How do I use Itertools to create all combinations of a range of Chrono dates?

Question

I'm trying to generate all combinations of a date range using Rust itertools but it says that trait bounds were not satisfied.

extern crate chrono;
extern crate itertools;

use itertools::Itertools;
use chrono::prelude::*;

fn main() {
    let min = NaiveDate::from_ymd(2018, 10, 1);
    let max = NaiveDate::from_ymd(2018, 10, 14);
    let combinations = (min..=max).combinations(5);
}

The error message:

error[E0599]: no method named `combinations` found for type `std::ops::RangeInclusive<chrono::NaiveDate>` in the current scope
  --> src/main.rs:46:36
   |
46 |     let combinations = (min..=max).combinations(5);
   |                                    ^^^^^^^^^^^^
   |
   = note: the method `combinations` exists but the following trait bounds were not satisfied:
           `std::ops::RangeInclusive<chrono::NaiveDate> : itertools::Itertools`
           `&std::ops::RangeInclusive<chrono::NaiveDate> : itertools::Itertools`
           `&mut std::ops::RangeInclusive<chrono::NaiveDate> : itertools::Itertools`

I would expect that Itertools were implemented for generic RangeInclusive. I am learning Rust so I might be missing something obvious.

Answer 1

Itertools, as the name suggests, works on Iterators. It is not possible to create an iterable range of a type from outside of the standard library in stable Rust (version 1.29).

Instead, we can create a custom iterator for a date range, based on looping over dates

extern crate chrono; // 0.4.6
extern crate itertools; // 0.7.8

use chrono::{Duration, NaiveDate};
use itertools::Itertools;
use std::mem;

struct DateRange(NaiveDate, NaiveDate);

impl Iterator for DateRange {
    type Item = NaiveDate;
    fn next(&mut self) -> Option<Self::Item> {
        if self.0 < self.1 {
            let next = self.0 + Duration::days(1);
            Some(mem::replace(&mut self.0, next))
        } else {
            None
        }
    }
}

fn main() {
    let min = NaiveDate::from_ymd(2018, 10, 1);
    let max = NaiveDate::from_ymd(2018, 10, 14);
    let combinations: Vec<_> = DateRange(min, max).combinations(5).collect();
    println!("{:?}", combinations)
}

• Iterating over a range of generic type
• Loop over date range