Type constraints example 1

type-constraints-example1.scala

/*
 * Problem - Constraining a parameter to only take a specific type, not its subtype
 * 
 * https://herringtondarkholme.github.io/2014/09/30/scala-operator/
 */
class Foo()
class Bar() extends Foo

def onlyTakesFoo[A](f: A)(implicit ev: A =:= Foo) = "Foo"
def onlyTakesBar[A](f: A)(implicit ev: A =:= Bar) = "Bar"

onlyTakesFoo(new Foo())
onlyTakesFoo(new Bar) //won't compile
onlyTakesBar(new Bar)
onlyTakesBar(new Foo) //won't compile