A generalization of NullObject.

Represent special values with an object of distinguished class.

For example:

  Date (or Timestamp) -> Past & Future - special cases that are before (after) every other date.

  Date (or Timestamp) -> Now - continuously updates

  Money -> Zero - an object representing no monetary value

Combining the date special cases with TimeInterval results in some interesting possibilities.

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SpecialCase is really a SpecialCase of ObjectOriented programming in general just applied to some well known base types, isn't it? Consider Boolean in SmallTalk. True and False are just special cases.

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See also http://www.martinfowler.com/eaaCatalog/specialCase.html (Part of PatternsOfEnterpriseApplicationArchitecture catalog)

CategoryPattern