Haskell 笔记：data, type, newtype

2018-11-16 | Tech

新类型有自己的 data constructor（literals 可以看成特殊的 data constructor），由这一点来区分是否创建了新类型。

data 创建了新类型，可以有多个 data constructor。
newtype 创建了新类型，只能有一个 data constructor，同时新类型的内存布局与原来的类型相同。
type 没有创建新类型，只是建立了 alias，没有新的 data constructor。

type

常用于语义化类型，是业务逻辑层的概念。

type ID = Int

a = 1 :: ID
b = a + 2 -- legal

showID :: ID -> IO ()
showID x = print x -- legal, since Int has already been an instance of class Show

-- illegal, since Int has already been instantiated
instance Show ID where
    -- ...

newtype

在编译期创建新类型，但差异在运行期被抹去。带有一个构造器。

newtype ID' = ID' Int

a = ID' 1
b = a + 2 -- illegal, since Int and ID' are totally different types

showID' :: ID' -> IO ()
showID' x = print x -- illegal, since ID' is not an instance of Show

-- either
showID' (ID' x) = print x
-- or
instance Show ID' where
    show (ID' x) = show x

作者：hsfzxjy
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Haskell

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