理解类型的高阶函数在斯卡拉

Question

我自学斯卡拉（从多年的Java的未来），我穿过下面的代码，我不能完全神交传来：

case class Email(subject: String, text: String, sender: String, recipient: String) 

type EmailFilter = Email => Boolean 
def newMailsForUser(mails: Seq[Email], f: EmailFilter) = mails.filter(f) 

val sentByOneOf: Set[String] => EmailFilter = 
    senders => email => senders.contains(email.sender) 

val notSentByAnyOf: Set[String] => EmailFilter = 
    senders => email => !senders.contains(email.sender) 

val minimumSize: Int => EmailFilter = n => email => email.text.size >= n 
val maximumSize: Int => EmailFilter = n => email => email.text.size <= n 

我找了份很难理解与缺少类型定义的丘壑：

val sentByOneOf: Set[String] => EmailFilter = 
    senders => email => senders.contains(email.sender) 

我创建使用def相同的功能，这我能理解：

def sentByOne(senders: Set[String], email: Email): (Set[String] => EmailFilter) = { 
    senders => email => senders.contains(email.sender) 
} 

这个类型来自哪个版本？另外，后一版本是否可取？

Answer 1

短的答案，在该definiton：

val sentByOneOf: Set[String] => EmailFilter = 
    senders => email => senders.contains(email.sender) 

扩展为：

val sentByOneOf: Set[String] => Email => Boolean = 
    senders => email => senders.contains(email.sender) 

类型:如Set[String] => EmailFilter之后定义。所以传递给sentByOneOf的第一个值是Set[String]，第二个值是EmailFilter，这是Email => Boolean的别名。

较长版本：

由于所有变量/ Scala中值类型可以冒号之后像它在本例中，或者它可以从它指的是表达来infered指定。所以在这里：

类型的 sentByOneOf

val sentByOneOf = (senders: Set[String]) => (email: Email) => 
    senders.contains(email.sender) 

也将infered到sentByOneOf: Set[String] => (Email => Boolean)其中Email => Boolean有定义为EmailFilter

Scala中function values的别名，这是你必须在这里实际上是在FunctioN现实情况其中N是变量的数量。所以，你的功能sentByOneOf其实是这样的：

val sentByOneOf = new Function2[Set[String], Email, Boolean] { 
    def apply(senders: Set[String], email: Email, pred: => Boolean): Boolean = senders.contains(email.sender) 
} 

顺便说一句，你选择了伟大的系列文章，但因为比较困难首次与斯卡拉:)