---
title: "SE-0030: Property Behaviors"
framework: swift-evolution
role: article
path: swift-evolution/0030-property-behavior-decls
---
# SE-0030: Property Behaviors
* Proposal: [SE-0030](0030-property-behavior-decls.md)
* Author: [Joe Groff](https://github.com/jckarter)
* Review Manager: [Doug Gregor](https://github.com/DougGregor)
* Status: **Withdrawn**
* Superseded by: [SE-0258](0258-property-wrappers.md)
* Decision Notes: [Rationale](https://forums.swift.org/t/rejected-se-0030-property-behaviors/1546)
## Introduction
There are property implementation patterns that come up repeatedly. Rather than hardcode a fixed set of patterns into the compiler, we should provide a general "property behavior" mechanism to allow these patterns to be defined as libraries.
[Swift Evolution Discussion](https://forums.swift.org/t/proposal-property-behaviors/594)
[Review](https://forums.swift.org/t/review-se-0030-property-behaviors/1385)
## Motivation
We've tried to accommodate several important patterns for properties with targeted language support, but this support has been narrow in scope and utility. For instance, Swift 1 and 2 provide `lazy` properties as a primitive language feature, since lazy initialization is common and is often necessary to avoid having properties be exposed as `Optional`. Without this language support, it takes a lot of boilerplate to get the same effect:
```swift class Foo { // lazy var foo = 1738 private var _foo: Int? var foo: Int { get { if let value = _foo { return value } let initialValue = 1738 _foo = initialValue return initialValue } set { _foo = newValue } } } ```
Building `lazy` into the language has several disadvantages. It makes the language and compiler more complex and less orthogonal. It's also inflexible; there are many variations on lazy initialization that make sense, but we wouldn't want to hardcode language support for all of them. For instance, some applications may want the lazy initialization to be synchronized, but `lazy` only provides single-threaded initialization. The standard implementation of `lazy` is also problematic for value types. A `lazy` getter must be `mutating`, which means it can't be accessed from an immutable value. Inline storage is also suboptimal for many memoization tasks, since the cache cannot be reused across copies of the value. A value-oriented memoized property implementation might look very different, using a class instance to store the cached value out-of-line in order to avoid mutation of the value itself.
There are important property patterns outside of lazy initialization. It often makes sense to have "delayed", once-assignable-then-immutable properties to support multi-phase initialization:
```swift class Foo { let immediatelyInitialized = "foo" var _initializedLater: String?
// We want initializedLater to present like a non-optional 'let' to user code; // it can only be assigned once, and can't be accessed before being assigned. var initializedLater: String { get { return _initializedLater! } set { assert(_initializedLater == nil) _initializedLater = newValue } } }
```
Implicitly-unwrapped optionals allow this in a pinch, but give up a lot of safety compared to a non-optional 'let'. Using IUO for multi-phase initialization gives up both immutability and nil-safety.
We also have other application-specific property features like `didSet`/`willSet` that add language complexity for limited functionality. Beyond what we've baked into the language already, there's a seemingly endless set of common property behaviors, including synchronized access, copying, and various kinds of proxying, all begging for language attention to eliminate their boilerplate.
## Proposed solution
I suggest we allow for **property behaviors** to be implemented within the language. A `var` declaration can specify its **behaviors** in square brackets after the keyword:
```swift var [lazy] foo = 1738 ```
which implements the property `foo` in a way described by the **property behavior declaration** for `lazy`:
```swift var behavior lazy: Value { var value: Value? = nil initialValue
mutating get { if let value = value { return value } let initial = initialValue value = initial return initial } set { value = newValue } } ```
Property behaviors can control the storage, initialization, and access of affected properties, obviating the need for special language support for `lazy`, observers, and other special-case property features.
## Examples
Before describing the detailed design, I'll run through some examples of potential applications for behaviors.
### Lazy
The current `lazy` property feature can be reimplemented as a property behavior.
```swift // Property behaviors are declared using the `var behavior` keyword cluster. public var behavior lazy: Value { // Behaviors can declare storage that backs the property. private var value: Value?
// Behaviors can bind the property's initializer expression with an // `initialValue` property declaration. initialValue
// Behaviors can declare initialization logic for the storage. // (Stored properties can also be initialized in-line.) init() { value = nil }
// Inline initializers are also supported, so `var value: Value? = nil` // would work equivalently.
// Behaviors can declare accessors that implement the property. mutating get { if let value = value { return value } let initial = initialValue value = initial return initial } set { value = newValue } } ```
Properties declared with the `lazy` behavior are backed by the `Optional`-typed storage and accessors from the behavior:
```swift var [lazy] x = 1738 // Allocates an Int? behind the scenes, inited to nil print(x) // Invokes the `lazy` getter, initializing the property x = 679 // Invokes the `lazy` setter ```
### Delayed Initialization
A property behavior can model "delayed" initialization behavior, where the DI rules for properties are enforced dynamically rather than at compile time. This can avoid the need for implicitly-unwrapped optionals in multi-phase initialization. We can implement both a mutable variant, which allows for reassignment like a `var`:
```swift public var behavior delayedMutable: Value { private var value: Value? = nil
get { guard let value = value else { fatalError("property accessed before being initialized") } return value } set { value = newValue } } ```
and an immutable variant, which only allows a single initialization like a `let`:
```swift public var behavior delayedImmutable: Value { private var value: Value? = nil
get { guard let value = value else { fatalError("property accessed before being initialized") } return value }
// Perform an initialization, trapping if the // value is already initialized. set { if let _ = value { fatalError("property initialized twice") } value = initialValue } } ```
This enables multi-phase initialization, like this:
```swift class Foo { var [delayedImmutable] x: Int
init() { // We don't know "x" yet, and we don't have to set it }
func initializeX(x: Int) { self.x = x // Will crash if 'self.x' is already initialized }
func getX() -> Int { return x // Will crash if 'self.x' wasn't initialized } } ```
### Property Observers
A property behavior can also approximate the built-in behavior of `didSet`/`willSet` observers, by declaring support for custom accessors:
```swift public var behavior observed: Value { initialValue
var value = initialValue
// A behavior can declare accessor requirements, the implementations of // which must be provided by property declarations using the behavior. // The behavior may provide a default implementation of the accessors, in // order to make them optional.
// The willSet accessor, invoked before the property is updated. The // default does nothing. mutating accessor willSet(newValue: Value) { }
// The didSet accessor, invoked before the property is updated. The // default does nothing. mutating accessor didSet(oldValue: Value) { }
get { return value }
set { willSet(newValue) let oldValue = value value = newValue didSet(oldValue) } } ```
A common complaint with `didSet`/`willSet` is that the observers fire on *every* write, not only ones that cause a real change. A behavior that supports a `didChange` accessor, which only gets invoked if the property value really changed to a value not equal to the old value, can be implemented as a new behavior:
```swift public var behavior changeObserved: Value { initialValue
var value = initialValue
mutating accessor didChange(oldValue: Value) { }
get { return value } set { let oldValue = value value = newValue if oldValue != newValue { didChange(oldValue) } } } ```
For example:
```swift var [changeObserved] x: Int = 1 { didChange { print("\(oldValue) => \(x)") } }
x = 1 // Prints nothing x = 2 // Prints 1 => 2 ```
(Note that, like `didSet`/`willSet` today, neither behavior implementation will observe changes through class references that mutate a referenced class instance without changing the reference itself. Also, as currently proposed, behaviors would force the property to be initialized in-line, which is not acceptable for instance properties. That's a limitation that can be lifted by future extensions.)
### Synchronized Property Access
Objective-C supports `atomic` properties, which take a lock on `get` and `set` to synchronize accesses to a property. This is occasionally useful, and it can be brought to Swift as a behavior. The real implementation of `atomic` properties in ObjC uses a global bank of locks, but for illustrative purposes (and to demonstrate referring to `self`) I'll use a per-object lock instead:
```swift // A class that owns a mutex that can be used to synchronize access to its // properties. public protocol Synchronizable: class { func withLock(@noescape body: () -> R) -> R }
// Behaviors can refer to a property's containing type using // the implicit `Self` generic parameter. Constraints can be // applied using a 'where' clause, like in an extension. public var behavior synchronized: Value { initialValue
var value: Value = initialValue
get { return self.withLock { return value } } set { self.withLock { value = newValue } } } ```
### `NSCopying`
Many Cocoa classes implement value-like objects that require explicit copying. Swift currently provides an `@NSCopying` attribute for properties to give them behavior like Objective-C's `@property(copy)`, invoking the `copy` method on new objects when the property is set. We can turn this into a behavior:
```swift public var behavior copying: Value { initialValue
// Copy the value on initialization. var value: Value = initialValue.copy()
get { return value } set { // Copy the value on reassignment. value = newValue.copy() } } ```
This is a small sampling of the possibilities of behaviors. Let's look at the proposed design in detail:
## Detailed design
### Property behavior declarations
A **property behavior declaration** is introduced by the `var behavior` contextual keyword cluster. The declaration is designed to resemble the syntax of a property using the behavior:
```text property-behavior-decl ::= attribute* decl-modifier* 'var' 'behavior' identifier // behavior name generic-signature? ':' type '{' property-behavior-member-decl* '}' ```
Inside the behavior declaration, standard initializer, property, method, and nested type declarations are allowed, as are **core accessor** declarations —`get` and `set`. **Accessor requirement declarations** and **initial value requirement declarations** are also recognized contextually within the declaration:
```text property-behavior-member-decl ::= decl property-behavior-member-decl ::= accessor-decl // get, set property-behavior-member-decl ::= accessor-requirement-decl property-behavior-member-decl ::= initial-value-requirement-decl ```
### Bindings within Behavior Declarations
Inside a behavior declaration, `self` is implicitly bound to the value that contains the property instantiated using this behavior. For a freestanding property at global or local scope, this will be the empty tuple `()`, and for a static or class property, this will be the metatype. Within the behavior declaration, the type of `self` is abstract and represented by the implicit generic type parameter `Self`. Constraints can be placed on `Self` in the generic signature of the behavior, to make protocol members available on `self`:
```swift protocol Fungible { typealias Fungus func funge() -> Fungus }
var behavior runcible: Value { get { return self.funge() } } ```
Lookup within `self` is *not* implicit within behaviors and must always be explicit, since unqualified lookup refers to the behavior's own members. `self` is immutable except in `mutating` methods, where it is considered an `inout` parameter unless the `Self` type has a class constraint. `self` cannot be accessed within inline initializers of the behavior's storage or in `init` declarations, since these may run during the container's own initialization phase.
Definitions within behaviors can refer to other members of the behavior by unqualified lookup, or if disambiguation is necessary, by qualified lookup on the behavior's name (since `self` is already taken to mean the containing value):
```swift var behavior foo: Value { var x: Int
init() { x = 1738 }
mutating func update(x: Int) { foo.x = x // Disambiguate reference to behavior storage } } ```
If the behavior includes *accessor requirement declarations*, then the declared accessor names are bound as functions with labeled arguments:
```swift var behavior fakeComputed: Value { accessor get() -> Value mutating accessor set(newValue: Value)
get { return get() } set { set(newValue: newValue) } } ```
Note that the behavior's own *core accessor* implementations `get { ... }` and `set { ... }` are *not* referenceable this way.
If the behavior includes an *initial value requirement declaration*, then the identifier `initialValue` is bound as a get-only computed property that evaluates the initial value expression for the property
### Properties and Methods in Behaviors
Behaviors may include property and method declarations. Any storage produced by behavior properties is expanded into the containing scope of a property using the behavior.
```swift var behavior runcible: Value { var x: Int = 0 let y: String = "" ... } var [runcible] a: Int
// expands to:
var `a.[runcible].x`: Int let `a.[runcible].y`: String var a: Int { ... } ```
For public behaviors, this is inherently *fragile*, so adding or removing storage is a breaking change. Resilience can be achieved by using a resilient type as storage. The instantiated properties must also be of types that are visible to potential users of the behavior, meaning that public behaviors must use storage with types that are either public or internal-with-availability, similar to the restrictions on inlineable functions.
Method and computed property implementations have only immutable access to `self` and their storage by default, unless they are `mutating`. (As with computed properties, setters are `mutating` by default unless explicitly marked `nonmutating`).
### `init` in Behaviors
The storage of a behavior must be initialized, either by inline initialization, or by an `init` declaration within the initializer:
```swift var behavior inlineInitialized: Value { var x: Int = 0 // initialized inline ... }
var behavior initInitialized: Value { var x: Int
init() { x = 0 } } ```
Behaviors can contain at most one `init` declaration, which must take no parameters. This `init` declaration cannot take a visibility modifier; it is always as visible as the behavior itself. Neither inline initializers nor `init` declaration bodies may reference `self`, since they will be executed during the initialization of a property's containing value.
### Initial Value Requirement Declaration
An *initial value requirement declaration* specifies that a behavior requires any property declared using the behavior to be declared with an initial value expression.
```swift initial-value-requirement-decl ::= 'initialValue' ```
The initial value expression from the property declaration is coerced to the property's type and bound to the `initialValue` identifier in the scope of the behavior. Loading from `initialValue` behaves like a get-only computed property, evaluating the expression every time it is loaded:
```swift var behavior evalTwice: Value { initialValue
get { // Evaluate the initial value twice, for whatever reason. _ = initialValue return initialValue } }
var [evalTwice] test: () = print("test")
// Prints "test" twice _ = evalTwice ```
A property declared with a behavior must have an initial value expression if and only if the behavior has an initial value requirement.
### Accessor Requirement Declarations
An *accessor requirement declaration* specifies that a behavior requires any property declared using the behavior to provide an accessor implementation. An accessor requirement declaration is introduced by the contextual `accessor` keyword:
```swift accessor-requirement-decl ::= attribute* decl-modifier* 'accessor' identifier function-signature function-body? ```
An accessor requirement declaration looks like, and serves a similar role to, a function requirement declaration in a protocol. A property using the behavior must supply an implementation for each of its accessor requirements that don't have a default implementation. The accessor names (with labeled arguments) are bound as functions within the behavior declaration:
```swift // Reinvent computed properties var behavior foobar: Value { accessor foo() -> Value mutating accessor bar(bas: Value)
get { return foo() } set { bar(bas: newValue) } }
var [foobar] foo: Int { foo { return 0 } bar { // Parameter gets the name 'bas' from the accessor requirement // by default, as with built-in accessors today. print(bas) } }
var [foobar] bar: Int { foo { return 0 } bar(myNewValue) { // Parameter name can be overridden as well print(myNewValue) } } ```
Accessor requirements can be made optional by specifying a default implementation:
```swift // Reinvent property observers var behavior observed: Value { // Requirements
initialValue mutating accessor willSet(newValue: Value) { // do nothing by default } mutating accessor didSet(oldValue: Value) { // do nothing by default }
// Implementation
init() { value = initialValue } get { return value } set { willSet(newValue: newValue) let oldValue = value value = newValue didSet(oldValue: oldValue) } } ```
Accessor requirements cannot take visibility modifiers; they are always as visible as the behavior itself.
Like methods, accessors are not allowed to mutate the storage of the behavior or `self` unless declared `mutating`. Mutating accessors can only be invoked by the behavior from other `mutating` contexts.
### Core Accessor Declarations
The behavior implements the property by defining its *core accessors*, `get` and optionally `set`. If a behavior only provides a getter, it produces read-only properties; if it provides both a getter and setter, it produces mutable properties (though properties that instantiate the behavior may still control the visibility of their setters). It is an error if a behavior declaration does not provide at least a getter.
### Using Behaviors in Property Declarations
Property declarations gain the ability to instantiate behavior, with arbitrary accessors:
```text property-decl ::= attribute* decl-modifier* core-property-decl core-property-decl ::= ('var' | 'let') behavior? pattern-binding ((',' pattern-binding)+ | accessors)? behavior ::= '[' visibility? decl-ref ']' pattern-binding ::= var-pattern (':' type)? inline-initializer? inline-initializer ::= '=' expr accessors ::= '{' accessor+ '}' | brace-stmt // see notes about disambiguation accessor ::= decl-modifier* decl-ref accessor-args? brace-stmt accessor-args ::= '(' identifier (',' identifier)* ')' ```
For example:
```swift public var [behavior] prop: Int { accessor1 { body() } behavior.accessor2(arg) { body() } } ```
If multiple properties are declared in the same declaration, the behavior apply to every declared property. `let` properties cannot yet use behaviors.
If the behavior requires accessors, the implementations for those accessors are taken from the property's accessor declarations, matching by name. To support future composition of behaviors, the accessor definitions can use qualified names `behavior.accessor`. If an accessor requirement takes parameters, but the definition in for the property does not explicitly name parameters, the parameter labels from the behavior's accessor requirement declaration are implicitly bound by default.
```swift var behavior foo: Value { accessor bar(arg: Int) ... }
var [foo] x: Int { bar { print(arg) } // `arg` implicitly bound }
var [foo] x: Int { bar(myArg) { print(myArg) } // `arg` explicitly bound to `myArg` } ```
If any accessor definition in the property does not match up to a behavior requirement, it is an error.
The shorthand for get-only computed properties is only allowed for computed properties that use no behaviors. Any property that uses behaviors with accessors must name all those accessors explicitly.
If a property with behaviors declares an inline initializer, the initializer expression is captured as the implementation of a computed, get-only property which is bound to the behavior's initializer requirement. If the behavior does not have a behavior requirement, then it is an error to use an inline initializer expression. Conversely, it is an error not to provide an initializer expression to a behavior that requires one.
Properties cannot be declared using behaviors inside protocols.
Under this proposal, even if a property with a behavior has an initial value expression, the type is always required to be explicitly declared. Behaviors also do not allow for out-of-line initialization of properties. Both of these restrictions can be lifted by future extensions; see the **Future directions** section below.
## Impact on existing code
By itself, this is an additive feature that doesn't impact existing code. However, with some of the future directions suggested, it can potentially obsolete `lazy`, `willSet`/`didSet`, and `@NSCopying` as hardcoded language features. We could grandfather these in, but my preference would be to phase them out by migrating them to library-based property behavior implementations. (Removing them should be its own separate proposal, though.)
## Alternatives considered
### Using a protocol (formal or not) instead of a new declaration
A previous iteration of this proposal used an informal instantiation protocol for property behaviors, desugaring a behavior into function calls, so that:
```swift var [lazy] foo = 1738 ```
would act as sugar for something like this:
```swift var `foo.[lazy]` = lazy(var: Int.self, initializer: { 1738 }) var foo: Int { get { return `foo.[lazy]`[varIn: self, initializer: { 1738 }] } set { `foo.[lazy]`[varIn: self, initializer: { 1738 }] = newValue } } ```
There are a few disadvantages to this approach:
- Behaviors would pollute the namespace, potentially with multiple global functions and/or types. - In practice, it would require every behavior to be implemented using a new (usually generic) type, which introduces runtime overhead for the type's metadata structures. - The property behavior logic ends up less clear, being encoded in unspecialized language constructs. - Determining the capabilities of a behavior relied on function overload resolution, which can be fiddly, and would require a lot of special case diagnostic work to get good, property-oriented error messages out of. - Without severely complicating the informal protocol, it would be difficult to support eager vs. deferred initializers, or allow mutating access to `self` concurrently with the property's own storage without violating `inout` aliasing rules. The code generation for standalone behavior decls can hide this complexity.
Making property behaviors a distinct declaration undeniably increases the language size, but the demand for something like behaviors is clearly there. In return for a new declaration, we get better namespacing, more efficient code generation, clearer, more descriptive code for their implementation, and more expressive power with better diagnostics. I argue that the complexity can pay for itself, today by eliminating several special-case language features, and potentially in the future by generalizing to other kinds of behaviors (or being subsumed by an all-encompassing macro system). For instance, a future `func behavior` could conceivably provide Python decorator-like behavior for transforming function bodies.
### "Template"-style behavior declaration syntax
John McCall proposed a "template"-like syntax for property behaviors, used in a previous revision of this proposal:
```swift behavior var [lazy] name: Value = initialValue { ... } ```
It's appealing from a declaration-follows-use standpoint, and provides convenient places to slot in name, type, and initial value bindings. However, this kind of syntax is unprecedented in Swift, and in initial review, was not popular.
### Declaration syntax for properties using behaviors
Alternatives to the proposed `var [behavior] propertyName` syntax include:
- A different set of brackets, `var (behavior) propertyName` or `var {behavior} propertyName`. Parens have the problem of being ambiguous with a tuple `var` declaration, requiring lookahead to resolve. Square brackets also work better with other declarations behaviors could be extended to apply to in the future, such as subscripts or functions - An attribute, such as `@behavior(lazy)` or `behavior(lazy) var`. This is the most conservative answer, but is clunky. - Use the behavior function name directly as an attribute, so that e.g. `@lazy` works. - Use a new keyword, as in `var x: T by behavior`. - Something on the right side of the colon, such as `var x: lazy(T)`. To me this reads like `lazy(T)` is a type of some kind, which it really isn't.
## Future directions
The functionality proposed here is quite broad, so to attempt to minimize the review burden of the initial proposal, I've factored out several aspects for separate consideration:
### Behaviors for immutable `let` properties
Since we don't have an effects system (yet?), `let` behavior implementations have the potential to invalidate the immutability assumptions expected of `let` properties, and it would be the programmer's responsibility to maintain them. We don't support computed `let`s for the same reason, so I suggest leaving `let`s out of property behaviors for now. `let behavior`s could be added in the future when we have a comprehensive design for immutable computed properties and/or functions.
### Type inference of properties with behaviors
There are subtle issues with inferring the type of a property using a behavior when the behavior introduces constraints on the property type. If you have something like this:
```swift var behavior uint16only: UInt16 { ... }
var [uint16only] x = 1738 ```
there are two, and possibly more, ways to define what happens:
- We type-check the initializer expression in isolation *before* resolving behaviors. In this case, `1738` would type-check by defaulting to `Int`, and then we'd raise an error instantiating the `uint16only` behavior, which requires a property to have type `UInt16`. - We apply the behaviors *before* type-checking the initializer expression, introducing generic constraints on the contextual type of the initializer. In this case, applying the `uint16only` behavior would constrain the contextual type of the initializer to `UInt16`, and we'd successfully type-check the literal as a `UInt16`.
There are merits and downsides to both approaches. To allow these issues to be given proper consideration, I'm subsetting them out by proposing to first require that properties with behaviors always declare an explicit type.
### Composing behaviors
It is useful to be able to compose behaviors, for instance, to have a lazy property with observers that's also synchronized. Relatedly, it is useful for subclasses to be able to `override` their inherited properties by applying behaviors over the base class implementation, as can be done with `didSet` and `willSet` today. Linear composition can be supported by allowing behaviors to stack, each referring to the underlying property beneath it by `super` or some other magic binding. However, this form of composition can be treacherous, since it allows for "incorrect" compositions of behaviors. One of `lazy • synchronized` or `synchronized • lazy` is going to do the wrong thing. This possibility can be handled somewhat by allowing certain compositions to be open-coded; John McCall has suggested that every composition ought to be directly implemented as an entirely distinct behavior. That of course has an obvious exponential explosion problem; it's infeasible to anticipate and hand-code every useful combination of behaviors. These issues deserve careful separate consideration, so I'm leaving behavior composition out of this initial proposal.
### Deferred evaluation of initialization expressions
This proposal does not suggest changing the allowed operations inside initialization expressions; in particular, an initialization of an instance property may not refer to `self` or other instance properties or methods, due to the potential for the expression to execute before the value is fully initialized:
```swift struct Foo { var a = 1 var b = a // Not allowed var c = foo() // Not allowed
func foo() { } } ```
This is inconvenient for behaviors like `lazy` that only ever evaluate the initial value expression after the true initialization phase has completed, and where it's desirable to reference `self` to lazily initialize. Behaviors could be extended to annotate the initializer as "deferred", which would allow the initializer expression to refer to `self`, while preventing the initializer expression from being evaluated at initialization time. (If we consider behaviors to be essentially always fragile, this could be inferred from the behavior implementation.)
### Out-of-line initialization with behaviors
This proposal also does not allow for behaviors that support out-of-line initialization, as in:
```swift func foo() { // Out-of-line local variable initialization var [behavior] x: Int x = 1 }
struct Foo { var [behavior] y: Int
init() { // Out-of-line instance property initialization y = 1 } } ```
This is a fairly serious limitation for instance properties. There are a few potential approaches we can take. One is to allow a behavior's `init` logic to take an out-of-line initialization as a parameter, either directly or by having a different constraint on the initializer requirement that *only* allows it to be referred to from `init` (the opposite of "deferred"). It can also be supported indirectly by linear behavior composition, if the default root `super` behavior for a stack of properties defaults to a plain old stored property, which can then follow normal initialization rules. This is similar to how `didSet`/`willSet` behave today. However, this would not allow behaviors to change the initialization behavior in any way.
### Binding the name of a property using the behavior
There are a number of clever things you can do with the name of a property if it can be referenced as a string, such as using it to look up a value in a map, to log, or to serialize. We could conceivably support a `name` requirement declaration:
```swift var behavior echo: Value { name: String
get { return name } }
var [echo] echo: String print(echo) // => echo ```
### Overloading behaviors
It may be useful for behaviors to be overloadable, for instance to give a different implementation to computed and stored variants of a concept:
```swift // A behavior for stored properties... var behavior foo: Value { initialValue
var value: Value = initialValue get { ... } set { ... } }
// Same behavior for computed properties... var behavior foo: Value { initialValue
accessor get() -> Value accessor set(newValue: Value)
get { ... } set { ... } } ```
We could resolve overloads by accessors, type constraints on `Value`, and/or initializer requirements. However, determining what this overload signature should be, and also the exciting interactions with type inference from initializer expressions, should be a separate discussion.
### Accessing "out-of-band" behavior members
It is useful to add out-of-band operations to a property that aren't normal members of its formal type, for instance, to `clear` a lazy property to be recomputed later, or to reset a property to an implementation-defined default value. This is useful, but it complicates the design of the feature. Aside from the obvious surface-level concerns of syntax for accessing these members, this also exposes behaviors as interface rather than purely an implementation detail, meaning their interaction with resilience, protocols, class inheritance, and other abstractions needs to be designed. It's also a fair question whether out-of- band members should be tied to behaviors at all--it could be useful to design out-of-band members as an independent feature independent with behaviors.