Hi, I have this datatype: data PeAux a = Folha Char | Nodo Char (PeAux a) (PeAux a) deriving Show And i need to reconstruct a tree from its post order visit.... example "12H3V" |'m trying something like this to create the nodes and leave what lefts: converteAux [] (x,y) = (x,y) converteAux (a:t) (x,y) | ((length (a:t))<3) = converteAux [] (x,y ++ (a:t)) converteAux (a:b:t) (x,y) | ((length (a:b:t))<3) = converteAux [] (x,y ++ (a:b:t)) converteAux (a:b:c:t) (x,y) | (isNumber a) && (isNumber b) && (isLetter c) = converteAux t (x ++ [(Nodo c (Folha a) (Folha b))],y) | otherwise = converteAux (b:c:t) (x,y++[a]) and i'm trying this to rebuild the tree but i dont know what to do in the case of [] since i dont have a empty constructer preenche (x:xs) = Nodo 'H' (preenche xs) x Can anyone help me? Many thx, Nuno