In Scala, how to use Ordering[T] with List.min or List.max and keep code readable

Question

In Scala 2.8, I had a need to call List.min and provide my own compare function to get the value based on the second element of a Tuple2. I had to write this kind of code:

Answer 1

You could always define your own implicit conversion:

implicit def funToOrdering[T,R <% Ordered[R]](f: T => R) = new Ordering[T] {
  def compare(x: T, y: T) = f(x) compare f(y)
}

val list = ("a", 5) :: ("b", 3) :: ("c", 2) :: Nil

list.min { t: (String,Int) => t._2 }  // (c, 2)

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EDIT: Per @Dario's comments.

Might be more readable if the conversion wasn't implicit, but using an "on" function:

def on[T,R <% Ordered[R]](f: T => R) = new Ordering[T] {
  def compare(x: T, y: T) = f(x) compare f(y)
}

val list = ("a", 5) :: ("b", 3) :: ("c", 2) :: Nil

list.min( on { t: (String,Int) => t._2 } ) // (c, 2)