This is a short example about how to use open recursive type definitions, written up as a refresher for myself.
The examples cover:
- defining open data types
- closing open recursive data types with Fix
- conventions for constructors of these types
- abstracting out recursion (cata)
- annotations
- composing open types
The commentary is meant jog my memory more than it is to be accurate, because the examples are about some techniques that I do not use very often, but find very valuable when I do use them. However, if you see something blatently wrong, let me know so that I can correct it, because the ideal situation is for the examples and commentary to be both good at reminding me and be accurate enough that anyone can use the examples as a reference.
Other people have written about the same things, but for some reason I'm never able to find the particular blog posts, videos, etc. that I am thinking of when I need them. These examples are based partly on examples I saw long ago and haven't been able to find again, and partly on refreshing my memory from:
- http://en.wikibooks.org/wiki/Haskell/Fix_and_recursion
- http://www.haskell.org/haskellwiki/Catamorphisms
Other sets of examples and blog posts that go into the subject more deeply can be found by searching Google for "haskell catamorphisms".