相关疑难解决方法(0)

我似乎真的不了解Map和FlatMap.我无法理解的是for-comprehension是如何嵌套调用map和flatMap的.以下示例来自Scala中的Functional Programming

def bothMatch(pat:String,pat2:String,s:String):Option[Boolean] = for {
            f <- mkMatcher(pat)
            g <- mkMatcher(pat2)
 } yield f(s) && g(s)

翻译成

def bothMatch(pat:String,pat2:String,s:String):Option[Boolean] = 
         mkMatcher(pat) flatMap (f => 
         mkMatcher(pat2) map (g => f(s) && g(s)))

mkMatcher方法定义如下:

  def mkMatcher(pat:String):Option[String => Boolean] = 
             pattern(pat) map (p => (s:String) => p.matcher(s).matches)

模式方法如下:

import java.util.regex._

def pattern(s:String):Option[Pattern] = 
  try {
        Some(Pattern.compile(s))
   }catch{
       case e: PatternSyntaxException => None
   }

如果有人能够阐明在这里使用map和flatMap背后的理由,那将会很棒.

为什么这种结构会导致Scala中出现类型不匹配错误？

for (first <- Some(1); second <- List(1,2,3)) yield (first,second)

<console>:6: error: type mismatch;
 found   : List[(Int, Int)]
 required: Option[?]
       for (first <- Some(1); second <- List(1,2,3)) yield (first,second)

如果我用List切换Some,它编译得很好:

for (first <- List(1,2,3); second <- Some(1)) yield (first,second)
res41: List[(Int, Int)] = List((1,1), (2,1), (3,1))

这也很好:

for (first <- Some(1); second <- Some(2)) yield (first,second)