Scala: specifying a generic type that is constructible from another - scala

Question

I would like to do something which, more or less, boils down to the following:
def foo[S](x: String): S = S(x) // S(x) does not compile
So that if I have:
case class S1(x:String)
case class S2(x:String)
...
case class Sn(x:String)
I can write foo[Sx]("bar") to get Sx("foo").
Is there any way to specify that an instance of a class should be constructible from an instance of another (in this example String) and to actually invoke the constructor in a generic way?

Answer 1

You may use reflection (see #Ben Reich answer for detailed answer)
def foo[S:ClassTag](x: String): S = {
val runtimeClass = implicitly[ClassTag[S]].runtimeClass
val constructor = runtimeClass.<get a constructor with single String argument>
constructor(x) .asInstanceOf[S]
}
Or a type class that can construct an instance:
trait CanConstruct[S] {
def apply(x:String):S
}
def foo[S:CanConstruct](x: String): S = {
val constructor = implicitly[CanConstruct[S]]
constructor(x).asInstanceOf[S]
}
UPD You would need an instance of the type class for every type you wish to construct:
implicit val s1constructor = new CanConstruct[S1] { def apply(x:String) = S1(x) }
...
Also it seems to be the case for conversion functions:
implicit val convertStringToS1 = S1(_)
implicit val convertStringToS2 = S2(_)
...

Answer 2

Using reflection:
import reflect.ClassTag
def foo[S: ClassTag](x: String) = implicitly[ClassTag[S]]
.runtimeClass
.getConstructors
.map(a => a -> a.getParameters)
.collectFirst {
case (constructor, Array(p)) if p.getType == classOf[String]
=> constructor.newInstance(x).asInstanceOf[S]
}
Which will return an Option[S], if the proper constructor is found.

Answer 3

I sort of solved it with a macro:
object Construct {
import scala.language.experimental.macros
import scala.reflect.macros.Context
def construct[A,B](x:B):A = macro constructImpl[A,B]
def constructImpl[A: c.WeakTypeTag,B](c:Context)(x:c.Expr[B]) = {
import c.universe._
c.Expr[A](q"""new ${c.weakTypeOf[A]}(${x.tree})""")
}
}
I now can write things like:
case class Foo(x:String)
case class Bar(x:Int)
construct[Foo,String]("foo")
construct[Bar,Int](42)
It would be nice to find a way to avoid having to write the second type parameter, though.