Intersection Types
Interface types cannot be used in type annotations directly; instead they must be used as part of intersection types. An intersection type represents a value that conforms to all of the interfaces listed in the intersection.
The syntax of a intersection type is {U1, U2, ... Un},
where the types U1 to Un are the interfaces that the type conforms to.
The members and functions of any of the set of interfaces are available.
Intersection types are useful for writing functions that work on a variety of different inputs. For example, by using an intersection type for a parameter's type, the function may accept any concrete value that implements all the interfaces in that intersection. The value is restricted to the functionality of the intersection; if the function accidentally attempts to access other functionality, this is prevented by the static checker.
_41access(all) struct interface HasID {_41 access(all) let id: String_41}_41_41access(all) struct A: HasID {_41 access(all) let id: String_41_41 init(id: String) {_41 self.id = id_41 }_41}_41_41access(all) struct B: HasID {_41 access(all) let id: String_41_41 init(id: String) {_41 self.id = id_41 }_41}_41_41// Create two instances, one of type `A`, and one of type `B`._41// Both types conform to interface `HasID`, so the structs can be assigned_41// to variables with type `{HasID}`: Some resource type which only allows_41// access to the functionality of resource interface `HasID`_41_41let hasID1: {HasID} = A(id: "1")_41let hasID2: {HasID} = B(id: "2")_41_41// Declare a function named `getID` which has one parameter with type `{HasID}`._41// The type `{HasID}` is a short-hand for `AnyStruct{HasID}`:_41// Some structure which only allows access to the functionality of interface `HasID`._41//_41access(all) fun getID(_ value: {HasID}): String {_41 return value.id_41}_41_41let id1 = getID(hasID1)_41// `id1` is "1"_41_41let id2 = getID(hasID2)_41// `id2` is "2"
If more than two interfaces are present in an intersection type, any concrete value of that type must implement both of them:
_22access(all) struct interface HasMetadata {_22 access(all) let metadata: AnyStruct_22}_22_22access(all) struct C: HasID, HasMetadata {_22 access(all) let id: String_22 access(all) var metadata: AnyStruct_22_22 init(id: String) {_22 self.id = id_22 }_22_22 fun setMetadata(_ data: AnyStruct) {_22 self.metadata = data_22 }_22}_22_22// valid, because `C` implements both `HasID` and `HasMetadata`._22let hasID3: {HasID, HasMetadata} = C(id: "3")_22_22// valid, because `A` implements only `HasID`._22let hasID4: {HasID, HasMetadata} = A(id: "4")