[Haskell-beginners] Either Monadic Trouble

[Henk-Jan van Tuyl]

Either is not a monad, you can check this by typing
   :i Either
in GHCi; you will not see a line like
   instance Monad Either
in the result. Compare this to
   :i Maybe

getConf could be something like:
   getConf :: FilePath -> IO (Either ParseError Config)
   getConf filePath =
     do
       c <- readConfig filePath  -- (Either ParseError ConfigMap)
       return $
         case c of
           Right c' -> Right $ Config $ Map.findWithDefault "web"  
"Document-Root" c'
           Left  _  -> c


Met vriendelijke groet,
Henk-Jan van Tuyl


--
http://Van.Tuyl.eu/
http://members.chello.nl/hjgtuyl/tourdemonad.html
--


On Mon, 09 Nov 2009 10:01:43 +0100, iæfai <iaefai at me.com> wrote:

> With the below code, I am getting an error that I cannot resolve…
>
>
> 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)
>          return Config { documentRoot = Map.findWithDefault "web"  
> "Document-Root" c }
>
>
> 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-haskell/   
> 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.
>
>


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