[Haskell-beginners] Handling a MonadError

[Christopher Done]

On 30 May 2011 05:05, Christopher Howard
<christopher.howard at frigidcode.com>wrote:

> Hi. I'm trying to learn Data.ConfigFile. I want to use the function
> readfile:
>
> [DESCRIPTION]
> readfile :: MonadError CPError m => ConfigParser -> FilePath -> IO (m
> ConfigParser)
>
> I understand that this function returns a monad, and I pull the value
> out of it with (<-). And I believe this is similar to a "Maybe"
> situation where I can do one thing if it provides a syntax error and
> another thing if it provides the ConfigParser. But I'm not sure what
> this would look like in actual code. Could somebody give me a small,
> simple example?
>

It's understandable that actual usage isn't clear, it wasn't clear to me
either when I first encountered it. There are a few ways to use it. It
returns an instance of error Monad, so you can either force it into an
Either value:

λ> do parser <- readfile emptyCP "../confy.conf"; return (get (forceEither
parser) "PATHS" "upload_path") :: IO (Either CPError String)
Right "uploads"

Or you can get it within the monad like everything else:

λ> do parser <- readfile emptyCP "../confy.conf"; return (do config <-
parser; get config "PATHS" "upload_path") :: IO (Either CPError String)
Right "uploads"

Because it returns any MonadError instance, and IO is an instance of
MonadError, you can skip the double level of monadery:

λ> let getUploads ::  IO (Either CPError String); getUploads = runErrorT $
do parser <- liftIO $ readfile emptyCP "../confy.conf"; config <- parser;
get config "PATHS" "upload_path" in getUploads
Right "uploads"

Or with join:

λ> :t join
join :: (Monad m) => m (m a) -> m a
λ> let getUploads ::  IO (Either CPError String); getUploads = runErrorT $
do cp <- join $ liftIO $ readfile emptyCP "../confy.conf"; get cp "PATHS"
"upload_path" in getUploads
Right "uploads"

Control.Applicative also works nicely for this.

instance Applicative (ErrorT CPError IO) where (<*>) = ap; pure = return

λ> let getCfg ::  IO (Either CPError (String,String));
       getCfg = runErrorT $ do
         cp <- join $ liftIO $ readfile emptyCP "../confy.conf";
         (,) <$> get cp "PATHS" "upload_path"
             <*> get cp "PATHS" "err_log"
   in getCfg
Right ("uploads","errors.log")
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