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a poorly thought out suggestion for cabal and packages that have lots of instances
by Jeremy Shaw 16 Mar '10

16 Mar '10

Hello, There are a number of libraries like syb-with-class, happstack-data, etc, which provide a new class that potentially has a lot of new instances. For example, let's say I want a 'Data ctx Text' instance. Where should that go? To avoid orphan instances it would need to go in the 'text' package where Text is defined. But it seems unlikely that I am going to convince the authors of text to add additional dependencies on things like syb-with-class, happstack-data, etc, just to satisfy a small number of users. A second place to put the instance would be in the syb-with-class package. They will be orphan instances, but as long as you can't import the Data class without also getting those instances, it doesn't seem like much could go wrong. But now syb-with-class has to depend on all the libraries that it is going to provide instances for. So, I have just shifted the problem from one library to another. A third option is to just define the instance in the library where I want to use it. But now I have opened up the possibility of conflicting orphan instances for the type -- which is no good. A fourth option is to create a new package which contains just the instance. For example: syb-with-class-text This still opens the possibility of conflicting orphan instances. But at least there is a version that is supposedly the 'official' one. The problem is that it seems like this might really clutter up hackage? Also, it is annoying to have to create and maintain and entire new cabal package just for one single class instance. And if i want to install all the instances, it would be annoying to have to do 20 different cabal installs. What might be nicer is if I could have a single project directory that somehow generated all those instances. And if cabal new how to work with them as a family. Something like: cabal upload syb-with-class* cabal install syb-with-class* Obviously, I have not really thought this through all the way. I just know that I am continually wondering what I should do when I want to do something like add a Text instance for syb-with-class? If we were to get really experimental I might suggest that when you install syb-with-class, you don't automatically get all the extra instances by default. But the package system and compiler would know they exist. So if you tried to write code that required the missing instance, it would suggest that you need to install the extra package. This would help reduce instances of people accidentally creating a conflicting instance. But I am sure it makes something else horrible happen instead ;) - jeremy

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Code that doesn't compile - but should :)
by Giuseppe Maggiore 16 Mar '10

16 Mar '10

Hi! Can anyone tell me why this code does not work? I cannot seem to figure why it is broken... {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances, FlexibleContexts, EmptyDataDecls, ScopedTypeVariables, TypeOperators, TypeSynonymInstances #-} data Data k = Pair Integer (() -> k) data RecData = RecData (Data RecData) mk_data x = RecData(Pair x (\() -> mk_data (x+1))) class Converter a f where convert :: a -> f a instance Converter RecData Data where convert (RecData r) = r class Selector s a where select :: s -> a f :: (Selector s (a->f a), Converter a f) => s -> (a->a) f s = let method = select s in (\x -> let res = method x in convert res) -- Giuseppe Maggiore Ph.D. Student (Languages and Games) Microsoft Student Partner Mobile: +393319040031 Office: +390412348444

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ANNOUNCE: feldspar-language-0.2, feldspar-compiler-0.2
by Emil Axelsson 16 Mar '10

16 Mar '10

We are happy to announce the new release of Feldspar and its compiler! http://feldspar.sourceforge.net/ Feldspar is an embedded domain-specific language for digital signal processing. It is developed as a joint project with Ericsson, ELTE university and Chalmers university. This is an intermediate release to make some of our ongoing work available. The most user-visible change from the previous version is that the vector types have got much simpler, leading to much simpler programs. There have also been some substantial changes under the hood: Most notably, the compiler has a completely new structure which allows defining various optimizations as plugins. The examples available are still very limited. We hope to make a new release quite soon with a reworked tutorial and many more examples. / Emil

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how to listen on a specific IP using the network library
by Jeremy Shaw 16 Mar '10

16 Mar '10

Hello, I would really like to modify happstack so that you can specific what IP address to listen on. So I think I want a function like: listenOnAddr :: SockAddr -> IO Socket The problem is that the user might want to specify IPv4 or IPv6 address. But support for IPv6 is only conditionally compiled into the network library. I can not figure out how to write my code so that it will compile regardless of whether network was compiled with ipv6 enabled. I got this far: -------------------------------- import qualified Control.Exception as Exception import Network.BSD (getProtocolNumber) import Network.Socket listenOnAddr :: Family -> SocketType -> SockAddr -> IO Socket listenOnAddr fam typ sockAddr = do proto <- getProtocolNumber "tcp" Exception.bracketOnError (socket fam typ proto) (sClose) (\sock -> do setSocketOption sock ReuseAddr 1 bindSocket sock sockAddr listen sock maxListenQueue return sock ) --------------------------------------- But I am not sure how to modify it to calculate 'fam' and 'typ' from 'sockAddr' with out making my code depend on IPv6 being enabled. If I was adding this function directly to the network package, there would be no problem. I could just use: #if defined(IPV6_SOCKET_SUPPORT) #endif But in a 3rd party library IPV6_SOCKET_SUPPORT is not defined. I could copy the autoconf code into my library -- but there is no guarantee that my library and the network library were compiled with the same value for IPV6_SOCKET_SUPPORT. In happstack we use a really horrible trick involving template haskell where we see if the SockAddrInet6 constructor exists at compile time and conditionally compile different versions of the code that way. But it is really ugly. We defined supportsIPv6 here: http://patch-tag.com/r/mae/happstack/snapshot/current/content/pretty/happst… And then use it in acceptLite here: http://patch-tag.com/r/mae/happstack/snapshot/current/content/pretty/happst… It wouldn't be that bad if we could actually use the [| |] mechanism. But we can't because it won't compile when ipv6 support is not enabled due to '(S.SockAddrInet6 _ _ ha _) -> showHostAddress6 ha'. Any suggestions? At this point I am thinking that the best solution is to add a new function to network, since it is not trivial to do it anywhere else in a portable way, and it seems like a fairly useful and common operation? - jeremy

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Can't cabal install darcs because hashed-storage needs 'System'
by Daniel McAllansmith 16 Mar '10

16 Mar '10

Hello. Short story: $ cabal install --global --constraint="old-time>=1.0.0.3.1" --reinstall -v darcs /tmp/hashed-storage-0.4.71127/hashed-storage-0.4.7/Setup.hs:13:7: Could not find module `System': Use -v to see a list of the files searched for. This is using GHC 6.12.1, cabal-install 0.8.0 and Cabal 1.8.0.2 How do I get darcs installed? Longer story: I've been trying to install darcs 2.4 built against a patched version of old- time, 1.0.0.3.1 is effectively 1.0.0.4 just that it's patched locally because 1.0.0.4 hasn't been released to hackage yet. I ran into various problems with global vs. user packages, no version of base being valid and a few other bits and pieces. So I am trying a global install and have had to reinstall Cabal (and various other packages), presumably because it was built against old-time 1.0.0.3. Looking at the hashed-storage code it imports System (system, exitWith). Is that old base-3 code? The debug messages show: selecting base-3.0.3.2 (installed) and 4.2.0.0 (installed) and /usr/local/bin/ghc --make /tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7/Setup.hs -o /tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7/dist/setup/setup -odir /tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7/dist/setup -hidir /tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7/dist/setup -i -i/tmp/hashed-storage-0.4.71127/hashed- storage-0.4.7 -package Cabal-1.8.0.2 so I'm not really sure what version of base it's actually trying to compile hashed-storage against. Any further information I can provide? Thanks Dan

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ANNOUNCE: tupleinstances-0.0.1
by Diego Echeverri 16 Mar '10

16 Mar '10

Hello. TupleInstances It's a really small library that provides instances for Functor, Applicative, Foldable and Traversable for tuples of the form (a,a...a). It uses newtypes to wrap the tuples and Template Haskell to generate the code. Here's an example of these instances in action: example6 :: (Int, Int) -> [(Int, Int)] example6 p = map unT $ liftA2 (+) (T2 p) <$> moves where moves = tail $ traverse (\x -> [0,x+1,x-1]) (pure 0 :: T2 Int) -- > Result with (1,1): [(1,2),(1,0),(2,1),(2,2),(2,0),(0,1),(0,2),(0,0)] More examples here: http://github.com/diegoeche/tupleinstances/raw/master/README PS: This is also my first package uploaded to Hackage. I would appreciate any feedback. Best Regards, Diego Echeverri

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definition of sum
by TeXitoi 16 Mar '10

16 Mar '10

After programming as an exercice the sum function, my version is faster than the Data.List's version. Looking at the source code, Data.List uses a foldl and not a foldl'. foldl' seems faster and allows to use very big lists. So, why is foldl used by Data.List for sum? -- Guillaume Pinot http://www.irccyn.ec-nantes.fr/~pinot/ « Les grandes personnes ne comprennent jamais rien toutes seules, et c'est fatigant, pour les enfants, de toujours leur donner des explications... » -- Antoine de Saint-Exupéry, Le Petit Prince () ASCII ribbon campaign -- Against HTML e-mail /\ http://www.asciiribbon.org -- Against proprietary attachments

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HTTP package freezes on Windows 7
by Phil 15 Mar '10

15 Mar '10

Hi, I'm using GHC 6.12.1 on Windows 7. I've built the latest Network package using Haskell's MinGW and installed HTTP package on top of this. The code below builds fine, but on execution it just sits there grabbing ever increasing amounts of memory. It's a simplified call that I've got working fine in Linux. Is this a known issue? Anyone else had success using HTTP from Windows? Thanks, Phil. import qualified Network.HTTP as HTTP main :: IO () main = do x <- HTTP.simpleHTTP(HTTP.getRequest "http://maps.google.com/maps/api/geocode/json?address=London&sensor=false") print x

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How to do the "permutation and combination" thing?
by Magicloud Magiclouds 15 Mar '10

15 Mar '10

Hi, For example, I have this: list1 = [a, b, c] list2 = [d, e, f] list3 = [g, h, i] Now I want: [ [(a, d, g), (b, e, h), (c, f, i)] , ... ] -- a list that contains all the combinations. How to do it pretty? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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How to define instance for type synonym with parameter.
by Vasyl Pasternak 15 Mar '10

15 Mar '10

Hello, I am start with example. Suppose I have the following type synonym: type SomeMonad a = ErrorT String (ReaderT String IO) a this is monad, and I want to make it instance of Applicative, so, the obvious way is to write the following: instance Applicative SomeMonad where pure = return (<*>) = ap GHCi warns me, that I have to use -XTypeSynonymInstances option to allow this construction, but than I have following error: Type synonym `SomeMonad' should have 1 argument, but has been given 0 In the instance declaration for `Applicative SomeMonad' Neither `instance Applicative (SomeMonad a)` nor `instance Applicative SomeMonad a` help. But works the following: instance Applicative (ErrorT String (ReaderT String IO)) where pure = return (<*>) = ap Which is the same (from my point of view). Could anyone tell me what is going on, and how to declare SomeMonad as instance of Applicative ? Thanks, Vasyl

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