Attachments
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:
_12access(all)_12attachment Foo for MyStruct {_12 // ..._12}_12_12attachment Bar for MyResource: MyResourceInterface {_12 // ..._12}_12_12attachment Baz for MyInterface: MyOtherInterface {_12 // ..._12}
Like all other type declarations, attachments may only be declared with all access.
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.
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.
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 reference to A, rather than A like in other composite declarations.
The specific entitlements that this reference has depends on the access modifier associated with the member function in which the self reference
appears, and is explained in more detail below.
If a resource with attachments on it is destroyed, all its attachments are destroyed in an unspecified order.
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:
_29access(all)_29resource R {_29_29 access(all)_29 let x: Int_29_29 init (_ x: Int) {_29 self.x = x_29 }_29_29 access(all)_29 fun foo() { ... }_29}_29_29access(all)_29attachment A for R {_29 _29 access(all)_29 let derivedX: Int_29_29 init (_ scalar: Int) {_29 self.derivedX = base.x * scalar_29 }_29_29 access(all)_29 fun foo() {_29 base.foo()_29 }_29}
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.
_31access(all)_31resource interface SomeInterface {_31_31 access(all)_31 let b: Bool_31_31 access(self)_31 let i: Int_31_31 access(all)_31 let a: [String]_31}_31access(all)_31attachment SomeAttachment for SomeContract.SomeStruct {_31_31 access(all)_31 let i: Int_31_31 init(i: Int) {_31 if base.b {_31 self.i = base.i // cannot access `i` on the `base` value_31 } else {_31 self.i = i_31 }_31 }_31_31 access(all)_31 fun foo() {_31 base.a.append("hello") // cannot mutate `a` outside of the composite where it was defined_31 }_31}
Within an attachment's member function, the base and self references are entitled to the same entitlements that the function's access modifier specifies.
E.g., in an attachment declared as access(all) attachment A for R, within a definition of a function access(E) fun foo(),
the type of base would be auth(E) &R, and the type of self would be auth(E) &A. Thus the following definition would work:
_14resource R {_14 access(E)_14 fun foo() {_14 //..._14 }_14}_14_14access(all)_14attachment A for R {_14 access(E)_14 fun bar() {_14 base.foo() // available because `E` is required above, and thus `base` is type `auth(E) &R`._14 }_14}
while this would not:
_16resource R {_16 access(E)_16 fun foo() {_16 //..._16 }_16}_16_16access(all)_16attachment A for R {_16_16 access(self)_16 fun bar() {_16 base.foo() // unavailable because this function has `self` access, and thus `base` only is type `&R`._16 }_16_16}
Note that as a result of how entitlements are propagated to the self and base values here, attachment definitions can only support
the same entitlements that their base values support; i.e. some attachment A defined for R can only use an entitlement E in its definition
if R also uses E in its definition (or the definition of any interfaces to which it conforms).
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.
_13access(all)_13resource R {}_13_13access(all)_13attachment A for R {_13 init(x: Int) {_13 //..._13 }_13}_13_13// ..._13let r <- create R()_13let 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.
_12access(all)_12resource interface I {}_12_12access(all)_12resource R: I {}_12_12access(all)_12attachment A for I {}_12_12// ..._12let r <- create R() // has type @R_12let 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.
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:
_10let 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.
The set of entitlements to which the result of an attachment access is authorized is the same as the set of entitlements to which the base
value is authorized. So, for example, given the following definition for A:
_25entitlement E _25entitlement F _25_25resource R {_25 access(E)_25 fun foo() {_25 // ... _25 }_25_25 access(F)_25 fun bar() { _25 // ... _25 }_25}_25_25attachment A for R {_25 access(E | F)_25 fun qux() { _25 // ... _25 }_25}_25_25// ... _25_25let a = v[A]!
When v has type &R, the resulting type of a will be an unauthorized &A.
Contrarily, if v has type auth(E) &R, then the type of a will be authorized to the same: auth(E) &A.
Finally, when v is not a reference (i.e. an owned value of type R), then a will be "fully entitled" to A; it will be granted
all the entitlements mentioned by A, i.e. in this case it will have type auth(E, F) &A.
This is roughly equivalent to the behavior of the Identity entitlement mapping; indeed, attachments can be thought of
as being Identity-mapped fields on their base value.
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.
E.g., to remove an A attachment from some resource r whose type supports that attachment:
_10remove A from r
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 the attachment that appears after the remove keyword, 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.