Abstract supertype of objects which associate values with keys. A Correspondence<Key,Item> may be a viewed as a partial function from domain Key to range Item, where some Keys have no Item.

Correspondence does not satisfy Category, since in some cases—List, for example—it is convenient to consider the subtype a Category of its indexed items, and in other cases—Map, for example—it is convenient to treat the subtype as a Category of its entries.

The item corresponding to a given key may be obtained from a Correspondence using the item operator:

value bg = settings["backgroundColor"] else white;

The get() operation and item operator result in an optional type, to reflect the possibility that there may be no item for the given key.