The following applies only to the first section, for now, which is all
I've read.
On 19/02/07, apfelmus@quantentunnel.de
Of course, comments, corrections and criticism are welcome!
I made a single correction and expanded on one quick point: http://en.wikibooks.org/w/index.php?title=Haskell/Zippers&diff=760692&oldid=760465 My comments, then: 1) Any reason you're not using the standard camelCase? 2) Any reason the maze datatypes take an extra parameter? For simplicity, I'd have expected: data Node = DeadEnd | Passage Node | Fork Node Node And so on. 3) You could do with splitting the first section into various subsections, it's a bit long and unwieldy as-is. 4) I'm not sure I like the typesetting of the conversation using lots of <br>s, but I can't think of a better way of doing it off the top of my head. 5) I don't really get this picture: http://en.wikibooks.org/wiki/Image:Laybrinth-Zipper.png Is the red bit meant to be one long, straight arrow? 6) I don't think you spend enough time with the zipper concept. I found when I read Huet's paper that zippers were one of those brain-exploding concepts, perhaps because everything is stored backwards. A few suggestions that would have helped me: * We store the entirety of the labyrinth in any zipper; by zipping all the way to the top so that the focus is at the entrance, the sub-labyrinth is the whole labyrinth. The reason this is done is so that we can backtrack and take an alternate path at any point. * We definitely need to show a zipper for another datatype, perhaps binary trees. I suggest with this one, we break the narrative and explain step by step what each constructor means. Something like the following: data BinTree a = Leaf | Node a (BinTree a) (BinTree a) data BinThread a = Root | Left a (BinTree a) | Right (BinTree a) a type BinZip a = (BinThread a, BinTree a) Root = The focus is at the top of the tree. L x t = We went left at a node where the value was x and the right subtree t. R t x = We went right at a node where the value was x and the left subtree t. Then give examples. The best way is probably to give an example BinTree in a picture, pick one element out, display the zipper for that element and explain what it means. This sound like a good plan? As a way for the reader to confirm their knowledge, show them the simple zipper for lists: data List a = End | Cons a (List a) data ListThread a = El a (List a) type ListZip a = (ListThread a, List a) Then set an exercise for the reader to explain to themselves how this works. I'd set this lot just before the 'half a year later', so we can return to the narrative as a nice ending. I'm sure I'll think of more, but that's it for now. -- -David House, dmhouse@gmail.com