Hi, I have this type which represents polish expressions (floorplan representation): data PeAux a = Folha Char | Nodo Char (PeAux a) (PeAux a) deriving Show The reverse polish expression are the result of doing a post order visit to the tree One example of this reverse polish expression is "12H3V" and a example of tree is pe5 = Node 'V' (Node 'H' (Folha '1') (Folha '2')) (Folha '3') the tree above is the same as the expression "12H3V" What i need and i cant do is turn the expression a tree again... I have done the following: 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]) The strategy followed is to recognize operand, operand, operator and then put it as a basic element in a list, for instance, 12H -> Node 'H' (Folha '1') (Folha '2') And at the end put it togheter in one tree. ( not done yet) But i'm getting to the conclusion that it doesnt cover every case... Can anybody give me a light here? Many thx, Best regards, Nuno Santos