Simplifying type signatures using higher-kinded types

gistfile1.scala

scala> def f[M[_]](x: M[Int]) = x
f: [M[_]](x: M[Int])M[Int]

scala> val cb = (x: Int) => println(x)
cb: Int => Unit = <function1>

scala> f[Function1](cb)
<console>:11: error: Function1 takes two type parameters, expected: one
              f[Function1](cb)

scala> type Callback[T] = Function1[T, Unit]
defined type alias Callback

scala> f[Callback](cb)
res4: Int => Unit = <function1>