具有许多类型参数的Type类

Question

I have function with a few type parameters. How to tell compiler that these types are actually in type class?

trait Sizeable[-T, +R] {
  def mySize(x: T): R
}

implicit object StringSize extends Sizeable[String, Int] {
  def mySize(s: String) = s.length
}

def sum[T, R: Sizeable[T, R]](xs: List[T]): R =  xs.map(x => x.mySize)

The error is:

Error:(9, 14) A$A40.this.Sizeable[T,R] does not take type parameters 
def sum[T, R: Sizeable[T, R]](xs: List[T]): R =  xs.map(x => x.mySize);}

Answer 1

You can't use a context bound in this case. You'll have to write the implicit parameter explicitly:

def foo[T, R](xs: List[T])(implicit sizeable: Sizeable[T,R]): List[R] =
  xs.map(x => sizeable.mySize(x))