Maybe常用方法

fromMaybe

描述：当 Maybe 值为 Nothing 时，返回一个默认值。

类型签名：

fromMaybe :: a -> Maybe a -> a

示例

import Data.Maybe (fromMaybe)

-- 如果值为 Nothing，将使用默认值 10
let result = fromMaybe 10 Nothing       -- 结果为 10
let result2 = fromMaybe 10 (Just 5)     -- 结果为 5

readMaybe

描述：解析字符串为指定类型（如 Int），如果解析失败则返回 Nothing。

类型签名：

readMaybe :: Read a => String -> Maybe a

示例

import Text.Read (readMaybe)

let parsedInt = readMaybe "123" :: Maybe Int  -- 结果为 Just 123
let parsedFail = readMaybe "abc" :: Maybe Int -- 结果为 Nothing

mapMaybe

描述：对列表中的每个元素进行转换，并过滤掉转换失败的元素。

类型签名：

mapMaybe :: (a -> Maybe b) -> [a] -> [b]

示例

import Data.Maybe (mapMaybe)

-- 将字符串列表尝试转换为 Int，仅保留成功转换的结果
let result = mapMaybe readMaybe ["123", "abc", "456"]  -- 结果为 [123, 456]

catMaybes

描述：将列表中所有 Just 的值提取出来，忽略 Nothing。

类型签名：

catMaybes :: [Maybe a] -> [a]

示例

import Data.Maybe (catMaybes)

let result = catMaybes [Just 1, Nothing, Just 2, Nothing, Just 3]  -- 结果为 [1, 2, 3]

maybe

描述：将 Maybe 转换为另一个值。第一个参数为 Nothing 的转换值，第二个参数为 Just 的转换函数。

类型签名：

maybe :: b -> (a -> b) -> Maybe a -> b

示例

let result = maybe 0 (*2) (Just 5)    -- 结果为 10
let result2 = maybe 0 (*2) Nothing    -- 结果为 0

hoistMaybe

描述：将一个普通值转换为 Maybe 值：如果条件满足则返回 Just，否则返回 Nothing。

类型签名：

hoistMaybe :: Bool -> a -> Maybe a

示例

hoistMaybe :: Bool -> a -> Maybe a
hoistMaybe condition value = if condition then Just value else Nothing

let result = hoistMaybe (5 > 3) "符合条件"     -- 结果为 Just "符合条件"
let result2 = hoistMaybe (5 < 3) "不符合条件"   -- 结果为 Nothing