Provide collections that take custom comparators

[yjbanov]

A common use-case in applications is to use objects as keys whose equals/hashCode is different from the use-case at hand. The most common example that I keep hearing is you want to use database entities as map keys and set elements but you want to have ID-based identity, so for example {id: 1, name: "foo"} and {id: 1, name: "bar"} are considered equal.

Feature request: provide implementations of Map, Set and List that take a comparator function as a constructor parameter and use it to compare keys and elements.

[seaneagan]

Looks like there is already some work on this in the collection package:

https://api.dartlang.org/apidocs/channels/stable/#collection/dart-pkg-collection-equality

So Equalitys would take the place of Comparators. The only thing left would be to Provide the actual collections which take Equalitys in their constructors. Maybe there are already plans to add those to the collection package ?

[seaneagan]

Looks like the HashMap and HashSet constructors already support this:

https://api.dartlang.org/apidocs/channels/stable/#dart-collection.HashMap@id_HashMap- https://api.dartlang.org/apidocs/channels/stable/#dart-collection.HashSet@id_HashSet-

They take custom equals and hashCode Functions, so something like:

new HashSet<DBEntity>(equals: (e1, e2) => e1.id == e2.id, hashCode: (e) => e.id.hashCode);