Hello list! I am trying to understand zipper concept using papers like [1] and [2]. Though main idea looks clear, I still have a problem in applying it for custom data types. Please help me with deriving Zipper-type from
data DTree a = P | D [(a, DTree)]
Looking in [1] ('Zippers via Differentiation' chapter) I tried to do the following: 1. Find a DTreeF type which is a pattern functor ([2], chapter 2.1) of my DTree 2. Write DTreeF in 'algebraic' form (using '+' and '*') 3. Find DTreeF' - "derivative" of DTreeF 4. Define my zipper type using list of DTreeF' Step 1 likely ends with
data DTreeF a x = P | D [(a,x)]
[2] says that using this pattern functor one could build a fixed-point version of DTree:
data Fix f = In {out :: (f (Fix f))} data DTreeFP = Fix DTreeF
but seems that I have nothing to do with it right now. Step 2 is my main question: In [1] authors did it for binary tree: data Tree a = Leaf a | Bin (Tree a) (Tree a) data TreeF a x = Leaf a | Bin x x and thus TreeF = a + x * x TreeF' = x + x My DTree has inner list of tuples. How should I rewrite it in terms of '+' and '*' ? Any comments are welcome! [1] - http://en.wikibooks.org/wiki/Haskell/Zippers [2] - http://people.cs.uu.nl/andres/Rec/MutualRec.pdf -- Thanks, Sergey