6.6 Traits🔗ℹ

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.

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.

(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.

(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.

• 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.