Consider this simplified snippet of some units of measurement functionality in Scala:
object UnitsEx {
case class Quantity[M <: MInt, T: Numeric](value: T) {
private val num = implicitly[Numeric[T]]
def *[M2 <: MInt](m: Quantity[M2, T]) =
Quantity[M, T](num.times(value, m.value))
}
implicit def measure[T: Numeric](v: T): Quantity[_0, T] = Quantity[_0, T](v)
implicit def numericToQuantity[T: Numeric](v: T): QuantityConstructor[T] =
new QuantityConstructor[T](v)
class QuantityConstructor[T: Numeric](v: T) {
def m = Quantity[_1, T](v)
}
sealed trait MInt
final class _0 extends MInt
final class _1 extends MInt
}
This snippet shows the usage and the compiler error I get currently:
import UnitsEx._
(1 m) * 1 // Works
1 * (1 m) // Doesn't work:
/*
<console>:1: error: overloaded method value * with alternatives:
(x: Double)Double <and>
(x: Float)Float <and>
(x: Long)Long <and>
(x: Int)Int <and>
(x: Char)Int <and>
(x: Short)Int <and>
(x: Byte)Int
cannot be applied to (UnitsEx.Quantity[UnitsEx._1,Int])
1 * (1 m)
^
*/
Wrapping the 1 with measure would fix the problem, but why isn't the implicit in scope applied?
If I remove the type parameter M like in the next snippet it starts working, although I can't see how that type parameter is related to implicit search:
object UnitsEx2 {
case class Quantity[T: Numeric](value: T) {
private val num = implicitly[Numeric[T]]
def *(m: Quantity[T]) = Quantity[T](num.times(value, m.value))
}
implicit def measure[T: Numeric](v: T): Quantity[T] = Quantity[T](v)
implicit def numericToQuantity[T: Numeric](v: T): QuantityConstructor[T] =
new QuantityConstructor[T](v)
class QuantityConstructor[T: Numeric](v: T) {
def m = Quantity[T](v)
}
}
Is this expected or a known limitation of the type checker?
