Abstract supertype of objects which associate values with keys. Correspondence does not satisfy Category, since in some cases—List, for example—it is convenient to consider the subtype a Category of its values, 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 is no item for the given key.