haskell 初学者

Question

我不明白为什么我会从 GHCi 得到以下回复。不是Maybe构造函数？

Prelude> :t Maybe

<interactive>:1:1: Not in scope: data constructor `Maybe'
Prelude> let e = Maybe 5

<interactive>:1:9: Not in scope: data constructor `Maybe'

Answer 1

Maybe是 type constructor , 及其两个可能的 data constructors是 Nothing和 Just .所以你不得不说Just 5而不是 Maybe 5 .

> let x = Just 5
> x
Just 5
> let y = Nothing
> y
Nothing
> :type x
x :: Maybe Integer
> :type y
y :: Maybe a
> :info Maybe
data Maybe a = Nothing | Just a     -- Defined in Data.Maybe
instance Eq a => Eq (Maybe a) -- Defined in Data.Maybe
instance Monad Maybe -- Defined in Data.Maybe
instance Functor Maybe -- Defined in Data.Maybe
instance Ord a => Ord (Maybe a) -- Defined in Data.Maybe
instance Read a => Read (Maybe a) -- Defined in GHC.Read
instance Show a => Show (Maybe a) -- Defined in GHC.Show

Maybe是一个类型构造函数，因为它用于构造新类型(结果类型取决于 a 中的 Maybe a 的类型)，其中这种类型可能是 Maybe Int (注意，不再有类型参数 a，即所有类型参数都已绑定(bind))。 Just a和 Nothing是数据构造器，因为它们用于构造某个 Maybe 的实例输入，例如 Just Int创建 Maybe Int 的实例.

另一个主要区别是您只能在模式匹配时使用数据构造函数。你不能说:

case foo of
     Maybe a -> ...

你不得不说:

case foo of
     Just a  -> ...
     Nothing -> ...