Function in Ascent's Relation Read/Write Traits should return iterator type or iterator's impl trait?

[StarGazerM]

Now trait like RelIndexReadAll need explicitly declare the returned iterator type. Can we return a impl of Iterator instead? come up with correct iterator type is painful, return impl should have the same function but much simpler code?

pub trait RelIndexReadAll<'a> {
   type Key: 'a;
   type Value;
   // type ValueIteratorType: Iterator<Item = Self::Value> + 'a;
   // type AllIteratorType: Iterator<Item = (Self::Key, Self::ValueIteratorType)> + 'a;
   // fn iter_all(&'a self) -> Self::AllIteratorType;
   fn iter_all(&'a self) -> impl Iterator<Item = (Self::Key, impl Iterator<Item = Self::Value> + 'a)> + 'a;
   fn combine_delta(&'a self) -> bool { true }
}

(commented are original code in Ascent)

[s-arash]

I agree that it is painful to have to write the type name, and I've thought about using RPITIT (return position impl trait in trait) instead, but I think the better solution long-term is impl_trait_in_assoc_type (see rust-lang/rust#63063).

For now, you can use a nightly compiler, and enable this feature to avoid having to spell out the type name:

#![feature(impl_trait_in_assoc_type)]

use ascent::internal::RelIndexRead;
use std::collections::BTreeMap;

struct TestRelIndex<K, V>(BTreeMap<K, Vec<V>>);

impl<'a, K: 'a, V: 'a> RelIndexReadAll<'a> for TestRelIndex<K, V>
where
    K: Eq + Ord,
{
    type Key = &'a K;
    type Value = &'a V;

    type ValueIteratorType = impl Iterator<Item = Self::Value> + 'a;
    type AllIteratorType = impl Iterator<Item = (Self::Key, Self::ValueIteratorType)> + 'a;

    fn iter_all(&'a self) -> Self::AllIteratorType {
        self.0.iter().map(|(k, vs)| (k, vs.iter()))
    }
}