Scala类型界限=：=

Question

While this compiles:

  implicit class Container[T](val value:T) extends AnyVal{
    def addInt(x:Int)(implicit ev:T=:=Int) = value+x
  }

This complains about type mismatch, expected T , actual Int , as if it ignores the type bound.

  implicit class Container[T](val value:T=>Int) extends AnyVal{
    def addInt(x:Int)(implicit ev:T=:=Int) = value(x)
  }

Why?

Answer 1

Your type constraint is backwards, actually. T =:= Int provides implicit evidence that T is Int , but not exactly that Int is T . If you look at the declaration if =:= , you'll see that it only goes one way:

sealed abstract class =:=[From, To] extends (From => To) with Serializable

Your first example works because value is a T , and the constraint is T =:= Int , which implicitly converts T to Int . But for the second example, we need to feed a T to value: T => Int , so we need the other direction.

This works:

implicit class Container[T](val value: T => Int) extends AnyVal {
    def addInt(x: Int)(implicit ev: Int =:= T) = value(x)
}

The reason why your second example using Int <:< T also works is because <:< provides the implicit conversion from Int => T .

Answer 2

Oh probably the issue is the contra-variance of the function. This works now:

implicit class Container[T](val value:T=>Int) extends AnyVal{
    def addInt(x:Int)(implicit ev:Int<:<T) = value(x)
  }