- Haskell-Cafe - Haskell.org

How to put a string into Data.Binary.Put
by C K Kashyap 06 Nov '10

06 Nov '10

Hi, I was trying to put a String in a ByteString import qualified Data.ByteString.Lazy as BS message :: BS.ByteString message = runPut $ do let string="SOME STRING" map (putWord8.fromIntegral.ord) string -- this ofcourse generates [Put] How can I convert the list of Put's such that it could be used in the Put monad? For now I used the workaround of first converting the string to ByteString like this - stringToByteString :: String -> BS.ByteString stringToByteString str = BS.pack (map (fromIntegral.ord) str) and then using putLazyByteString inside the Put monad. -- Regards, Kashyap

4 4

Packagse for a simple database application
by jean-christophe mincke 06 Nov '10

06 Nov '10

Hello Cafe, More as an exercice than anything else I would like to write a small "database application" which would allow a user to read/write data from/to a DB. The user would interact with the application via a web browser. Which (reliable/stable/workable) packages are the best for this task? I was thinking of Happstack and HaskellDB. Any others? Thank you Regards J-C

1 0

vector-space and standard API for vectors
by Alexey Khudyakov 06 Nov '10

06 Nov '10

Hello everyone! It's well known that Num & Co type classes are not adequate for vectors (I don't mean arrays). I have an idea how to address this problem. Conal Elliott wrote very nice set of type classes for vectors. (Definition below). I used them for some time and quite pleased. Code is concise and readable. > class AdditiveGroup v where > zeroV :: v > (^+^) :: v -> v -> v > negateV :: v -> v > > class AdditiveGroup v => VectorSpace v where > type Scalar v :: * > (*^) :: Scalar v -> v -> v > > class VectorSpace v => InnerSpace v where > (<.>) :: v -> v -> Scalar v There are few problems though. Fisrt one is inlining. Functions are not inlined so if client code uses uses stream fusion it breaks. Second is dependecies. Vector-space doesn't have big chain of dependencies. People may be reluctant to use vector-spaces just for API for small packages. My proposal is to split type classes above into separate package which provide only type classes, doesn't have any dependencies. Second point is to lean more on performance side in performance vs elegance. Add INLINE pragmas everywhere (unfortunate feature of stream fusion), move ^-^ to AdditiveGroup with default implementation. I'd like to know opinion of haskellers on this and specifically opinion of Conal Eliott as author and maintainer (I CC'ed him)

11 24

Rigid types fun
by Mitar 06 Nov '10

06 Nov '10

Hi! I have much fun with rigid types, type signatures and GADTs. And I would like to invite also others in and share my joy. ;-) Please see the attached file and chase a solution to how to make it compile. I would like to have a function which I would call like: createNerve (Axon undefined) (AxonAny undefined) and it would return proper Nerve. Similar to how asTypeOf works. I would like to do that to remove repeating code like: from <- newChan for <- newChan let nerve = Nerve (Axon from) (AxonAny for) which I have to write again and again just to make types work out. Why I cannot move that into the function? I am using GHC 6.12.3. Is this something which will work in 7.0? Mitar

4 8

What is simplest extension language to implement?
by Permjacov Evgeniy 06 Nov '10

06 Nov '10

Let us think, that we need some scripting language for our pure haskell project and configure-compile-run is not a way. In such a case a reasonably simple, yet standartized and wide known language should be implemented. What such language may be? R(4/5/6)RS ? EcmaScript ? Some other ?

16 17

Cabal and using a throw-away package database during distro package building
by Magnus Therning 05 Nov '10

05 Nov '10

Recently I experimented a little with putting several Hackage packages into a single Linux distro package, but I ran into a slight problem with registering Haskell packages temporarily. These are the basic steps taken to configure, compile, and stage a Haskell package in ArchLinux: runhaskell Setup configure --prefix=/usr --docdir=/usr/share/doc/${pkgname} runhaskell Setup build runhaskell Setup haddock runhaskell Setup register --gen-script runhaskell Setup unregister --gen-script runhaskell Setup copy --destdir=${pkgdir} This works fine for a single Hackage package, but if I want to compile two or more of them into a single distro package, and there are dependencies between them, then it won't do. I also don't want to fully install and/or register the Hackage packages during the build for obvious reasons of containment and system contamination. So, looking at the Cabal help I found the --package-db flag for configure. The steps now become: runhaskell Setup configure --prefix=/usr --docdir=/usr/share/doc/${pkgname} --package-db=my-temp-db runhaskell Setup build runhaskell Setup haddock runhaskell Setup register --inplace runhaskell Setup register --gen-script runhaskell Setup unregister --gen-script runhaskell Setup copy --destdir=${pkgdir} That works and I can satisfy dependencies between the Hackage packages, without affecting the system-wide database. Great. Except that the generated register/unregister scripts now also point to my-temp-db, and there seems to be no way to prevent this. I solved it for now by using sed, but I'd love to leave sed out, if at all possible. So, have I missed something, or is this a use case that wasn't considered when developing Cabal? Would it be worth raising a bug for this at all? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

2 1

Cabal and using a throw-away package database during distro package building
by Dimitry Golubovsky 05 Nov '10

05 Nov '10

Magnus, You might try Capri which operates Cabal-related stuff privately on a local-to-project package database not touching global or user databases. http://hackage.haskell.org/package/capri http://www.haskell.org/haskellwiki/Capri -- Dimitry Golubovsky Anywhere on the Web

1 0

Re: Haskell on ancient machines (Was: "Haskell is a scripting language inspired by Python.")
by Malcolm Wallace 05 Nov '10

05 Nov '10

>> I haven't checked how much RAM nhc98 needs for bootstrapping >> recently, but the >> Makefile suggests 16Mb of heap + 2Mb of stack is more than >> sufficient - >> it could probably manage with less. > > If it was possible to save resources in these days - why isn't it > possible today anymore? > Too many Haskell extensions? Too much auxiliary code for interfacing > with OS? Increased safety? Simply that more people care about speed than about space. Regards, Malcolm

1 0

Who manages <http://hackage.haskell.org/trac/>?
by Peter Simons 05 Nov '10

05 Nov '10

Hi guys, a while ago, I created an account on Trac. Now, it seems that I've forgotten both the password and the e-mail address that I used at the time. I cannot log in, and I cannot make Trac send me the password either. Clearly, I need the help of a human being with administrator privileges to figure that out. Can someone give me a pointer about who I'd want to contact regarding that issue? Take care, Peter

2 1

Reference for technique wanted
by Richard O'Keefe 05 Nov '10

05 Nov '10

There's a long-known technique in functional languages where [x1,...,xn] => \tail -> x1:...xn:tail xs ++ ys => f . g xs => f [] A correspondent mentioned to me that he couldn't find a reference to the idea (which I gather he had independently rediscovered). I know I've read about it somewhere. Can anyone provide a reference?

6 22