Hi Doaitse,
that is very interesting, and I'll take a precise look at the uu-
parsinglib. Regarding my original question there exist (I believe)
one serious problem: existing code is written exclusively using
Parsec and it's already quite complex. At first glimpse I don't see
an obvious way to use both libraries in one parsing module
simulatiously. However, these are a very good news indeed, thank you
2009/8/9 S. Doaitse Swierstra <doai...@swierstra.net>
The uu-parsinglib:
http://hackage.haskell.org/packages/archive/uu-parsinglib/2.2.0/doc/html/Text-ParserCombinators-UU-Core.html
contains a combinator to achieve just this:
-- parsing two alternatives and returning both rsults
pAscii = pSym ('\000', '\254')
pIntList = pParens ((pSym ';') `pListSep` (read <$> pList
(pSym ('0', '9'))))
parseIntString = pList (pAscii)
parseBoth = pPair pIntList parseIntString
pPair p q = amb (Left <$> p <|> Right <$> q)
The amb combinator tells you that it's parser parameter is
ambiguous, and returns you all the possible results. Amazingly it
still maintains its online behaviour. The only problem is that if
either one of the parsers fails then you will get only a single
result.
I have added the code above to the Examples.hs contained in the uu-
parsinglib (so it will show up in due time when I release a new
version) which I am attaching. Just load this file, and call the
function main to see what are the results of the different parsers
and correction strategies. The only problem is that if either one of
the parsers fails you will only get one of the results. If both
fail you will get the result which fails latest and if both fail at
the same place, the one which fails with the least repair costs.
If you really want both results, even if the input is erroneaous,
things become more complicated, especially if you want to embed this
parser in a larger one, since then we have to check whether both
parse the same prefix. If needed I could put some work into this, by
making a slightly different version of the amb combinator.
Doaitse
On 6 aug 2009, at 21:03, Dan Weston wrote:
Paul,
Arrows (and category theory in general) are interesting, but you
certainly don't need to understand them for this.
The only arrow in this code is the lowly function arrow (->). (&&&)
and (|||) are duals of each other and mean, respectively, "both" and
"either" (though for some bizarre reason, "both" is usually called
"fanout"!)
This style of pointfree (or "pointless") code is clearer to me
because I don't have a bunch of variable names to invent and have
lying around.
Anyway, if you prefer, don't import Control.Arrow at all, and just
use:
-- |Both: Apply two functions to same argument and tuple the results
infixr 3 &&&
(&&&) :: (a -> b) -> (a -> c) -> a -> (b,c)
(f &&& g) x = (f x, g x)
-- |Either: If argument is Left, apply Left function, else apply
Right function
infixr 2 |||
(|||) :: (a -> c) -> (b -> c) -> Either a b -> c
(|||) = either
either is implicitly imported from the Prelude and is defined as:
-- | Case analysis for the 'Either' type.
-- If the value is @'Left' a@, apply the first function to @a@;
-- if it is @'Right' b@, apply the second function to @b...@.
either :: (a -> c) -> (b -> c) -> Either a b -> c
either f _ (Left x) = f x
either _ g (Right y) = g y
Dan
Paul Sujkov wrote:
Hi Dan,
thank you for the solution. It looks pretty interesting and usable,
however I'll have to spend some time understanding arrows: I never
had an opportunity to use them before. Anyway, it looks very close
to what I actually need, and in any case much less ugly than
breaking the GenParser encapsulation
2009/8/6 Dan Weston <weston...@imageworks.com <mailto:weston...@imageworks.com
>>
Of course, since ParsecT s u m is a functor, feel free to use fmap
instead of parsecMap. Then you don't need to import from
Text.Parsec.Prim.
And in hindsight, I might prefer the name (<:>) or cons to (<>) for
the first function, but now I'm just obsessing. :)
Dan
Dan Weston wrote:
I think parsecMap does the job here:
-----------------------
import Text.ParserCombinators.Parsec hiding ((<|>))
import Text.Parsec.Prim(parsecMap)
import Control.Applicative((<|>))
import Control.Arrow((|||),(&&&))
-- Tagged (:)
(<>) :: Either Char Char -> Either String String -> Either
String String
Left a <> Left b = Left (a:b)
Left a <> Right b = Left (a:b)
Right a <> Left b = Left (a:b)
Right a <> Right b = Right (a:b)
-- Tagged concat
stringParser :: [Either Char Char] -> Either String String
stringParser = foldr (<>) (Right "")
-- Parse Integer if properly tagged, keeping unparsed string
maybeToInteger :: Either String String -> (Maybe Integer,
String)
maybeToInteger = (const Nothing ||| Just . read) &&& (id ||| id)
-- Tagged-choice parser
intOrStringParser = parsecMap (maybeToInteger . stringParser)
$ many1 (parsecMap Right digit <|> parsecMap Left (noneOf
";)"))
-- Parse between parentheses
intOrStringListParser = between (char '(')
(char ')')
(sepBy1 intOrStringParser (char
';'))
-----------------------
Then you get a tagged version of each string, along with the
string itself:
*P> parseTest intOrStringListParser $ "(1;2w4;8;85)"
[(Just 1,"1"),(Nothing,"2w4"),(Just 8,"8"),(Just 85,"85")]
There may be some parsecMap-fold fusion optimization possible,
though I haven't looked into that.
Dan
Paul Sujkov wrote:
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 result:
*Parlog> parseTest parseIntList "(1;2;3;4)"
[1,2,3,4]
*Parlog> parseTest parseIntString "(1;2;3;4)"
"(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
--
Regards, Paul Sujkov
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Regards, Paul Sujkov
