Hi,

   Could some one point out what I'm doing wrong below. This is a parser which takes an arithmetic expression built up from single digits. Turns 3*4+2 into (3*4)+2 etc.It giving me the last expression in a do statement or parse error on ->

 

(+++) :: Parser a -> Parser a -> Parser a

expr :: term( '+' expr |"")

term -> factor('*' term |"")

expr :: Parser Int
expr  = do t <- term
   do char '+'
      e <- expr
      return (t + e)
    +++ return t
   
term :: Parser Int
term = do f <- factor
      do char '*'
         t <- term
      return (f * t)
   +++ return f
   
factor :: Parser Int
factor = do d <- digit
             return (digitToInt d)
  +++ do char '('
         e <- expr
      char ')'
      return e
     
eval :: String -> Int
eval xs = fst(head(parse expr xs))

 

 

John