A trait is a named collection of related attibutes ( fields ). Traits are not intended to hold data, but simply model a group of fields. Container UDTs such as entity, key, record, and union can use traits by using the includes keyword.

Multiple assertAll blocks can be specified to perform validation. Asserts defined as part of a trait will be part of asserts in definitions that include the trait.

trait Shape {
    Colour : string

record Rectangle includes Shape {
    Width : int
    Height : int

record Circle includes Shape {
    Radius : int

By includes of Shape, Rectangle and Circle will contain the Colour attribute.

Traits can be used extensively to model UDTs with common attributes, or as simply a marker.

trait Datasource {

record File includes Datasource {
    Name : string

record Database includes Datasource {
    ConnString : string

A trait can include one or many traits.

trait HealthFacts {
    CalorieCount : int

trait FoodItem {
    ExpiryDate : date

trait FrozenFood {
    StorageTemperature : int

record FrozenPizza includes HealthFacts, FoodItem, FrozenFood {
    Toppings : list< string >

When combining traits in this way fields get aggregated, so FrozenPizza will contain 4 fields.

Diamond-shaped Dependencies

Consider the following change on the declaration above.

trait FrozenFood includes HealthFacts {
    StorageTemperature : int

By doing so, FrozenPizza has 2 possible paths to the CalorieCount field - via FrozenFood and direct.

ALFA emits a warning message when diamond-shaped dependency cases are encountered, however it will continue with a single CalorieCount field and it is not considered an error. In order to stop the warning, the field can be explicitly declared in the base of the diamond types - FrozenPizza in the example.

Multiple assertAll blocks can be specified to perform validation.