hi Bob,

I believe the type of foldTree is implementable, but what confuses me is

the output example:

...

concatT (Leaf t) = t concatT (Branch (Leaf t1) (Leaf t2)) = Branch t1 t2

they seem to be inconsistent with the type of concatT which return a Tree only, but in the Leaf case, a atomic value is returned?

anything important I am missing?

You need to remember what type is stored in that Leaf – remember that this is a Tree (Tree a) – i.e. the variable t in the line above has the type Tree a. Bob