まあ、いくつかのゆがみを気にしないのであれば、結局のところ、関数はクラスであるという事実を利用できます...
// abstract class MyIntToString extends (Int => String) // declare here if you want
// to use from different classes
// EDIT: f1 and f2 are now val instead of def as per comment below
// by @Régis Jean-Gilles
class A {
abstract class MyIntToString private[A]() extends (Int => String)
// if MyIntToString is declared here
// with a constructor private to the enclosing class
// you can ensure it's used only within A (credit goes to @AlexeyRomanov
// for his comment below)
val f1 = new MyIntToString {
def apply(i: Int) = i.toString + " f1"
}
val f2= new MyIntToString {
def apply(i: Int) = i.toString + " f2"
}
}
def foo(f: A#MyIntToString) = f(42) // f: MyIntToString if MyIntToString not nested in A
val a = A
今、あなたはできる:
scala> foo((new A).f1)
res1: String = 42 f1
scala> foo((new A).f2)
res2: String = 42 f2
Int => String
しかし foo は署名を受け入れません
scala> val itos = (i:Int) => i.toString
itos: Int => String = <function1>
scala> foo(itos)
<console>:11: error: type mismatch;
found : Int => String
required: MyIntToString
foo(itos)
^