Structs

A struct is the simplest of the three structural types: a record of named values. To say that a point has an x and a y, use a struct.

STRUCT Point:
    PUBLIC x: Integer
    PUBLIC y: Integer

A STRUCT declaration is the keyword, a name, a colon and the members on the lines below. Give a member a struct type to nest structs. By convention the name starts with an upper-case letter, like every type in the language.

Members and visibility

Every member of a structural type (struct, PROTOCOL or CLASS) begins with a visibility keyword. The next two pages reuse this rule, so it is introduced here.

Keyword	Reachable from
PUBLIC	anywhere
PRIVATE	only inside the declaring type
PROTECTED	inside the type and its subtypes

Visibility keywords are always upper-case and always come first:

STRUCT UUID:
    PRIVATE raw: String

After the visibility comes the member name, then the rest of the member. There are two kinds:

• a property: visibility name: Type
• a method: visibility name(params): ReturnType, marked by the parentheses

PUBLIC id: Integer                 # property
PUBLIC vertices(): Array[Vertex]   # method

Structs hold data

A struct may only hold properties. It describes state, a set of typed fields and nothing more. Methods belong to protocols and classes.

STRUCT Domain/Entity/User:
    PUBLIC id: Integer
    PUBLIC name: String
    PUBLIC email: String
    PUBLIC roles: Array[String]
    PUBLIC manager: ?Integer

Every member above is a property. The composite types appear here: roles is an array of strings and manager is optional, so it may be absent. Named fields with precise, composable types are what a struct is for.

note

To attach behaviour to data, use a CLASS, which holds both properties and methods. Keep STRUCT for data on its own. The restriction tells a reader that the type is state and nothing else.