flag pattern

Choice, exclusion.

The interpretation of a data element is controlled by another element.

At least two data elements, one acting as switch, the other as target.

Flags allow to easily add or enrich interpretations and to resolve ambiguity.

• One value from a list of possible values (encoding).
• A default value (void) which can be overridden.
• Markup to turn flags on and off, for instance <b>bold</b>.
• An identifier in a key-value record structure.
• An embedding that shows which elements are switches and which elements are targets.

• The bit as basic unit of all digital data is a choice between two possible values. It can be encoded by 0 and 1, by a given value and absence as default.
• Boolean and enumerated data types with type as switch and instance variable as target.
• An exclusive-or constraint in a schema to enforce choice of one possibility.
• A qualified value such as “Dublin, Ohio”.
• The statement “license: CC-BY-SA” in contrast to “CC-BY-SA” only.
• Annotations that modify interpretation, such as uncertainty and temporal flags.
• General rules how to read a data element (schema).
• A namespace is a flag that gives context to local identifiers.

• Possible values of the switch must be clear. If the first element is ambiguous, interpretation of the target element will also be.
• Default values are not always known or different values are assumed.
• Some flags don’t have an independent interpretation, for instance the value “miscellaneous” in a classification.
• In practice flags are created or extended ad-hoc, for instance as additional annotation or as additional switch value to add more interpretations.
• A flag may switch more then one element and an element may be influenced by a combination of flags. It should be made clear, what elements a flag refers to,
• Some flags allow nesting (for instance a qualifier of a qualifier), others do not make a difference when nested (<b><b>...</b></b> is simply bold).

• Most flags are based on a dependence between switch and target element.
• Instead of exclusion one could also disallow specific combinations of elements prohibition.
• A separator such as ‘:’ can simultaneously indicate a flag.
• The target data element may not be usable without the flag. In this case, a third element is derived from flag and target (derivation).
• If the switch consists of conditions which must be met to make use of the target element, a schema is more appropriate.

A flag is a form of derivation as the target’s interpretation is implied by the switch.

A schema defines which choices and exclusions are possible for some data elements. Schemas can further act as flags by telling how to read data.