Interface Declaration

Interfaces are declared using the struct, resource, or contract keyword, followed by the interface keyword, the name of the interface, and the requirements, which must be enclosed in opening and closing braces.

Field requirements can be annotated to require the implementation to be a variable field, by using the var keyword; require the implementation to be a constant field, by using the let keyword; or the field requirement may specify nothing, in which case the implementation may either be a variable field, a constant field, or a synthetic field.

Field requirements and function requirements must specify the required level of access. The access must be at least be public, so the pub keyword must be provided. Variable field requirements can be specified to also be publicly settable by using the pub(set) keyword.

Interfaces can be used in types. This is explained in detail in the section Interfaces in Types. For now, the syntax {I} can be read as the type of any value that implements the interface I.

// Declare a resource interface for a fungible token.
// Only resources can implement this resource interface.
//
pub resource interface FungibleToken {

    // Require the implementing type to provide a field for the balance
    // that is readable in all scopes (`pub`).
    //
    // Neither the `var` keyword, nor the `let` keyword is used,
    // so the field may be implemented as either a variable field,
    // a constant field, or a synthetic field.
    //
    // The read balance must always be positive.
    //
    // NOTE: no requirement is made for the kind of field,
    // it can be either variable or constant in the implementation.
    //
    pub balance: Int {
        set(newBalance) {
            pre {
                newBalance >= 0:
                    "Balances are always set as non-negative numbers"
            }
        }
    }

    // Require the implementing type to provide an initializer that
    // given the initial balance, must initialize the balance field.
    //
    init(balance: Int) {
        pre {
            balance >= 0:
                "Balances are always non-negative"
        }
        post {
            self.balance == balance:
                "the balance must be initialized to the initial balance"
        }

        // NOTE: The declaration contains no implementation code.
    }

    // Require the implementing type to provide a function that is
    // callable in all scopes, which withdraws an amount from
    // this fungible token and returns the withdrawn amount as
    // a new fungible token.
    //
    // The given amount must be positive and the function implementation
    // must add the amount to the balance.
    //
    // The function must return a new fungible token.
    // The type `{FungibleToken}` is the type of any resource
    // that implements the resource interface `FungibleToken`.
    //
    pub fun withdraw(amount: Int): @{FungibleToken} {
        pre {
            amount > 0:
                "the amount must be positive"
            amount <= self.balance:
                "insufficient funds: the amount must be smaller or equal to the balance"
        }
        post {
            self.balance == before(self.balance) - amount:
                "the amount must be deducted from the balance"
        }

        // NOTE: The declaration contains no implementation code.
    }

    // Require the implementing type to provide a function that is
    // callable in all scopes, which deposits a fungible token
    // into this fungible token.
    //
    // No precondition is required to check the given token's balance
    // is positive, as this condition is already ensured by
    // the field requirement.
    //
    // The parameter type `{FungibleToken}` is the type of any resource
    // that implements the resource interface `FungibleToken`.
    //
    pub fun deposit(_ token: @{FungibleToken}) {
        post {
            self.balance == before(self.balance) + token.balance:
                "the amount must be added to the balance"
        }

        // NOTE: The declaration contains no implementation code.
    }
}

Note that the required initializer and functions do not have any executable code.

Struct and resource Interfaces can only be declared directly inside contracts, i.e. not inside of functions. Contract interfaces can only be declared globally and not inside contracts.