[Haskell-cafe] [Haskell Cafe] Parsec: using two different parser for the same string

Paul Sujkov psujkov at gmail.com
Wed Aug 5 13:42:16 EDT 2009

Hi everybody,

suppose I have two different parsers: one just reads the string, and another
one parses some values from it. E.g.:

parseIntList :: Parser [Integer]
parseIntList = do
  char '('
  res <- liftM (map read) (sepBy1 (many1 digit) (char ';'))
  char ')'
  return res

parseIntString :: Parser String
parseIntString = manyTill anyChar eof

so for some input like this - "(1;2;3;4)" - I will have two different

*Parlog> parseTest parseIntList "(1;2;3;4)"
*Parlog> parseTest parseIntString "(1;2;3;4)"

but the thing that I actually want is something like Parser ([Integer],
String) - results from both parsers at a time, no matter whether one of them
fails or not:

*Parlog> parseTest parseIntListAndString "(1;2;3;4)"
([1,2,3,4], "(1;2;3;4)")

it is impossible at first sight, because first parser to use will consume
all the input, and there will be nothing to parse for the second one

Parsec contains "choice" function, but it is implemented via <|> and that is
mplus - so it tries second alternative only if the first one fails. Is it
possible to use two parsers for the same string (with try-like backtracking,
no input actually consumed till the second parser finishes)? I can assume
only dirty hacks with the GenParser internals - manual position storing and
backtracking - but that is obviously not good

however, my first attempt to solve the problem was kind a like that: to
parse string to String, and then to use it as an input for the next level
parse call:

parseIntListAndString :: Parser ([Integer], String)
parseIntListAndString = do
  str <- parseIntString
  return (res str, str)
      where res str = case (parse parseIntList "" str) of
                        Left  err -> []
                        Right val -> val

but the problems with such a method began when I switched from Parser to
GenParser with user state: function parseIntList have to update the state,
but it can't have the same state as the parseIntListAndString any more: it
has it's own. I can explicitly pass the state from parseIntListAndString to
parseIntList, but I see no suitable way for the parseIntList to update it. I
can return the updated state value from the parseIntList function, and call
setState on a result - but it seems rather ugly to mee. However, if nothing
else will do, that is an alternative

it is of course possible to use two different parsers sequentially, but it
is also very ineffective: I need to use such multiple parsing on a
relatively small substring of the actual input, so little backtracking would
be a much nicier approach. Any suggestions?

Regards, Paul Sujkov
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