Question

Suppose I have a list of functions as so:

val funcList = List(func1: A => T, func2: B => T, func2: C => T)

(where func1, et al. are defined elsewhere)

I want to write a method that will take a value and match it to the right function based on exact type (match a: A with func1: A => T) or throw an exception if there is no matching function.

Is there a simple way to do this?

This is similar to what a PartialFunction does, but I am not able to change the list of functions in funcList to PartialFunctions. I am thinking I have to do some kind of implicit conversion of the functions to a special class that knows the types it can handle and is able to pattern match against it (basically promoting those functions to a specialized PartialFunction). However, I can’t figure out how to identify the “domain” of each function.

Thank you.

Answer 1

The usual answer to work around type erasure is to use the help of manifests. In your case, you can do the following:

abstract class TypedFunc[-A:Manifest,+R:Manifest] extends (A => R) {
  val retType: Manifest[_] = manifest[R]
  val argType: Manifest[_] = manifest[A]
}
object TypedFunc {
  implicit def apply[A:Manifest, R:Manifest]( f: A => R ): TypedFunc[A, R] = {
    f match {
      case tf: TypedFunc[A, R]  => tf
      case _ => new TypedFunc[A, R] { final def apply( arg: A ): R = f( arg ) }
    }
  }
}

def applyFunc[A, R, T >: A : Manifest]( funcs: Traversable[TypedFunc[A,R]] )( arg: T ): R = {
  funcs.find{ f => f.argType <:< manifest[T] } match {
    case Some( f ) => f( arg.asInstanceOf[A] )
    case _ => sys.error("Could not find function with argument matching type " + manifest[T])
  }
}

val func1 = { s: String => s.length }
val func2 = { l: Long => l.toInt }
val func3 = { s: Symbol => s.name.length }
val funcList = List(func1: TypedFunc[String,Int], func2: TypedFunc[Long, Int], func3: TypedFunc[Symbol, Int])

Testing in the REPL:

scala> applyFunc( funcList )( 'hello )
res22: Int = 5
scala> applyFunc( funcList )( "azerty" )
res23: Int = 6
scala> applyFunc( funcList )( 123L )
res24: Int = 123
scala> applyFunc( funcList )( 123 )
java.lang.RuntimeException: Could not find function with argument matching type Int
        at scala.sys.package$.error(package.scala:27)
        at .applyFunc(<console>:27)
        at .<init>(<console>:14)
        ...