November 2009 - Beginners

Finding documentation when Hackage is down.
by aditya siram 02 Nov '09

02 Nov '09

Hi all, I use Hackage (through Hayoo sometimes) primarily for viewing API documentation. How can I store API docs for cabal-installed packages locally so I am not slowed down when Hackage goes down? -deech

4 4

Error Handling and case statements
by iæfai 02 Nov '09

02 Nov '09

I have been trying to work out a problem for the last few hours with little success. In the following code, using ConfigFile, I obtain the results of the configuration file, but in the main function I am trying to get the Config type out of the case statement. I need to be able to generate that error, but it means the two branches of the case are not the same type. I am not particularly attached to this direction, I am quite willing to do any way that works. I might be adding more configuration in the future. Any ideas? iæfai -- import Network.Shed.Httpd import Network.URI import Data.Either import Data.ConfigFile as C import Control.Monad.Error import Control.Applicative import ChessBoard data Config = Config { documentRoot :: String } deriving (Read, Show) main :: IO () main = do opt <- getConf "./config" config <- case opt of Left (_, err) -> ioError (userError err) Right (config) -> config docPath <- documentRoot config putStrLn "Starting up httpd." server <- initServer 6666 request return () request :: Request -> IO Response request req = do putStrLn $ "Recieved " ++ (show $ uriPath $ reqURI req) return $ Response 404 [] "Not found." -- 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"

3 2

Multiple type numeric data
by Didier Jacquemart 02 Nov '09

02 Nov '09

Hello, I'm just learning Haskell and have problems with a simple calculation: Here is my code below : data Figure = Carre Int Int Int | Rond (Int, Int, Integer) -- i hope Integer allows fromInteger function for r surface_carre :: Figure -> Int surface_carre (Carre x y c)= c * c surface_rond :: Figure -> Float surface_rond (Rond (x, y, r))= 3.14 * r * r surface x = case x of Carre a b c -> surface_carre x Rond (a, b, c) -> surface_rond x a::Figure a=Carre 10 10 5 b::Figure b=Rond (20, 20, 3) When i load this program, i get the following error for the line 3.14 * r * r Couldn't match expected type Float against inferred type Integer In the expression : 3.14 *r * r ... Further, i think there's a problem with the surface function, since it's not typed. Thanks for your help. Didier.

3 3

several questions: multi-param typeclasses, etc.
by Michael Mossey 02 Nov '09

02 Nov '09

I'm trying to understand multi-param typeclasses; a particular one (MonadError); and functional dependencies. For example, throwError in in the MonadError class definition: class Monad m => MonadError e m | m -> e where throwError :: e -> m a The concept of a multi-parameter class is a litte tricky. It's no longer so simple to say that "ONE type is AN instance of ONE class." Instead, it seems more appropriate to say that "in a context where e and m fulfill the criteria for class MonadError, the function throwError is available." In a sense, there is no single type that is an instance of MonadError. Is this the right way to put it? So I went looking for an instance of MonadError, in particular with use with Either. I couldn't find actual source code with an instance, but the Haddock documentation lists this: Instances: Error e => MonadError e (Either e) The code doesn't give the definition, but I suppose it would be: throwError e = Left e ??? Now I'm interested in understanding this functional dependency between m and e. For the compiler to decide that a particular instance's definition of throwError is available, it must decide that e is of class Error (it is given by class constraint) m is a Monad (Either e is such) and then this m -> e thing: I don't know how to put this into words Any explanation/clarification/correction appreciated. Thanks, Mike

3 7

Class definition syntax
by Shawn Willden 01 Nov '09

01 Nov '09

I have a program that makes use of various data types built on top of Arrays. In some cases, they're data types that contain an Array plus some additonal information, in others, they're just "newtype" Arrays, so that I can use typechecking to make sure that I'm not using the wrong kind of object. I'd really like to define an "ArrayOps" class with all of the operations I need, and define instances for all of the specific types. I also use some "raw" Array objects, so it would be even better if I could make an instance of my class for Array. And, ideally, I'd like to use the Array operations for my class operations. So, I want something like: class ArrayOps a where (!) :: a -> i -> e (//) :: a -> (i,e) -> a bounds :: a -> (i,i) range :: a -> [i] 'i' and 'e' are the index and element types, respectively. Obviously, the signatures above reference type variables that aren't declared, and really must be constrained to be the 'i' and 'e' that were used in building the type 'a' (which is an Array i e). Something like the following (though this obviously doesn't work): class ((Array.Array i e) a) => ArrayOps a where ... I'm sure there must be a way to do this, but I can't figure out what the syntax would look like. Thanks, Shawn.

3 9

ErrorT Identity
by Michael Mossey 31 Oct '09

31 Oct '09

Does using ErrorT Identity have advantages over Either? -Mike

1 0