tokenize :: Parser String
tokenize =do op <- many1 $ letter   <|>
                           char '+' <|>
                           char '^' <|>
                           char '-' <|>
                           char '>' <|>
                           char '*'
             return  op

curveOpt inval = case tokenize of
                      (Parser "++")   ->
                        (do  c  <- curve
                             curveOpt $ Connect inval c)

Couldn't match type `[Char]'
                  with `ParsecT String () Data.Functor.Identity.Identity String'
    Expected type: Parser String
      Actual type: [Char]
    In the pattern: "++"
    In a case alternative:
        "++"
          -> (do { c <- curve;
                   curveOpt $ Connect inval c })
    In the expression:
      case tokenize of {
        "++"
          -> (do { c <- curve;
                   curveOpt $ Connect inval c }) }