Hello, I'm trying to understand how I would manipulate a Data.Tree in Haskell. I came across this StackOverflow answer and accompanying blog post, but it doesn't run locally for me: * http://stackoverflow.com/a/15489761/410759 * https://www.fpcomplete.com/user/davorak/code-snippets/zipper-tree-examples Here's a gist with the code and the error I'm seeing: https://gist.github.com/zmoazeni/0049f3ab365ae600b131 I just want to start slow and be able to do three actions: * add +10 to all values in the tree * add a node to an existing node's subForest * remove a node (along with its subForest) from the tree I understand that this won't actually "edit in place" and my code would need to call a function which would return an updated version of the original tree. Finally, Lens wasn't my first choice. I was trying to understand Zippers from http://learnyouahaskell.com/zippers and my brain seized. I'm just falling back to Lens because it seems like the simplest/dsl-ish way to manipulate structures like this. Thanks for any help! -Zach