6.6Traits🔗 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 t1t2))

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.