"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 `Key`s 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|Entry]]. 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." see (`interface Map`, `interface List`, `interface Category`) by ("Gavin") tagged("Collections") shared interface Correspondence<in Key, out Item=Anything> given Key satisfies Object { "Returns the value defined for the given key, or `null` if there is no value defined for the given key." see (`function Correspondence.getAll`) shared formal Item? get(Key key); "Determines if there is a value defined for the given key." see (`function Correspondence.definesAny`, `function Correspondence.definesEvery`, `value Correspondence.keys`) shared formal Boolean defines(Key key); /*"Return a boolean value indicating whether there is an item with the given [[key]], together with the [[item|Item]] for that key, if any. value found -> item = map.lookup(key); if (found) { print(item); } This operation is useful for correspondences which contain [[null]] items, where a null return value for [[get]] does not distinguish between: - the case that there is no item for the given key, and - the case that `null` is the item for the given key." shared default see (`function defines`, `function get`) Boolean->Item? lookup(Key key) => if (defines(key)) then true -> get(key) else false -> null;*/ "The `Category` of all keys for which a value is defined by this `Correspondence`." see (`function Correspondence.defines`) shared default Category<Key> keys => object satisfies Category<Key> { contains(Key key) => defines(key); }; "Determines if this `Correspondence` defines a value for every one of the given keys." see (`function Correspondence.defines`) shared default Boolean definesEvery({Key*} keys) { for (key in keys) { if (!defines(key)) { return false; } } else { return true; } } "Determines if this `Correspondence` defines a value for any one of the given keys." see (`function Correspondence.defines`) shared default Boolean definesAny({Key*} keys) { for (key in keys) { if (defines(key)) { return true; } } else { return false; } } "Returns the items defined for the given keys, in the same order as the corresponding keys. For any key which does not have an item defined, the resulting stream contains the value `null`." see (`function Correspondence.get`) shared default Iterable<Item?,Absent> getAll<Absent> (Iterable<Key,Absent> keys) given Absent satisfies Null => { for (key in keys) get(key) }; }