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 来实现。

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)

这是相同的函数 verEmails，我在其中使用了 Maybe：

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

问题来自于尝试做e : verEmails n xs，因为verEmails n xs不返回列表，而是包含在Maybe. 处理此问题的最简单方法是使用该Data.Maybe.fromMaybe函数：

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

在这里，我假设你希望回到Just aList这里aList包含了从过滤过的所有电子邮件Agenda中传递，这意味着，只有这样，verEmails将返回Nothing时传递的议程是空的。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verEmails n xs从Maybe [String]to转换[String]，默认为空列表，在 prepends e，然后将其包装在 a 中Just。

作为旁注，您的功能并未涵盖所有可能的情况，如果我要运行会发生什么verEmails n ((nome, []):xs)？甚至verEmails n ((nome, [Casa 1]):xs)？