Thomas Conway wrote:
> One of the things that gets messy is that in lots of places you can
> put either a thing or a reference to a thing (i.e. the name of a thing
> defined elsewhere). For example, consider the production:
>
> NamedNumber ::= identifier "(" SignedNumber ")"
> | identifier "(" DefinedValue ")"
>
I like solving this with either a (WriterT Parser) or using the Parsec state to
lazily access the final mapping. Here is a working Toy example where 'finalMap'
is used during the parsing. Parsec was a bit too strict with the return of
'parseVal' so I had to use a (data Box) to make it lazy:
> import Text.ParserCombinators.Parsec
>
> import Data.Maybe
> import qualified Data.Map as M
>
> data Box a = Box {unBox :: a}
>
> input = unlines $
> [ "name(ref)"
> , "ref=7"
> ]
>
> data Toy = Toy String Int deriving (Show)
>
> myParse s = toys where
> result = runParser parser M.empty "Title" s
> toys = either Left (Right . fst) result
>
> lookupRef r = Box (finalMap M.! r)
> where finalMap = either undefined snd result
>
> parser = do
> maybeToyList <- many parseLine
> defMap <- getState
> return (catMaybes maybeToyList,defMap)
>
> parseLine = try parseToy <|> parseDef <|> (char '\n' >> return Nothing)
>
> parseToy = do
> name <- many1 letter
> val <- between (char '(') (char ')') (try parseVal <|> parseRef)
> return (Just (Toy name (unBox val)))
>
> parseVal = do
> s <- many1 digit
> return (Box (read s))
>
> parseRef = do
> s <- many1 letter
> return (lookupRef s)
>
> parseDef = do
> s <- many1 letter
> char '='
> v <- parseVal
> defMap <- getState
> let defMap' = M.insert s (unBox v) defMap
> setState $! defMap'
> return Nothing
When I run it in ghci I get:
> *Main> myParse input
> Right [Toy "name" 7]
Cheers,
Chris
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