Modelling of C++ concepts with Scala traits - scala

Question

There are some examples of using Scala traits like C++ concepts and Haskell type classes in paper «Type Classes as Objects and Implicits». I try to write something like InputIterator concept and find function in Scala:
concept InputIterator<typename Iter> {
typename value_type;
value_type operator*(Iter);
...
};
template<typename Iter, typename V>
requires InputIterator<Iter> && EqualityComparable<Iter::value_type, V>
Iter find(Iter first, Iter last, V v) {
while (first < last && *first != v)
++first;
return first;
}
I'm not sure I understand traits correctly. But still... There is InputIterator trait written in Scala (or more exactly — it's simplified analogue with methods used in find function):
trait InputIterator[Iter] {
type value_type
def <(a: Iter, b: Iter): Boolean
def ++(it: Iter): Unit
def *(it: Iter): value_type
}
EqualityComparable is clear:
trait EqualityComparable[S, T] {
def ==(s: S, t: T): Boolean
def !=(s: S, t: T): Boolean = !(s == t)
}
But what should we do with find? I would like to write something like this:
def find[Iter, V](first: Iter, last: Iter, x: V)(implicit iterator: InputIterator[Iter],
cmp: EqualityComparable[iterator.value_type, V]): Iter =
{
while (iterator.<(first, last) && cmp.!=(iterator.*(first), x))
iterator.++(first)
first
}
But it causes an error «illegal dependent method type». And I don't know how to «extract» abstract type value_type other way. So as a result I have got this code:
trait EqualityComparable[S, T] {
def ==(s: S, t: T): Boolean
def !=(s: S, t: T): Boolean = !(s == t)
}
trait InputIterator[Iter] {
type value_type
def <(a: Iter, b: Iter): Boolean
def ++(it: Iter): Unit
def *(it: Iter): value_type
}
trait VTInputIterator[Iter, VT] extends InputIterator[Iter] {
type value_type = VT
}
class ArrayListIterator[T](a: ArrayList[T], i: Int) {
val arr: ArrayList[T] = a
var ind: Int = i
def curr(): T = arr.get(ind)
def ++(): Unit = { ind += 1 }
override def toString() = "[" + ind.toString() + "]"
}
class InputIterArrList[T] extends VTInputIterator[ArrayListIterator[T], T]{
def <(a: ArrayListIterator[T], b: ArrayListIterator[T]) = {
if (a.arr == b.arr) a.ind < b.ind
else throw new IllegalArgumentException()
}
def ++(it: ArrayListIterator[T]): Unit = it.++()
def *(it: ArrayListIterator[T]) = it.curr()
}
object TestInputIterator extends Application{
def find[Iter, VT, V](first: Iter, last: Iter, x: V)(implicit iterator: VTInputIterator[Iter, VT],
cmp: EqualityComparable[VT, V]): Iter =
{
while (iterator.<(first, last) && cmp.!=(iterator.*(first), x))
iterator.++(first)
first
}
implicit object EqIntInt extends EqualityComparable[Int, Int] {
def ==(a: Int, b: Int): Boolean = { a == b }
}
implicit object inputIterArrListInt extends InputIterArrList[Int]{}
val len = 10;
var arr: ArrayList[Int] = new ArrayList(len);
for (i: Int <- 1 to len)
arr.add(i)
var arrFirst = new ArrayListIterator(arr, 0)
var arrLast = new ArrayListIterator(arr, len)
var r = find(arrFirst, arrLast, 7)
println(r)
}
Instead of abstract type we used type parameter VT in def find[Iter, VT, V].
So the question is: how it can be done better? And is it possible to use abstract type value_type without additional type parameter VT?

Answer 1

Change the signature of find to:
def find[Iter, V, II <: InputIterator[Iter]](first: Iter, last: Iter, x: V)(
implicit iterator: II, cmp: EqualityComparable[II#value_type, V]): Iter
That's probably what you want to express.
Note that your Scala code is really not the same as the C++ code. In C++ find uses Iter::value_type but in Scala you use InputIterator[Iter]#value_type.