I am trying to parse an XML dump of Wikipedia to find certain links on each page using the Haskell Parsec library. Links are denoted by double brackets: texttext[[link]]texttext. To simplify the scenario as much as possible, let's say I am looking for the first link not enclosed in double curly braces (which can be nested): {{ {{ [[Wrong Link]] }} [[Wrong Link]] }} [[Right Link]]. I have written a parser to discard links which are enclosed in non-nested double braces:
import Text.Parsec
getLink :: String -> Either ParseError String
getLink = parse linkParser "Links"
linkParser = do
beforeLink
link <- many $ noneOf "]"
string "]]"
return link
beforeLink = manyTill (many notLink) (try $ string "[[")
notLink = try doubleCurlyBrac <|> (many1 normalText)
normalText = noneOf "[{"
<|> notFollowedByItself '['
<|> notFollowedByItself '{'
notFollowedByItself c = try ( do x <- char c
notFollowedBy $ char c
return x)
doubleCurlyBrac = between (string "{{") (string "}}") (many $ noneOf "}")
getLinkTest = fmap getLink testList
where testList = [" [[rightLink]] " --Correct link is found
, " {{ [[Wrong_Link]] }} [[rightLink]]" --Correct link is found
, " {{ {{ }} [[Wrong_Link]] }} [[rightLink]]" ] --Wrong link is found
I have tried making the doubleCurlyBrac parser recursive to also discard links in nested curly braces, without success:
doubleCurlyBrac = between (string "{{") (string "}}") betweenBraces
where betweenBraces = doubleCurlyBrac <|> (many $ try $ noneOf "}")
This parser stops consuming input after the first }}, rather than the final one, in a nested example. Is there an elegant way to write a recursive parser to (in this case) correctly ignore links in nested double curly braces? Also, can it be done without using try? I have found that since try does not consume input, it often causes the parser to hang on unexpected, ill-formed input.
Here's a more direct version that doesn't use a custom lexer. It does use try though, and I don't see how to avoid it here. The problem is that it seems we need a non-committing look ahead to distinguish double brackets from single brackets; try is for non-committing look ahead.
The high level approach is that same as in
my first answer. I've been careful
to make the three node parsers commute -- making the code more robust
to change -- by using both try and notFollowedBy:
{-# LANGUAGE TupleSections #-}
import Text.Parsec hiding (string)
import qualified Text.Parsec
import Control.Applicative ((<$>) , (<*) , (<*>))
import Control.Monad (forM_)
import Data.List (find)
import Debug.Trace
----------------------------------------------------------------------
-- Token parsers.
llink , rlink , lbrace , rbrace :: Parsec String u String
[llink , rlink , lbrace , rbrace] = reserved
reserved = map (try . Text.Parsec.string) ["[[" , "]]" , "{{" , "}}"]
----------------------------------------------------------------------
-- Node parsers.
-- Link, braces, or string.
data Node = L [Node] | B [Node] | S String deriving Show
nodes :: Parsec String u [Node]
nodes = many node
node :: Parsec String u Node
node = link <|> braces <|> string
link , braces , string :: Parsec String u Node
link = L <$> between llink rlink nodes
braces = B <$> between lbrace rbrace nodes
string = S <$> many1 (notFollowedBy (choice reserved) >> anyChar)
----------------------------------------------------------------------
parseNodes :: String -> Either ParseError [Node]
parseNodes = parse (nodes <* eof) "<no file>"
----------------------------------------------------------------------
-- Tests.
getLink :: [Node] -> Maybe Node
getLink = find isLink where
isLink (L _) = True
isLink _ = False
parseLink :: String -> Either ParseError (Maybe Node)
parseLink = either Left (Right . getLink) . parseNodes
testList = [ " [[rightLink]] "
, " {{ [[Wrong_Link]] }} [[rightLink]]"
, " {{ {{ }} [[Wrong_Link]] }} [[rightLink]]"
, " [[{{[[someLink]]}}]] {{}} {{[[asdf]]}}"
-- Pathalogical example from comments.
, "{{ab}cd}}"
-- A more pathalogical example.
, "{ [ { {asf{[[[asdfa]]]}aasdff ] ] ] {{[[asdf]]}}asdf"
-- No top level link.
, "{{[[Wrong_Link]]asdf[[WRong_Link]]{{}}}}{{[[[[Wrong]]]]}}"
-- Too many '{{'.
, "{{ {{ {{ [[ asdf ]] }} }}"
-- Too many '}}'.
, "{{ {{ [[ asdf ]] }} }} }}"
-- Too many '[['.
, "[[ {{ [[{{[[asdf]]}}]]}}"
]
main =
forM_ testList $ \ t -> do
putStrLn $ "Test: ^" ++ t ++ "$"
let parses = ( , ) <$> parseNodes t <*> parseLink t
printParses (n , l) = do
putStrLn $ "Nodes: " ++ show n
putStrLn $ "Link: " ++ show l
printError = putStrLn . show
either printError printParses parses
putStrLn ""
The output is the same in the non-error cases:
Test: ^ [[rightLink]] $
Nodes: [S " ",L [S "rightLink"],S " "]
Link: Just (L [S "rightLink"])
Test: ^ {{ [[Wrong_Link]] }} [[rightLink]]$
Nodes: [S " ",B [S " ",L [S "Wrong_Link"],S " "],S " ",L [S "rightLink"]]
Link: Just (L [S "rightLink"])
Test: ^ {{ {{ }} [[Wrong_Link]] }} [[rightLink]]$
Nodes: [S " ",B [S " ",B [S " "],S " ",L [S "Wrong_Link"],S " "],S " ",L [S "rightLink"]]
Link: Just (L [S "rightLink"])
Test: ^ [[{{[[someLink]]}}]] {{}} {{[[asdf]]}}$
Nodes: [S " ",L [B [L [S "someLink"]]],S " ",B [],S " ",B [L [S "asdf"]]]
Link: Just (L [B [L [S "someLink"]]])
Test: ^{{ab}cd}}$
Nodes: [B [S "ab}cd"]]
Link: Nothing
Test: ^{ [ { {asf{[[[asdfa]]]}aasdff ] ] ] {{[[asdf]]}}asdf$
Nodes: [S "{ [ { {asf{",L [S "[asdfa"],S "]}aasdff ] ] ] ",B [L [S "asdf"]],S "asdf"]
Link: Just (L [S "[asdfa"])
Test: ^{{[[Wrong_Link]]asdf[[WRong_Link]]{{}}}}{{[[[[Wrong]]]]}}$
Nodes: [B [L [S "Wrong_Link"],S "asdf",L [S "WRong_Link"],B []],B [L [L [S "Wrong"]]]]
Link: Nothing
but the parse error messages are not as informative in the cases of unmatched openings:
Test: ^{{ {{ {{ [[ asdf ]] }} }}$
"<no file>" (line 1, column 26):
unexpected end of input
expecting "[[", "{{", "]]" or "}}"
Test: ^{{ {{ [[ asdf ]] }} }} }}$
"<no file>" (line 1, column 26):
unexpected "}}"
Test: ^[[ {{ [[{{[[asdf]]}}]]}}$
"<no file>" (line 1, column 25):
unexpected end of input
expecting "[[", "{{", "]]" or "}}"
and I couldn't figure out how to fix them.