import ParsecExpr
 
expr    :: Parser Integer
expr    = buildExpressionParser table factor
        <?> "expression"
 
table   = [[op "*" (*) AssocLeft, op "/" div AssocLeft]
          ,[op "+" (+) AssocLeft, op "-" (-) AssocLeft]
          ]          
        where
          op s f assoc
             = Infix (do{ string s; return f}) assoc
 
factor  = do{ char '('
            ; x <- expr
            ; char ')'
            ; return x 
            }
        <|> number
        <?> "simple expression"
 
number  :: Parser Integer
number  = do{ ds <- many1 digit
            ; return (read ds)
            }
        <?> "number"