Ornaments in practice

Ornaments have been introduced as a way to describe some changes in datatype definitions that preserve their recursive structure, reorganizing, adding, or dropping some pieces of data. After a new data structure has been described as an ornament of older one, some functions operating on the bare structure can be partially or sometimes totally lifted into functions operating on the ornamented structure. We explore the feasibility and the interest of using ornaments in practice by applying these notions in an ML-like programming language. We propose a concrete syntax for defining ornaments of datatypes and the lifting of bare functions to their ornamented counterparts, describe the lifting process, and present several interesting use cases of ornaments.