haskell - 在 Haskell 中使用 Maybe [String]

Question

我想使用 Maybe [String] 返回一个字符串，但我无法使用 Maybe 来完成它。

我应该定义一个实例吗？

data Contacto = Casa Integer
              | Trab Integer
              | Tlm Integer
              | Email String
              deriving (Show)
type Nome = String
type Agenda = [(Nome, [Contacto])]

addEmail :: Nome -> String -> Agenda -> Agenda
addEmail n email agenda = (n, [Email email]):(agenda)


verEmails :: Nome -> Agenda -> [String]
verEmails n [] = []
verEmails n ((nome, ((Email e):ls)):xs) = if n == nome then (e:(verEmails n xs))
                                                       else (verEmails n xs)

这是我使用 Maybe 的同一个函数 verEmails:

verEmails :: Nome -> Agenda -> Maybe [String]
verEmails n [] = Nothing
verEmails n ((nome, ((Email e):ls)):xs) = if n == nome then Just (e:(verEmails n xs))
                                                       else (verEmails n xs)

GHCi 给我的错误:

Couldn't match expected type `[String]'
                with actual type `Maybe [String]'
    In the return type of a call of `verEmails'
    In the second argument of `(:)', namely `(verEmails n xs)'
    In the first argument of `Just', namely `(e : (verEmails n xs))'

Answer 1

问题来自尝试执行 e : verEmails n xs，因为 verEmails n xs 不返回列表，而是包含在 Maybe< 中的列表。处理此问题的最简单方法是使用 Data.Maybe.fromMaybe 函数:

fromMaybe :: a -> Maybe a -> a
fromMaybe onNothing Nothing = onNothing
fromMaybe onNothing (Just a) = a

这里我假设您想要返回 Just aList，其中 aList 包含从传入的 Agenda 中过滤的所有电子邮件。这意味着 verEmails 将返回 Nothing 的唯一方法是传入的议程为空。所以我们有

verEmails n [] = Nothing
verEmails n ((nome, ((Email e):ls)):xs)
    = if n == nome
        then Just $ e : (fromMaybe [] $ verEmails n xs)
        else verEmails n xs

这只是简单地将 verEmails n xs 从 Maybe [String] 转换为 [String]，默认为空列表，在 e，然后将其包装在 Just 中。

附带说明一下，您的函数并未涵盖所有可能的情况，如果我运行 verEmails n ((nome, []):xs) 会发生什么情况？甚至 verEmails n ((nome, [Casa 1]):xs)？