implement contains_key, update, update_or

[keithamus]

PR Type

Feature

PR Checklist

• Tests for the changes have been added / updated.
• Documentation comments have been added / updated.
• A changelog entry has been made for the appropriate packages.
• Format code with the nightly rustfmt (cargo +nightly fmt).

Overview

It's quite common to want to update a value based on its existence in a map, and its current value. The typical case being incrementing counters. Right now this is a bit cumbersome, as you have to do a handle error states for serialize/deserialize and then an insert/update.

This PR adds contains_key, update, and update_or methods.

• contains_key checks if a key exists in the map, delegating to the underlying HashMap. This makes it a little easier than using .entries().
• update takes a key and a closure that takes the current value and sets the returned value. If the key doesn't exist, the closure is not called and the key is not inserted.
• update_or is similar to update, but takes a default value to insert if the key doesn't exist, kind of like an "emplace" or "upsert" or equivalent.

My core use case is probably update_or, but the other two feel like a natural fit in terms of ergonomics, and are kind of required to make update_or work.

I come with an open mind and I'm happy to hear feedback on the API design, or otherwise if you don't think this is a good fit feel free to close the PR.

[keithamus]

Thoughts on this one?

[robjtede]

to match std's map_or i'd like to have default value before the updater parameter