> 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/nested-type-requirements.md).

# Nested Type Requirements

> 🚧 Status: Currently only contracts and contract interfaces support nested type requirements.

Interfaces can require implementing types to provide concrete nested types. For example, a resource interface may require an implementing type to provide a resource type.

```
// Declare a resource interface named `FungibleToken`.
//
// Require implementing types to provide a resource type named `Vault`
// which must have a field named `balance`.
//
resource interface FungibleToken {

    pub resource Vault {
        pub balance: Int
    }
}

// Declare a resource named `ExampleToken` that implements the `FungibleToken` interface.
//
// The nested type `Vault` must be provided to conform to the interface.
//
resource ExampleToken: FungibleToken {

    pub resource Vault {
        pub var balance: Int

        init(balance: Int) {
            self.balance = balance
        }
    }
}
```