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Tour of Scala Type Bounds
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
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