Polymorphic ranges

Any type that provides certain typeclasses supports range notation: For example, 2...<5 stands for the numbers at least 2 and smaller than 5. Such ranges support iteration with for .. in and can be converted into a list with PRange.toList. After importing Std.Data.Iterators, there will also be PRange.iter, which provides an iterator over the elements of the range.

In order to support ranges of a certain type α, multiple instances need to be implemented. An example of how this plays out can be found in Init.Data.Range.Polymorphic.Nat.

The typeclass system is experimental and will change soon, so at this point it is not recommended to provide custom ranges outside of the standard library.