alias ~ Pointer.
idea ~ An element is used to refer to another.
context ~ An element as pointer and another element from a set of possible targets.
motivation ~ Identifiers help establishing uniqueness and allow to refer to elements which not are directly available or impractical to express.
implementations ~ Identifier systems defines which identifiers exist and what data elements they refer to.
examples
~ - Keys or field names in key-value structures or records.
- Queries and requests expressed in data, for instance an XPath
expression.
- Data elements originally created for identification, such as
URIs and link anchors.
- Parts of an encoding, for instance the byte 0x41 that
encodes the letter "A" in ASCII.
- Computable hash codes which directly transform the content of an
element into an identifier.
- The identity which distinguishes a data element from any other
data elements, can only be expressed by an identifier. Every
metadata that uniquely refers to this single element is an
identifier.
- A geocode, given as WGS 84 coordinate, can identify a place
or an address.
difficulties ~ - An identifier must only refer to one element. If it refers to multiple elements, it is not clear whether this is an error (for instance collisions of hash codes), or whether all referenced elements are equal, or whether the collection of all referenced elements is actually identified. - Multiple identifiers may point to the same element, making it difficult to reverse the relation. - The existence of an identifier does not tell the kind of relation it is used for (e.g. as representation or to indicate a type or membership). - Identifiers may be meaningless (for instance inode numbers of files or memory addresses), it may be used as label or it may be a descriptive identifier with embedding. - The practical requirements of an identifier (unambiguity, uniqueness, persistence, readability, scope, actionability) contradict each other.
related patterns ~ An identifier may simultaneously act as label or it may have a structure with content that can further be analyzed (embedding). If positions are used as identifiers, there must be a sequence to refer to.
specialized patterns ~ Every encoding is based on a set of identifiers.