Tree Traversal, Depth first and Breadth first in Haskell

Lately I'm really digging Functional Programming, and especially Haskell. I've been reading Real World Haskell, which is a very nice free book about Haskell.

Today I was wondering what Breadth First traversal was. Of course I should know this and it's stupid I forgot. To make sure I wouldn't forget in the future I made a little exercise to improve my Haskell skills, and to make sure I wouldn't forget the Breadth First algorithm anymore.