Scala factory method with generics
I have several objects for which I am trying to write a factory method.
Simplified:
case class a[a1,a2](j:a1, k:a2) {}
case class b[b1,b2](j:b1, k:b2) {}
I would like to create a method that would allow me to pass a type and get an instance of this class. I am trying to find something like this:
class myfactory[T] {
def make[K,L](p1: K, p2: L): T[K,L] = {
new T(p1,p2)
}
}
This clearly does not work (for various reasons, including "T cannot take parameters"), but is there an elegant solution to create something like this?
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0__ the answer is almost there. If you make a Factory[A[_,_]]
class, you're all set. Here's an example with standardized names:
// enable higher kinded types to prevent warnings
import scala.language.higherKinds
// our case classes
case class A[A1,A2](j:A1, k:A2)
case class B[B1,B2](j:B1, k:B2)
// Define our factory interface
trait Factory[T[_,_]] {
def make[P1,P2](p1: P1, p2: P2): T[P1,P2]
}
// Companion class makes factory easier to use
object Factory {
def apply[T[_, _]](implicit ev: Factory[T]) = ev
}
// Add implicit implementations of Factory[A]
implicit object AFactory extends Factory[A] {
def make[P1,P2](p1: P1, p2: P2): A[P1,P2] = A(p1, p2)
}
// Add implicit implementations of Factory[B]
implicit object BFactory extends Factory[B] {
def make[P1,P2](p1: P1, p2: P2): B[P1,P2] = B(p1, p2)
}
Now test the factory test in the REPL
scala> val a = Factory[A].make("one", 2)
a: A[String,Int] = A(one,2)
scala> val b = Factory[B].make(1, "two")
b: B[Int,String] = B(1,two)
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The keyword new
cannot be used with a type parameter, only a specific class, as the object instance is checked at compile time. You can get around this with reflection .
Probably the best approach is to provide specific factories.
eg.
trait Factory[A[_,_]] {
def make[P, Q](p: P, q: Q): A[P, Q]
}
case class a[a1,a2](j:a1, k:a2) {}
object AFactory extends Factory[a] {
def make[P, Q](p: P, q: Q): a[P, Q] = a(p, q)
}
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