Type class pattern in Scala doesn't consider inheritance?

Question

I am designing an API using type classes in some cases however I have encountered a problem with implicit resolution. As shown below, if there is an implicit object for type A but an object of type B extends A is passed to the method, then an implicit object cannot be found. Is there a way to make this work or do callers have to put implicit objects into scope for each subclass?

Here is an example:

class A
class B extends A

class T[+X]

object T {
  implicit object TA extends T[A]
}

def call[X:T](x:X) = println(x)

// compiles
call(new A)
// doesn't compile
call(new B)

var a = new A
// compiles
call(a)

a = new B
// compiles
call(a)

val b = new B
// doesn't compile
call(b)

This fails to compile with the following output:

/private/tmp/tc.scala:16: error: could not find implicit value for evidence parameter of type this.T[this.B]
call(new B)
    ^
/private/tmp/tc.scala:28: error: could not find implicit value for evidence parameter of type this.T[this.B]
call(b)

Answer 1

The call call(new B) means call[B](new B)(tB) such that tb is of type T[B] or subclass of it. (A method that expects argument of type T can only expect T or subclass of T, e.g., def foo(s: String) cannot be called with argument of type Any). T[A] is not a subtype of T[B]

To fix, you can change T to be defined T[-X]. This means that the compiler will consider T[A] to be a subtype of T[B]

Answer 2

The following works fine:

scala> def call[X](x: X)(implicit evidence: T[X]<:<T[X])  = println(x)
call: [X](x: X)(implicit evidence: <:<[T[X],T[X]])Unit

scala> call(new A)
line0$object$$iw$$iw$A@1d869b2

scala> call(new B)
line2$object$$iw$$iw$B@b3a5d1

scala> val b = new B
b: B = B@30e4a7

scala> call(b)
line2$object$$iw$$iw$B@30e4a7

In your case compilation fails, because def call[X:T](x:X) = println(x) is treated as call: [X](x: X)(implicit evidence$1: T[X])Unit. In order to pass the subtype, you may use generalized type constraints.

Answer 3

Try this:

object T {
  implicit def TA[X <: A] = new T[X]
}

import T._

or simply:

implicit def TA[X <: A] = new T[X]