About attributes

Attributes are the most important properties of classes. They are defined by four aspects (plus an optional Description used to document the attribute):

• Type, which can be Single (where the attribute has once value), or Set (where the attribute can have any number of values).

• Syntax, which determines the acceptable values for an attribute. The syntax of values is not checked by the framework, but it is important in defining the class. The syntax can take three values:

  
  None

  there are no restrictions on the value of the attribute.

  
  Boolean

  the value can either be "true" or "false".

  
  Enum

  the value can take one of a number of specified values.

• Operations, which determine the object operations that can be performed on a per-attribute basis.

  For single-valued attributes, possible operations are the following:

  
  get

  return the value of the attribute.

  
  replace

  replace the value with a provided attribute.

  
  replace with default

  replace the current value with a default value.

  For set-valued attributes, the following are also possible:

  
  add

  add a value to the set of values for the attribute.

  
  remove

  remove a value from the set.

• Filters. Object operations have the option to filter the set of objects on which they operate by comparing their attributes with given values. Each attribute can specify the filters that can be applied to it.

  For single-valued attributes, the possible filters are:

  
  Equal

  the value is equal to a given value.

  
  Greater

  the value is greater than a given value.

  
  Less

  the value is less than a given value.

  Set-valued attributes have the following possible filters:

  
  Present

  a given value is present in the attribute's value set.

  
  Superset

  a given set is a superset of the attribute's value set.

  
  Subset

  a given set is a subset of the attribute's value set.

  
  Intersect

  a given set has a non-empty intersection with the attribute's value set.