Hi Daryoush,

You could add another case to label, importing Debug.Trace:

data Tree = Leaf | Node Tree Int Tree deriving Show

label t | trace (show $ "label " ++ show t) False = undefined
label (Node ln _ rn) ((h:r):rest) = (Node lr h rr, r:r2) where
                                        (lr, r1) = label ln rest
                                        (rr, r2) = label rn r1
label _ _  = (Leaf, [])
lt t = let (r, unused) = label t ([1..]:unused)
       in r

This will output one line per each call to label. Except for one thing: your show function will never actually terminate, if the tree is cyclic. You can fix this by defining your own show function.

HTH,
Ozgur