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Data

data type:

data SelfDefinedTypeName =
  ValueConstructorName ValueType .. | .. deriving (TypeClass, ..)
data Point = Point Float Float deriving (Show)
data Shape = Circle Point Float | Rectangle Point Point deriving (Show)

Export data type:

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