Interfaces

An interface is an abstract type that specifies the behavior of types that implement the interface. Interfaces declare the required functions and fields, the access control for those declarations, and preconditions and postconditions that implementing types need to provide.

There are three kinds of interfaces:

• Structure interfaces: implemented by structures

• Resource interfaces: implemented by resources

• Contract interfaces: implemented by contracts

Structure, resource, and contract types may implement multiple interfaces.

Nominal typing applies to composite types that implement interfaces. This means that a type only implements an interface if it has explicitly declared it.

Interfaces consist of the function and field requirements that a type implementing the interface must provide implementations for. Interface requirements, and therefore also their implementations, must always be at least public.

Variable field requirements may be annotated to require them to be publicly settable.

Function requirements consist of the name of the function, parameter types, an optional return type, and optional preconditions and postconditions.

Field requirements consist of the name and the type of the field. Field requirements may optionally declare a getter requirement and a setter requirement, each with preconditions and postconditions.

Calling functions with preconditions and postconditions on interfaces instead of concrete implementations can improve the security of a program, as it ensures that even if implementations change, some aspects of them will always hold.