- Beginners - Haskell.org

I have created an ugly Haskell program..
by Philip Scott 04 Nov '09

04 Nov '09

.. and I am positive there must be a way of beautifying it, but I am struggling. I bet there is just some lovely way of making this all shrink to three lines.. So here's the problem. I have two lists of tuples: (timestamp, value) What I would like to do in do a kind of 'zip' on two of these lists to make a list of (timestamp, (value1, value2)) with the following rules: - If the timestamps are equal it's easy - make your new element an move on - If one of the lists has a timestamp that the other doesn't, repeat an old value from the other list - If we don't have an old value yet, then don't create an element in the new list. e.g. if I ran my algorithm on these two lists d1 = [ (1,"a"), (2,"b"), (3,"c") ] d2 = [ (2,"b'"), (4,"d'") ] I would like to get result = [ (2, (b,b')), (3, (c,b')), (4, (c,d')) ] e.g. there was no data in d2 for our first element so we skipped it. Okay, so here is my code.. It works, but makes me feel a bit dirty. To explain my nomenclature 't' is 'timestamp of', 'v' is 'value of'. vx' and vy' are the 'old' values from the previous iteration in case a repeat is needed. They are Maybes because at the beginning there may be no old value. d1 = [ (1,"a"), (2,"b"), (3,"c") ] d2 = [ (2,"b'"), (4,"d'") ] t (x,y) = x v (x,y) = y js vx' vy' (x:xs) (y:ys) | t x == t y = ( (t x), (v x, v y) ) : js (Just (v x)) (Just (v y)) xs ys | t x < t y = maybe (js (Just (v x)) Nothing xs (y:ys)) (\z -> ( t x, (v x, z ) ) : ( js (Just (v x)) (Just z) xs (y:ys))) vy' | t x > t y = maybe (js Nothing (Just (v y)) (x:xs) ys) (\z -> ( t y, (z, v y ) ) : ( js (Just z) (Just (v y)) (x:xs) ys)) vx' js vx' vy' (x:xs) [] = maybe [] (\z -> ( t x, (v x, z ) ) : ( js (Just (v x)) (Just z) xs [])) vy' js vx' vy' [] (y:ys) = maybe [] (\z -> ( t y, (z, v y ) ) : ( js (Just z) (Just (v y)) [] ys )) vx' js _ _ [] [] = [] You call it with the first two arguments as Nothing to kick it off (I have a trivial wrapper function to do this) It works fine: > :t js js :: (Ord t) => Maybe a1 -> Maybe a -> [(t, a1)] -> [(t, a)] -> [(t, (a1, a))] > js Nothing Nothing d1 d2 [(2,("b","b'")),(3,("c","b'")),(4,("c","d'"))] But it just feels gross. Any advice on how to tame this beast would be greatly appreciated :) All the best, Philip

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order of monad transformers
by Michael Mossey 04 Nov '09

04 Nov '09

How does one consider the best ordering of monad transformers? For example, if I'm combining ErrorT, StateT (or State), and WriterT (or Writer)? But not just this specific example---what principles can one consult to determine ordering? Thanks, Mike

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about model checking of haskell program
by Liu Jian 03 Nov '09

03 Nov '09

Dear, Is there any work about "model checking haskell program"? in my opinion, think the haskell as a specification language, then we can check it or transform it to other specification language (such as, promela), so can be analyzed. cheers, Liu Jian ---- email to: gjk.liu(a)gmail.com

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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

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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"

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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.

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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

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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.

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ErrorT Identity
by Michael Mossey 01 Nov '09

01 Nov '09

Does using ErrorT Identity have advantages over Either? -Mike

1 0

Ord [] instance
by Keith Sheppard 31 Oct '09

31 Oct '09

the compare function for lists seems to be work like (leaving off class details): > compare [] [] = EQ > compare [] _ = LT > compare _ [] = GT > compare (x:xt) (y:yt) = case x `compare` y of > EQ -> xt `compare` yt > _ -> x `compare` y I poked around to see if I could find where this was defined in the spec or if it was an implementation specific thing (I need to be able to rely on this definition across implementations). I found this text in the report: -- Lists data [a] = [] | a : [a] deriving (Eq, Ord) -- Not legal Haskell; for illustration only Is this basically saying the same thing as my compare definition? Thanks Keith -- keithsheppard.name

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