自定义类型和类型类

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-- 方法1 使用data定义类型
-- 左边为类型名， 右边有类型值构造器｀
data Bool = False | True


    

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-- 构造自己的类型
data Shape = Circle Float Float Float | Rectangle Float Float Float Float


    

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:t Circle


    

    





Circle :: Float -> Float -> Float -> Shape

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:t Rectangle


    

    





Rectangle :: Float -> Float -> Float -> Float -> Shape

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-- 值构造器类似函数调用


    

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-- 这里Shape是类型，而Shape/Circle不是
area :: Shape -> Float
area (Circle _ _ r) = pi * r ^ 2
area (Rectangle x1 y1 x2 y2) = (abs $ x2 - x1) * (abs $ y2 - y1)


    

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area $ Circle 10 20 10


    

    






314.15927

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area $ Rectangle 0 0 100 100


    

    






10000.0

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-- 实现show类型类才能显示在终端，类似__str__方法
data Shape = Circle Float Float Float | Rectangle Float Float Float Float
    deriving (Show)


    

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Circle 10 20 5


    

    






Circle 10.0 20.0 5.0

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Rectangle 50 230 60 90


    

    






Rectangle 50.0 230.0 60.0 90.0

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-- 值构造器本质是函数，所以可以部分应用
map (Circle 10 20) [4..7]


    

    






[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 7.0]

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-- point类型，类型组合
-- 值构造器可以和类型同名
data Point = Point Float Float deriving (Show)
data Shape = Circle Point Float | Rectangle Point Point deriving (Show)


    

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area :: Shape -> Float
area (Circle _ r) = pi * r ^ 2
area (Rectangle (Point x1 y1) (Point x2 y2)) = (abs $ x2 - x1) * (abs $ y2 - y1)


    

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area (Rectangle (Point 0 0) (Point 100 100))


    

    






10000.0

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area (Circle (Point 0 0) 24)


    

    






1809.5574

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nudge :: Shape -> Float -> Float -> Shape
nudge (Circle (Point x y) r) a b = Circle (Point (x+a) (y+b)) r
nudge (Rectangle (Point x1 y1) (Point x2 y2)) a b = Rectangle (Point (x1+a) (y1+b)) (Point (x2 + a) (y2 + b))


    

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nudge (Circle (Point 34 34) 10) 5 10


    

    






Circle (Point 39.0 44.0) 10.0

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baseCircle :: Float -> Shape
baseCircle r = Circle (Point 0 0) r


    

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baseRect :: Float -> Float -> Shape
baseRect width height = Rectangle (Point 0 0) (Point width height)


    

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nudge (baseRect 40 100) 60 23


    

    






Rectangle (Point 60.0 23.0) (Point 100.0 123.0)

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-- 模块中可以导出类型, 方便外部使用
-- module Shapes (Shape(..), Point(..), area, nudge) where
-- Shape(..)导出shape所有值构造器, 等价于Shape(Circle, Rectangle)
-- Shape会不导出值构造器.

-- 也可不导出，抽象度更高. 但无法使用类型匹配


    

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-- 第二种数据类型创建方法
-- 非纪录语法
data Person = Person String String deriving (Show)
let guy = Person "Buddy" "Frink"
guy


    

    






Person "Buddy" "Frink"

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firstName :: Person -> String
firstName (Person first _) = first


    

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lastName :: Person -> String
lastName (Person _ lastname) = lastname


    

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firstName guy


    

    






"Buddy"

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lastName guy


    

    






"Frink"

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-- 记录语法, 可以直接通过字段取值，类型ruby extends
-- show更好看
data Person = Person {firstName :: String
                      , lastName :: String
                      , age :: Int } deriving (Show)


    

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:t age


    

    





age :: Person -> Int

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data Car = Car String String Int deriving (Show)


    

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:t Car


    

    





Car :: String -> String -> Int -> Car

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Car "Ford" "Must" 1978


    

    






Car "Ford" "Must" 1978

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-- XXX errors
data Car ＝ Car { company :: String
                 , model :: String
                 , year :: Int } deriving (Show)


    

    





Parse error (line 1, column 6): Unexpected type ‘Car’
In the data declaration for ‘＝’
A data declaration should have form
  data ＝ a b = ...

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:t Car


    

    





Car :: String -> String -> Int -> Car

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-- 类型参数, 类型可以取参数产生新类型
-- Maybe变为类型构造器，
data Maybe a = Nothing | Just a


    

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Just 3 :: Maybe Int


    

    





Unshowable:Maybe IntNo instance for (Show (Maybe Int)) arising from a use of ‘print’
In a stmt of an interactive GHCi command: print it

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-- 列表是类型参数


    

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Just "Haha"


    

    





Unshowable:Maybe StringNo instance for (Show (Maybe String)) arising from a use of ‘print’
In a stmt of an interactive GHCi command: print it

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-- 配合记录语法
data Car = Car { company :: a
                , model :: b
                , year :: c } deriving (Show)


    

    





Not in scope: type variable ‘a’


Not in scope: type variable ‘b’


Not in scope: type variable ‘c’

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