Scala：在参数通配符上调用方法/访问值

Question

How do I call a method on or access a value of an argument wildcard? Eg in this example I want to find the max "rev" value of all F objects.

scala> case class F(rev:Long)
defined class F

scala> List(F(1),F(2),F(3))
res3: List[F] = List(F(1), F(2), F(3))

scala> res3.foldLeft(0L){math.max(_,_.rev)}
<console>:11: error: wrong number of parameters; expected = 2
              res3.foldLeft(0L){math.max(_,_.rev)}
                                    ^

Answer 1

You can't use wildcards here and need to give names to the arguments:

res3.foldLeft(0L){(x,y) => math.max(x,y.rev)}

Note that it would be the same if you had a function foo taking 1 argument instead of math.max : foo(_.rev) is the same as foo(x => x.rev) and not x => foo(x.rev) .

The problem is the scope of wildcards. math.max(_,_.rev) expands to (I think) x => math.max(x, y => y.rev) . Since this function only has one parameter, you get this error.