Unfolds generate data structures,and folds consume them.
A hylomorphism is a fold after an unfold,generating then consuming a
virtual data structure. A metamorphism is the opposite composition,an
unfold after a fold; typically,it will convert from one data representation
to another. In general, metamorphisms are less interesting than hylomorphisms: there is no automatic fusion to deforest the intermediate virtual data structure. However, under certain conditions fusion is possible: some of the work of the unfold can be done before all of the work of the fold is complete. This permits streaming metamorphisms, and among other things allows conversion of infinite data representations. We present the theory of metamorphisms and outline some examples.
The paper presents some nice examples (reformatting lines, radix conversion and continued fractions).
Closely related to other recent papers by Gibbons that were mentioned here before, but natrually the discussion of metamorphisms here is more focused and detailed.
Posted to functional by Ehud Lamm on 3/16/04; 7:39:43 AM