6.6 Traits

► |  | The Racket Reference

1 | Language Model
2 | Notation for Documentation
3 | Syntactic Forms
4 | Datatypes
5 | Structures
6 | Classes and Objects
7 | Units
8 | Contracts
9 | Pattern Matching
10 | Control Flow
11 | Concurrency and Parallelism
12 | Macros
13 | Input and Output
14 | Reflection and Security
15 | Operating System
16 | Memory Management
17 | Unsafe Operations
18 | Running Racket
| Bibliography
| Index

▼ | 6 | Classes and Objects

6.1 | Creating Interfaces
6.2 | Creating Classes
6.3 | Creating Objects
6.4 | Field and Method Access
6.5 | Mixins
6.6 | Traits
6.7 | Object and Class Contracts
6.8 | Object Equality and Hashing
6.9 | Object Serialization
6.10 | Object Printing
6.11 | Object, Class, and Interface Utilities
6.12 | Surrogates

On this page:

trait
trait?
trait->mixin
trait-sum
trait-exclude
trait-exclude-field
trait-alias
trait-rename
trait-rename-field

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### 6.6 Traits🔗ℹ

(require racket/trait) | package: base

The bindings documented in this section are provided by the racket/trait library, not racket/base or racket.

A trait is a collection of methods that can be converted to a mixin and then applied to a class. Before a trait is converted to a mixin, the methods of a trait can be individually renamed, and multiple traits can be merged to form a new trait.

> syntax (trait trait-clause ...)
> trait-clause |  | = |  | (public maybe-renamed ...)
> |  | | |  | (pubment maybe-renamed ...)
> |  | | |  | (public-final maybe-renamed ...)
> |  | | |  | (override maybe-renamed ...)
> |  | | |  | (overment maybe-renamed ...)
> |  | | |  | (override-final maybe-renamed ...)
> |  | | |  | (augment maybe-renamed ...)
> |  | | |  | (augride maybe-renamed ...)
> |  | | |  | (augment-final maybe-renamed ...)
> |  | | |  | (inherit maybe-renamed ...)
> |  | | |  | (inherit/super maybe-renamed ...)
> |  | | |  | (inherit/inner maybe-renamed ...)
> |  | | |  | method-definition
> |  | | |  | (field field-declaration ...)
> |  | | |  | (inherit-field maybe-renamed ...)

Creates a trait. The body of a trait form is similar to the body of a class* form, but restricted to non-private method definitions. In particular, the grammar of maybe-renamed, method-definition, and field-declaration are the same as for class*, and every method-definition must have a corresponding declaration (one of public, override, etc.). As in class, uses of method names in direct calls, super calls, and inner calls depend on bringing method names into scope via inherit, inherit/super, inherit/inner, and other method declarations in the same trait; an exception, compared to class is that overment binds a method name only in the corresponding method, and not in other methods of the same trait. Finally, macros such as public* and define/public work in trait as in class.

External identifiers in trait, trait-exclude, trait-exclude-field, trait-alias, trait-rename, and trait-rename-field forms are subject to binding via define-member-name and define-local-member-name. Although private methods or fields are not allowed in a trait form, they can be simulated by using a public or field declaration and a name whose scope is limited to the trait form.

> procedure (trait? v) → boolean?
> v : any/c

Returns #t if v is a trait, #f otherwise.

> procedure (trait->mixin tr) → (class? . -> . class?)
> tr : trait?

Converts a trait to a mixin, which can be applied to a class to produce a new class. An expression of the form

> (trait->mixin
> (trait
> trait-clause ...))

is equivalent to

> (lambda (%)
> (class %
> trait-clause ...
> (super-new)))

Normally, however, a trait’s methods are changed and combined with other traits before converting to a mixin.

> procedure (trait-sum tr ...+) → trait?
> tr : trait?

Produces a trait that combines all of the methods of the given trs. For example,

> (define t1
> (trait
> (define/public (m1) 1)))
> (define t2
> (trait
> (define/public (m2) 2)))
> (define t3 (trait-sum t1 t2))

creates a trait t3 that is equivalent to

> (trait
> (define/public (m1) 1)
> (define/public (m2) 2))

but t1 and t2 can still be used individually or combined with other traits.

When traits are combined with trait-sum, the combination drops inherit, inherit/super, inherit/inner, and inherit-field declarations when a definition is supplied for the same method or field name by another trait. The trait-sum operation fails (the exn:fail:contract exception is raised) if any of the traits to combine define a method or field with the same name, or if an inherit/super or inherit/inner declaration to be dropped is inconsistent with the supplied definition. In other words, declaring a method with inherit, inherit/super, or inherit/inner, does not count as defining the method; at the same time, for example, a trait that contains an inherit/super declaration for a method m cannot be combined with a trait that defines m as augment, since no class could satisfy the requirements of both augment and inherit/super when the trait is later converted to a mixin and applied to a class.

> syntax (trait-exclude trait-expr id)

Produces a new trait that is like the trait result of trait-expr, but with the definition of a method named by id removed; as the method definition is removed, either an inherit, inherit/super, or inherit/inner declaration is added:

- A method declared with public, pubment, or public-final is replaced with an inherit declaration.

- A method declared with override or override-final is replaced with an inherit/super declaration.

- A method declared with augment, augride, or augment-final is replaced with an inherit/inner declaration.

- A method declared with overment is not replaced with any inherit declaration.

If the trait produced by trait-expr has no method definition for id, the exn:fail:contract exception is raised.

> syntax (trait-exclude-field trait-expr id)

Produces a new trait that is like the trait result of trait-expr, but with the definition of a field named by id removed; as the field definition is removed, an inherit-field declaration is added.

> syntax (trait-alias trait-expr id new-id)

Produces a new trait that is like the trait result of trait-expr, but the definition and declaration of the method named by id is duplicated with the name new-id. The consistency requirements for the resulting trait are the same as for trait-sum, otherwise the exn:fail:contract exception is raised. This operation does not rename any other use of id, such as in method calls (even method calls to identifier in the cloned definition for new-id).

> syntax (trait-rename trait-expr id new-id)

Produces a new trait that is like the trait result of trait-expr, but all definitions and references to methods named id are replaced by definitions and references to methods named by new-id. The consistency requirements for the resulting trait are the same as for trait-sum, otherwise the exn:fail:contract exception is raised.

> syntax (trait-rename-field trait-expr id new-id)

Produces a new trait that is like the trait result of trait-expr, but all definitions and references to fields named id are replaced by definitions and references to fields named by new-id. The consistency requirements for the resulting trait are the same as for trait-sum, otherwise the exn:fail:contract exception is raised.
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