在Haskell使用Maybe [String]

Question

我想使用Maybe [String]返回一个字符串，但是我无法使用Maybe做到这一点。

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)

这是相同的功能verEmails，我在其中使用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给我的错误是：

 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不会返回列表，而是返回Maybe中包含的列表。 解决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

在这里，我假设您想返回Just aList ，其中aList包含从传入的Agenda筛选出的所有电子邮件。这意味着，当传入的议程为空时， verEmails唯一不会返回Nothing内容的方法。 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

这只是将verEmails n xs从Maybe [String]为[String] ，默认为空列表，在e加上e ，然后将其包装回Just 。

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