Interface Implementation

Declaring that a type implements (conforms) to an interface is done in the type declaration of the composite type (e.g., structure, resource): The kind and the name of the composite type is followed by a colon (:) and the name of one or more interfaces that the composite type implements.

This will tell the checker to enforce any requirements from the specified interfaces onto the declared type.

A type implements (conforms to) an interface if it declares the implementation in its signature, provides field declarations for all fields required by the interface, and provides implementations for all functions required by the interface.

The field declarations in the implementing type must match the field requirements in the interface in terms of name, type, and declaration kind (e.g. constant, variable) if given. For example, an interface may require a field with a certain name and type, but leaves it to the implementation what kind the field is.

The function implementations must match the function requirements in the interface in terms of name, parameter argument labels, parameter types, and the return type.

// Declare a resource named `ExampleToken` that has to implement
// the `FungibleToken` interface.
//
// It has a variable field named `balance`, that can be written
// by functions of the type, but outer scopes can only read it.
//
pub resource ExampleToken: FungibleToken {

    // Implement the required field `balance` for the `FungibleToken` interface.
    // The interface does not specify if the field must be variable, constant,
    // so in order for this type (`ExampleToken`) to be able to write to the field,
    // but limit outer scopes to only read from the field, it is declared variable,
    // and only has public access (non-settable).
    //
    pub var balance: Int

    // Implement the required initializer for the `FungibleToken` interface:
    // accept an initial balance and initialize the `balance` field.
    //
    // This implementation satisfies the required postcondition.
    //
    // NOTE: the postcondition declared in the interface
    // does not have to be repeated here in the implementation.
    //
    init(balance: Int) {
        self.balance = balance
    }

    // Implement the required function named `withdraw` of the interface
    // `FungibleToken`, that withdraws an amount from the token's balance.
    //
    // The function must be public.
    //
    // This implementation satisfies the required postcondition.
    //
    // NOTE: neither the precondition nor the postcondition declared
    // in the interface have to be repeated here in the implementation.
    //
    pub fun withdraw(amount: Int): @ExampleToken {
        self.balance = self.balance - amount
        return create ExampleToken(balance: amount)
    }

    // Implement the required function named `deposit` of the interface
    // `FungibleToken`, that deposits the amount from the given token
    // to this token.
    //
    // The function must be public.
    //
    // NOTE: the type of the parameter is `{FungibleToken}`,
    // i.e., any resource that implements the resource interface `FungibleToken`,
    // so any other token – however, we want to ensure that only tokens
    // of the same type can be deposited.
    //
    // This implementation satisfies the required postconditions.
    //
    // NOTE: neither the precondition nor the postcondition declared
    // in the interface have to be repeated here in the implementation.
    //
    pub fun deposit(_ token: @{FungibleToken}) {
        if let exampleToken = token as? ExampleToken {
            self.balance = self.balance + exampleToken.balance
            destroy exampleToken
        } else {
            panic("cannot deposit token which is not an example token")
        }
    }
}

// Declare a constant which has type `ExampleToken`,
// and is initialized with such an example token.
//
let token <- create ExampleToken(balance: 100)

// Withdraw 10 units from the token.
//
// The amount satisfies the precondition of the `withdraw` function
// in the `FungibleToken` interface.
//
// Invoking a function of a resource does not destroy the resource,
// so the resource `token` is still valid after the call of `withdraw`.
//
let withdrawn <- token.withdraw(amount: 10)

// The postcondition of the `withdraw` function in the `FungibleToken`
// interface ensured the balance field of the token was updated properly.
//
// `token.balance` is `90`
// `withdrawn.balance` is `10`

// Deposit the withdrawn token into another one.
let receiver: @ExampleToken <- // ...
receiver.deposit(<-withdrawn)

// Run-time error: The precondition of function `withdraw` in interface
// `FungibleToken` fails, the program aborts: the parameter `amount`
// is larger than the field `balance` (100 > 90).
//
token.withdraw(amount: 100)

// Withdrawing tokens so that the balance is zero does not destroy the resource.
// The resource has to be destroyed explicitly.
//
token.withdraw(amount: 90)

The access level for variable fields in an implementation may be less restrictive than the interface requires. For example, an interface may require a field to be at least public (i.e. the pub keyword is specified), and an implementation may provide a variable field which is public, but also publicly settable (the pub(set) keyword is specified).

pub struct interface AnInterface {
    // Require the implementing type to provide a publicly readable
    // field named `a` that has type `Int`. It may be a constant field,
    // a variable field, or a synthetic field.
    //
    pub a: Int
}

pub struct AnImplementation: AnInterface {
    // Declare a publicly settable variable field named `a` that has type `Int`.
    // This implementation satisfies the requirement for interface `AnInterface`:
    // The field is at least publicly readable, but this implementation also
    // allows the field to be written to in all scopes.
    //
    pub(set) var a: Int

    init(a: Int) {
        self.a = a
    }
}