A Meta-Language for Ornamentation in ML

Ornaments are a way to describe changes in datatype definitions that preserve their recursive structure, reorganizing, adding, or dropping some pieces of data so that functions operating on the bare definition can be partially and sometimes totally lifted into functions operating on the ornamented structure. We propose an extension of ML with higherorder ornaments. We introduce a meta-language above ML in which we can elaborate a most generic lifting of bare code, so that ornamented code can then be obtained by instantiation of the generic lifting, followed by staged reduction and some remaining simplifications. We use logical relations to closely relate the ornamented code to the bare code.