[ballot parser
Evan Martin <martine@danga.com>**20050430204358] {
hunk ./Demo.hs 8
-import Parsec
+import Text.ParserCombinators.Parsec
+import Control.Monad.Error
+
+instance Error ParseError where
+  
+{- |Parses a simple form of ballots as strings.
+Ballot strings are comma-separated list in order of preference;
+equally-ranked candidates are separated with an equals sign.
+E.g., "3=4=5,1,2"
+-}
+parseBallot :: String -> Either ParseError Condorcet.Ballot
+parseBallot input = parse ballot "" input where
+  ballot :: Parser [[Int]]
+  ballot = do { b <- sepBy1 rank (char ','); eof; return b }
+  rank :: Parser [Int]
+  rank = sepBy1 number (char '=')
+  number :: Parser Int
+  number = do { ds <- many1 digit; return (read ds) }
+
+-- parse a set of ballots, halting on failure (using the Error monad).
+parseBallots :: [String] -> Either String [Condorcet.Ballot]
+parseBallots bs = mapM pb bs >>= return where
+  pb ballot = case parseBallot ballot of
+                Left err -> throwError (show err)
+                Right x  -> return x
hunk ./Demo.hs 36
-  let votes = [[ [1],[2],[3] ],
-               [ [2],[1],[3] ],
-               [ [2],[1],[3] ]]
-  let winners = Condorcet.run votes
-  putStrLn "winners:"
-  putStrLn $ show winners
+  case parseBallots ["1,2,3", "2=1,3", "2,1,3"] of
+    Left e -> putStrLn e
+    Right ballots -> do
+      putStrLn $ "parsed " ++ (show (length ballots)) ++ " ballots."
+      let winners = Condorcet.run ballots
+      putStrLn "winners:"
+      putStrLn $ show winners
}