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