在 Haskell 中使用 Maybe [String]

Question

我想使用 Maybe [String] 返回一个

string

，但我无法使用

Maybe

来做到这一点。

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)

这是相同的函数

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：

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

问题来自于尝试执行

e : verEmails n xs

，因为

verEmails n xs

不返回列表，而是返回

Maybe

中包含的列表。 处理这个问题最简单的方法是使用

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

过滤出来的所有电子邮件。这意味着

verEmails

返回

Nothing

的唯一方法是议程通过时中是空的。 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

这只是简单地将

verEmails n xs

从

Maybe [String]

转换为

[String]

，默认为空列表，在前面添加

e

，然后将其包装回

Just

中。

顺便说一句，你的函数并没有涵盖所有可能的情况，如果我运行

verEmails n ((nome, []):xs)

会发生什么？ 或者甚至

verEmails n ((nome, [Casa 1]):xs)

？