Scala: Traversable foreach definition

问题

I suppose foreach method in Traversable trait is defined as follows:

def foreach[U](f: Elem => U)

Should it be defined as foreach[U,Elem](f:Elem =>U) as there are two types, Elem and U ?

回答1:

foreach[U](A => U) is a method in the trait Traversable[+A].

trait Traversable[+A] extends TraversableLike[A, ...] {
  ...
}

trait TraversableLike[+A, ...] extends ... {
  ...

  def foreach[U](f: A => U): Unit

  ...
}

https://github.com/scala/scala/blob/2.12.x/src/library/scala/collection/TraversableLike.scala#L124

回答2:

The Elem type comes from the generic type of the Traversable. The input to the function must the the same type as the elements of the Traversable so it doesn't make sense to redefine it at the method call site.

来源：https://stackoverflow.com/questions/46835288/scala-traversable-foreach-definition