Factory method with dependent type - scala

Question

I am struggling with dependent types in Scala 2.11.7. Here is the context:
trait Counter {
type T
def zero: T
def incr( t: T ): T
}
object IntCounter extends Counter {
type T = Int
val zero = 0
def incr( t: Int ) = t + 1
}
case class Foo( counter: Counter )
def twice( foo: Foo )( cntr: foo.counter.T ): foo.counter.T =
foo.counter.incr( foo.counter.incr( cntr ) )
So far so good, everything compiles. However I would like to add an object which contains both a Foo instance and a corresponding counter state. For instance:
trait Bar {
val foo: Foo
val current: foo.counter.T
}
The definition is OK (provided I use abstract vals). But I cannot define a factory method (aka smart constructors). All my naive attempts fail to compile. For example, the definition:
def bar( f: Foo )( cntr: f.counter.T ): Bar = new Bar {
val foo = f
val current = cntr
}
fails to compile with the error:
xxx: overriding value current in trait Bar of type this.foo.counter.T;
value current has incompatible type
val current = cntr
^
How can I force the compiler to understand that both type are indeed the same ? I could solve the problem with generics instead but I prefer to avoid this option if possible.

Answer 1

If bar is single constructor for Bar, you could solve it like this:
sealed trait Foo { //can't be case class because you can't even call `twice` method then
type Ctr <: Counter
type Z <: Ctr#T
val counter: Ctr
}
def foo[Ct <: Counter](ctr: Ct): Foo{type Ctr = Ct} = new Foo {
type Ctr = Ct
type Z = ctr.T
val counter = ctr
}
sealed trait Bar {
type Ctrr <: Counter
type TT <: Counter#T
val foo: Foo {type Ctr = Ctrr}
val current: TT
}
def bar[Ct <: Counter]( f: Foo{type Ctr = Ct} )( cntr: f.counter.T )(implicit ev: Ct =:= f.Ctr): Bar {type Ctrr = Ct; type TT = f.counter.T} = new Bar {
type Ctrr = Ct
type TT = f.counter.T
val foo = f
val current = cntr
}
Usage:
scala> val br = bar(foo(IntCounter))(5)
br: Bar{type Ctrr = IntCounter.type; type TT = Int} = $anon$1#35267fd4
scala> br.foo.counter.incr(br.current)
res41: Int = 6
The disadvantage here is that you have to specify (and maintain) the same root-type between TT and foo members wherever you create a new Foo instance.