Scala: type arguments do not conform to class type parameter bounds

Question

My code

object TestApp extends App {   
  class A[T <: Ordered[T]] {
    def check(f: T, s: T) = f > s
  }    
  object A {
    def apply[T] = new A[T]
  }   
  A[String].check("ab", "aa")    
}

throws an exception:

Error:(13, 9) type arguments [T] do not conform to class A's type parameter bounds [T <: Ordered[T]]
    def apply[T] = new A[T]
        ^

But I don't understand why. For me, it seems ok. I have a class with type variable bounded with Ordered. In companion object, I'm trying to create an instance of A. But I'm not sure if my companion object has an access to the type variable of class A.

How can I fix it?

Ende Neu · Accepted Answer · 2016-01-28 21:43:34Z

7

String doesn't extend Ordered in scala, it's just an alias for java.lang.String, plus your apply doesn't conform to the context bound you have in A (you accept any T there), you can work around requiring an implicit Ordering for that type:

scala> :pa
// Entering paste mode (ctrl-D to finish)

class A[T](implicit ord: Ordering[T]) {
  def check(f: T, s: T) = ord.gt(f, s)
}

object A {
  def apply[T](implicit ord: Ordering[T]) = new A[T]
}

A[String].check("ab", "aa")

// Exiting paste mode, now interpreting.

defined class A
defined object A
res0: Boolean = true

To show you that implicit conversions don't work for context bounds I made a small example:

scala> :pa
// Entering paste mode (ctrl-D to finish)

trait A
case class AA() extends A
case class CC()
implicit def conv(a: CC): A = AA() 

def f[T <: A](x: T): T = x
f(CC())

// Exiting paste mode, now interpreting.

<console>:23: error: inferred type arguments [CC] do not conform to method f's type parameter bounds [T <: A]
     f(CC())

As you can see the implicit I defined wasn't applied to CC to match the context bound.

edited Jan 28, 2016 at 21:43

answered Jan 28, 2016 at 21:02

Ende Neu

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5 Comments

Finkelson Over a year ago

But String has implicit converter to StringOps which does extend Ordered trait.

Finkelson Over a year ago

implicit def augmentString(x: String): StringOps = new StringOps(x) in Predef

Finkelson Over a year ago

I can write class B { def check(f: String, s: String) = f > s } and it works.

Ende Neu Over a year ago

@Finkelson Agreed, and in fact if you do something like val p: StringOps = "123" the implicit conversion from String to StringOps kicks in, but it won't to satisfy a context bound.

Ende Neu Over a year ago

@Finkelson I added a small example showing that implicits don't kick in to satisfy context bounds.

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