,6,8,10]
ghci> let notNull x = not (null x) in filter notNull [[1,2,3],[],[3,4,5],[2,2],[],[],[]]
[[1,2,3],[3,4,5],[2,2]]
ghci> filter (`elem` ['a'..'z']) "u LaUgH aT mE BeCaUsE I aM diFfeRent"
"uagameasadifeent"
ghci> filter (`elem` ['A'..'Z']) "i lauGh At You BecAuse u r aLL the Same"
"GAYBALLS"

fold 函数与 scan 函数

如图中所示,左折叠时将 acc 视为第 0 个元素;右折叠时将 acc 视为最后一个元素.

• 三要素:
  • 二元函数 \acc x -> function 或 \x acc -> function
  • 初始累加值
  • 待折叠 List
• 工作原理:
  • 不断从 List 中取出元素,进行二元函数调用,直至 List 被取空
  • 调用参数分别为 新取出元素 x 与 之前 n 次调用后的累加值 acc
  • 返回值作为下次调用的累加值 acc
• 左折叠函数
  • foldl \acc x ->
  • foldl1: 取 List 首元素作为初始累加值

foldl :: (Foldable t) => (b -> a -> b) -> b -> t a -> b

sum' :: (Num a) => [a] -> a
sum' xs = foldl (\acc x -> acc + x) 0 xs

• 右折叠函数
  • foldr \x acc ->
  • foldr1: 取 List 尾元素作为初始累加值

foldr :: (Foldable t) => (a -> b -> b) -> b -> t a -> b

map' :: (a -> b) -> [a] -> [b]
map' f xs = foldr (\x acc -> f x : acc) [] xs

• 更多范例

maximum' :: (Ord a) => [a] -> a
maximum' = foldr1 (\x acc -> if x > acc then x else acc)

reverse' :: [a] -> [a]
reverse' = foldl (\acc x -> x : acc) []

product' :: (Num a) => [a] -> a
product' = foldr1 (*)

filter' :: (a -> Bool) -> [a] -> [a]
filter' p = foldr (\x acc -> if p x then x : acc else acc) []

head' :: [a] -> a
head' = foldr1 (\x _ -> x)

last' :: [a] -> a
last' = foldl1 (_ x -> x)

• scanl/scanr/scanl1/scanr1 函数会将每次折叠的结果都记录在一个 List 中

ghci> scanl (+) 0 [3,5,2,1]
[0,3,8,10,11]
ghci> scanr (+) 0 [3,5,2,1]
[11,8,3,1,0]
ghci> scanl1 (\acc x -> if x > acc then x else acc) [3,4,5,3,7,9,2,1]
[3,4,5,5,7,9,9,9]
ghci> scanl (flip (:)) [] [3,2,1]
[[],[3],[2,3],[1,2,3]]

• 逆波兰表达式

import Data.List

solveRPN :: String -> Float
solveRPN = head . foldl foldingFunction [] . words
where   foldingFunction (x:y:ys) "*" = (x * y):ys
        foldingFunction (x:y:ys) "+" = (x + y):ys
        foldingFunction (x:y:ys) "-" = (y - x):ys
        foldingFunction (x:y:ys) "/" = (y / x):ys
        foldingFunction (x:y:ys) "^" = (y ** x):ys
        foldingFunction (x:xs) "ln" = log x:xs
        foldingFunction xs "sum" = [sum xs]
        foldingFunction xs numberString = read numberString:xs

lambda 表达式

\args -> function

ghci> zipWith (\a b -> (a * 30 + 3) / b) [5,4,3,2,1] [1,2,3,4,5]
[153.0,61.5,31.0,15.75,6.6]

ghci> map (\(a,b) -> a + b) [(1,2),(3,5),(6,3),(2,6),(2,5)]
[3,8,9,8,7]

flip' :: (a -> b -> c) -> b -> a -> c
flip' f = \x y -> f y x

$ 函数

($) :: (a -> b) -> a -> b

• 特性: 优先级最低,右结合
• 功能: 改变优先级,相当于在右方添加括号

f (g (z x)) <-> f $ g $ z x

.函数与 Function composition(函数组合)

(.) :: (b -> c) -> (a -> b) -> a -> c

f . g = \x -> f (g x) - f 的参数类型必须与 g 的返回值类型相同

• 功能: 可以去除函数调用括号

fn x = ceiling (negate (tan (cos (max 50 x))))
-> 去括号: fn x = ceiling . negate . tan . cos . max 50 x
-> Curry化: fn = ceiling . negate . tan . cos . max 50

• Best Practice: 三种不同的函数写法

oddSquareSum :: Integer
oddSquareSum = sum (takeWhile (<10000) (filter odd (map (^2) [1..])))

oddSquareSum :: Integer
oddSquareSum = sum . takeWhile (<10000) . filter odd . map (^2) $ [1..]

oddSquareSum :: Integer
oddSquareSum =
    let oddSquares = filter odd $ map (^2) [1..]
        belowLimit = takeWhile (<10000) oddSquares
    in  sum belowLimit

常用函数

无参函数

“定义”(或者“名字”)

sabertaz = "It's a-me, sabertaz!"

前缀函数

> succ 8
9

`FunctionName` - 可使前缀函数变为中缀函数

> 92 `div` 10
9

中缀函数

+: 从类型定义可以看出,+左右两边参数必须为同类型

ghci> :t (+)
(+) :: (Num a) => a -> a -> a

++: List 连接符,遍历前一 List

ghci> [1,2,3,4] ++ [9,10,11,12]
[1,2,3,4,9,10,11,12]
ghci> "hello" ++ " " ++ "world"
"hello world"

:: 连接单个元素

ghci> 'A':" SMALL CAT"
"A SMALL CAT"
ghci> 5:[1,2,3,4,5]
[5,1,2,3,4,5]

!!: 引用符

ghci> [9.4,33.2,96.2,11.2,23.25] !! 1
33.2

数学函数

• x mod y
• even arg
• odd arg

System Random

• random :: (RandomGen g, Random a) => g -> (a, g)
• getStdGen
• newStdGen

数字函数

• fromInteger 函数 (Num a) => Integer -> a
• fromIntegral 函数 (Integral a, Num b) => a -> b

模块

import

import Data.List
import Data.List (nub，sort)
import Data.List hiding (nub)

import qualified Data.Map

调用 Map 中的 filter 时,必须使用 Data.Map.filter.

import qualified Data.Map as M

Data.Map.filter <-> M.filter

建立模块

单一模块

新建一个 Geometry.hs 的文件

module Geometry
( sphereVolume
，sphereArea
，cubeVolume
，cubeArea
，cuboidArea
，cuboidVolume
) where

sphereVolume :: Float -> Float
sphereVolume radius = (4.0 / 3.0) * pi * (radius ^ 3)

sphereArea :: Float -> Float
sphereArea radius = 4 * pi * (radius ^ 2)

cubeVolume :: Float -> Float
cubeVolume side = cuboidVolume side side side

cubeArea :: Float -> Float
cubeArea side = cuboidArea side side side

cuboidVolume :: Float -> Float -> Float -> Float
cuboidVolume a b c = rectangleArea a b * c

cuboidArea :: Float -> Float -> Float -> Float
cuboidArea a b c =
  rectangleArea a b * 2 + rectangleArea a c * 2 + rectangleArea c b * 2

rectangleArea :: Float -> Float -> Float
rectangleArea a b = a * b

• 使用

import Geometry

子模块

把 Geometry 分成三个子模块

建立一个 Geometry 文件夹(注意首字母要大写),新建三个文件

• sphere.hs

module Geometry.Sphere
( volume
，area
) where

volume :: Float -> Float
volume radius = (4.0 / 3.0) * pi * (radius ^ 3)

area :: Float -> Float
area radius = 4 * pi * (radius ^ 2)

• cuboid.hs

module Geometry.Cuboid
( volume
，area
) where

volume :: Float -> Float -> Float -> Float
volume a b c = rectangleArea a b * c

area :: Float -> Float -> Float -> Float
area a b c =
  rectangleArea a b * 2 + rectangleArea a c * 2 + rectangleArea c b * 2

rectangleArea :: Float -> Float -> Float
rectangleArea a b = a * b

• cube.hs

module Geometry.Cube
( volume
，area
) where

import qualified Geometry.Cuboid as Cuboid

volume :: Float -> Float
volume side = Cuboid.volume side side side

area :: Float -> Float
area side = Cuboid.area side side side

• 使用

import Geometry.Sphere

import qualified Geometry.Sphere as Sphere
import qualified Geometry.Cuboid as Cuboid
import qualified Geometry.Cube as Cube

常用基础模块

Learn you a haskell for great good

Data.List

• intersperse
• intercalate
• transpose

ghci> transpose [[1,2,3],[4,5,6],[7,8,9]]
[[1,4,7],[2,5,8],[3,6,9]]
ghci> transpose ["hey","there","guys"]
["htg","ehu","yey","rs","e"]

ghci> map sum $ transpose [[0,3,5,9],[10,0,0,9],[8,5,1,-1]]
[18,8,6,17]

foldl' 和 foldl1' 是它们各自惰性实现的严格版本,可防止溢出问题.

concat - 移除一级嵌套

ghci> concat ["foo","bar","car"]
"foobarcar"
ghci> concat [[3,4,5],[2,3,4],[2,1,1]]
[3,4,5,2,3,4,2,1,1]

concatMap 函数与 map 一个 List 之后再 concat 它等价

ghci> concatMap (replicate 4) [1..3]
[1,1,1,1,2,2,2,2,3,3,3,3]

and

ghci> and $ map (>4) [5,6,7,8]
True
ghci> and $ map (==4) [4,4,4,3,4]
False

or

ghci> or $ map (==4) [2,3,4,5,6,1]
True
ghci> or $ map (>4) [1,2,3]
False

any 和 all 使用 any 或 all 会更多些

ghci> any (==4) [2,3,5,6,1,4]
True
ghci> all (>4) [6,9,10]
True
ghci> all (`elem` ['A'..'Z']) "HEYGUYSwhatsup"
False
ghci> any (`elem` ['A'..'Z']) "HEYGUYSwhatsup"
True

iterate 取一个函数和一个值作参数。它会用该值去调用该函数并用所得的结果再次调用该函数，产生一个无限的 List.

ghci> take 10 $ iterate (*2) 1
[1,2,4,8,16,32,64,128,256,512]
ghci> take 3 $ iterate (++ "haha") "haha"
["haha","hahahaha","hahahahahaha"]

splitAt

ghci> splitAt 3 "heyman"
("hey","man")
ghci> splitAt 100 "heyman"
("heyman","")
ghci> splitAt (-3) "heyman"
("","heyman")
ghci> let (a,b) = splitAt 3 "foobar" in b ++ a
"barfoo"

takeWhile 一旦遇到不符合条件的某元素就停止

ghci> takeWhile (>3) [6,5,4,3,2,1,2,3,4,5,4,3,2,1]
[6,5,4]
ghci> takeWhile (/=' ') "This is a sentence"
"This"

ghci> sum $ takeWhile (<10000) $ map (^3) [1..]
53361

dropWhile 扔掉符合条件的元素。一旦限制条件返回 False，它就返回 List 的余下部分

ghci> dropWhile (/=' ') "This is a sentence"
" is a sentence"
ghci> dropWhile (<3) [1,2,2,2,3,4,5,4,3,2,1]
[3,4,5,4,3,2,1]

span - 扩展takeWhile

ghci> let (fw，rest) = span (/=' ')
\ "This is a sentence" in "First word:" ++ fw ++ "，the rest:" ++ rest
"First word: This，the rest: is a sentence"

break - 取反span

ghci> break (==4) [1,2,3,4,5,6,7]
([1,2,3],[4,5,6,7])
ghci> span (/=4) [1,2,3,4,5,6,7]
([1,2,3],[4,5,6,7])

sort

ghci> sort [8,5,3,2,1,6,4,2]
[1,2,2,3,4,5,6,8]
ghci> sort "This will be sorted soon"
" Tbdeehiillnooorssstw"

group

ghci> group [1,1,1,1,2,2,2,2,3,3,2,2,2,5,6,7]
[[1,1,1,1],[2,2,2,2],[3,3],[2,2,2],[5],[6],[7]]

ghci> map (\l@(x:xs) -> (x,length l)) . group . sort $ [1,1,1,1,2,2,2,2,3,3,2,2,2,5,6,7]
[(1,4),(2,7),(3,2),(5,1),(6,1),(7,1)]

inits 和 tails

ghci> inits "w00t"
["","w","w0","w00","w00t"]
ghci> tails "w00t"
["w00t","00t","0t","t",""]
ghci> let w = "w00t" in zip (inits w) (tails w)
[("","w00t"),("w","00t"),("w0","0t"),("w00","t"),("w00t","")]

isInfixOf 从一个 List 中搜索一个子 List

search :: (Eq a) => [a] -> [a] -> Bool
search needle haystack =
  let nLen = length needle
  in foldl (\acc x -> if take nLen x == needle then True else acc) False (tails haystack)

ghci> "cat" `isInfixOf` "im a cat burglar"
True
ghci> "Cat" `isInfixOf` "im a cat burglar"
False
ghci> "cats" `isInfixOf` "im a cat burglar"
False

isPrefixOf 与 isSuffixOf

ghci> "hey" `isPrefixOf` "hey there!"
True
ghci> "hey" `isPrefixOf` "oh hey there!"
False
ghci> "there!" `isSuffixOf` "oh hey there!"
True
ghci> "there!" `isSuffixOf` "oh hey there"
False

elem 与 notElem

partition

ghci> partition (`elem` ['A'..'Z']) "BOBsidneyMORGANeddy"
("BOBMORGAN","sidneyeddy")
ghci> partition (>3) [1,3,5,6,3,2,1,0,3,7]
([5,6,7],[1,3,3,2,1,0,3])

find

ghci> find (>4) [1,2,3,4,5,6]
Just 5
ghci> find (>9) [1,2,3,4,5,6]
Nothing
ghci> :t find
find :: (a -> Bool) -> [a] -> Maybe a

elemIndex '可能' (Maybe)返回我们找的元素的索引,若这一元素不存在，就返回 Nothing.

ghci> :t elemIndex
elemIndex :: (Eq a) => a -> [a] -> Maybe Int
ghci> 4 `elemIndex` [1,2,3,4,5,6]
Just 3
ghci> 10 `elemIndex` [1,2,3,4,5,6]
Nothing

elemIndices 与elemIndex相似.

ghci> ' ' `elemIndices` "Where are the spaces?"
[5,9,13]

findIndex 与 find 相似. findIndices 返回所有符合条件的索引.

ghci> findIndex (==4) [5,3,2,1,6,4]
Just 5
ghci> findIndex (==7) [5,3,2,1,6,4]
Nothing
ghci> findIndices (`elem` ['A'..'Z']) "Where Are The Caps?"
[0,6,10,14]

ghci> zipWith3 (\x y z -> x + y + z) [1,2,3] [4,5,2,2] [2,2,3]
[7,9,8]
ghci> zip4 [2,3,3] [2,2,2] [5,5,3] [2,2,2]
[(2,2,5,2),(3,2,5,2),(3,2,3,2)]

在处理来自文件或其它地方的输入时，lines 会非常有用.

ghci> lines "first line\nsecond line\nthird line"
["first line","second line","third line"]

unlines 是 lines 的反函数.

ghci> unlines ["first line"，"second line"，"third line"]
"first line\nsecond line\nthird line\n"

words 和 unwords 可以把一个字串分为一组单词或执行相反的操作.

ghci> words "hey these are the words in this sentence"
["hey","these","are","the","words","in","this","sentence"]
ghci> words "hey these are the words in this\nsentence"
["hey","these","are","the","words","in","this","sentence"]
ghci> unwords ["hey","there","mate"]
"hey there mate"

nub 可以将一个 List 中的重复元素全部筛掉.

ghci> nub [1,2,3,4,3,2,1,2,3,4,3,2,1]
[1,2,3,4]
ghci> nub "Lots of words and stuff"
"Lots fwrdanu"

delete

ghci> delete 'h' "hey there ghang!"
"ey there ghang!"
ghci> delete 'h' . delete 'h' $ "hey there ghang!"
"ey tere ghang!"
ghci> delete 'h' . delete 'h' . delete 'h' $ "hey there ghang!"
"ey tere gang!"

\ 差集

ghci> [1..10] \\ [2,5,9]
[1,3,4,6,7,8,10]
ghci> "Im a big baby" \\ "big"
"Im a  baby"

union 并集

ghci> "hey man" `union` "man what's up"
"hey manwt'sup"
ghci> [1..7] `union` [5..10]
[1,2,3,4,5,6,7,8,9,10]

intersection 交集

ghci> [1..7] `intersect` [5..10]
[5,6,7]

insert

ghci> insert 4 [1,2,3,5,6,7]
[1,2,3,4,5,6,7]
ghci> insert 'g' $ ['a'..'f'] ++ ['h'..'z']
"abcdefghijklmnopqrstuvwxyz"
ghci> insert 3 [1,2,4,3,2,1]
[1,2,3,4,3,2,1]

修正 Prelude 模块

length，take，drop，splitAt，!! 和 replicate Data.List 中包含了更通用的替代版,如: genericLength，genericTake，genericDrop，genericSplitAt，genericIndex 和 genericReplicate

nub, delete, union, intsect 和 group 函数 也有各自的通用替代版 nubBy，deleteBy，unionBy，intersectBy 和 groupBy， 它们的区别就是前一组函数使用 (==) 来测试是否相等，而带 By 的那组则取一个函数作参数来判定相等性.

ghci> let values = [-4.3，-2.4，-1.2，0.4，2.3，5.9，10.5，29.1，5.3，-2.4，-14.5，2.9，2.3]
ghci> groupBy (\x y -> (x > 0) == (y > 0)) values
[[-4.3,-2.4,-1.2],[0.4,2.3,5.9,10.5,29.1,5.3],[-2.4,-14.5],[2.9,2.3]]

on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
f `on` g = \x y -> f (g x) (g y)

ghci> groupBy ((==) `on` (> 0)) values
[[-4.3,-2.4,-1.2],[0.4,2.3,5.9,10.5,29.1,5.3],[-2.4,-14.5],[2.9,2.3]]

sort，insert，maximum 和 min 都有各自的通用版本。 如 groupBy 类似，sortBy，insertBy，maximumBy 和 minimumBy 都取一个函数来比较两个元素的大小.

ghci> let xs = [[5,4,5,4,4],[1,2,3],[3,5,4,3],[],[2],[2,2]]
ghci> sortBy (compare `on` length) xs
[[],[2],[2,2],[1,2,3],[3,5,4,3],[5,4,5,4,4]]

Data Char

• isControl 判断一个字符是否是控制字符。
• isSpace 判断一个字符是否是空格字符，包括空格，tab，换行符等.
• isLower 判断一个字符是否为小写.
• isUpper 判断一个字符是否为大写。
• isAlpha 判断一个字符是否为字母.
• isAlphaNum 判断一个字符是否为字母或数字.
• isPrint 判断一个字符是否是可打印的.
• isDigit 判断一个字符是否为数字.
• isOctDigit 判断一个字符是否为八进制数字.
• isHexDigit 判断一个字符是否为十六进制数字.
• isLetter 判断一个字符是否为字母.
• isMark 判断是否为 unicode 注音字符，你如果是法国人就会经常用到的.
• isNumber 判断一个字符是否为数字.
• isPunctuation 判断一个字符是否为标点符号.
• isSymbol判断一个字符是否为货币符号.
• isSeparator 判断一个字符是否为 unicode 空格或分隔符.
• isAscii 判断一个字符是否在 unicode 字母表的前 128 位。
• isLatin1 判断一个字符是否在 unicode 字母表的前 256 位.
• isAsciiUpper 判断一个字符是否为大写的 ascii 字符.
• isAsciiLower 判断一个字符是否为小写的 ascii 字符.

ghci> all isAlphaNum "bobby283"
True
ghci> all isAlphaNum "eddy the fish!"
False

ghci> words "hey guys its me"
["hey","guys","its","me"]
ghci> groupBy ((==) `on` isSpace) "hey guys its me"
["hey"," ","guys"," ","its"," ","me"]
ghci>

ghci> filter (not . any isSpace) . groupBy ((==) `on` isSpace) $ "hey guys its me"
["hey","guys","its","me"]

ghci> generalCategory ' '
Space
ghci> generalCategory 'A'
UppercaseLetter
ghci> generalCategory 'a'
LowercaseLetter
ghci> generalCategory '.'
OtherPunctuation
ghci> generalCategory '9'
DecimalNumber
ghci> map generalCategory " \t\nA9?|"
[Space,Control,Control,UppercaseLetter,DecimalNumber,OtherPunctuation,MathSymbol]

• toUpper 将一个字符转为大写字母，若该字符不是小写字母，就按原值返回.
• toLower 将一个字符转为小写字母，若该字符不是大写字母，就按原值返回.
• toTitle 将一个字符转为 title-case，对大多数字元而言，title-case 就是大写.
• digitToInt 将一个字符转为 Int 值，而这一字符必须得在 '1'..'9','a'..'f'或'A'..'F' 的范围之内.

ghci> map digitToInt "34538"
[3,4,5,3,8]
ghci> map digitToInt "FF85AB"
[15,15,8,5,10,11]

ghci> intToDigit 15
'f'
ghci> intToDigit 5
'5'

ord 与 char 函数可以将字符与其对应的数字相互转换.

ghci> ord 'a'
97
ghci> chr 97
'a'
ghci> map ord "abcdefgh"
[97,98,99,100,101,102,103,104]

encode :: Int -> String -> String
encode shift msg =
  let ords = map ord msg
      shifted = map (+ shift) ords
  in map chr shifted

decode :: Int -> String -> String
decode shift msg = encode (negate shift) msg

Data Map

findKey :: (Eq k) => k -> [(k,v)] -> v
findKey key xs = snd . head . filter (\(k,v) -> key == k) $ xs

findKey :: (Eq k) => k -> [(k,v)] -> Maybe v
findKey key [] = Nothing
findKey key ((k,v):xs) =
     if key == k then
         Just v
     else
         findKey key xs

findKey :: (Eq k) => k -> [(k,v)] -> Maybe v
findKey key = foldr (\(k,v) acc -> if key == k then Just v else acc) Nothing

fromList 取一个关联列表，返回一个与之等价的 Map。

Map.fromList :: (Ord k) => [(k，v)] -> Map.Map k v

若其中存在重复的键,就将其忽略.

ghci> Map.empty
fromList []

insert

ghci> Map.insert 5 600 (Map.insert 4 200 ( Map.insert 3 100  Map.empty))
fromList [(3,100),(4,200),(5,600)]
ghci> Map.insert 5 600 . Map.insert 4 200 . Map.insert 3 100 $ Map.empty
fromList [(3,100),(4,200),(5,600)]

fromList' :: (Ord k) => [(k,v)] -> Map.Map k v
fromList' = foldr (\(k,v) acc -> Map.insert k v acc) Map.empty

null

ghci> Map.null Map.empty
True

size

ghci> Map.size $ Map.fromList [(2,4),(3,3),(4,2),(5,4),(6,4)]
5

singleton

ghci> Map.singleton 3 9
fromList [(3,9)]
ghci> Map.insert 5 9 $ Map.singleton 3 9
fromList [(3,9),(5,9)]

lookup

member

ghci> Map.member 3 $ Map.fromList [(3,6),(4,3),(6,9)]
True
ghci> Map.member 3 $ Map.fromList [(2,5),(4,5)]
False

map 与 filter 与其对应的 List 版本相似

toList 是 fromList 的反函数

ghci> Map.toList . Map.insert 9 2 $ Map.singleton 4 3
[(4,3),(9,2)]

keys 与 elems

phoneBook =
    [("betty","555-2938")
    ,("betty","342-2492")
    ,("bonnie","452-2928")
    ,("patsy","493-2928")
    ,("patsy","943-2929")
    ,("patsy","827-9162")
    ,("lucille","205-2928")
    ,("wendy","939-8282")
    ,("penny","853-2492")
    ,("penny","555-2111")
    ]

phoneBookToMap :: (Ord k) => [(k, String)] -> Map.Map k String
phoneBookToMap xs = Map.fromListWith
  (\number1 number2 -> number1 ++ ", " ++ number2) xs

ghci> Map.lookup "patsy" $ phoneBookToMap phoneBook
"827-9162, 943-2929, 493-2928"
ghci> Map.lookup "wendy" $ phoneBookToMap phoneBook
"939-8282"
ghci> Map.lookup "betty" $ phoneBookToMap phoneBook
"342-2492，555-2938"

phoneBookToMap :: (Ord k) => [(k，a)] -> Map.Map k [a]
phoneBookToMap xs = Map.fromListWith (++) $ map (\(k,v) -> (k,[v])) xs
ghci> Map.lookup "patsy" $ phoneBookToMap phoneBook
["827-9162","943-2929","493-2928"]

在遇到重复元素时，单选最大的那个值.

ghci> Map.fromListWith max [(2,3),(2,5),(2,100),(3,29),(3,22),(3,11),(4,22),(4,15)]
fromList [(2,100),(3,29),(4,22)]

将相同键的值都加在一起.

ghci> Map.fromListWith (+) [(2,3),(2,5),(2,100),(3,29),(3,22),(3,11),(4,22),(4,15)]
fromList [(2,108),(3,62),(4,37)]

insertWith

ghci> Map.insertWith (+) 3 100 $ Map.fromList [(3,4),(5,103),(6,339)]
fromList [(3,104),(5,103),(6,339)]

Data Set

内部元素排序且唯一

import qualified Data.Set as Set

fromList

ghci> let set1 = Set.fromList text1
ghci> let set2 = Set.fromList text2
ghci> set1
fromList " .?AIRadefhijlmnorstuy"
ghci> set2
fromList " !Tabcdefghilmnorstuvwy"

ghci> Set.intersection set1 set2
fromList " adefhilmnorstuy"

difference 差集

ghci> Set.difference set1 set2
fromList ".?AIRj"
ghci> Set.difference set2 set1
fromList "!Tbcgvw"

union 并集

ghci> Set.union set1 set2
fromList " !.?AIRTabcdefghijlmnorstuvwy"

null，size，member，empty，singleton，insert，delete

ghci> Set.null Set.empty
True
ghci> Set.null $ Set.fromList [3,4,5,5,4,3]
False
ghci> Set.size $ Set.fromList [3,4,5,3,4,5]
3
ghci> Set.singleton 9
fromList [9]
ghci> Set.insert 4 $ Set.fromList [9,3,8,1]
fromList [1,3,4,8,9]
ghci> Set.insert 8 $ Set.fromList [5..10]
fromList [5,6,7,8,9,10]
ghci> Set.delete 4 $ Set.fromList [3,4,5,4,3,4,5]
fromList [3,5]

ghci> Set.fromList [2,3,4] `Set.isSubsetOf` Set.fromList [1,2,3,4,5]
True
ghci> Set.fromList [1,2,3,4,5] `Set.isSubsetOf` Set.fromList [1,2,3,4,5]
True
ghci> Set.fromList [1,2,3,4,5] `Set.isProperSubsetOf` Set.fromList [1,2,3,4,5]
False
ghci> Set.fromList [2,3,4,8] `Set.isSubsetOf` Set.fromList [1,2,3,4,5]
False

执行 map 和 filter:

ghci> Set.filter odd $ Set.fromList [3,4,5,6,7,2,3,4]
fromList [3,5,7]
ghci> Set.map (+1) $ Set.fromList [3,4,5,6,7,2,3,4]
fromList [3,4,5,6,7,8]

• 删除重复元素

ghci> let setNub xs = Set.toList $ Set.fromList xs
ghci> setNub "HEY WHATS CRACKALACKIN"
" ACEHIKLNRSTWY"
ghci> nub "HEY WHATS CRACKALACKIN"
"HEY WATSCRKLIN"

输入与输出

IO action

name <- IO action: 将 action 绑定至名字上,IO String -> String

name <- getLine
name <- return String

name <- putStrLn String

在一个do block中,最后一个action不能绑定任何名字,它会被绑定成为do block的结果值.

main = do
    foo <- putStrLn "Hello, what's your name?"
    name <- getLine
    putStrLn ("Hey " ++ name ++ ", you rock!")

return

• return 功能:将 pure value 包成 I/O actions,不会终止函数/程序
• return 作用:
  • if condition then I/O action else I/O action
  • 改变do block形成的I/O action的结果值: otherIOAction -> return pureValue

e.g return "haha" - String -> IO String

main = do
    line <- getLine
    if null line
        then return ()
        else do
            putStrLn $ reverseWords line
            main

reverseWords :: String -> String
reverseWords = unwords . map reverse . words

Command Line

System.Environment

• getArgs: getArgs :: IO [String]
• getProgName: getProgName :: IO String

import System.Environment
import Data.List

main = do
    args <- getArgs
    progName <- getProgName
    mapM putStrLn args
    putStrLn progName

import System.Environment
import System.Directory
import System.IO
import Data.List

dispatch :: [(String, [String] -> IO ())]
dispatch =  [ ("add", add)
            , ("view", view)
            , ("remove", remove)
            ]

main = do
    (command:args) <- getArgs
    let (Just action) = lookup command dispatch
    action args

add :: [String] -> IO ()
add [fileName, todoItem] = appendFile fileName (todoItem ++ "\n")

view :: [String] -> IO ()
view [fileName] = do
    contents <- readFile fileName
    let todoTasks = lines contents
        numberedTasks = zipWith (\n line -> show n ++ " - " ++ line) [0..] todoTasks
    putStr $ unlines numberedTasks

remove :: [String] -> IO ()
remove [fileName, numberString] = do
    handle <- openFile fileName ReadMode
    (tempName, tempHandle) <- openTempFile "." "temp"
    contents <- hGetContents handle
    let number = read numberString
        todoTasks = lines contents
        newTodoItems = delete (todoTasks !! number) todoTasks
    hPutStr tempHandle $ unlines newTodoItems
    hClose handle
    hClose tempHandle
    removeFile fileName
    renameFile tempName fileName

ByteString

• Data.ByteString
• Data.ByteString.Lazy

lazy byteStrings 像装了一堆大小为 64K 的 strict byteStrings 的 list

import qualified Data.ByteString.Lazy as B
import qualified Data.ByteString as S

• pack/unpack
• fromChunks/toChunks
• cons/empty/head/tail/init/null/length/map/reverse/foldl/foldr/concat/takeWhile/filter

常用输入输出函数

输出

Output String

putChar/putStr/putStrLn:

putStr :: String -> IO ()
putStr [] = return ()
putStr (x:xs) = do
    putChar x
    putStr xs

print

print = putStrLn . show

File System IO

• hPutStr
• hPutStrLn

Write File

writefile :: FilePath -> String -> IO () - WriteMode, not AppendMode

Append File

输入

getChar :: IO Char

main = do
    c <- getChar
    if c /= ' '
        then do
            putChar c
            main
        else return ()

Get Contents

getContents :: IO String (Lazy I/O) - 内容暂存在文件,需要使用时读取至内存区

import Data.Char

main = do
    contents <- getContents
    putStr (map toUpper contents)

File

handle

• data IOMode = ReadMode | WriteMode | AppendMode | ReadWriteMode

• openFile :: FilePath(String) -> IOMode -> IO Handle

• hGetContents :: Handle -> IO String

• hClose :: Handle -> IO ()

• hGetChar

• hGetLine

import System.IO

main = do
    handle <- openFile "girlfriend.txt" ReadMode
    contents <- hGetContents handle
    putStr contents
    hClose handle

With File

withFile :: FilePath -> IOMode -> (Handle -> IO a) -> IO a

import System.IO

main = do
    withFile "girlfriend.txt" ReadMode (\handle -> do
            contents <- hGetContents handle
            putStr contents)

Read File

contents <- readFile "girlfriend.txt"

Action

When

Control.Monad.when :: (Applicative f) => Bool -> f () -> f ()

when bool表达式 I/O-Action - 真时返回 Action,假时return ()

import Control.Monad

main = do
    c <- getChar
    when (c /= ' ') $ do
        putChar c
        main

sequence

sequence :: [IO a] -> IO [a]

main = do
    rs <- sequence [getLine, getLine, getLine]
    print rs

Map

mapM, Control.Monad.forM:

= sequence . map

ghci> mapM print [1,2,3]
1
2
3
[(),(),()]
ghci> mapM_ print [1,2,3]
1
2
3

Control Monad forever

接受一个 I/O action 并回传一个永远作同一件事的 I/O action

以下代码实现了循环结构:

import Control.Monad
import Data.Char

main = forever $ do
    putStr "Give me some input: "
    l <- getLine
    putStrLn $ map toUpper l

hSetBuffering

data BufferMode = NoBuffering | LineBuffering | BlockBuffering (Maybe Int) hSetBuffering :: Handle -> BufferMode -> IO ()

main = do
    withFile "something.txt" ReadMode (\handle -> do
        hSetBuffering handle $ BlockBuffering (Just 2048)
        contents <- hGetContents handle
        putStr contents)

hFlush

Other

lines

lines :: String -> [String] - 按换行符将段落切割成句子

interact

• arguments: String -> String 的函数
• return: 一个 I/O action
• function: I/O action 会读取输入，调用提供的函数，然后把函数的结果打印出来

main = interact shortLinesOnly

shortLinesOnly :: String -> String
shortLinesOnly input =
    let allLines = lines input
        shortLines = filter (\line -> length line < 10) allLines
        result = unlines shortLines
    in result

main = interact $ unlines . filter ((<10) . length) . lines

respondPalindromes = unlines . map (\xs ->
    if isPalindrome xs then "palindrome" else "not a palindrome") . lines
        where isPalindrome xs = xs == reverse xs

System Directory

• removeFile
• renameFile

异常

System.IO.Error

catch

catch :: IO a -> (IOError -> IO a) -> IO a

import System.Environment
import System.IO
import System.IO.Error

main = toTry `catch` handler

toTry :: IO ()
toTry = do (fileName:_) <- getArgs
            contents <- readFile fileName
            putStrLn $ "The file has " ++ show (length (lines contents)) ++ " lines!"

handler :: IOError -> IO ()
handler e
    | isDoesNotExistError e = putStrLn "The file doesn't exist!"
    | isFullError e = freeSomeSpace
    | isIllegalOperation e = notifyCops
    | otherwise = ioError e

Advanced Monad

Writer Monad

Writer 可以让我们在计算的同时搜集所有 log 纪录，并汇集成一个 log 并附加在结果上

applyLog :: (a,String) -> (a -> (b,String)) -> (b,String)
applyLog (x,log) f = let (y,newLog) = f x in (y,log ++ newLog)

ghci> (30, "A freaking platoon.") `applyLog` isBigGang
(True,"A freaking platoon.Compared gang size to 9")
ghci> ("BathCat","Got outlaw name.") `applyLog` (\x -> (length x, "Applied length"))
(7,"Got outlaw name.Applied length")

Control Monad Writer

instance (Monoid w) => Monad (Writer w) where
    return x = Writer (x, mempty)
    (Writer (x,v)) >>= f = let (Writer (y, v')) = f x in Writer (y, v `mappend` v')

import Control.Monad.Writer

logNumber :: Int -> Writer [String] Int
logNumber x = Writer (x, ["Got number: " ++ show x])

multiWithLog :: Writer [String] Int
multiWithLog = do
    a <- logNumber 3
    b <- logNumber 5
    return (a*b)

Reader Monad

instance Monad ((->) r) where
    return x = _ -> x
    h >>= f = \w -> f (h w) w

State Monad

Control Monad State

newtype State s a = State { runState :: s -> (a,s) }

instance Monad (State s) where
    return x = State $ \s -> (x,s)
    (State h) >>= f = State $ \s -> let (a, newState) = h s
                                        (State g) = f a
                                    in  g newState

get = State $ \s -> (s,s)
put newState = State $ \s -> ((),newState)

State Monad Case

import Control.Monad.State

pop :: State Stack Int
pop = State $ \(x:xs) -> (x,xs)

push :: Int -> State Stack ()
push a = State $ \xs -> ((),a:xs)

stackManipulation :: State Stack Int
stackManipulation = do
  push 3
  a <- pop
  pop

Error Monad

instance (Error e) => Monad (Either e) where
    return x = Right x
    Right x >>= f = f x
    Left err >>= f = Left err
    fail msg = Left (strMsg msg)

ghci> :t strMsg
strMsg :: (Error a) => String -> a
ghci> strMsg "boom!" :: String
"boom!"

liftM

join

filterM

foldM

<=<(组合函数)

Self-Defined Monad

注释

符号

• -- 单行注释
• {- -} 块注释
• {-# #-} (文件头部)编译器参数

Reference

• Haskell Book