Am Montag 09 November 2009 10:01:43 schrieb iæfai:
With the below code, I am getting an error that I cannot resolve…
Everybody was so busy discussing whether Either (or rather (Either e)) is a monad that nobody looked at the code, so:
Chess.hs:52:82: Couldn't match expected type `Map [Char] [Char]' against inferred type `Either ParseError ConfigMap' In the third argument of `findWithDefault', namely `c' In the `documentRoot' field of a record In the first argument of `return', namely `Config {documentRoot = (findWithDefault "web" "Document- Root" c)}'
The specific code is:
getConf :: FilePath -> IO (Either ParseError Config) getConf filePath = return $ do c <- readConfig filePath -- (Either ParseError ConfigMap)
I believe the type of readConfig is FilePath -> IO (Either ParseError ConfigMap) , thus the binding of c, (c <-), still takes place in IO and c is one of (Left parseerror) or (Right configmap), hence c is not a suitable argument for findWithDefault.
return Config { documentRoot = Map.findWithDefault "web" "Document-Root" c }
The inner return also lives in IO, so had c a suitable type, your getConf would have type FilePath -> IO (IO something). I think you want getConf filePath = do r <- readConfig filePath return $ do c <- r -- *now* we're using the monad (Either ParseError) return Config{ documentRoot = Map.findWithDefault "web" "Document-Root" c } (if you have instance Monad (Either ParseError) in scope) or the equivalent using Pattern matching on the result of readConfig filePath.
The type of c should be Either ParseError ConfigMap, which by my understanding of the Either monad would cause the c to be the Right side stripped, or skipped if Left.
Full source for the module is below, and full project is hosted at http://patch-tag.com/r/iaefai/chess
For some general information, I am replacing ConfigFile dependancy with a Parsec based config parser (I call it SimpleConfig) that suits my needs - it came from
http://www.serpentine.com/blog/2007/01/31/parsing-a-simple-config-file-in-h askell/ originally and I modified it. On windows ConfigFile's dependancy on a posix regex library was causing trouble, so this is the effort to get rid of that dependancy.
Any thoughts would be useful.
There is one associated thought…
The original function used to get configuration back to the program is -- Mostly from Chris Done's Blog getConf :: FilePath -> IO (Either (C.CPErrorData, String) Config) getConf filePath = runErrorT $ do let cp = C.emptyCP { optionxform = id } contents <- liftIO $ readFile filePath config <- C.readstring cp contents let get = C.get config "DEFAULT" Config <$> get "Document-Root"
I noted it used <$> and in the code that I retrieved originally from Chris Done's blog (no longer able to find it) used <*> for additional items. I would like some easy method of constructing the new Config structure in my new code, especially if it can be done without the record syntax like this thing gets away with. I am not sure how this thing associated "Document-Root" with documentRoot mind you.
Thank you again. iæfai.