- Haskell-Cafe - Haskell.org

Vector: blit'ing a source into a destination
by Nicolas Trangez 02 Sep '12

02 Sep '12

All, For some code I need to alter an (unboxed or storable) vector by blit'ing (copying) data from another (smaller) vector into it, overwriting the existing data. I couldn't find a function for this in the vector API, so I hacked something up which cuts the jobs (see below), but it feels really ugly/dirty. Any ideas about a better way to tackle this, and wouldn't this be a useful addition to the standard API? Thanks, Nicolas import Data.Vector.Unboxed (Unbox, Vector) import qualified Data.Vector.Unboxed as UV import qualified Data.Vector.Unboxed.Mutable as MUV blit :: Unbox a => Vector a -> Vector a -> Int -> Vector a blit src dest offset = UV.modify act dest where act dest' = do let slice = MUV.slice offset len dest' src' <- UV.unsafeThaw src MUV.copy slice src' len = UV.length src main :: IO () main = do let v = UV.fromList [0 :: Int .. 10] b = UV.fromList [20, 30, 40] putStrLn $ "v = " ++ show v putStrLn $ "b = " ++ show b putStrLn $ "blit b v 2 = " ++ show (blit b v 2) -- v = fromList [0,1,2,3,4,5,6,7,8,9,10] -- b = fromList [20,30,40] -- blit b v 2 = fromList [0,1,20,30,40,5,6,7,8,9,10]

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Is there a tool for automatic update of other-extensions and other-modules sections in a .cabal file?
by Tsuyoshi Ito 01 Sep '12

01 Sep '12

Hello, I have a *.cabal file which contains a package description for several executables (which share some of the source files). Is there a tool which updates other-extensions section and other-modules section in the .cabal file automatically to synchronize them to the source files? If it matters, I am using Haskell Platform 2012.2.0.0 on Windows. (I am aware that neither “cabal configure” nor “cabal build” requires these sections, and therefore it is not strictly necessary to update them during development of a program. But when I use a version control system such as Git, I find it ugly to store a .cabal file which is not synchronized to the latest source files… this is the primary reason why I would like such a tool.) Best regards, Tsuyoshi

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: Perdure
by Felipe Almeida Lessa 31 Aug '12

31 Aug '12

[moving discussion to haskell-cafe] Congratulations and thanks for your new open source contribution! I hope you feel at home =). Your library looks really interesting but I'm completely overwhelmed by its size. Its Cabal description is huge and there's no example of how to use the library (it actually took me some time to understand what the library tries to accomplish). I took a quick look at the modules but there are so many that I couldn't really make sense of anything in a short time. So, could you perhaps write a full example of how to use the library? It looks like a nice library but I need a starting point =). Cheers! -- Felipe.

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Yesod a href render issue
by Thiago Negri 31 Aug '12

31 Aug '12

I'm following Yesod tutorial that gives this as the first example for "type-safe URLs": | getHomeR = defaultLayout [whamlet|<a href=@{Page1R}>Go to page 1!|] Worked fine, the "a href" generated looks perfect. Then I tried this: | getHomeR = defaultLayout [whamlet|Hello!<a href=@{Page1R}>Go to page 1!|] And got a "a href" referecing a URL without quotes and with no end tag for "a", something like this: | <html><head><title></title></head><body>Hello<a href=/page1>Go to page 1!</body></html> I tried in many different ways, and the only way to get it working properly was to set the "a" tag in a line by itself: | getHomeR = defaultLayout [whamlet|Hello! | <a href=@{Page1R}>Go to page 1! | |] Is this a failure of Yesod quasiquotation or am I missing something? I know Yesod clearly states that the best approach is to use external files instead of quasiquotes, but as I'm making my first steps into Yesod, I would like to use the practical embedded quasiquotes.

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Build regressions due to GHC 7.6
by Bryan O'Sullivan 31 Aug '12

31 Aug '12

Since the release of the GHC 7.6 RC, I've been going through my packages and fixing up build problems so that people who upgrade to 7.6 will have a smooth ride. Sad to say, my experience of 7.6 is that it has felt like a particularly rough release for backwards incompatibility. I wanted to quantify the pain, so I did some research, and here's what I found. I maintain 25 open source Haskell packages. Of these, the majority have needed updates due to the GHC 7.6 release: - base16-bytestring - blaze-textual - bloomfilter - configurator - criterion - double-conversion - filemanip - HDBC-mysql - mwc-random - pcap - pool - riak-haskell-client - snappy - text - text-format - text-icu That's 16 out of 25 packages I've had to update. I've also either reported bugs on, or had to fix, several other people's packages along the way (maybe four?). So let's say I've run into problems with 20 out of the combined 29 packages of mine and my upstreams. The reasons for these problems fall into three bins: - Prelude no longer exports catch, so a lot of "import Prelude hiding (catch)" had to change. - The FFI now requires constructors to be visible, so "CInt" has to be imported as "CInt(..)". - bytestring finally got bumped to 0.10, so many upper bounds had to be relaxed (*cf* my suggestion that the upper-bounds-by-default policy is destructive). It has been a lot of work to test 29 packages, and then modify, rebuild, and release 20 of them. It has consumed most of my limited free time for almost two weeks. Worse, this has felt like make-work, of no practical benefit to anyone beyond scrambling to restore the status quo ante. If over half of my packages needed fixing, I'm alarmed at the thought of the effects on the rest of Hackage. I'm torn over this. I understand and agree with the impetus to improve the platform by tidying things up, and yet just two seemingly innocuous changes (catch and FFI) have forced me to do a bunch of running to stand still. I don't have any suggestions about what to do; I know that it's hard to estimate the downstream effects of what look like small changes. And so I'm not exactly complaining. Call this an unhappy data point.

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Function names in Haskell lib not first-class on web!
by damodar kulkarni 31 Aug '12

31 Aug '12

Hi Cafe, It seems, the function names in Haskell libs are not first-class objects, AT LEAST when it comes to searching for them of the net! I was trying to search for the following Haskell functions in the mailing list archives. Here is a summary of the responses I have had from various servers upon searching various "valid and normally used" Haskell function identifiers from well-known libraries. Responses from the http://www.mail-archive.com >>=<http://www.mail-archive.com/search?l=haskell-cafe%40haskell.org&q=%3E%3E%3D>, It seems the script searched for the string "&*gt*; &*gt*;" * -> *<http://www.mail-archive.com/search?l=haskell-cafe%40haskell.org&q=*+-%3E+*>There are 0 results. and so on... Responses from the http://search.gmane.org >>=<http://search.gmane.org/search.php?group=gmane.comp.lang.haskell.cafe&query…> Here, too, it seems the script searched for the string "&*gt*; &*gt*;" * -> *<http://search.gmane.org/?query=*+-%3E+*&author=&group=gmane.comp.lang.haske…> Here it seems the script searched for the string "&*gt*;" and so on... *Note: google badly fails to search these functions identifiers.* My current solution to this problem is: download compressed text archives and do a simple grep on it on my machine. But the archives up to year 2000 only are found at http://www.haskell.org/pipermail/haskell-cafe/. So, as an aside: Where do I find the earlier messages for downloading? Thanks, Damodar

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Data.Text UTF-8 question
by jeff p 31 Aug '12

31 Aug '12

Hello, I have a sample file (attached) which I cannot read into Text: Prelude Control.Applicative> Data.Text.IO.readFile "foo" *** Exception: utf8.txt: hGetContents: invalid argument (invalid byte sequence) Prelude Control.Applicative> Data.Text.Encoding.decodeUtf8 <$> Data.ByteString.Char8.readFile "foo" "*** Exception: Cannot decode byte '\x6e': Data.Text.Encoding.decodeUtf8: Invalid UTF-8 stream So it seems that foo doesn't contain valid UTF-8. However, System.IO.UTF8 has no problem reading the data: Prelude Control.Applicative> System.IO.UTF8.readFile "foo" "3591,,,dihigma99h,1905,5,25,CUBA,,Matanzas,1971,5,20,CUBA,,Cienfuegos,Martin,Dihigo,,Mart\65533n Magdaleno Dihigo (Llanos),,190,74,R,R,,,,dihigma99,dihigma99,dihim001,dihigma99,dihigma99\r\n" Shouldn't these all have the same behavior? I am running on Mac OS X 10.8.1, with GHC 7.4.2 and text-0.11.2.3. thanks for any insight, Jeff

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Is it worth adding Gaussian elimination and eigenvalues to REPA?
by KC 31 Aug '12

31 Aug '12

I realize if one wants speed you probably want to use the hMatrix interface to GSL, BLAS and LAPACK. Worth it in the sense of have a purely functional implementation. -- -- Regards, KC

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Code review request - using ErrorT
by C K Kashyap 31 Aug '12

31 Aug '12

Dear gentle Haskellers, I'd appreciate it very much if you could please take a look the my code here and give me some feedback - https://github.com/ckkashyap/haskell-websocket/blob/master/src/lib/Net/WebS… I am particularly looking for advice around what would be a good type for a server handler function that does IO and needs to short circuit if it fails at any point also needs to thread a state along. I am considering this - ErrorT String (StateT Int IO) Data Regards, Kashyap

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why do I need class context in declaring data constructor?
by Paul Liu 31 Aug '12

31 Aug '12

I had a toy program that encodes simply typed lambda in types. It used to work fine with GHC prior to 7.2. But now it no longer compiles. Here is a minimal fragment that demonstrates this problem. > {-# LANGUAGE GADTs, > MultiParamTypeClasses, > FlexibleInstances, > FlexibleContexts #-} > > data Abs env t v where > Abs :: g (a, env) h v -> Abs env (g (a, env) h v) (a -> v) > > class Eval g env t v where > eval :: env -> g env t v -> v > > instance Eval g (a, env) h v => > Eval Abs env (g (a, env) h v) (a -> v) where > eval env (Abs e) = \x -> eval (x, env) e The type Abs has 3 parameters: its environment, sub term (encoded in types), and type. The constructor Abs has 1 parameter: its sub term. The code loads fine in GHC 7.0.3. Here is the error reported by GHC 7.2.2 (and later): test.lhs:14:30: Could not deduce (Eval g1 (a1, env) h1 v1) arising from a use of `eval' from the context (Eval g (a, env) h v) bound by the instance declaration at test.lhs:(12,12)-(13,49) or from (g (a, env) h v ~ g1 (a1, env) h1 v1, (a -> v) ~ (a1 -> v1)) bound by a pattern with constructor Abs :: forall env (g :: * -> * -> * -> *) a h v. g (a, env) h v -> Abs env (g (a, env) h v) (a -> v), in an equation for `eval' at test.lhs:14:15-19 Possible fix: add (Eval g1 (a1, env) h1 v1) to the context of the data constructor `Abs' or the instance declaration or add an instance declaration for (Eval g1 (a1, env) h1 v1) In the expression: eval (x, env) e In the expression: \ x -> eval (x, env) e In an equation for `eval': eval env (Abs e) = \ x -> eval (x, env) e However, if I move the class context to the data constructor of definition, then it compiles fine in GHC 7.2.2 (and later): > data Abs env t v where > Abs :: Eval g (a, env) h v => g (a, env) h v -> Abs env (g (a, env) h v) (a -> v) But this is very troublesome because for every new class instance I want to make Abs of, I have to make a new class context to the data constructor. It totally defeats the purpose of making class instances to extend usage of data types. Did I missed a language extension when moving code from GHC 7.0.3 to GHC 7.2.2? What can I do to fix it for newer GHCs? -- Regards, Paul Liu

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