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
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