使用apply方法的泛型类型的Scala工厂？

Question

假设我有以下的特征定义的接口，并需要几个类型参数...

trait Foo[A, B] { 

    // implementation details not important 

} 

我想使用的同伴对象的一个​​工厂性状的具体实现。我也想强迫用户使用Foo接口，而不是子类所以我躲在同伴对象的具体实现，像这样：

object Foo { 

    def apply[A, B](thing: Thing): Foo[A, B] = { 
    ??? 
    } 

    private case class FooImpl[A1, B1](thing: Thing) extends Foo[A1, B1] 

    private case class AnotherFooImpl[A2, B1](thing: Thing) extends Foo[A2, B1] 

} 

我希望能够使用工厂如下：

val foo = Foo[A1, B1](thing) // should be an instance of FooImpl 

val anotherFoo = Foo[A2, B1](thing) // should be an instance of AnotherFooImpl 

如何实现apply方法来做到这一点？这SO post似乎接近标记。

Answer 1

使用TypeTags（为了克服类型参数的擦除），我们可以根据传入apply方法的类型参数调用相应的隐藏实现，如下所示。它正确地实例化相应的实施方案，而是为Foo类型信息丢失，实际上它的未来的一些垃圾一样_202等？我不知道为什么会发生这种情况，以及如何保留Foo的正确类型。也许有人可以抛开这一点。

trait Foo[A,B] 
object Foo { 
    def apply[A: TypeTag, B: TypeTag](thing: Thing) = 
    if(typeTag[A] == typeTag[Int]) 
     FooImpl(thing) 
    else if(typeTag[A] == typeTag[String]) 
     AnotherFooImpl(thing) 
    else 
     new Foo[Double,Double] {} 

    private case class FooImpl(thing: Thing) extends Foo[Int, String] 
    private case class AnotherFooImpl(thing: Thing) extends Foo[String, String] 
    } 

Foo[Int,String](new Thing) // Foo[_202, _203] = FooImpl($sess.cmd123$Thing@50350b75) 

The actual types for _203 and _203 are: ??? 
// type _203 >: String with _201, type _202 >: Int with _200 


Foo[String,String](new Thing) //Foo[_202, _203] = AnotherFooImpl($sess.cmd123$Thing@51d80d6) 

Answer 2

如何：

trait Foo[A, B] 
trait Factory[A, B] { 
    def make(thing: Thing): Foo[A, B] 
} 

class Thing 

object Foo { 
def apply[A, B](thing: Thing)(implicit ev: Factory[A, B]) = ev.make(thing) 

private case class FooImpl[A, B](thing: Thing) extends Foo[A, B] 
private case class AnotherFooImpl[A, B](thing: Thing) extends Foo[A, B] 

implicit val fooImplFactory: Factory[Int, String] = new Factory[Int, String] { 
    override def make(thing: Thing): Foo[Int, String] = new FooImpl[Int, String](thing) 
} 

implicit val anotherFooImplFactory: Factory[String, String] = new Factory[String, String] { 
    override def make(thing: Thing): Foo[String, String] = new AnotherFooImpl[String, String](thing) 
} 

现在：

def main(args: Array[String]): Unit = { 
    import Foo._ 

    val fooImpl = Foo[Int, String](new Thing) 
    val anotherFooImpl = Foo[String, String](new Thing) 

    println(fooImpl) 
    println(anotherFooImpl) 
} 

产量：

FooImpl(testing.X$Thing@4678c730) 
AnotherFooImpl(testing.X$Thing@c038203)