[Haskell-beginners] Use Lens to manipulate Data.Tree

[Zach Moazeni]

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
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