Show Expression Tree with parenthesis

gistfile1.hs

instance Show Expression where
    show e = showWithParen 0 e where 
        showWithParen _ (Symbol x)       = x
        showWithParen _ (Number x)       = show x
        showWithParen _ (Func name e)    = name ++ "(" ++ show e ++ ")"
        showWithParen  w (Op op e1 e2)   = head ++ showWithParen curOpWeight e1 ++ show op ++ showWithParen curOpWeight e2 ++ foot where
            head = if w > curOpWeight then "(" else ""
            foot = if w > curOpWeight then ")" else ""
            curOpWeight = getOpWeight op