Iterating over a range of generic type - iterator

Question

I have a trait
trait B {
type Index: Sized + Copy;
fn bounds(&self) -> (Self::Index, Self::Index);
}
I want to get all the Indexes within bounds:
fn iterate<T: B>(it: &T) {
let (low, high) = it.bounds();
for i in low..high {}
}
This won't work since there's no constraint that the type T can be "ranged" over, and the compiler says as much:
error[E0277]: the trait bound `<T as B>::Index: std::iter::Step` is not satisfied
--> src/main.rs:8:5
|
8 | for i in low..high {}
| ^^^^^^^^^^^^^^^^^^^^^ the trait `std::iter::Step` is not implemented for `<T as B>::Index`
|
= help: consider adding a `where <T as B>::Index: std::iter::Step` bound
= note: required because of the requirements on the impl of `std::iter::Iterator` for `std::ops::Range<<T as B>::Index>`
I tried adding the Step bound to Index
use std::iter::Step;
trait B {
type Index: Sized + Copy + Step;
fn bounds(&self) -> (Self::Index, Self::Index);
}
but apparently it isn't stable:
error: use of unstable library feature 'step_trait': likely to be replaced by finer-grained traits (see issue #42168)
--> src/main.rs:1:5
|
1 | use std::iter::Step;
| ^^^^^^^^^^^^^^^
error: use of unstable library feature 'step_trait': likely to be replaced by finer-grained traits (see issue #42168)
--> src/main.rs:4:32
|
4 | type Index: Sized + Copy + Step;
| ^^^^
Am I missing something or is it just not possible to do so right now?

Answer 1

If you want to require that a Range<T> can be iterated over, just use that as your trait bound:
trait Bounded {
type Index: Sized + Copy;
fn bounds(&self) -> (Self::Index, Self::Index);
}
fn iterate<T>(it: &T)
where
T: Bounded,
std::ops::Range<T::Index>: IntoIterator,
{
let (low, high) = it.bounds();
for i in low..high {}
}
fn main() {}

Answer 2

To do this kind of thing generically the num crate is helpful.
extern crate num;
use num::{Num, One};
use std::fmt::Debug;
fn iterate<T>(low: T, high: T)
where
T: Num + One + PartialOrd + Copy + Clone + Debug,
{
let one = T::one();
let mut i = low;
loop {
if i > high {
break;
}
println!("{:?}", i);
i = i + one;
}
}
fn main() {
iterate(0i32, 10i32);
iterate(5u8, 7u8);
iterate(0f64, 10f64);
}