Fields assigned to Literals or Expressions

ALFA fields can be assigned a default value or an expression. The value can be any ALFA type - scalars, maps, sets, UDTs etc. The ALFA parser will read and validate the value to ensure it complies with the declared typed.

A default value can be over-written with a different value at object construction time. Expression fields cannot be assigned a value - they are read-only. Expressions are evaluated as the last step before the object is constructed.

Fields can be assigned to service functions, allowing custom implementation to return a value that is assigned to the field. This is particularly useful in the case of values that are complex or can only be accessed natively

(See the VersionHost field in the Auditable user-defined type below).

See the example below on fields being assigned to various values.

trait Auditable {
  VersionTimestamp : datetime = timestamp()
  VersionHost : string = AuditHelper::getHost()
}

service AuditHelper() {
  getHost() : string
}

enum RequestType {
  New Cancel
}

record Request includes Auditable {
  Type : RequestType = RequestType.New
  Metadata : map< string, string > = { "source" : "unknown", "type" : "unknown" }
  TimeToLiveSecs : int = 600
  Payload : string
  MessageSize : long = len( Payload ) + 64
}