Version: Next

Attachments

warning

⚠️ This section describes a feature that is not yet released on Mainnet.

Attachments are a feature of Cadence designed to allow developers to extend a struct or resource type (even one that they did not declare) with new functionality, without requiring the original author of the type to plan or account for the intended behavior.

Declaring Attachments

Attachments are declared with the attachment keyword, which would be declared using a new form of composite declaration: attachment <Name> for <Type>: <Conformances> { ... }, where the attachment functions and fields are declared in the body. As such, the following would be examples of legal declarations of attachments:

access(all) attachment Foo for MyStruct {
    // ...
}
attachment Bar for MyResource: MyResourceInterface {
    // ...
}
attachment Baz for MyInterface: MyOtherInterface {
    // ...
}

Specifying the kind (struct or resource) of an attachment is not necessary, as its kind will necessarily be the same as the type it is extending. Note that the base type may be either a concrete composite type or an interface. In the former case, the attachment is only usable on values specifically of that base type, while in the case of an interface the attachment is usable on any type that conforms to that interface.

Unlike other type declarations, attachments may use either an access(all) access modifier, or an access(M) modifier, where M is the name of an entitlement mapping. When attachments are defined with an access(all) modifier, members on the attachment may not use any entitlements in their access modifiers, and any references to that attachment are always unauthorized. When attachments are defined with an an entitlement mapping, members on the attachments may use any entitlements in the range of that mapping, and any references to that attachments will have their authorization depend on the entitlements present on the base type on which they are accessed.

The body of the attachment follows the same declaration rules as composites. In particular, they may have both field and function members, and any field members must be initialized in an initializer. Only resource-kinded attachments may have resource members, and such members must be explicitly handled in the destroy function. The self keyword is available in attachment bodies, but unlike in a composite, self is a reference type, rather than a composite type: In an attachment declaration for A, the type of self would be &A, rather than A like in other composite declarations. If the attachment declaration uses an access(all) access modifier, the self reference is always unauthorized, whereas if it uses an access(M) access modifier, the self reference is fully-entitled to the range of M.

If a resource with attachments on it is destroyed, the destroy functions of all its attachments are all run in an unspecified order; destroy should not rely on the presence of other attachments on the base type in its implementation. The only guarantee about the order in which attachments are destroyed in this case is that the base resource will be the last thing destroyed.

Within the body of an attachment, there is also a base keyword available, which contains a reference to the attachment's base value; that is, the composite to which the attachment is attached. Its type, therefore, is a reference to the attachment's declared base type. So, for an attachment declared access(all) attachment Foo for Bar, the base field of Foo would have type &Bar.

So, for example, this would be a valid declaration of an attachment:

access(all) resource R {
    access(all) let x: Int
    init (_ x: Int) {
        self.x = x
    }
    access(all) fun foo() { ... }
}
access(all) attachment A for R {
    access(all) let derivedX: Int
    init (_ scalar: Int) {
        self.derivedX = base.x * scalar
    }
    access(all) fun foo() {
        base.foo()
    }
}

For the purposes of external mutation checks or access control, the attachment is considered a separate declaration from its base type. A developer cannot, therefore, access any access(self) fields (or access(contract) fields if the base was defined in a different contract to the attachment) on the base value, nor can they mutate any array or dictionary typed fields.

access(all) resource interface SomeInterface {
    access(all) let b: Bool
    access(self) let i: Int
    access(all) let a: [String]
}
access(all) attachment SomeAttachment for SomeContract.SomeStruct {
    access(all) let i: Int
    init(i: Int) {
        if base.b {
            self.i = base.i // cannot access `i` on the `base` value
        } else {
            self.i = i
        }
    }
    access(all) fun foo() {
        base.a.append("hello") // cannot mutate `a` outside of the composite where it was defined
    }
}

By default, the base reference is unauthorized, and thus entitled-access members on the base type are inaccessible in the attachment. If the author of the attachment wishes to have access to entitled-access members on the base type, they must declare that their attachment requires certain entitlements to the base, using require entitlement E syntax. Required entitlements must be valid entitlements for the base type, and requiring entitlements in the attachment declaration will impose additional requirements when the attachment is attached, as described below. So, for example:

entitlement mapping M {
    E -> F
}
resource R {
    access(E) fun foo() {
        //...
    }
}
access(M) attachment A for R {
    require entitlement E
    access(all) fun bar() {
        base.foo() // available because `E` is required above, and thus `base` is type `auth(E) &R`.
    }
}

Attachment Types

An attachment declared with access(all) attachment A for C { ... } will have a nominal type A.

It is important to note that attachments are not first class values, and as such their usage is limited in certain ways. In particular, their types cannot appear outside of a reference type. So, for example, given an attachment declaration attachment A for X {}, the types A, A?, [A] and fun(): A are not valid type annotations, while &A, &A?, [&A] and fun(): &A are valid.

Creating Attachments

An attachment is created using an attach expression, where the attachment is both initialized and attached to the base value in a single operation. Attachments are not first-class values; they cannot exist independently of a base value, nor can they be moved around on their own. This means that an attach expression is the only place in which an attachment constructor can be called. Tightly coupling the creation and attaching of attachment values helps to make reasoning about attachments simpler for the user. Also for this reason, resource attachments do not need an explicit <- move operator when they appear in an attach expression.

An attach expression consists of the attach keyword, a constructor call for the attachment value, the to keyword, and an expression that evaluates to the base value for that attachment. Any arguments required by the attachment's initializer are provided in its constructor call.

access(all) resource R {}
access(all) attachment A for R {
    init(x: Int) {
        //...
    }
}
// ...
let r <- create R()
let r2 <- attach A(x: 3) to <-r

The expression on the right-hand side of the to keyword must evaluate to a composite value whose type is a subtype of the attachment's base, and is evaluated before the call to the constructor on the left side of to. This means that the base value is available inside of the attachment's initializer, but it is important to note that the attachment being created will not be accessible on the base (see the accessing attachments section below) until after the constructor finishes executing.

access(all) resource interface I {}
access(all) resource R: I {}
access(all) attachment A for I {}
// ...
let r <- create R() // has type @R
let r2 <- attach A() to <-r // ok, because `R` is a subtype of `I`, still has type @R

Because attachments are stored on their bases by type, there can only be one attachment of each type present on a value at a time. Cadence will raise a runtime error if a user attempts to add an attachment to a value when one it already exists on that value. The type returned by the attach expression is the same type as the expression on the right-hand side of the to; attaching an attachment to a value does not change its type.

If an attachment has required entitlements to its base, those entitlements must be explicitly provided in the attach expression using an additional with syntax. So, for example, if an attachment A declared for R requires entitlements E and F, it can be attached to an r of type @R like so:

let rWithA <- attach A() to <-r with (E, F)

Accessing Attachments

Attachments are accessed on composites via type-indexing: composite values function like a dictionary where the keys are types and the values are attachments. So given a composite value v, one can look up the attachment named A on v using indexing syntax:

let a = v[A] // has type `&A?`

This syntax requires that A is a nominal attachment type, and that v has a composite type that is a subtype of A's declared base type. As mentioned above, attachments are not first-class values, so this indexing returns a reference to the attachment on v, rather than the attachment itself. If the attachment with the given type does not exist on v, this expression returns nil.

Because the indexed value must be a subtype of the indexing attachment's base type, the owner of a resource can restrict which attachments can be accessed on references to their resource using interface types, much like they would do with any other field or function. E.g.

struct R: I {}
struct interface I {}
attachment A for R {}
fun foo(r: &{I}) {
    r[A] // fails to type check, because `{I}` is not a subtype of `R`
}

Hence, if the owner of a resource wishes to allow others to use a subset of its attachments, they can create a capability to that resource with a borrow type that only allows certain attachments to be accessed.

If an attachment is declared with an access(all) modifier, accessing one this way will always produce an unauthorized reference to the attachment. However, if the attachment is declared with an access(M) modifier, where M is some entitlement mapping, then the authorization of the resulting reference will depend on the authorization of the accessed value.

So, for example, given a declaration

entitlement E
entitlement F
entitlement G
entitlement H
entitlement mapping M {
    E -> F
    G -> H
}
resource R {}
access(M) attachment A for R {}

when A is accessed on an owned value of type @R, it will be fully-authorized to the domain of M, having a type of auth(F, H) &A. However, if A is accessed on an auth(E) &R reference, then it will only have a auth(F) &A type. If A is accessed on an unauthorized &R reference, then it will yield an unauthorized &A type.

Removing Attachments

Attachments can be removed from a value with a remove statement. The statement consists of the remove keyword, the nominal type for the attachment to be removed, the from keyword, and the value from which the attachment is meant to be removed.

The value on the right-hand side of from must be a composite value whose type is a subtype of the attachment type's declared base.

Before the statement executes, the attachment's destroy function (if present) will be executed. After the statement executes, the composite value on the right-hand side of from will no longer contain the attachment. If the value does not contain t, this statement has no effect.

Attachments may be removed from a type in any order, so users should take care not to design any attachments that rely on specific behaviors of other attachments, as there is no to require that an attachment depend on another or to require that a type has a given attachment when another attachment is present.

If a resource containing attachments is destroyed, all its attachments will be destroyed in an arbitrary order.

Declaring Attachments​

Attachment Types​

Creating Attachments​

Accessing Attachments​

Removing Attachments​

Declaring Attachments

Attachment Types

Creating Attachments

Accessing Attachments

Removing Attachments