map/try_map for BoundedBTreeMap

[benluelo]

It would be useful if there was a way to iterate over an &mut BoundedBTreeMap and mutate each key in place. This wouldn't affect the length of the map at all, so it would be safe. Perhaps a try_map or something similar would be good as well, to allow for short circuiting.

I would be happy to take this on if it's deemed useful.

(BoundedBTreeMap link for the lazy:

substrate/primitives/runtime/src/bounded/bounded_btree_map.rs

Line 33 in 9a140a6

pub struct BoundedBTreeMap<K, V, S>(BTreeMap<K, V>, PhantomData<S>);

)

EDIT:

After some experimentation, I think this is a better API:

pub fn map<T>(self, mut f: impl FnMut((&K, V)) -> T) -> BoundedBTreeMap<K, T, S> {
    self.0
        .into_iter()
        .map(|(k, v)| {
            let t = f((&k, v));
            (k, t)
        })
        .try_collect()
        // SAFETY: No entries were added or removed.
        .unwrap()
}

pub fn try_map<T, E>(
    self,
    mut f: impl FnMut((&K, V)) -> Result<T, E>,
) -> Result<BoundedBTreeMap<K, T, S>, E> {
    Ok(self
        .0
        .into_iter()
        .map(|(k, v)| (f((&k, v)).map(|t| (k, t))))
        .collect::<Result<BTreeMap<_, _>, _>>()?
        .try_into()
        // SAFETY: No entries were added or removed.
        .unwrap())
}

[shawntabrizi]

Sounds reasonable. Would you like to work on this?

[benluelo]

Absolutely! I will open a PR shortly 👍

[benluelo]

#11869 implemented similar behaviour to this (adding an iter_mut() method to the Bounded* collections), but as per my comment (#11869 (comment)) I still think that this would be useful functionality to have. I often create one bounded collection from another with the same length, which results in unsightly .expect(...)s throughout the code.

[ggwpez]

One issue with iter_mut is that it doesn't allow you to transform the item's type in the collection […]

Yea… I guess that is a valid concern. Do you want to implement map/try_map as well?

[benluelo]

Yeah I can definitely do that! (#11869 actually started out as that 😅) I'll open a new PR for it 👍

[ggwpez]

I often create one bounded collection from another with the same length, which results in unsightly .expect(...)s throughout the code.

FYI there is defensive_truncate_from which can at-least get rid of these expects.