- Haskell-Cafe - Haskell.org

http-enumerator: redirects, streaming and keep-alive
by Michael Snoyman 07 Feb '11

07 Feb '11

Hi all, There are a number of new feature requests for http-enumerator, and I wanted to discuss some possible API changes with everyone: * Allow keep-alive requests, which will reuse the same connection. * Allow client code to determine whether it accepts a server's SSL certificate. * Allow the request body to be an Enumerator so we can send large request bodies without using lazy IO. This is already implemented via streamingHttp, but it would make more sense to modify the Request datatype to replace L.ByteString with (Int, Enumerator ByteString IO ()). * Fix redirect behavior. It seems that the right thing to do is change a POST into a GET and remove the request body for a 303, and resend the same request for all other 3xx codes. However, when using an Enumerator for the request body, it is not guaranteed that we can resend an Enumerator. For addressing the last two, my plan is to: * Change the Request datatype as mentioned, but provide a convenience method for converting a lazy ByteString into a (Int, Enumerator ByteString IO ()). * Implement redirecting as I have described, and explain in the documentation that automatic redirecting can only work with Enumerators that can be run multiple times. AFAIK, this works with most standard Enumerators, such as enumFile, and with the helper function I will provide to convert lazy ByteStrings to Enumerators. * Remove streamingHttp, as it will no longer be necessary. As far as keep-alive goes, I still need to do a bit more research, but my basic idea (with credit to Bryan O'Sullivan): * http (and family) will all take an extra argument, Maybe Manager. * Manager will be an abstract type that will keep an MVar (Map (Host, Port, IsSecure) Socket). * If http is provided with a Manager, then it uses the Socket available in the Manager. If none is available, it creates a new Socket and places it in the Manager. * If http is *not* provided with a Manager, then it creates a new socket and closes it before returning. * There will be a newManager :: IO Manager, and a closeManager :: Manager -> IO (), which closes all Sockets in the Manager and empties out the inner Map. Some open questions are: * Who is responsible to recover if a server is no longer responding on a Socket from the Manager: http or the client code? I would assume http. * I haven't fully thought through how this will work with secure connections: most likely in addition to storing the Socket in the Manager I will need to store some certificate data. And speaking of certificates, the main concern I have here is data types: since http-enumerator can use either tls or OpenSSL, there isn't a single certificate datatype I can use. If someone wants to help out and write some code to convert an OpenSSL X509 datatype into a Certificate from the certificate package (which tls uses), I will be very grateful. Assuming we had such a unified datatype, I would recommend changing Request's secure record to be Certificate -> IO Bool, where returning True means the certificate is accepted and False means it is rejected. To get the current functionality of trusting any certificate, we would use (const $ return True). Any thoughts? Michael

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ANN: case-insensitive-0.1
by Bas van Dijk 07 Feb '11

07 Feb '11

Dear all, I had this old module laying around that I wrote some time ago. I dusted it off and uploaded it to Hackage: http://hackage.haskell.org/package/case-insensitive-0.1 The package provides the module Data.CaseInsensitive which exports the CI type constructor which can be parameterised by a string-like type like: String, ByteString, Text, etc.. Comparisons of values of the resulting type are then insensitive to cases. It is my hope that one day packages like snap-core[1], http-types[2] or wai[3] all use this common way of making strings case insensitive. Regards, Bas [1] http://hackage.haskell.org/packages/archive/snap-core/latest/doc/html/Data-… [2] http://hackage.haskell.org/packages/archive/http-types/0.3.0/doc/html/Netwo… [3] http://hackage.haskell.org/packages/archive/wai/latest/doc/html/Network-Wai…

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hsc2hs not using foreign.c types?
by Edward Amsden 07 Feb '11

07 Feb '11

I'm just now trying to figure out how to use the FFI, along with hsc2hs. When I use the #type ... directive in my .hsc files to pull in a c typedef that is aliased to an integer, it puts in "Word32" instead of CInt. I was hoping to use this in my foreign imports: Two questions: - Can I declare a foreign import of a c function that takes a c 'int' to have a 'word32' input type in Haskell and will that function correctly? - Why doesn't hsc2hs use 'CInt' instead? -- Edward Amsden Undergraduate Computer Science Rochester Institute of Technology www.edwardamsden.com

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Extending GHCi
by C K Kashyap 07 Feb '11

07 Feb '11

Hi, I am looking for a way to extend GHCI such that I can do something like this $ ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> startMyFunction Prelude> startMyFunction will do a forkIO and listen on a network port for interaction with a remote process and will drop back to GHCI prompt where I can invoke haskell functions that'll control the way the interaction with the remote process occurs. Can this be done? Regards, Kashyap

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fetching BLOBs
by Dmitry Olshansky 07 Feb '11

07 Feb '11

Hello, cafe, Is there a way to get BLOB fields from OracleDB using Takusen? What about other possibilities (through HDBC - ODBC)? Dima

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Re: [Haskell-cafe] Extending GHCi
by Kashyap C 07 Feb '11

07 Feb '11

I tried a fresh install of Haskell platform on my Vista box and I find that starting the thread does not return to the prompt. Is there some setting I need to do? Sent from my Windows Phone ------------------------------ From: Steffen Schuldenzucker Sent: Monday, 7 February 2011 6:46 PM To: C K Kashyap Cc: Haskell Cafe Subject: Re: [Haskell-cafe] Extending GHCi On 02/07/2011 12:45 PM, C K Kashyap wrote: > $ ghci > GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help > Loading package ghc-prim ... linking ... done. > Loading package integer-gmp ... linking ... done. > Loading package base ... linking ... done. > Loading package ffi-1.0 ... linking ... done. > Prelude> :m +Data.IORef Control.Concurrent Control.Monad > Prelude Data.IORef Control.Concurrent Control.Monad> msg <- newIORef > "Hello" > Prelude Data.IORef Control.Concurrent Control.Monad> let echo = > forever $ readIORef msg >>= putStrLn >> threadDelay 3000000 > Prelude Data.IORef Control.Concurrent Control.Monad> t <- forkIO echo > Hello > Prelude Data.IORef Control.Concurrent Control.Monad> Hello > Hello > writeIORefHello msg "World" > Prelude Data.IORef Control.Concurrent Control.Monad> World > World > On my mac, this works..but on Linux, the moment I do t <- forkIO ... , it starts off a thread in the foreground and does not return to the prompt. Strange. Works for me (ghc 6.12.1 on Debian squeeze).

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Re: [Haskell-cafe] Byte Histogram
by John Lato 07 Feb '11

07 Feb '11

> > From: Ivan Lazar Miljenovic <ivan.miljenovic(a)gmail.com> > > On 7 February 2011 12:30, wren ng thornton <wren(a)freegeek.org> wrote: > > On 2/6/11 4:53 PM, Ivan Lazar Miljenovic wrote: > >> > >> On 7 February 2011 08:12, Alexey Khudyakov<alexey.skladnoy(a)gmail.com> > >> ?wrote: > >>> > >>> Also there is a container-classes package which provide set of type > class > >>> for containers. > >>> > >>> [1] http://hackage.haskell.org/package/container-classes > >> > >> Don't use that package: it's broken. ?I did start on a re-write that > >> works (and there are similar libraries around that I believe do work), > >> but gave up because it was a) too fiddly, and b) I decided that the > >> use case I had for them wasn't worth it (as it would just make that > >> library more complicated and difficult to use, possibly also less > >> efficient). ?If anyone wants it I can certainly send them what I've > >> got though. > > > > Any chance you could push a 0.0.0.1 version which tells people this, so > that > > they know to avoid it? > > Didn't think about that; I'll try to do so tonight. > Perhaps a better approach would be to ask the hackage maintainer (Ross Patterson, if I'm not mistaken) to deprecate container-classes. John

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Snap 0.4 Released!
by Gregory Collins 06 Feb '11

06 Feb '11

The Snap team is proud to announce the release of Snap 0.4, containing a whole bunch of nifty new features. Here is what we've been up to: New Features ------------ - Heist now uses the brand new xmlhtml HTML5 parser for parsing and rendering templates. This eliminates our dependency on XML and makes it possible to include inline javascript/css in templates. Check out Chris Smith's blog post for more information about xmlhtml. - Along with the change to xmlhtml, we decided to convert Heist to use Text instead of ByteString. This is a backwards-incompatible change which breaks old code, but which we feel is the right thing to do. - Snap now has support for file uploads (!) and the multipart/form-data content type. Snap's file upload support uses iteratees to stream uploaded data and comes with a convenience function to writeuploaded files to a temporary directory. We put significant effort into preventing denial of service attacks and providing policy controls for things like maximum allowable file size, upload timeouts, minimum upload speed, etc. - The web server now uses blaze-builder in the output response body Enumerator. Besides being significantly faster for most workloads than ByteString enumeration, this allowed us to save several copies within the server code, giving us a moderate performance improvement. However, this will break any existing code making direct use of the output enumerator rather than using convenience functions like writeBS. We encourage users who are building up large responses out of lots of little bytestring chunks to consider switching their code to using Builder and writeBuilder to get a speed boost. - The "development mode" of projects built using our snap project starter is now quite a bit smarter. The 0.3 version used hint to interpret web handlers on-the-fly, but it re-interpreted the code on each request, making keeping in-memory state between requests impossible. The development mode now only rebuilds the project when files actually change. State will now be preserved across requests as long as the project files are not changed on disk. - Our file serving code has been substantially improved/rewritten. The new code can automatically generate stylable/themable directory indexes, provides configurable lists of index files, and allows the user to plug in dynamic handlers. These handlers can be used to perform arbitrary transformations based on file type or file content on the fly. The new code also correctly handles trailing slashes for relative path resolution. - It is now possible for user handlers to modify socket timeouts, using the setTimeout function. This (finally) makes it possible to have long-running request handlers. Bugfixes/Improvements --------------------- - Debugging support in snap-core is now turned off by default, as it involves too much of a performance impact. Turning on debug support results in a ~25% performance penalty. To turn debugging output back on, pass the -f debug flag when installing snap-core. - Bugfix: we no longer log spurious "thread killed" messages in the simple backend under HTTP/1.0. - Added catchFinishWith function. - Changed cookie interface to expose cookies in the response as a map for easier manipulation. As always, please let us know if you find any issues with the release. Thanks! -- Gregory Collins <greg(a)gregorycollins.net>

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Concurrency best practices?
by wren ng thornton 06 Feb '11

06 Feb '11

So I'm working on a project that uses STM to run a lot of things in parallel without the headaches of locks. So far it's working beautifully, STM rocks. But there's one snag... Sometimes I need those threads to do some IO like printing logging info. I'd like to make these IO chunks atomic with respect to one another so that the logging output isn't garbage. Since I'm using STM everywhere else, I'd love to use it for this too (instead of mixing STM and non-STM concurrency) except you can't embed IO in STM. I could just use STM vars as locks to force the atomicity, but locks are ugly and bug-prone. So what's the current best practice for doing this kind of thing? -- Live well, ~wren

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ANN: monad-control-0.1
by Bas van Dijk 06 Feb '11

06 Feb '11

Dear all, Several attempts have been made to lift control operations (functions that use monadic actions as input instead of just output) through monad transformers: MonadCatchIO-transformers[1] provided a type class that allowed to overload some often used control operations (catch, block and unblock). Unfortunately that library was limited to those operations. It was not possible to use, say, alloca in a monad transformer. More importantly however, the library was broken as was explained[2] by Michael Snoyman. In response Michael created the MonadInvertIO type class which solved the problems. Then Anders Kaseorg created the monad-peel library which provided an even nicer implementation. monad-control is a rewrite of monad-peel that uses CPS style operations and exploits the RankNTypes language extension to simplify and speedup most functions. A very preliminary and not yet fully representative, benchmark shows that monad-control is on average about 2.6 times faster than monad-peel: bracket: 2.4 x faster bracket_: 3.1 x faster catch: 1.8 x faster try: 4.0 x faster mask: 2.0 x faster Note that, although the package comes with a test suite that passes, I still consider it highly experimental. API DOCS: http://hackage.haskell.org/package/monad-control INSTALLING: $ cabal update $ cabal install monad-control TESTING: The package contains a copy of the monad-peel test suite written by Anders. You can perform the tests using: $ cabal unpack monad-control $ cd monad-control $ cabal configure -ftest $ cabal test BENCHMARKING: $ darcs get http://bifunctor.homelinux.net/~bas/bench-monad-peel-control/ $ cd bench-monad-peel-control $ cabal configure $ cabal build $ dist/build/bench-monad-peel-control/bench-monad-peel-control DEVELOPING: The darcs repository will be hosted on code.haskell.org ones that server is back online. For the time being you can get the repository from: $ darcs get http://bifunctor.homelinux.net/~bas/monad-control/ TUTORIAL: This short unpolished tutorial will explain how to lift control operations through monad transformers. Our goal is to lift a control operation like: foo ∷ M a → M a where M is some monad, into a transformed monad like 'StateT M': foo' ∷ StateT M a → StateT M a The first thing we need to do is write an instance for the MonadTransControl type class: class MonadTrans t ⇒ MonadTransControl t where liftControl ∷ (Monad m, Monad n, Monad o) ⇒ (Run t n o → m a) → t m a If you ignore the Run argument for now, you'll see that liftControl is identical to the 'lift' method of the MonadTrans type class: class MonadTrans t where lift ∷ Monad m ⇒ m a → t m a So the instance for MonadTransControl will probably look very much like the instance for MonadTrans. Let's see: instance MonadTransControl (StateT s) where liftControl f = StateT $ \s → liftM (\x → (x, s)) (f run) So what is this run function? Let's look at its type: type Run t n o = ∀ b. t n b → n (t o b) The run function executes a transformed monadic action 't n b' in the non-transformed monad 'n'. In our case the 't' will be a StateT computation. The only way to run a StateT computation is to give it some state and the only state we have lying around is the one from the outer computation: 's'. So let's run it on 's': instance MonadTransControl (StateT s) where liftControl f = StateT $ \s → let run t = ... runStateT t s ... in liftM (\x → (x, s)) (f run) Now that we are able to run a transformed monadic action, we're almost done. Look at the type of Run again. The function should leave the result 't o b' in the monad 'n'. This 't o b' computation should contain the final state after running the supplied 't n b' computation. In case of our StateT it should contain the final state s': instance MonadTransControl (StateT s) where liftControl f = StateT $ \s → let run t = liftM (\(x, s') → StateT $ \_ → return (x, s')) (runStateT t s) in liftM (\x → (x, s)) (f run) This final computation, "StateT $ \_ → return (x, s')", can later be used to restore the final state. Now that we have our MonadTransControl instance we can start using it. Recall that our goal was to lift "foo ∷ M a → M a" into our StateT transformer yielding the function "foo' ∷ StateT M a → StateT M a". To define foo', the first thing we need to do is call liftControl: foo' t = liftControl $ \run → ... This captures the current state of the StateT computation and provides us with the run function that allows us to run a StateT computation on this captured state. Now recall the type of liftControl ∷ (Run t n o → m a) → t m a. You can see that in place of the ... we must fill in a value of type 'm a'. In our case this will be a value of type 'M a'. We can construct such a value by calling foo. However, foo expects an argument of type 'M a'. Fortunately we can provide one if we convert the supplied 't' computation of type 'StateT M a' to 'M a' using our run function of type ∀ b. StateT M b → M (StateT o b): foo' t = ... liftControl $ \run → foo $ run t However, note that the run function returns the final StateT computation inside M. So the type of the right hand side is now 'StateT M (StateT o b)'. We would like to restore this final state. We can do that using join: foo' t = join $ liftControl $ \run → foo $ run t That's it! Note that because it's so common to join after a liftControl I provide an abstraction for it: control = join ∘ liftControl Allowing you to simplify foo' to: foo' t = control $ \run → foo $ run t Probably the most common control operations that you want to lift through your transformers are IO operations. Think about: bracket, alloca, mask, etc.. For this reason I provide the MonadControlIO type class: class MonadIO m ⇒ MonadControlIO m where liftControlIO ∷ (RunInBase m IO → IO a) → m a Again, if you ignore the RunInBase argument, you will see that liftControlIO is identical to the liftIO method of the MonadIO type class: class Monad m ⇒ MonadIO m where liftIO ∷ IO a → m a Just like Run, RunInBase allows you to run your monadic computation inside your base monad, which in case of liftControlIO is IO: type RunInBase m base = ∀ b. m b → base (m b) The instance for the base monad is trivial: instance MonadControlIO IO where liftControlIO = idLiftControl idLiftControl directly executes f and passes it a run function which executes the given action and lifts the result r into the trivial 'return r' action: idLiftControl ∷ Monad m ⇒ ((∀ b. m b → m (m b)) → m a) → m a idLiftControl f = f $ liftM $ \r -> return r The instances for the transformers are all identical. Let's look at StateT and ReaderT: instance MonadControlIO m ⇒ MonadControlIO (StateT s m) where liftControlIO = liftLiftControlBase liftControlIO instance MonadControlIO m ⇒ MonadControlIO (ReaderT r m) where liftControlIO = liftLiftControlBase liftControlIO The magic function is liftLiftControlBase. This function is used to compose two liftControl operations, the outer provided by a MonadTransControl instance and the inner provided as the argument: liftLiftControlBase ∷ (MonadTransControl t, Monad base, Monad m, Monad (t m)) ⇒ ((RunInBase m base → base a) → m a) → ((RunInBase (t m) base → base a) → t m a) liftLiftControlBase lftCtrlBase = \f → liftControl $ \run → lftCtrlBase $ \runInBase → f $ liftM (join ∘ lift) ∘ runInBase ∘ run Basically it captures the state of the outer monad transformer using liftControl. Then it captures the state of the inner monad using the supplied lftCtrlBase function. If you recall the identical definitions of the liftControlIO methods: 'liftLiftControlBase liftControlIO' you will see that this lftCtrlBase function is the recursive step of liftLiftControlBase. If you use 'liftLiftControlBase liftControlIO' in a stack of monad transformers a chain of liftControl operations is created: liftControl $ \run1 -> liftControl $ \run2 -> liftControl $ \run3 -> ... This will recurse until we hit the base monad. Then liftLiftControlBase will finally run f in the base monad supplying it with a run function that is able to run a 't m a' computation in the base monad. It does this by composing the run and runInBase functions. Note that runInBase is basically the composition: '... ∘ run3 ∘ run2'. However, just composing the run and runInBase functions is not enough. Namely: runInBase ∘ run ∷ ∀ b. t m b → base (m (t m b)) while we need to have ∀ b. t m b → base (t m b). So we need to lift the 'm (t m b)' computation inside t yielding: 't m (t m b)' and then join that to get 't m b'. Now that we have our MonadControlIO instances we can start using them. Let's look at how to lift 'bracket' into a monad supporting MonadControlIO. Before we do that I define a little convenience function similar to 'control': controlIO = join ∘ liftControlIO Bracket just calls controlIO which captures the state of m and provides us with a runInIO function which allows us to run an m computation in IO: bracket ∷ MonadControlIO m ⇒ m a → (a → m b) → (a → m c) → m c bracket before after thing = controlIO $ \runInIO → E.bracket (runInIO before) (\m → runInIO $ m >>= after) (\m → runInIO $ m >>= thing) I welcome any comments, questions or patches. Regards, Bas [1] http://hackage.haskell.org/package/MonadCatchIO-transformers [2] http://docs.yesodweb.com/blog/invertible-monads-exceptions-allocations/ [3] http://hackage.haskell.org/package/monad-peel

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community server & hasktags learned recursing into directories
by Marc Weber 06 Feb '11

06 Feb '11

hasktags learned about how to recurse into subdirectories itself. This is especially useful for windows because writing scripts can be done but is less well known (http://stackoverflow.com/questions/4865391/answer/submit) Now I'd like to push the latest changes to the community server which was hosting the repo in the past but my account does no longer work? Also there are some dead links now. Eg http://www.haskell.org/haskellwiki/Haskell.org_domain still references http://community.haskell.org/admin/ (404) Do you know what happened and where I should host the repo now? (uploading it to github would take seconds only) Moreover I fonud that I was no longer subscribed to haskell-cafe. I can't remember having unsubscribed ? Let's hope this mail get's through now after having resubscribed. Marc Weber

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Storing passwords securely
by Peter Scott 06 Feb '11

06 Feb '11

The usual advice on how to store passwords securely is "use bcrypt", but since there seem to be no Haskell bindings for bcrypt, the other good option is to iterate a salted hash function at least 1000 times. In order for people to get this right, there should be a library with a really simple API that makes it Just Work. I think I have such an API, but I'd like to hear if anybody else has suggestions before I go releasing it onto Hackage. The code is here: https://github.com/PeterScott/pwstore The part of the API that people have to care about is two functions. makePassword creates a hashed, salted password that you can store in a database. verifyPassword takes this hashed, salted password and a user's password input, and tells you if it matches. Like this: >>> makePassword (B.pack "hunter2") 12 "sha256|12|lMzlNz0XK9eiPIYPY96QCQ==|1ZJ/R3qLEF0oCBVNtvNKLwZLpXPM7bLEy/Nc6QBxWro=" >>> verifyPassword (B.pack "wrong guess") passwordHash False >>> verifyPassword (B.pack "hunter2") passwordHash True There's also a function for increasing the number of hash iterations on stored password hashes, to compensate for Moore's law. Does this sound reasonable? Also, I have a pure-Haskell version and a version which depends on some C code, for speed (about 25x difference). Does anybody care about the pure Haskell version, or should I just drop it and require the faster C/Haskell mixed version? Thanks, -Peter

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Re: [Haskell-cafe] combined parsing & pretty-printing
by Ozgur Akgun 05 Feb '11

05 Feb '11

Great! That was pretty fast :) Are you going to update invertible-syntax to use partial-isomorphisms-0.2? 2011/2/5 Tillmann Rendel <tillmann(a)rendel.net> > Hi Ozgur, > > Ozgur Akgun wrote: > >> I've already implemented a toy example and it worked great. Now I am >> trying >> to use your library in a more serious piece of code, and I've realised >> that >> defineIsomorphisms doesn't support record constructors. To be precise, you >> handle the case for NormalC in the code, however you do not handle RecC. I >> don't think this will be a difficult thing to add. Is there any reason for >> you not to support RecC? >> > > No reason at all. I just generalized the Template Haskell macros. They now > support: > > * newtypes > * infix constructors > * record constructors > > Infix constructors are only support by constructorIso, though, not by > defineIsomorphisms, because I can't think of a good automatic naming scheme. > Suggestions welcome. > > The code is on hackage. > > There are still some constructors which are not supported: constructors of > GADTs as well as data or newtype instances, and constructors with > existentials. I would need to think about typing issues before supporting > these. > > Tillmann > -- Ozgur Akgun

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combined parsing & pretty-printing
by Ozgur Akgun 05 Feb '11

05 Feb '11

Dear Café, I working on a DSL represented by a algebraic data type with many constructors. I can write (separately) a parser and a pretty-printer for it, and I am doing so at the moment. However, this way it feels like repeating the same information twice. Is there any work to combine the two? Best, Ozgur

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Problem with xhtml 1.5.1.1: html tags are split
by Gwern Branwen 05 Feb '11

05 Feb '11

On Thu, May 13, 2010 at 11:38 AM, Gwern Branwen <gwern0(a)gmail.com> wrote: > On Tue, May 11, 2010 at 3:00 AM, Bjorn Bringert <bjorn(a)bringert.net> wrote: >> I support finding a new maintainer. > > Alright; as the old maintainer, I guess it falls on you to advertise > on -cafe and libraries. Has a request gone out yet? I didn't notice anything (maybe it got lost in all the recent haskell.org downtime), and the current Hackage page of xhtml is still listing Bjorn. Anyway, John MacFarlane has found a workaround which he's implemented in HEAD Pandoc (http://code.google.com/p/pandoc/issues/detail?id=134) from http://groups.google.com/group/pandoc-discuss/browse_frm/thread/915586b5c5e… : > Many people have wanted pandoc to produce "more normal" HTML output, > without nesting, but with line breaks between block elements (where > they aren't semantically significant). Something like this: > > <ul> > <li>a <a href="/foo">link</a></li> > <li>another list item</li> > </ul> > > I've found that I can get this kind of output, even using Text.XHtml's > renderer, by modifying the writer to insert raw newlines after block > elements, and using 'showHtmlFragment' rather than 'prettyHtmlFragment'. This may be useful for other people to know. (There are quite a few users of xhtml: http://bifunctor.homelinux.net/~roel/cgi-bin/hackage-scripts/revdeps/xhtml-… ) -- gwern http://www.gwern.net

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Re: [Haskell-cafe] Byte Histogram
by Janis Voigtländer 05 Feb '11

05 Feb '11

On 2/5/11 4:26 AM, Claus Reinke wrote: >> Lately I've been trying to go the other direction: make a large >> section of formerly strict code lazy. > > There used to be a couple of tools trying to make suggestions when a > function could be made less strict (Olaf Chitil's StrictCheck and > another that escapes memory at the moment). Probably Sloth: http://korsika.informatik.uni-kiel.de/~jac/wordpress/ http://korsika.informatik.uni-kiel.de/~jac/wordpress/wp-content/uploads/PAD… Best, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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MonadPeelIO instance for monad transformers on top of "forall"
by Max Bolingbroke 05 Feb '11

05 Feb '11

Hi Anders, I'm using your monad-peel package to good effect in my project, but I'm having trouble lifting peelIO through a particular monad. What follows is a simplified description of my problem. Say I have this monad: {{{ data M a = M { unM :: forall m. MonadPeelIO m => Reader.ReaderT () m a } instance Monad M where return x = M (return x) M mx >>= fxmy = M $ mx >>= unM . fxmy instance MonadIO M where liftIO io = M (liftIO io) }}} It seems "clear" that there should be a MonadPeelIO instance for M, but I can't for the life of me figure it out. I've tried: {{{ instance MonadPeelIO M where peelIO = M (fmap (\peel (M mx) -> liftM M (peel mx)) peelIO) }}} But this is not polymorphic enough: the peelIO gives me back a function (ReaderT () m a -> IO (ReaderT () m a)) for some *fixed* m, so I can't pack the result of that function into an M again using (liftM M). Have you (or the big brains on Haskell-Cafe, who are CCed) come across this before? Is there an obvious solution I am missing? Cheers, Max

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Haskell wiki admins -- who are they?
by Jason Dusek 04 Feb '11

04 Feb '11

I am having some trouble getting in to my account on the Haskell wiki. I requested a password ~24 hours ago reset but have not received the email yet. Who should I contact about this? -- Jason Dusek Linux User #510144 | http://counter.li.org/

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ANN: HackageOneFive: Reverse dependency lookup for all packages on Hackage
by Simon Hengel 04 Feb '11

04 Feb '11

Hello, I wrote a tiny Snap app that provides reverse dependency lookup for all packages on Hackage. A git repository is at: https://github.com/sol/HackageOneFive Setup instructions are provide in the README[1] file. It uses a PostgreSQL database for storage but it should be trivial to port this to e.g. SQLite. As always feedback and patches are gladly welcome! btw: Is there still progress on Hackage 2.0? Cheers, Simon [1] https://github.com/sol/HackageOneFive#readme

2 2

Re: [Haskell-cafe] OSX i386/x86 and x86_64 - time to switch supported platforms?
by John Lato 04 Feb '11

04 Feb '11

> > From: Max Bolingbroke <batterseapower(a)hotmail.com> > > On 4 February 2011 02:35, Steve Severance <steve(a)medwizard.net> wrote: > > Wholly support moving OSX to x64. x86 should be supported only on a > > best effort basis for legacy. > > Moving from x86 to x64 has advantages and disadvantages from my POV. > Advantages: > * Able to address more memory > * More registers for code generation > * Haskell dependencies wouldn't need to be built for x86 on Snow > Leopard (though if we swapped to x64 on Leopard as well, the Leopard > users would start having to build 64-bit libraries specially) > > Disadvantages: > * Pointers become wider, and Haskell data structures mostly consist > of pointers. This will bloat memory use of Haskell programs. > * Generated binaries won't work on older Macs that don't have a > 64-bit OS/CPU. This is important if you are distributing compiled > Haskell binaries, which is not something I personally do but which is > probably important to support > > Did I miss anything? > I'm moderately in favor of making the primary supported platform x86_64, except for the distribution issue. If i386 were maintained but not necessarily targeted for improvement that would be fine by me. > I don't know if anyone using a 64-bits GHC on e.g. Linux has reported > experience of whether moving to 64-bits is a net win or not from a > performance perspective. My guess is that it is a win for certain > classes of programs (numerically intensive, "high performance > Haskell"), and a loss for programs making extensive use of laziness, > boxed data structures etc. > For my code, x86_64 (OS X) is a net win. It's sometimes significantly faster, and hasn't ever been noticeably slower. The biggest gains seems to be I/O, although I haven't really investigated this. Cheers, John

2 1

Problems with iteratees
by wren ng thornton 04 Feb '11

04 Feb '11

I'm working on a project that's using John Lato's old implementation of iteratees (iteratee >= 0.3.5 && < 0.4; I'm hoping to migrate to 0.7 soon, but that's a ways off yet) and I'm running into some issues I haven't been able to untangle. Maybe a new set of eyes can help... The overarching program brings three things together for doing some interprocess communication: the medium is Posix FIFOs, the messages themselves are encoded with Google's Protocol Buffers[1], and the control flow for getting and processing the messages is handled by iteratees. The error message indicates iteratees are at fault, though it could be an error elsewhere instead. First, some boilerplate. -- For messageWithLengthEnumeratee only {-# LANGUAGE ScopedTypeVariables #-} import qualified Text.ProtocolBuffers.Reflections as R import qualified Text.ProtocolBuffers.WireMessage as W import qualified Text.ProtocolBuffers.Get as G import qualified Data.ByteString as S import qualified Data.ByteString.Lazy as L import qualified Data.Iteratee as I import Data.Iteratee.WrappedByteString import Data.Word (Word8) import Control.Monad (liftM) -- | Return a final value, and the remainder of the stream. idone :: a -> c el -> I.IterGV c el m a idone a xs = I.Done a (I.Chunk xs) {-# INLINE idone #-} -- | Convert a continuation into 'I.IterGV'. icontinue :: (I.StreamG c el -> m (I.IterGV c el m a)) -> I.IterGV c el m a icontinue k = I.Cont (I.IterateeG k) Nothing {-# INLINE icontinue #-} -- | Throw an error message. ifail :: (Monad m) => String -> I.IterGV c el m a ifail msg = ierror (I.Err msg) {-# INLINE ifail #-} -- | An 'I.IterGV' variant of 'I.throwErr'. ierror :: (Monad m) => I.ErrMsg -> I.IterGV c el m a ierror err = I.Cont (I.throwErr err) (Just err) {-# INLINE ierror #-} toLazyBS :: S.ByteString -> L.ByteString toLazyBS = L.fromChunks . (:[]) {-# INLINE toLazyBS #-} toStrictBS :: L.ByteString -> S.ByteString toStrictBS = S.concat . L.toChunks {-# INLINE toStrictBS #-} Now we have the code for converting the Get monad used by protocol buffers into an iteratee. This should be correct, and it's pretty straightforward. -- | Convert a 'G.Result' iteratee state into a 'I.IterGV' -- iteratee state. result2iterv :: (Monad m) => G.Result a -> I.IterGV WrappedByteString Word8 m a result2iterv (G.Finished rest _ a) = idone a (WrapBS $ toStrictBS rest) result2iterv (G.Failed _ msg) = ifail msg result2iterv (G.Partial k) = I.Cont (iterify k) Nothing -- | Convert a protobuf-style continuation into an -- iteratee-style continuation. iterify :: (Monad m) => (Maybe L.ByteString -> G.Result a) -> I.IterateeG WrappedByteString Word8 m a iterify k = I.IterateeG $ \s -> return $! case s of I.Chunk (WrapBS xs) -> result2iterv $ k (Just $ toLazyBS xs) I.EOF Nothing -> result2iterv $ k Nothing I.EOF (Just err) -> ierror err -- | A variant of 'G.runGet' as an iteratee. runGetIteratee :: (Monad m, R.ReflectDescriptor a, W.Wire a) => G.Get a -> I.IterateeG WrappedByteString Word8 m a runGetIteratee g = I.IterateeG $ \s -> return $! case s of I.Chunk (WrapBS xs) -> result2iterv $ G.runGet g (toLazyBS xs) I.EOF Nothing -> result2iterv $ G.runGet g L.empty I.EOF (Just err) -> ierror err Okay, now we have an iteratee which consumes a stream of bytestrings and will render a protocol buffer message. But what we really want is an enumeratee to do this repeatedly so we can use an iteratee to consume the stream of messages. I have the following definition which typechecks, but doesn't seem to work. The call to convStream seems like it always hangs: -- | A variant of 'G.runGet' as an enumeratee. runGetEnumeratee :: (Monad m, R.ReflectDescriptor a, W.Wire a) => G.Get a -> I.EnumeratorN WrappedByteString Word8 [] a m b runGetEnumeratee = I.convStream . liftM (Just . (:[])) . runGetIteratee Once we have a working definition of runGetEnumeratee, then we can define the specific enumeratee we need: -- | An enumeratee for converting bytestrings into protocol -- buffer messages. messageWithLengthEnumeratee :: forall m msg a . (Monad m, R.ReflectDescriptor msg, W.Wire msg) => I.EnumeratorN WrappedByteString Word8 [] msg m a messageWithLengthEnumeratee = runGetEnumeratee (W.messageWithLengthGetM :: G.Get msg) And then at the use site we have the following: let processRequest = ... :: msg -> IO () I.run -- run the [()], sending EOF . I.joinIM -- push monadic effects inside . I.enumFdFollow fifo_in -- read from the Fd forever . I.joinI -- when EOF bytestrings, EOF msgs . messageWithLengthEnumeratee -- ByteStrings -> messages $ I.mapM_ processRequest -- process messages I think this part is correct too, but just to be sure... the goal is that we should read bytestrings from the FIFO forever (or until the other process closes their end), and then we read off the messages one by one, handing them off to processRequest to interpret them and respond accordingly. When I put this all together, the process is killed with: control message: Just (Err "endOfInput") Data.Iteratee.Base.run is the origin of the "control message:" part of the error, but I don't know where (Err "endOfInput") is coming from since Data.Iteratee.Base only uses (Err "EOF") or (Err "Divergent Iteratee"). I believe runGetEnumeratee is where the problem is, though it could also be the use site or something in one of the libraries. Any help would be appreciated. [1] http://hackage.haskell.org/package/protocol-buffers http://hackage.haskell.org/package/hprotoc -- Live well, ~wren

3 6

How to #include into .lhs files?
by Conal Elliott 04 Feb '11

04 Feb '11

Does anyone have a working example of #include'ing Haskell code into a bird-tracks-style .lhs file with GHC? Every way I try leads to parsing errors. Is there documentation about how it's supposed to work? Help much appreciated. - Conal

4 4

OSX i386/x86 and x86_64 - time to switch supported platforms?
by Max Cantor 04 Feb '11

04 Feb '11

I originally posted this on haskell-GHC-users, but was curious how the wider community felt. The last 32-bit, Intel Mac was the Mac Mini, discontinued in August 2007. The bulk of them were discontinued in 2006, along with PowerPC Macs. Does it make sense to relegate OSX x86_64 to community status while the 32-bit version is considered a supported platform? Given that I'm far from experienced enough to be able to contribute meaningfully to GHC, I'm not complaining about anyone's efforts, just that those efforts might be a bit misallocated. I'd venture a guess that far more people are interested in running 64-bit GHC on OSX than in running GHC on what is now fairly antiquated hardware. mc

6 9

Instancing "Typeable" for monad transformers?
by John Millikin 04 Feb '11

04 Feb '11

Is there any reasonable way to do this if I want to cast a monadic value? For example: > castState :: (Typeable a, Typeable s, Typeable1 m, Typeable b) => a -> Maybe (StateT s m b) > castState = Data.Typeable.cast None of the common monad transformers declare instances of Typeable, so I don't know if the concept itself even works. The use case here is one of my library users wants to return an Iteratee from code running in "hint", which requires any extracted values be typeable. My first attempt at an extension-free instance is something like this: > import Data.Enumerator > import Data.Typeable > > instance (Typeable a, Typeable1 m) => Typeable1 (Iteratee a m) where > typeOf1 i = rep where > typed :: (a -> b) -> b -> a -> a > typed _ _ a = a > > ia :: a -> Iteratee a m b > ia = undefined > > im :: m c -> Iteratee a m b > im = undefined > > rep = mkTyConApp (mkTyCon "Data.Enumerator.Iteratee") [tyA, tyM] > tyA = typeOf (typed ia i undefined) > tyM = typeOf1 (typed im i undefined) which, besides being ugly, I have no idea if it's correct.

4 3

Problem cabal installing wxcore with ghc 7.0.1 on linux x86_64
by Alexander McPhail 04 Feb '11

04 Feb '11

zen:~# cabal install wxcore --enable-documentation --global Resolving dependencies... [1 of 1] Compiling Main ( /tmp/wxcore-0.12.1.617977/wxcore-0.12.1.6/Setup.hs, /tmp/wxcore-0.12.1.617977/wxcore-0.12.1.6/dist/setup/Main.o ) Linking /tmp/wxcore-0.12.1.617977/wxcore-0.12.1.6/dist/setup/setup ... generating: src/haskell/Graphics/UI/WXCore/WxcClassTypes.hs reading class definitions: parsing: src/include/wxc.h parsing: src/include/wxc_types.h parsing: src/include/wxc_glue.h parsing: src/include/db.h parsing: src/include/dragimage.h parsing: src/include/graphicscontext.h parsing: src/include/sound.h parsing: src/include/managed.h parsing: src/include/mediactrl.h parsing: src/include/previewframe.h parsing: src/include/printout.h parsing: src/include/textstream.h parsing: src/include/stc.h parsing: src/include/stc_gen.h generated 539 class definitions. ok. generating: src/haskell/Graphics/UI/WXCore/WxcClassInfo.hs reading class definitions: parsing: src/include/wxc.h parsing: src/include/wxc_types.h parsing: src/include/wxc_glue.h parsing: src/include/db.h parsing: src/include/dragimage.h parsing: src/include/graphicscontext.h parsing: src/include/sound.h parsing: src/include/managed.h parsing: src/include/mediactrl.h parsing: src/include/previewframe.h parsing: src/include/printout.h parsing: src/include/textstream.h parsing: src/include/stc.h parsing: src/include/stc_gen.h generated 381 class info definitions ok. parsing: src/include/wxc.h parsing: src/include/wxc_types.h parsing: src/include/wxc_glue.h parsing: src/include/db.h parsing: src/include/dragimage.h parsing: src/include/graphicscontext.h parsing: src/include/sound.h parsing: src/include/managed.h parsing: src/include/mediactrl.h parsing: src/include/previewframe.h parsing: src/include/printout.h parsing: src/include/textstream.h parsing: src/include/stc.h parsing: src/include/stc_gen.h ignore: parse error : //WXCOLORREF wxColour_GetPixel( TSelf(wxColour) _obj ); generating: src/haskell/Graphics/UI/WXCore/WxcClassesAL.hs reading class definitions: parsing: src/include/wxc.h parsing: src/include/wxc_types.h parsing: src/include/wxc_glue.h parsing: src/include/db.h parsing: src/include/dragimage.h parsing: src/include/graphicscontext.h parsing: src/include/sound.h parsing: src/include/managed.h parsing: src/include/mediactrl.h parsing: src/include/previewframe.h parsing: src/include/printout.h parsing: src/include/textstream.h parsing: src/include/stc.h parsing: src/include/stc_gen.h generated 1560 methods for 120 classes. generating: src/haskell/Graphics/UI/WXCore/WxcClassesMZ.hs generated 2184 methods for 123 classes. generating: src/haskell/Graphics/UI/WXCore/WxcClasses.hs generated 3744 total methods for 243 total classes. ok. parsing: src/eiffel/wxc_defs.e parsing: src/eiffel/wx_defs.e parsing: src/eiffel/stc.e generating: src/haskell/Graphics/UI/WXCore/WxcDefs.hs generated 2454 constant definitions ok. setup: failed cabal: Error: some packages failed to install: wxcore-0.12.1.6 failed during the configure step. The exception was: ExitFailure 1

1 0

Re: [Haskell-cafe] Problems with iteratees
by John Lato 04 Feb '11

04 Feb '11

> > From: wren ng thornton <wren(a)freegeek.org> > On 2/2/11 11:25 PM, Maciej Wos wrote: > > I think the problem is that the iteratee you give to I.convStream > > always returns Just [something] while you should return Nothing on > > EOF. > > That makes sense for the hanging problem (which I only noticed during > debugging). Though I still get the the same error message when running > the whole program... > I don't have too much to add to Maciej and Oleg's reply, except that I'd recommend looking at the Wave codec over the Tiff reader in those versions of iteratee. I don't think that's the only problem, though, because then you'd be getting a "Divergent iteratee" error. The "endOfInput" error is suspicious, and I think you'll need to track it down to solve this problem. It doesn't appear to be from either iteratee or protocol-buffers. Could it be coming from your IO library? I wonder if the enumerator is trying to force a read after EOF has been reached for some reason? As an experiment, you could try using a custom convStream function like this: convStream2 :: Monad m => IterateeG s el m (Maybe (s' el')) -> EnumeratorN s el s' el' m aconvStream2 fi iter = fi >>= check where check (Just xs) = lift (runIter iter (Chunk xs)) >>= docase check (Nothing) = return iter docase (Done a _) = return . return $ a docase (Cont k Nothing) = convStream2 fi k docase (Cont k (Just "endOfInput")) = convStream2 (return Nothing) k docase (Cont _ (Just e)) = return $ throwErr e This may help determine if it's a problem with IO or with the message parsing. John > > On Thu, Feb 3, 2011 at 10:06 AM, wren ng thornton<wren(a)freegeek.org> > wrote: > >> When I put this all together, the process is killed with: > >> > >> control message: Just (Err "endOfInput") > >> > >> Data.Iteratee.Base.run is the origin of the "control message:" part of > the > >> error, but I don't know where (Err "endOfInput") is coming from since > >> Data.Iteratee.Base only uses (Err "EOF") or (Err "Divergent Iteratee"). > I > >> believe runGetEnumeratee is where the problem is, though it could also > be > >> the use site or something in one of the libraries. Any help would be > >> appreciated. > > -- > Live well, > ~wren >

2 1

Haskell AES
by tsuraan 04 Feb '11

04 Feb '11

I'd like to use AES in a haskell program that I'm writing. I've come across three libraries in hackage that do AES: the library named AES, the Crypto library, and the SimpleAES library. The library named AES says in its documentation for the "crypt" function that it is not thread-safe. What exactly does this mean in haskell? Does it mean that if you have separate AESCtx instances in different forkIO "threads", that things will break? Or does it just mean that using the same AESCtx in multiple forkIO threads is a bad idea? I'm asking about that one comment because it looks like SimpleAES is probably a wrapper around AES, and the Crypto library also says it uses a library named AES, although one by an author not listed in the hackage AES library entry. Can anybody shed some light on the state of AES encryption in haskell? What is the preferred library, especially for us in a program that has many forkIO'd "threads"?

1 0

($) not as transparent as it seems
by Steffen Schuldenzucker 04 Feb '11

04 Feb '11

Dear cafe, does anyone have an explanation for this?: >>> error (error "foo") *** Exception: foo >>> error $ error "foo" *** Exception: *** Exception: foo -- Steffen

9 10

hsmagick on OSX 10.6
by Max Cantor 03 Feb '11

03 Feb '11

Just out of curiosity, has anyone gotten this to work? I'm getting bus errors whenever I call readImage. If anyone out there has gotten it working and would tell me which versions they're using of the relevant bits, that would be very much appreciated. Also, I'll post all my debug info if there's someone who's gotten this to work. thanks, mc

2 1

Reader monad wrapping State monad
by michael rice 03 Feb '11

03 Feb '11

Given the first program, it seems that the unchanging first element of the tuple could be handled by a Reader monad, leading to the second program, where b becomes the state, but how do I get the constant a from the Reader monad? Michael ================== import Control.Monad.State type GeneratorState = State (Double,Double) sqrtST :: GeneratorState Double sqrtST = do (a,b0) <- get let b1 = (b0**2.0+a)/(2.0*b0) (if (abs (a-b1**2.0)) < 0.000001 then return b1 else do put (a,b1) sqrtST) mySqrt a = let b = a/2.0 in fst ( runState sqrtST (a,b) ) {- *Main> mySqrt 2.0 1.4142135623746899 -} ================== import Control.Monad.Reader import Control.Monad.State type GeneratorState = State Double sqrtST :: GeneratorState Double sqrtST = do b0 <- get let a = ? b1 = (b0**2.0+a)/(2.0*b0) (if (abs (a-b1**2.0)) < 0.000001 then return b1 else do put b1 sqrtST) mySqrt a = let b = a/2.0 in runReaderT (runState sqrtST b) a

4 6

Re: [Haskell-cafe] Haskell Functions
by Ozgur Akgun 03 Feb '11

03 Feb '11

(CCing haskell-cafe again) Is this an homework question? On 3 February 2011 20:30, Manolache Andrei-Ionut <andressocrates90(a)yahoo.com > wrote: > http://pastebin.com/GxQBh3hx <http://pastebin.com/GxQBh3hx> > > --- On *Thu, 2/3/11, Ozgur Akgun <ozgurakgun(a)gmail.com>* wrote: > > > From: Ozgur Akgun <ozgurakgun(a)gmail.com> > Subject: Re: [Haskell-cafe] Haskell Functions > To: "Manolache Andrei-Ionut" <andressocrates90(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Thursday, February 3, 2011, 10:59 AM > > > On 3 February 2011 18:33, Manolache Andrei-Ionut < > andressocrates90(a)yahoo.com<http://mc/compose?to=andressocrates90@yahoo.com> > > wrote: > > first this is the curent code :http://pastebin.com/UPATJ0r > > > There is no code on that page. (It has expired, probably?) > > -- > Ozgur Akgun > > > -- Ozgur Akgun

3 2

Another Question
by Navin Rustagi 03 Feb '11

03 Feb '11

hi , I am stuck in the following problem. I am maintaining a list of tuples of the form ([Char],Int,Int, Int,Int) . The purpose of maintaining the tuples is that the program reads from a file line by line , Matches the contents with the first element of the tuple and updates the tuple respectively. The precise function I am using is as follows tupup::Bool->[Char]->Int->Int->Int->Int->Char->([Char],Int,Int,Int,Int) tupup val elf els elr ell elx ys= if val then case ys of 'A' -> (elf, els+1,elr,ell,elx) 'G' -> (elf,els, elr +1,ell,elx) 'C' -> (elf,els,elr, ell +1,elx) 'T' -> (elf,els,elr,ell, elx +1) else (elf,els,elr,ell,elx) uptable::[[Char]]->[([Char],Int,Int,Int,Int)]->[([Char],Int,Int,Int,Int)] uptable (xf:xs) main_array = map (\(x,y,z,r,t)-> tupup (x==xf) x y z r t (secvalue xs) ) main_array It gives the error ERROR - Control stack overflow. I assume it is because of the lazy evaluation . Is there a way to enforce strict evaluation only for the function tupup. Thanks, Navin

4 3

Haskell Functions
by Manolache Andrei-Ionut 03 Feb '11

03 Feb '11

I need help with two functions....first this is the curent code :http://pastebin.com/UPATJ0r ->Function 1)removeTautologies :: Formula->Formula If in a clause, a literal and its negation are found, it means that the clause will be true, regardless of the value finally assigned to that propositional variable. Consider the following example: (A v B v -A) ^ (B v C v A) The first clause contains the literals A and -A. This means that the clause will always be true, in which case it can be simplify the whole set to simply (B v C v A) . I was tinking of using something like removeTautologies (f:fs)=filter rTf:removeTautologies fs where rT-is supposed to take the firs Literal from the clasue and search for a similar one,if one si found we compare the values if not the we go to the second literal. ->Function 2)pureLiteralDeletion :: Formula->Formula This is a little bit complicate but from What I get this function is suppose to implement a simplification step that assumes as true any atom in a formula that appears exclusively in a positive or negative form (not both). Consider the formula: (P v Q v R) ^ (P v Q v -R) ^ (-Q v R) Note that in this formula P is present but -P is not. Using Pure Literal Deletion it can be assumed that the value of P will be True thus simplifying the formula to (-Q v R). If the literal were false then the literal would simply be deleted from the clauses it appears in. In that case any satisfying model for the resulting formula would also be a satisfying model for the formula when we assume that the literal is true.

2 1

ANNOUNCE: Database Supported Haskell (DSH)
by George Giorgidze 03 Feb '11

03 Feb '11

I am pleased to announce that Database Supported Haskell (DSH) has been released on Hackage [1]. DSH is a Haskell library for database-supported program execution. Using this library a relational database management system (RDBMS) can be used as a coprocessor for the Haskell programming language, especially for those program fragments that carry out data-intensive and data-parallel computation. Database executable program fragments can be written using the list comprehension notation (with modest syntax changes due to quasiquoting) and list processing combinators from the Haskell list prelude. Note that rather than embedding a relational language into Haskell, we turn idiomatic Haskell programs into SQL queries. DSH faithfully represents list order and nesting, and compiles the list processing combinators into relational queries. The implementation avoids unnecessary data transfer and context switching between the database coprocessor and the Haskell runtime by ensuring that the number of generated relational queries is only determined by the program fragment's type and not by the database size. DSH can be used to allow existing Haskell programs to operate on large scale data (e.g., larger than the available heap) or query existing database resident data with Haskell. Note that this package is flagged experimental and therefore not suited for production use. This is a proof of concept implementation only. To learn more about DSH, our paper entitled as "Haskell boards the Ferry: Database-supported program execution for Haskell" is a recommended reading [2]. The package includes a couple of examples that demonstrate how to use DSH. [1] http://hackage.haskell.org/package/DSH [2] http://www-db.informatik.uni-tuebingen.de/files/publications/ferryhaskell.p…

1 0

Yesod and concurrency
by Ertugrul Soeylemez 03 Feb '11

03 Feb '11

Hello everybody, how well do WAI, Yesod and the 'persistent' package play with concurrency? For example, I'd like to write a program, which concurrently provides two related sites as well as a few background workers, which do something with the database. My idea would look something like this: main :: IO () main = withMyAppPool $ \pool -> do forkIO $ worker1 ... -- background worker forkIO $ worker2 ... -- background worker forkIO $ worker3 ... -- background worker forkIO $ toWaiApp ... -- site 1 forkIO $ toWaiApp ... -- site 2 Will I run into problems with this? Thanks. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

2 5

PhD Studentships in Functional Programming, Nottingham
by Henrik Nilsson 03 Feb '11

03 Feb '11

Apologies for multiple copies. Best, /Henrik -- Henrik Nilsson School of Computer Science The University of Nottingham nhn(a)cs.nott.ac.uk

1 0

problem with instance method
by Patrick Browne 03 Feb '11

03 Feb '11

Hi, I am studying type classes using examples from the literature [1]. The attached code is a formalization of basic object oriented ideas. The particular approach seems to be built on the concepts of: thing, object, and identifier. I have no intension to implement anything or significantly change the code below. Rather, I am trying to understand the code as it stands. I include a number of test runs which seem OK, but I cannot get the *obj* function to work. obj :: t -> i -> o t i obj t i = Obj t i Any hints would be appreciated. Thanks, Pat [1] ftp://ftp.geoinfo.tuwien.ac.at/frank/frank97executableAxiomaticSpecification.pdf -- A property of a thing data Color = Blue | Green | Red | White deriving Show -- A thing class Cars c where car :: Color -> c getColor :: c -> Color putColor :: Color -> c -> c paint :: Color -> c -> c paint color car = putColor color car data Car = Car Color deriving Show instance Cars Car where car c = Car c putColor color (Car c) = Car color getColor (Car c) = c -- Identifiers for objects class (Integral i) => IDs i where startId :: i newId :: i -> i newId i = succ i sameId, notSameId :: i -> i -> Bool sameId i j = i == j notSameId i j = not (sameId i j) instance IDs Integer where startId = 1 -- Objects consist of Things, with Identifiers class (IDs i, Show i) => Objects o t i where obj :: t -> i -> o t i getId :: o t i -> i getThing :: o t i -> t doThing :: (t -> t) -> o t i -> o t i doThing f o = obj (f (getThing o)) (getId o) same :: o t i -> o t i -> Bool same i j = sameId (getId i) (getId j) isId :: i -> o t i -> Bool isId i o = sameId i (getId o) -- A general type of Obj data Object t i = Obj t i deriving Show -- A particular Car Obj, which an instance of Objects class (in Haskell terms, not OO terms) instance Objects Object Car Integer where obj t i = Obj t i getId (Obj t i) = i getThing (Obj t i) = t -- Create some actual Objects x = (Obj (Car Blue) (startId::Integer)) y = (Obj (Car Green) (newId startId::Integer)) -- Some tests on car thing, seem OK -- getColor (Car Blue) -- putColor Green (Car Blue) -- getColor (putColor Green (Car Blue)) -- Some tests on objects, seem OK -- same x y -- Obj (Car Blue) (newId startId::Integer) -- Obj (Car Blue) (startId::Integer) -- getThing (Obj (Car Blue) (startId::Integer)) -- getId (Obj (Car Blue) (startId::Integer)) -- isId 2 (Obj (Car Blue) (startId::Integer)) This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

3 2

Re: [Haskell-cafe] Inheritance and Wrappers
by John Lato 03 Feb '11

03 Feb '11

> > From: Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de> > > On 01/31/2011 08:58 PM, MattMan wrote: > > [...] > > > > data Wrapper a = Wrap a > > instance (Num a) => AbGroup (Wrapper a) where > > add (Wrap i) (Wrap j) = Wrap(i+j) > > > > However, this is clumsy. Is there something else I can do? Thanks > This is the normal approach. You can do funny things with the > OverlappingInstances extension, but it is probably not what you want. > > The problem is that the compiler only considers the heads of the > instance declarations when determining which instance to use for a > specific type. So an instance like this: > > > instance (Num a) => AbGroup a where ... > > means: Some type matching 'a' (that is, any type) is an instance of > 'AbGroup' if and only if it is an instance of 'Num'. > I would word this differently. I would say this instance means: Some type matching 'a' (that is, any type) is an instance of 'AbGroup'. It is an error to use AbGroup methods if 'a' does not have a Num instance in scope. The important point is that this declares an AbGroup instance for every type, not just types with Num instances. Which means that this instance is much less useful than you would hope. I would probably use either Henning's approach of separate functions or an automatic deriving mechanism. I've never understood why Haskellers are so averse to writing new instances anyway. John

5 8

Haskell procedure
by Houdini 03 Feb '11

03 Feb '11

One :Thank you Carsten Schultz,Daniel Fischer,Mihai and all the other for your help. Two:After my last post I wrote some function that should help me in the future,but I need some help with the followint as I'm tired and have to fit in a schedule.I know it is long so I'll try and explain it as short and consice as I can. The DPLL procedure has two main stages: a simplification stage and a search stage. In the simplification stage, functions are applied to the formula to assign truth values to certain propositional variables. The simplifications are made because if there is a satisfying assignment for the simplified formula then it is also a satisfying assignment for the original formula. This reduces the need for search which can take a long time. The search stage is performed when no more simplifications can be made. In this stage a literal is chosen and is assigned true or false leading to two new branches in the search space. I wrote some function wich should be helpfull,but I have to fit in a schedule and I'm getting tired I need some additional help if possible. module Algorithm where import System.Random import Data.Maybe import Data.List type Atom = String type Literal = (Bool,Atom) type Clause = [Literal] type Formula = [Clause] type Model = [(Atom, Bool)] type Node = (Formula, ([Atom], Model)) -- This function takess a Clause and return the set of Atoms of that Clause. atomsClause :: Clause -> [Atom] -- This function takes a Formula returns the set of Atoms of a Formula atoms :: Formula -> [Atom] -- This function returns True if the given Literal can be found within -- the Clause. isLiteral :: Literal -> Clause -> Bool -- this function takes a Model and an Atom and flip the truthvalue of -- the atom in the model flipSymbol :: Model -> Atom -> Model -- is this ok? Additional functions that I wrote: remove :: (Eq a) )a ->[a] ->[a] -This function removes an item from a list. neg :: Literal->Literal -This function flips a literal (ie. from P to :P and from :P to P). falseClause :: Model -> Clause -> Bool -This function takes a Model and a Clause and returns True if the clause is unsatisfied by the model or False otherwise. falseClauses :: Formula -> Model -> [Clause] -This function takes a Formula and a Model and returns the list of clauses of the formula that are not satisfied. assignModel :: Model -> Formula -> Formula -This function applies the assign function for all the assignments of a given model. checkFormula :: Formula -> Maybe Bool This function checks whether a formula can be decided to be satisfiable or unsatisfiable based on the effects of the assign function. satisfies :: Model -> Formula -. Bool This function checks whether a model satisfies a formula. This is done with the combination of the assignModel and checkFormula functions. --Where do I need help: removeTautologies :: Formula->Formula This function should output a simplified formula if tautologies can be found in one or more clauses in the input Notes: If in a clause, a literal and its negation are found, it means that the clause will be true, regardless of the value finally assigned to that propositional variable. Consider the following example: (A v B v -A) ^ (B v C v A) The first clause contains the literals A and -A. This means that the clause will always be true, in which case it can be simplify the whole set to simply (B v C v A) (the second clause alone) pureLiteralDeletion :: Formula->Formula This function is suppose to implement a simplification step that assumes as true any atom in a formula that appears exclusively in a positive or negative form (not both). Consider the formula: (P v Q v R) ^ (P v Q v -R) ^ (-Q v R) Note that in this formula P is present but -P is not. Using Pure Literal Deletion it can be assumed that the value of P will be True thus simplifying the formula to (-Q v R). If the literal were false then the literal would simply be deleted from the clauses it appears in. In that case any satisfying model for the resulting formula would also be a satisfying model for the formula when we assume that the literal is true. Hence this simplification is sound in that if there is a solution to the simplified formula then there is a solution to the original formula. propagateUnits :: Formula->Formula If a clause in a propositional formula contains only one literal, then that literal must be true (so that the particular clause can be satisfied). When this happens,we can remove the unit clauses (the ones that contain only one literal), all the clauses where the literal appears and also, from the remaining clauses, we can delete the negation of the literal (because if P is true, -P will be false).For example, in the formula (P v Q v R) ^ (-P v Q v -R) ^ (P) we have one unit clause (the third clause(P) ). Because this one has to be true for the whole formula to be true we assign True to P and try to find a satisfying assignment for the remaining formula. Finally because -P cannot be true (given the assigned value of P) then the second clause is reduced by eliminating the symbol -P . This simplification results in the revised formula (Q v -R). The resulting simplification can create other unit clauses. For example in the formula (-P v Q) ^ (P) is simplified to (Q) when the unit clause (P) is propagated. This makes (Q) a unit clause which can now also be simplified to give a satisfying assignment to the formula. The function should apply unit propagation until it can no longer make any further simplifications. Note that if both P and -P are unit clauses then the formula is unsatisfiable. In this case the function should return a formula with an empty clause in it to indicate that the formula could not be satisfied. update :: Node -> [Node] The update function should take in a Node and return a list of the Nodes that result from assigning True to an unassigned atom in one case and False in the other (ie. a case split). So the list returned should have two nodes as elements. One node should contain the formula with an atom assigned True and the model updated with this assignment, and the other should contain the formula with the atom assigned False and the model updated to show this. The lists of unassigned atoms of each node should also be updated accordingly. This function should use your implemented assign function to make the assignments. It should also use the chooseAtom function provided to select the literal to assign. search :: (Node -> [Node]) -> [Node] -> Int -> (Bool, Int) The search function should perform a backtracking search. The function takes the update function as input and uses it to generate nodes in the search space. The search function also takes in a list which has one element, the initial node consisting of the formula along with an initial model. It should generate nodes using the update function and check nodes using the given check function. If a node is unsatisfiable then it should abandon that branch of the search. If a node is satisfiable then a satisfying assignment has been found and so it should return True. If a node is neither satisfiable or unsatisfiable then it should generate new nodes from this node. If all possible branches of the search space have been tried, i.e. the list of nodes to try has become the empty list, then it should return False since all possible assignments have been tried. The search function also has an Int argument which should be an integer that tracks how many calls to the update function have been made. The search function should return a pair consisting of the truth value of the formula and the number of calls to update made. Thank you(because ,I'm anticipating your help),and until I get a reply I will continue to try an squize my exhausted brain. -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-procedure-tp3369005p3369005.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Problems with iteratees
by oleg＠okmij.org 03 Feb '11

03 Feb '11

I agree with Maciej Wos' diagnosis and solution. I'd like to point out that the Tiff library has to do a similar processing: use an iteratee to read bytes from the outer stream and to produce elements for the inner stream (e.g., signed 32-bit integers). In your version of the Iteratee library, please look at Data/Iteratee/Codecs/Tiff.hs and search for the following fragment: -- Read the array of long integers -- of 1 element: the offset field contains the value read_value typ e' 1 | typ == TT_long || typ == TT_slong = ... -- of n elements read_value typ e' count | typ == TT_long || typ == TT_slong = do offset <- endianRead4 e' return . Just . TEN_INT $ \iter_int -> return $ do Iter.seek (fromIntegral offset) let iter = convStream (liftM (either (const Nothing) (Just . (:[]) . conv_long typ)) (checkErr (endianRead4 e'))) iter_int Iter.joinI $ Iter.joinI $ Iter.takeR (4*count) iter Of relevance is the line that contains convStream. Please notice (const Nothing) that Maciej was talking about. The newer version has a somewhat more convenient sequence_stream function, which creates a nested stream using the supplied iteratee to read from the outer stream and produce an element for the inner stream. The function is particularly useful if you read the inner stream all the way, until it is exhausted. Here is how the above read_value clause looks now -- of n elements read_value typ count | typ == TT_long || typ == TT_slong = do offset <- endian_read4 let converter = endian_read4 >>= return . conv_long typ return . Just . TEN_INT $ \iter_int -> do seek_stream (fromIntegral offset) runI =<< takeR (4*count) (sequence_stream converter iter_int)

1 0

timely shutdown of timer threads
by Warren Harris 03 Feb '11

03 Feb '11

I have an application that forks a thread to run an activity on a timer. (The activity happens to be Berkeley DB checkpointing, but that's actually beside the point here.) The problem is that when the application wants to quit, I would like my main thread to be able to tell the timer thread to shut down in a timely way. However, I don't see a primitive in haskell that allows me to both wait for a timeout, or a notification. (If I were to do this in java, I would use wait/notify.) Here's the code I wrote to attempt this shutdown procedure: import Control.Exception import Database.Berkeley.Db import Data.IORef import System.Posix.Unistd ... closeEnv :: Env -> IO () closeEnv env = do threadId <- readIORef (envCheckpointThread env) case threadId of Just tid -> killThread tid Nothing -> return () dbEnv_close [] (envDbEnv env) startCheckpointing :: Env -> IO ThreadId startCheckpointing env = do forkOS run where run = catch checkpoint handler checkpoint = checkpoint1 >> checkpoint checkpoint1 = unblock $ do let dbenv = envDbEnv env _ <- sleep checkpointInterval putStrLn "# checkpoint" dbEnv_txn_checkpoint [] dbenv 0 0 handler ThreadKilled = return () handler exn = throw exn However, there are several problems here: First, the checkpointInterval is 15 sec, so it takes at least 15 seconds for the thread to wake up and be interrupted. Second, it isn't clear to me whether the killThread call should interrupt the sleep or not. In practice, several checkpoints occur before the ThreadKilled message is delivered, and this can take up to 2 minutes to shut down. Is there a better way to do this? Suggestions would be greatly appreciated, Warren

3 5

Haskell Weekly News: Issue 167 - February 02, 2011
by Daniel Santa Cruz 03 Feb '11

03 Feb '11

Welcome to issue 167 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of January 23 to 29, 2011. This week I've added a section which compiles a list of the top answers given in StackOverflow during the week. These seem to be of more interest to more experienced Haskell users than the newbie questions usually posted on the site. See what you think. Announcements R. Emre Basar [2]annouced the release of follower, a tool that allows you to follow people's tweets without having an account with Twitter. Alberto Ruiz [3]announced hmatrix version 0.11, a package for numerical computation based on LAPACK and SGL. Michael Snoyman [4]made some point updates for WAI and Warp. "They fix some documentation issues, add some status aliases, make it possible to handle exceptions more easily, and make things more responsive on Windows." Antoine Latter [5]announced a minor point release for Parsec 3.1.0. John Meacham [6]announced version 0.7.7 of jhc is out! "This release fixes a large number of bugs that cropped up when compiling haskell out in the wild as well as adds some more features. A major one being the garbage collector is enabled by default." Quotes of the Week * Twey: Maybe Bool: the Haskell equivalent to True/False/FileNotFound * elliott: Only two things in the universe are certain: Death, and two of the libraries you've decided to use taking different types of ByteString. * shapr: my personal heuristics judge Haskell as totally practical, perhaps too practical. * monochrom: the guys that I see when I look into the irc window believe that I'm just a bunch of text. * bernardh: Refactoring: spend a week to get where you were * Question: How do I use "fix", and how does it work? Answer (by TomMD): The prank answer is "fix has no real use, it's just there so you can type 'fix error' in ghci and feel good about yourself." Top Reddit Stories * Haskell at Bump Domain: devblog.bu.mp, Score: 44, Comments: 9 On Reddit: http://www.reddit.com/r/haskell/comments/f8cr8/haskell_at_bump/ Original: http://devblog.bu.mp/haskell-at-bump * How to write a simple TCP Server in Haskell Domain: catonmat.net, Score: 35, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/faa80/how_to_write_a_simple_tcp_se… Original: http://catonmat.net/blog/simple-haskell-tcp-server * Snap Framework article in IEEE Internet Computing Domain: steve.vinoski.net, Score: 32, Comments: 1 On Reddit: http://www.reddit.com/r/haskell/comments/fa0e0/snap_framework_article_in_ie… Original: http://steve.vinoski.net/blog/2011/01/21/column-on-the-snap-framework/ * My Dream GHCi Session, Take 2 Domain: cdsmith.wordpress.com, Score: 23, Comments: 5 On Reddit: http://www.reddit.com/r/haskell/comments/f8im5/my_dream_ghci_session_take_2/ Original: http://cdsmith.wordpress.com/2011/01/24/my-dream-ghci-session-take-2/ * Text: a visual programming language based on Haskell, demo Domain: vimeo.com, Score: 20, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/fbgxd/text_a_visual_programming_la… Original: http://vimeo.com/19273744 * Hoogle Embed Domain: neilmitchell.blogspot.com, Score: 16, Comments: 3 On Reddit: http://www.reddit.com/r/haskell/comments/f7iqa/hoogle_embed/ Original: http://neilmitchell.blogspot.com/2011/01/hoogle-embed.html * What does this mean for Haskell on Android? Domain: reddit.com, Score: 15, Comments: 12 On Reddit: http://www.reddit.com/r/haskell/comments/f9o67/what_does_this_mean_for_hask… Original: http://www.reddit.com/r/Android/comments/f97zh/google_not_happy_with_number… * Evolving a computer with Genprog (exercises from the TECS book) Domain: jpmoresmau.blogspot.com, Score: 13, Comments: On Reddit: http://www.reddit.com/r/haskell/comments/f7y6a/evolving_a_computer_with_gen… Original: http://jpmoresmau.blogspot.com/2011/01/evolving-computer-with-tecs-and-genp… * All about MVars : Inside T5 Domain: blog.ezyang.com, Score: 13, Comments: 4 On Reddit: http://www.reddit.com/r/haskell/comments/fduxb/all_about_mvars_inside_t5/ Original: http://blog.ezyang.com/2011/02/all-about-mvars/ * What textbooks would be useful to start approaching the low-hanging fruit on the ghc project? Domain: self.haskell, Score: 12, Comments: 8 On Reddit: http://www.reddit.com/r/haskell/comments/f8dm5/what_textbooks_would_be_usef… Original: /r/haskell/comments/f8dm5/what_textbooks_would_be_useful_to_start/ Top StackOverflow Answers * What is an idiomatic way to add lists in Haskell? Votes: 14 Read on SO: http://stackoverflow.com/questions/4776750/what-is-an-idiomatic-way-to-add-… There is a zipWith library function that combines two lists by using a supplied function. It does exactly what you want here and you get: addLists = zipWith (+) This uses (+) to combine the elements of lists given as further arguments. * Why don't the Haskell standard libraries make more use of polymorphism? Votes: 15 Read on SO: http://stackoverflow.com/questions/4779028/why-dont-the-haskell-standard-li… There is a very good pragmatic reason that "standard" Haskell (Prelude + base + maybe some more) doesn't use more polymorphism: Designing general-use type classes is hard. Good designs for classes that abstract over container types like lists, arrays and "bytestrings" (personally I don't really consider Bytestring a container) aren't floating round waiting … * How do I use fix, and how does it work? Votes: 17 Read on SO: http://stackoverflow.com/questions/4787421/how-do-i-use-fix-and-how-does-it… You are doing nothing wrong. fix id is an infinite loop. When we say that fix returns the least fixed point of a function, we mean that in the domain theory sense. So fix (\x -> 2*x-1) is not going to return 1, because although 1 is a fixed point of that function, it is not the least one in the domain ordering. I can't describe the domain ordering in … Top StackOverflow Questions * Why don't the Haskell standard libraries make more use of polymorphism? votes: 20, answers: 4 Read on SO: http://stackoverflow.com/questions/4779028/why-dont-the-haskell-standard-li… * How do I use fix, and how does it work? votes: 11, answers: 3 Read on SO: http://stackoverflow.com/questions/4787421/how-do-i-use-fix-and-how-does-it… * Why wrapping the Data.Binary.Put monad creates a memory leak? votes: 11, answers: 2 Read on SO: http://stackoverflow.com/questions/4828902/why-wrapping-the-data-binary-put… * Is there any kind of statistical natural language processing library for Haskell? votes: 9, answers: 1 Read on SO: http://stackoverflow.com/questions/4808702/is-there-any-kind-of-statistical… * How do you structure a stateful module in Haskell? votes: 8, answers: 2 Read on SO: http://stackoverflow.com/questions/4785379/how-do-you-structure-a-stateful-… About the Haskell Weekly News You can find an html version of this newsletter at my blog: http://contemplatecode.blogspot.com/2011/02/haskell-weekly-news-issue-167.h… Special thanks to Jean-Philippe Gariepy for sending in a quote for this week's quotes! We need more eyes! To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85566 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85648 4. https://groups.google.com/d/topic/haskell-cafe/Yd2ADZzonSI/discussion 5. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/85761 6. https://groups.google.com/d/topic/fa.haskell/T6EM4jbhj8w/discussion

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SYB: extending a generic reader with a type class
by Sugar Bzzz 02 Feb '11

02 Feb '11

Dear -cafe, Is it possible to extend a generic reader (extR / ext1R from syb) with a type class? For example, let foo :: (Integral a) => SomeMonad a I could write: reader = ... `extR` (foo :: SomeMonad Int) `extR` (foo :: SomeMonad Integer) However, that is tedious. Could I do something like reader = ... `extR` foo and have it apply to all instances of Integral? I hope I am being clear. Thank you.

2 1

Loading bitmap with xlib
by Francesco Mazzoli 02 Feb '11

02 Feb '11

I am trying to load a bitmap and display it with the Xlib bindings, but I wasn't able to find a way. The C Xlib functions would be XReadBitmapFile() of XReadBitmapFileData() to read the data, but it seems that those functions haven't been included in the haskell bindings. From http://hackage.haskell.org/packages/archive/X11/latest/doc/html/src/Graphic…: -- XCreateBitmapFromData omitted (awkward looking type) -- XReadBitmapFileData omitted (awkward looking type) Does anybody know how to do that?

3 4

Haskell DPLL
by Houdini 02 Feb '11

02 Feb '11

One :Thank you Carsten Schultz,Daniel Fischer and all the other for your help. Two:After my last post I wrote some function that should help me in the future,but I need some help with the followint as I'm tired and have to fit in a schedule.I know it is long so I'll try and explain it as short and consice as I can. The DPLL procedure has two main stages: a simplification stage and a search stage. In the simplification stage, functions are applied to the formula to assign truth values to certain propositional variables. The simplifications are made because if there is a satisfying assignment for the simplified formula then it is also a satisfying assignment for the original formula. This reduces the need for search which can take a long time. The search stage is performed when no more simplifications can be made. In this stage a literal is chosen and is assigned true or false leading to two new branches in the search space. I wrote some function wich should be helpfull,but I have to fit in a schedule and I'm getting tired I need some additional help if possible. module Algorithm where import System.Random import Data.Maybe import Data.List type Atom = String type Literal = (Bool,Atom) type Clause = [Literal] type Formula = [Clause] type Model = [(Atom, Bool)] type Node = (Formula, ([Atom], Model)) -- This function takess a Clause and return the set of Atoms of that Clause. atomsClause :: Clause -> [Atom] -- This function takes a Formula returns the set of Atoms of a Formula atoms :: Formula -> [Atom] -- This function returns True if the given Literal can be found within -- the Clause. isLiteral :: Literal -> Clause -> Bool -- this function takes a Model and an Atom and flip the truthvalue of -- the atom in the model flipSymbol :: Model -> Atom -> Model -- is this ok? Additional functions that I wrote: remove :: (Eq a) )a ->[a] ->[a] -This function removes an item from a list. neg :: Literal->Literal -This function flips a literal (ie. from P to :P and from :P to P). falseClause :: Model -> Clause -> Bool -This function takes a Model and a Clause and returns True if the clause is unsatisfied by the model or False otherwise. falseClauses :: Formula -> Model -> [Clause] -This function takes a Formula and a Model and returns the list of clauses of the formula that are not satisfied. assignModel :: Model -> Formula -> Formula -This function applies the assign function for all the assignments of a given model. checkFormula :: Formula -> Maybe Bool This function checks whether a formula can be decided to be satisfiable or unsatisfiable based on the effects of the assign function. satisfies :: Model -> Formula -. Bool This function checks whether a model satisfies a formula. This is done with the combination of the assignModel and checkFormula functions. --Where do I need help: removeTautologies :: Formula->Formula This function should output a simplified formula if tautologies can be found in one or more clauses in the input Notes: If in a clause, a literal and its negation are found, it means that the clause will be true, regardless of the value finally assigned to that propositional variable. Consider the following example: (A v B v -A) ^ (B v C v A) The first clause contains the literals A and -A. This means that the clause will always be true, in which case it can be simplify the whole set to simply (B v C v A) (the second clause alone) pureLiteralDeletion :: Formula->Formula This function is suppose to implement a simplification step that assumes as true any atom in a formula that appears exclusively in a positive or negative form (not both). Consider the formula: (P v Q v R) ^ (P v Q v -R) ^ (-Q v R) Note that in this formula P is present but -P is not. Using Pure Literal Deletion it can be assumed that the value of P will be True thus simplifying the formula to (-Q v R). If the literal were false then the literal would simply be deleted from the clauses it appears in. In that case any satisfying model for the resulting formula would also be a satisfying model for the formula when we assume that the literal is true. Hence this simplification is sound in that if there is a solution to the simplified formula then there is a solution to the original formula. propagateUnits :: Formula->Formula If a clause in a propositional formula contains only one literal, then that literal must be true (so that the particular clause can be satisfied). When this happens,we can remove the unit clauses (the ones that contain only one literal), all the clauses where the literal appears and also, from the remaining clauses, we can delete the negation of the literal (because if P is true, -P will be false).For example, in the formula (P v Q v R) ^ (-P v Q v -R) ^ (P) we have one unit clause (the third clause(P) ). Because this one has to be true for the whole formula to be true we assign True to P and try to find a satisfying assignment for the remaining formula. Finally because -P cannot be true (given the assigned value of P) then the second clause is reduced by eliminating the symbol -P . This simplification results in the revised formula (Q v -R). The resulting simplification can create other unit clauses. For example in the formula (-P v Q) ^ (P) is simplified to (Q) when the unit clause (P) is propagated. This makes (Q) a unit clause which can now also be simplified to give a satisfying assignment to the formula. The function should apply unit propagation until it can no longer make any further simplifications. Note that if both P and -P are unit clauses then the formula is unsatisfiable. In this case the function should return a formula with an empty clause in it to indicate that the formula could not be satisfied. update :: Node -> [Node] The update function should take in a Node and return a list of the Nodes that result from assigning True to an unassigned atom in one case and False in the other (ie. a case split). So the list returned should have two nodes as elements. One node should contain the formula with an atom assigned True and the model updated with this assignment, and the other should contain the formula with the atom assigned False and the model updated to show this. The lists of unassigned atoms of each node should also be updated accordingly. This function should use your implemented assign function to make the assignments. It should also use the chooseAtom function provided to select the literal to assign. search :: (Node -> [Node]) -> [Node] -> Int -> (Bool, Int) The search function should perform a backtracking search. The function takes the update function as input and uses it to generate nodes in the search space. The search function also takes in a list which has one element, the initial node consisting of the formula along with an initial model. It should generate nodes using the update function and check nodes using the given check function. If a node is unsatisfiable then it should abandon that branch of the search. If a node is satisfiable then a satisfying assignment has been found and so it should return True. If a node is neither satisfiable or unsatisfiable then it should generate new nodes from this node. If all possible branches of the search space have been tried, i.e. the list of nodes to try has become the empty list, then it should return False since all possible assignments have been tried. The search function also has an Int argument which should be an integer that tracks how many calls to the update function have been made. The search function should return a pair consisting of the truth value of the formula and the number of calls to update made. Thank you(because ,I'm anticipating your help),and until I get a reply I will continue to try an squize my exhausted brain. -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-DPLL-tp3368123p3368123.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

1 1

Gedit Haskell mode plugin indentation
by yukkuri yu!yu!yu! 02 Feb '11

02 Feb '11

Here is my first try to write a Haskell mode layout indentation plugin for gedit - GNOME Text Editor: ftp://neverb.net/soft/mine/gedit_haskell_mode/haskell_indentation-latest.ta… Curent version: v0.4 To install the plugin 1) Create directory ~/.gnome2/gedit/plugins 2) Copy tarball/plugin/* contents to ~/.gnome2/gedit/plugins/ 3) In gedit go to Edit -> Preferences -> Plugins 4) Check the box left from "Haskell Indentation" plugin 5) (Possibly needed) Restart gedit.

1 4

Haskell function help
by Houdini 02 Feb '11

02 Feb '11

I have a Question about haskell 1. is a something more complicated and I need some help with it if possible.. In order to check the satisfiability of a formula in a given model we propagate the effects of assigning a truth value to an atom in a formula. Assume an atom to which we assign the value True. The following effects can be applied to the formula: - The positive literals have the same True value and, therefore, any clauses that contain them are removed from the formula. This is to indicate that these clauses could be satisfied and hence no longer affect the satisfiability of the formula. - The negated literals have a value of False and are, therefore, removed from any clause they are in. This is to indicate that these clauses are still not satisfied and can only be made true by one of the other literals obtaining a value of True. In the case where False is assigned to the atom, the positive literals will now be false and should be removed from their clauses while the negative literals will become true and have their clauses removed from the formula. For example, in the formula (P _ Q _ R) ^ (:P _ Q _ :R) ^ (P _ :Q), assume we assign True to P. Then the clauses containing P, ie. (P _ Q _ R) and (P _ :Q) are removed from the formula, whereas :P is removed from any clause it is in, ie. (:P _ Q _ :R). This results in the formula (Q _ :R). On the other hand, if we assign False to P, we then remove (:P _ Q _ :R) from the formula and P from its clauses, thus obtaining (Q _ R) ^ (:Q). The overall formula is satisfiable if it can be reduced to the empty list since in this case all the clauses were satisfied. If there is an empty list within the overall formula then that means that a clause was not satisfied and hence the formula can not be satisfied with the assignment that led to this state. -assign :: (Atom,Bool)->Formula->Formula The assign function should take an (Atom,Bool) pair and a formula and propagate the effects of assigning the given truth value to the atom in the formula as described above The code(on which I received help from here also): module Algorithm where import System.Random import Data.Maybe import Data.List type Atom = String type Literal = (Bool,Atom) type Clause = [Literal] t ype Formula = [Clause] type Model = [(Atom, Bool)] t ype Node = (Formula, ([Atom], Model)) atomsClause :: Clause -> [Atom] -This function takess a Clause and return the set of Atoms of that Clause. atomsClause = nub . map snd atoms :: Formula -> [Atom] atoms = nub . map snd -----This function takes a Formula returns the set of Atoms of a Formula isLiteral :: Literal -> Clause -> Bool isLiteral = isLiteral = any . (==)----This function returns True if the given Literal can be found within the Clause. flipSymbol :: Model -> Atom -> Model--- f lipSymbol m a = map f m - where f (atom, value) = if a == atom then (atom, not value) else (atom, value)-this function takes a Model and an Atom and flip the truthvalue of the atom in the model assign :: (Atom,Bool)->Formula->Formula assign = undefined--------any advice here? Thank you -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-function-help-tp3365994p336599… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Instancing "Typeable" for monad transformers?
by oleg＠okmij.org 02 Feb '11

02 Feb '11

One can do something a bit shorter > instance (Typeable a, Typeable1 m) => Typeable1 (Iteratee a m) where > typeOf1 i = mkTyConApp (mkTyCon "Data.Enumerator.Iteratee") > [typeOf a, typeOf1 m] > where > (a,m) = peel i > peel :: Iteratee a m w -> (a, m ()) > peel = undefined still a bother to write every time one needs it, but tolerable. Only the signature of 'peel' matters.

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Haskell Platform 2011.x - planned release date?
by Max Cantor 02 Feb '11

02 Feb '11

January has come and gone and HP 2011 has not come with it. Is there an updated timetable for the next version of the HP? I'm not complaining or upset or whining, just trying to plan. Great work so far, looking forward to HP 2011! mc

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plugins and internal error: stg_ap_v_ret
by Michael Snoyman 02 Feb '11

02 Feb '11

Hi all, I'm trying to convert wai-handler-devel to use plugins instead of hint, but cannot even get some basic usages to work properly. I've put together a minimal example that loads a WAI application from a separate file and runs it, but this immediately causes the program to crash saying: loader: internal error: stg_ap_v_ret (GHC version 6.12.3 for i386_unknown_linux) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Is this an actual bug in GHC, or am I misusing the plugins package? The two source files: MyModule.hs {-# LANGUAGE OverloadedStrings #-} module MyModule where import Network.Wai import Data.ByteString.Lazy.Char8 () myapp _ = responseLBS status200 [("Content-Type", "text/plain")] "myapp" loader.hs import System.Plugins.Make import System.Plugins.Load import Network.Wai.Handler.Warp (run) main :: IO () main = do MakeSuccess _ obj <- makeAll "MyModule.hs" [] LoadSuccess _ app <- load_ obj [] "myapp" run 3000 app Thanks, Michael

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