- Beginners - Haskell.org

mailing list question
by Gregory Guthrie 01 Feb '12

01 Feb '12

I notice that the mailing list digest form is text only, so special symbols (Greek - alpha, beta, ..) all get mangled into "?". This is not so common on the Beginners list, but not infrequent on the Café list. To find the original version, one needs to go to the archives, but they are indexed by date, and the digest is by Vol, No, Topic. So, Is there any way to get an HTML version of the digest? And/or, to add some indexing to the archives? (an indexed page into the archives?) I presume the answer to these is no, since I did not see any such options on the mailman archive page; so likely this is just either a comment or suggestion, or both. :)

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Immutable refs for the functional code to get emulation of o.o. inner/nested classes
by Gabriel Riba 31 Jan '12

31 Jan '12

I have worked out a solution with System.IO.Unsafe.unsafePerformIO / unsafeDupablePerformIO. I would like any criticism in order to get a good solution. The idea is to have immutable refs readable safely from the functional code. Possible use: -------------------------- import Data.ImmIORef (ImmIORef, newImmIORef, readImmIORef) data Framework = Framework {prop :: Int} deriving (Eq, Show) -- ''Inner'' element with ref. to the framework data FrameworkElement = FrameworkElement {dta::Int, frameworkRef :: (ImmIORef Framework)} deriving (Eq, Show) getElement'sFramework_Prop :: FrameworkElement -> Int getElement'sFramework_Prop elem = k where k = prop $ readImmIORef $ frameworkRef elem -------------------------- -- Data.ImmIORef adapted from GHC.IORef -- module Data.ImmIORef ( ImmIORef, newImmIORef, readImmIORef ) where import GHC.Base import GHC.STRef import GHC.IO import Text.Show (Show, show) import System.IO.Unsafe -- |An immutable variable in the 'IO' monad newtype ImmIORef a = ImmIORef (STRef RealWorld a) -- explicit instance instance Eq (ImmIORef a) where ImmIORef x == ImmIORef y = x == y -- |Build a new 'ImmIORef' newImmIORef :: a -> IO (ImmIORef a) newImmIORef v = stToIO (newSTRef v) >>= \ var -> return (ImmIORef var) -- |Read the value of an 'ImmIORef' via unsafePerformIO {-# NOINLINE readImmIORef #-} -- recommended in System.IO.Unsafe doc readImmIORef :: ImmIORef a -> a #if __GLASGOW_HASKELL__>=721 readImmIORef (ImmIORef var) = unsafeDupablePerformIO $ stToIO (readSTRef var) #else readImmIORef (ImmIORef var) = unsafePerformIO $ stToIO (readSTRef var) #endif instance (Show a) => Show (ImmIORef a) where show xRef = "ImmIORef->(" ++ show (readImmIORef xRef) ++ ")"

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arrays
by Dennis Raddle 30 Jan '12

30 Jan '12

I've got a problem that is linear time with mutable arrays, and something god-awful complicated (not sure of time complexity) without them. What's the easiest way to use and understand mutable arrays? is it Data.Array that I want? Any good tutorials? Thanks, Dennis

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Iterating a monadic action with memoization
by Tim Cowlishaw 29 Jan '12

29 Jan '12

Hi all, I was wondering if any of you knew whether the following was possible, (I discussed this a little on #haskell at the weekend but didn't quite get to the bottom of it): Say I have a monadic action: > f :: a -> m a and an initial value of type a. I'd like to iterate this action and collect the results of each iteration in a list, as with 'iterate' on normal functions: > iterate (>>= f) . return $ a However, I would also like to memoize the intermediate results of the iteration, such that the entire iteration runs in O(n) time rather than O(n^2). This also implies a slight semantic difference, as side-effecting or non-deterministic actions will not be repeated in each iteration, while maintaining the laziness of the iteration function. Is it possible to do this? Many thanks, Tim

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data at different stages
by Dennis Raddle 28 Jan '12

28 Jan '12

I'm working on an application for reading Music XML and manipulating it, including translating it into midi events. Storing music data can take place from different perspectives or requirements. - a literal translation of the MusicXML would store each note and marking on the page - another representation might relate to higher concepts: for instance, instead of storing individual dynamic markings and indications for swelling and ebbing of dynamics, it might create a single data structure which is a dynamic profile over time. Instead of storing all the markings over the notes individually (staccato, accents, slur marks, etc.) it might examine all the markings at once and compute concrete performance instructions note by note. There are about four distinct stages in processing the music, in which things become either more abstract (grouping together details into higher concepts) or more concrete (such as more literal performance instructions). One of the stages, by the way, tries to represent notes in a way such that they can be moved in time or "sliced out" without disturbing the continuity of the performance instructions. At first I created several different data structures, one for each useful stage. Many of the fields are copied literally. There are similar fields which need distinct names. This is starting to seem like a confusing mess. Another approach is to create one data structure which contains some "Maybe" type fields. When the processing reaches a certain stage, they are set to "Just <value>" if applicable to that stage. Otherwise they are set to Nothing. I didn't want to do this at first because it's too much like a sloppy script language. The type system of the compiler would no longer assist me in verifying the validity of the data at each stage. But it's turning into a mess- and the key reason it's a mess, I think, is that I DON'T have a clear model of the stages. It keeps changing as I expand the requirements of my program. What seemed like a useful representation last month needs to be changed today. Any ideas welcome. Dennis

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Testing IO based monads and functions
by David Hinkes 27 Jan '12

27 Jan '12

Hey Haskellers, I had a design question: I'm using Haskell for the first time for something other than a simple toy application. I notice I'm coming up with a lot of IO based APIs that looks something like this: type UserInfo = (String, String) -- (name, password) getMagicNumberFromHTTPServer :: UserInfo -> IO (Maybe Int) getMagicNumberFromHTTPServer user = do let curlHTTPRequest = ... --Setup curl HTTP request httpRepsponse <- ... -- Make curl HTTP request return $ tryGetMagicNumberFromResponse httpResponse I like this API, because it's easy to understand. However, I find it hard to test. In OO land, I'd create a mock CurlHTTPRequest object and pass that into the magic number logic and test for the correct usage of the curl library. Is there a FP way of achieving the same goals? I suppose given the example above I could break down the above into several smaller pure functions and test them. I don't know if this strategy will hold-up when dealing with more complex monads. Thanks for your time, Dave

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HDBC-ODBC and Windows-1251 encoding
by Sergiy Nazarenko 26 Jan '12

26 Jan '12

Hi, guys! I use HDBC-ODBC module to work with windows data base and I got following problem: when I try to do INSERT > import Codec.Text.IConv (convert) > import qualified Data.ByteString.Lazy as BSL > ... > let name = convert "UTF-8" "WINDOWS-1251" $ BSL.pack "bla" > quickQuery "INSERT INTO Mytable (name,descript) VALUES (?, ?)" [toSql name), toSql name] and this also not worked: > quickQuery "INSERT INTO Mytable (name,descript) VALUES (?, ?)" [toSql ( BS.pack "bla "), toSql ( BS.pack "blabla" )] MSSQL server response me error: ... [ODBC SQL Server Driver][SQL Server]Invalid character value for cast specification It seems to be different encoding then HDBC-ODBC module can work with. I read documentation for HDBC: > To accomplish this, whenever a ByteString must be converted to or from a String, the ByteString is assumed to be in UTF-8 encoding, and will be decoded or encoded as appropriate. > Database drivers will generally present text or string data they have received from the database as a SqlValue holding a ByteString, which fromSql will automatically convert to a String, and thus automatically decode UTF-8, when you need it. In the other direction, database drivers will generally convert a SqlString to a ByteString in UTF-8 encoding before passing it to the database engine. How can I work with HDBC-ODBC module with non UTF-8 encoding? Cheers, Sergiy

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Another install problem
by Tom Murphy 19 Jan '12

19 Jan '12

Hi, Sorry to copy-paste more install errors, but this one has been confusing me for a while: Summary: "The pkg-config package poppler-glib version >=0.12.4 is required but it could not be found." I don't know how to fix this, as "poppler-glib" is not a package (and not a dependency). Thanks in advance! Tom $ cabal install poppler Resolving dependencies... /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/Gtk2HsSetup.hs:25:0: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [1 of 2] Compiling Gtk2HsSetup ( /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/Gtk2HsSetup.hs, /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/dist/setup/Gtk2HsSetup.o ) [2 of 2] Compiling Main ( /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/Setup.hs, /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/dist/setup/Main.o ) Linking /var/folders/+p/+pUVycVKFDWsYP8YiYnM3k+++TM/-Tmp-/poppler-0.12.277867/poppler-0.12.2/dist/setup/setup ... Configuring poppler-0.12.2... setup: The pkg-config package poppler-glib version >=0.12.4 is required but it could not be found. cabal: Error: some packages failed to install: poppler-0.12.2 failed during the configure step. The exception was: ExitFailure 1

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Is there an "unscan" function?
by Jeffrey Thornton 19 Jan '12

19 Jan '12

Hello- Is there standard function in Haskell that effectively does an inverse scan? For example, scanl1 (\ x y -> x+y) [1,2,3,4] == [1,3,6,10]. So is there a very simple built-in way to do this hypothetical example?: unscanl1 (\ x y -> y-x) [1,3,6,10] == [1,2,3,4] Thanks, Jeffrey

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questions about product types
by Tom Doris 18 Jan '12

18 Jan '12

My application is a server written in Haskell that receives uploads of structured data and persists them to disk and redistributes them on demand, kind of a persistent messaging middleware. Anyway, I started out using Haskell record syntax for everything, hit the usual problems with name clashes and ended up with field label names that looked like hungarian notation, everything looked a bit grimy and verbose etc,etc. As an experiment I got rid of the labels, and anywhere I had a product type that had more than one field of the same type (usually these were primitive types I hadn't bothered to create wrapper types for, e.g. Price and Quantity were both Double), I took that to be a "code smell" and created types that more accurately captured the use of the field's value in the program domain. Next I created a class Extractor to provide a standard way to pull individual fields from the product type: class Extract a b where extract :: a -> b e.g.: instance Extract Upload ClientID where extract (Upload c _ _ _ _ _) = c instance Extract Upload RecordPeriod where extract (Upload _ p _ _ _ _) = p So I can pass an Upload to extract and thanks to type inference it will usually be unambiguous, and I can annotate with a type signature to make it clearer what field I'm extracting as I see fit. I was missing record update syntax so I added a class to make it easier to do that: class Transform a b where transform :: a -> (b->b) -> a e.g.: instance Transform Upload ClientID where transform (Upload c p ab ar lr br) f = (Upload (f c) p ab ar lr br) i.e. take a function that updates a contained value and apply it to the appropriate field in an otherwise identical copy of the original. Apart from the verbosity of the instance declarations for every field I was pleased with the result, and the imposed discipline of giving each field a distinct type significantly improved the codebase. My questions are: 1) I presume this approach has been explored and probably there is a well thought through library of helper classes and functions already out there, can someone please point me to it? 2) I don't know much about Haskell generics, but for product types in which all contained values are distinct types, it should be possible to automatically derive instances of Extract, since there's never ambiguity as to which field you want from a container if you've specified the type of value you want, is this possible, and if so does it already exist? 3) In type theory, are product types consisting of distinct member types considered special in any way? i.e. does the concept have a name and what special properties do they have? Thanks for your help. T

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