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