alias ~ Possibility, required, mandatory.
idea ~ An element may be present but it can also be absent.
context ~ A data element as part of a schema or embedding.
motivation ~ Express constraints and possibilities and allow for flexibility.
implementations ~ Either optional or mandatory elements must be marked by a special flag.
examples ~ - Optional parts in regular expressions are indicated by a question mark or with an asterisk. - Mandatory roles in ORM are marked by a dot. - Requirement keywords defined in RFC 2119 (MUST, MUST NOT, SHOULD, SHOULD NOT, MAY). - Elements in regular grammars are mandatory by default. In other systems, such as RDF Schemas, elements are optional by default. - The end-tag matching to a start-tag in XML is mandatory. The same applies to closing brackets in JSON and other nested structures. - With a fixed size a specific number of elements is mandatory. - Annotations and qualifiers as optional additional elements (flag).
counter examples ~ - Default values, for instance default XML namespaces, make optional elements impossible because a default value cannot be omitted.
difficulties
~ - Optionality adds deontic logic to the realm of data, with all of
its difficulties: constraints on mandatory or optional data
elements are not true or false but they can only be fulfilled or
violated. Formalization of deontic rules may also lead to
unexpected logic results.
- Different levels of obligation may exist: in practice some
elements are more optional or more mandatory than others.
- Optional elements can be made mandatory and mandatory elements
can be made optional by introducing special garbage
elements, such as /, n/a, -, 0 etc. Such null values are
also created ad-hoc to trick mandatory constraints. For instance
12345 in a number field may indicate that the actual number
was not available.
- In a schema it is common to either explicitly mark only
mandatory only optional, assuming the other case as default. One
needs to know which is the default in which context.
related patterns ~ - If optional parts are irrelevant, they can also be garbage. - An absent element can still be data as void element. - Instead of or in addition to being mandatory, elements can also be derivable from other elements (derivation). - Prohibition can be used in a schema to express that specific elements must not be present.
implied patterns ~ Every optionality is either part of a schema or it constitutes a virtual schema consisting of this single optionality.