[Haskell-cafe] Troubles with Exercise 4.9 from YAHT

[michael at schmong.org]

from YAHT

Exercise 4.9 Write a function elements which returns the elements in a
BinaryTree
in a bottom-up, left-to-right manner (i.e., the ���rst element
returned in the left-most leaf,
followed by its parent���s value, followed by the other
child���s value, and so on). The re-
sult type should be a normal Haskell list.

here's the data type BinaryTree 
data BinaryTree a
   = Leaf a
      | Branch (BinaryTree a) a (BinaryTree a)

      Here's my first attempt

       
       nodeCount :: BinaryTree a -> [b]
       nodeCount = map putStrLn (show a)

       a is out scope.

       I started looking at what types putStrLn and map are.
       First thing that occurs to me is that I can't use map unless I
       pass it a list.
       I'm not allow to use fold yet, that's the next assignment.

       Any clues on how to proceed?