prohibition pattern

Forbidden element, exception.

Exclude specific elements.

A data element with embedding in another element.

Define what is not allowed instead of listing all possibilities.

• Explicitly list all disallowed elements (creating a container).
• Refer to a schema that tells which elements to avoid.
• Use an encoding that does not include prohibited elements.

• File systems disallow specific characters in file names, such as quotes, brackets, dot, colon, bar, asterisk, and question mark.
• A null-terminated character string must not contain null-bytes.
• Unicode and languages build on top, such as XML and RDF, disallow specific character code points.
• A separator element cannot occur as normal content.
• With Closed World Assumption everything is disallowed unless defined as allowed. With Open World Assumption one needs to explicitly state disallowed elements.
• Formal grammars extended by difference operator or negation in boolean grammars allow to express arbitrary forbidden elements in a schema.
• In mandatory fields (optionality) empty elements are prohibited.
• Specific graph types disallow some kinds of vertices, such as loops and circles.

• Prohibitions as “exceptions from a rule” are easy to grasp for human beings but they are more difficult to detect and compute algorithmically. Boolean grammars which support formal expression of exceptions via a negation operator are still more research topic rather than a practical tool for data description.
• Exceptions can have their own exceptions (the world is complex).
• Some prohibitions are not stated explicitly but implied by external constraints (derivation). For instance numbers in JSON can have arbitrary precision but in practice they are limited to standard floating point and integer representations.

• If the prohibition depends on existence of another element, it is rather an instance of the flag pattern.
• Optionality and mandatory constraints can be used in a schema to express whether an element must be present.

Every prohibition is either part of a schema or it constitutes a virtual schema consisting of this single prohibition.