type-bounds.scala

class A
class A2 extends A
class B

trait M[X]

//
// Upper Type Bound
//
def upperTypeBound[AA <: A](x: AA): A = x

// equivalently, require a implicit parameter of <:<[AA, A].
// This technique must be used if A is an abstract type from the
// enclosing scope.
def upperTypeBound2[AA](x: AA)(implicit ev: AA <:< A): A = x // compiles to ev(x)


//
// Lower Type Bound
//
def lowerTypeBound[AA >: A](x: AA, pred: (AA => Boolean)): Boolean = pred(x)

// equivalently, require a implicit parameter of <:<[A, AA].
// This technique must be used if A is an abstract type from the
// enclosing scope.
def lowerTypeBound2[AA >: A](x: AA, pred: (AA => Boolean))(implicit ev: A <:< AA): Boolean = pred(x)


//
// View Bound
//
def viewBound[AA <% A](x: AA): A = x

//compiles to:
def viewBound$[AA](x: AA)(implicit ev1$ : AA => A) = ev1$(x)


// 
// Context Bound
//
def contextBound[X: M](x: X) = x;

//compiles to:
def contextBound$[X](x: X)(implicit ev1$ : M[X]) = x

// In combination:

def combo[AA <: A >: B <% String <% Int : M : Ordering] = 0

// compiles to:
def combo[AA >: B <: A](implicit 
    evidence$1: (AA) => String,
    evidence$2: (AA) => Int,
    evidence$3: M[AA],
    evidence$4: Ordering[AA]
): Int = 0

//
// Usage
//
{
  {

    upperTypeBound(new A)
    upperTypeBound(new A2)
  }

  {
    val predA = (a: A) => true
    val predAny = (a: Any) => true
    lowerTypeBound(new A, predA)
    lowerTypeBound(new {}, predAny)
  }

  {
    implicit def String2A(s: String): A = new A
    viewBound("")
  }
  {
    implicit def MofA: M[A] = new M[A] {}
    contextBound(new A)
  }
}


implicit def int2str(i: Int): String = i.toString
def f1[T <% String](t: T) = 0
f1(1)

// This could also be expressed with a Context Bound, 
// with the help of a type alias representing functions 
// from type `F` to type `T`.
    
trait To[T] { type From[F] = F => T }           
def f2[T : To[String]#From](t: T) = 0           
f2(1)

// A context bound must be used with a type constructor of kind `* => *`. 
// However the type constructor `Function1` is of kind `(*, *) => *`. 
// The use of the type alias partially applies second type parameter 
// with the type `String`, yielding a type constructor of the correct kind
// for use as a context bound.

// There is a proposal to allow you to directly express partially applied types
// in Scala, without the use of the type alias inside a trait. You could then write:
//
//  def f3[T : [X](X => String)](t: T) = 0