Pattern Guards and Transformational Patterns

Pattern mat hing is a well-appre iated feature of languages like ML or Haskell; it greatly simpli es the task of inspe ting values of stru tured data types and fa ilitates su in t fun tion de nitions that are easy to understand. In its basi form, pattern mat hing tries to identify a ertain stru ture of a value to be pro essed by a fun tion. This stru ture is spe i ed by a pattern, and if it an be re overed in a value, orresponding parts of the value are usually bound to variables. These bindings are exploited on the right-hand side of the de nition. There are numerous proposals for extending the apabilities of patterns and pattern mat hing; in parti ular, the problems with pattern mat hing on abstra t data types have stimulated a lot of resear h [19, 16, 3, 12, 4, 6, 11℄. Other aspe ts have also been subje t to extensions and generalisations of pattern mat hing [8, 1, 9, 7, 17℄.