HashSet<E> constructor

Create a hash set using the provided equals as equality.

The provided equals must define a stable equivalence relation, and hashCode must be consistent with equals.

If equals or hashCode are omitted, the set uses the elements' intrinsic Object.== and Object.hashCode.

If you supply one of equals and hashCode, you should generally also supply the other.

Some equals or hashCode functions might not work for all objects. If isValidKey is supplied, it's used to check a potential element which is not necessarily an instance of E, like the argument to contains which is typed as Object?. If isValidKey returns false, for an object, the equals and hashCode functions are not called, and no key equal to that object is assumed to be in the map. The isValidKey function defaults to just testing if the object is an instance of E, which means that:

HashSet<int>(equals: (int e1, int e2) => (e1 - e2) % 5 == 0,
             hashCode: (int e) => e % 5)

does not need an isValidKey argument because it defaults to only accepting int values which are accepted by both equals and hashCode.

If neither equals, hashCode, nor isValidKey is provided, the default isValidKey instead accepts all values. The default equality and hashcode operations are assumed to work on all objects.

Likewise, if equals is identical, hashCode is identityHashCode and isValidKey is omitted, the resulting set is identity based, and the isValidKey defaults to accepting all keys. Such a map can be created directly using HashSet.identity.