Scala中的编译错误 - 类型不匹配

Question

This`s compile without problem :

class Tweet(val user: String, val text: String, val retweets: Int) { 
  override def toString: String = "User: " + user + "\n" + "Text: " + text + " [" + retweets + "]" 
}

var max: Tweet = elem
def most(cur: Tweet) {
  if (cur.retweets > max.retweets) max=cur
}
foreach(most( _ ))

But this not compile :

var max: Tweet = elem
foreach( if ( _.retweets > max.retweets) max=_ )

Why ?

I got this error:

Error:(157, 19) missing parameter type for expanded function ((x$1) => x$1.retweets.$greater(max.retweets))
    foreach( if ( _.retweets > max.retweets) max=_ )

Error:(157, 14) type mismatch;
 found   : Unit
 required: objsets.Tweet => Unit
    foreach( if ( _.retweets > max.retweets) max=_ )

Answer 1

The reason for that, is that each underscore refers to the next argument, so for instance, if you have a function defined like this:

def foo(f: (Int, Int) => Int)

You can use anonymous function using exactly two underscores:

// Valid:
foo(_ + _)

// Invalid:
foo(_ + 1)
foo(_ + _ + _)

So in your example, if you want to reuse the argument in the closure given to foreach function, you must call it this way:

foreach(i => if (i.retweets > max.retweets) max = i)