> For the complete documentation index, see [llms.txt](https://max-daunarovich.gitbook.io/flow-network/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://max-daunarovich.gitbook.io/flow-network/introduction/interfaces.md). # 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. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://max-daunarovich.gitbook.io/flow-network/introduction/interfaces.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.