Question

I observed a difference in Scala’s type inference when applied to def and val.

Using def, I can define an abstract nullary method const returning some value of type Int => Int. When implementing const with a function literal, I need not supply a parameter type, as it can be inferred by the compiler:

trait D {
  def const: Int => Int
}
object D extends D {
  def const = i => i + 1
}

This is fine. (On the downside, a new function instance is being created for every access to D.const.)

Now consider an analogous construction using val:

trait V {
  val const: Int => Int
}
object V extends V {
  val const = i => i + 1
}

This will not compile, failing with

error: missing parameter type
   val const = i => i + 1
               ^

Why?

Answer 1

As of Scala 2.9.1, this is “as speced”. Citing Martin Odersky from SI-2742:

For methods, return types in inherited abstract methods are taken as expected type of the right hand side. For values there is no such rule. So this would be a spec enhancement request, as I see it.

The ticket is low priority and has remained unchanged since its first discussion in late 2009, so it seems unlikely to change anytime soon.