June 2009 - Haskell-Cafe

Network.CGI -- practical web programming example.
by Edward Ing 02 Jul '09

02 Jul '09

I am somewhat new to haskell. It is amazing that I can actually write a CGI program using Network.CGI without really being comfortable with the Haskell type system. Especially when it involves monad transformations. So I decided that I better understand this. I looked at the Practical Web Programming examples to try to understand what is going on. I came up with a problem that might demonstrate my misunderstanding. I am wondering if you can answer questions I have. Code 1 is the example from PWP, Code 2 is my variation and it works so I am stumped by what the liftM is required. code 1>>> #!/usr/bin/runghc import Network.CGI import Text.XHtml import qualified Data.ByteString.Lazy as BS import Control.Monad (liftM) import Data.Maybe (fromJust) uploadDir = "../upload" fileForm = form ! [method "post", enctype "multipart/form-data"] << [afile "file", submit "" "Upload"] saveFile n = do cont <- (liftM fromJust) $ getInputFPS "file" let f = uploadDir ++ "/" ++ basename n liftIO $ BS.writeFile f cont return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +++ ".") page t b = header << thetitle << t +++ body << b basename = reverse . takeWhile (`notElem` "/\\") . reverse cgiMain = do mn <- getInputFilename "file" h <- maybe (return fileForm) saveFile mn output $ renderHtml $ page "Upload example" h main = runCGI $ handleErrors cgiMain Code 2 (modifier) >>> import Network.CGI import Text.XHtml import qualified Data.ByteString.Lazy as BS import Control.Monad (liftM) import Data.Maybe (fromJust) uploadDir = "../upload" fileForm = form ! [method "post", enctype "multipart/form-data"] << [afile "file", submit "" "Upload"] saveFile n = do cont <- getInputFPS "file" let f = uploadDir ++ "/" ++ basename n liftIO $ BS.writeFile f (fromJust cont) return $ paragraph << ("Saved as " +++ anchor ! [href f] << f +++ ".") page t b = header << thetitle << t +++ body << b basename = reverse . takeWhile (`notElem` "/\\") . reverse cgiMain = do mn <- getInputFilename "file" h <- maybe (return fileForm) saveFile mn output $ renderHtml $ page "Upload example" h main = runCGI $ handleErrors cgiMain Questions === 1) Why did the author choose to insert "liftM" in function saveFile? It doesn't seem necessary in my version. 2) My background mainly is Java but here is my understanding of Monad Transforms. The CGIT m type carries around with it the CGI Request context and response contexts. The transformations (lifts) is similar to casting so that you can use the functions for specific manifestations but it also encapsulates the data. Is this correct? Edward

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Suggestions for simulating Object ID
by Hemanth Kapila 01 Jul '09

01 Jul '09

Hello all, Can some one please suggest something on simulating some sort of "object-Ids" in Haskell? I mean, something like the following relation will hold: > a1 == a2 = (objectID a1) == (objectID a2) Currently I have something like > import Data.Hash >import qualified Data.ByteString.Char8 as B > newtype UniqueID = UniqueID (Hash, B.ByteString) I have a type class called Identifiable, the instances of which are supposed to give a unqiue bytestring representation of the instance which can be arbitrarily long but should satisfy the condition mentioned earlier. > class Identifiable a where > instanceID :: a -> B.ByteString and finally, the required funtion objectID is defined as : > objectID :: (Hashable a, Identifiable a) => a -> UniqueID > objectID a1 = UniqueID (hash a1, instanceID a1) while comparing object Ids, I compare the hash values first (which can be faster for arbitrary values). Only if they are equal then I compare the instanceIDs and count on laziness for avoiding the construction of the bytestrings where they are not needed. Can you please suggest a better approach? I also toyed with the idea of storing all the data 'objects' in an array and use the index as the identifier but the trouble is I am not aware of all the number of instances before hand under all the circumstances. Many thanks Hemanth K

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List spine traversal
by Tony Morris 01 Jul '09

01 Jul '09

Is there a canonical function for traversing the spine of a list? I could use e.g. (seq . length) but this feels dirty, so I have foldl' (const . const $ ()) () which still doesn't feel right. What's the typical means of doing this? -- Tony Morris http://tmorris.net/

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Haskell on JVM
by Greg Meredith 30 Jun '09

30 Jun '09

Simon, et al, It might be interesting to look at CAL<http://labs.businessobjects.com/cal/>as a non-blank-slate beginning for Haskell on the JVM. To my mind there are three things that this needs to make it a real winner: - Much, much better Java interop. Basically, the bar to meet here is Scala/Java interop. - Better support for "std" Haskell syntax - and better support for some of the basic (semantic and syntactic) extensions - Figuring out what of Hackage it is reasonable to support Best wishes, --greg Date: Tue, 23 Jun 2009 15:16:03 +0100 > From: Simon Peyton-Jones <simonpj(a)microsoft.com> > Subject: RE: [Haskell-cafe] Haskell on the iPhone > To: Rick R <rick.richardson(a)gmail.com>, Haskell Cafe > <haskell-cafe(a)haskell.org> > Message-ID: > < > 638ABD0A29C8884A91BC5FB5C349B1C33F4BAAFA41(a)EA-EXMSG-C334.europe.corp.microsoft.com > > > > Content-Type: text/plain; charset="us-ascii" > > Good news about the iPhone port! > > There seems to be quite a bit more interest now in supporting platforms > other than win/*nix on x86 these days*. Maybe now there will be sufficient > motivation to make the fundamental changes required. Caveat: I have > absolutely no idea of the scope or complexity of said changes. I will look > through the LambdaVM code (and iPwn when available) to get an idea. > > There is definitely an opportunity here for someone to make an impact. > There is no reason in principle why Haskell can't run on a JVM or .net or > other platform. But it's not just a simple matter of absorbing some patch > or other. Here's a summary I wrote a little while ago: > > http://haskell.org/haskellwiki/GHC:FAQ#Why_isn.27t_GHC_available_for_.NET_o… > > The short summary is: > > * There is interesting design work to do; and then interesting > engineering work to make it a reality. > > * We (at GHC HQ) would love to see that happen, but are not likely > to drive it. > > * If someone, or a small group, wanted to take up the cudgels and > work on it, we'd be happy to advise. > > * We'd certainly consider folding the results into the HEAD, > provided the author(s) are willing to maintain it, and we feel that the > result is comprehensible and maintainable. > > * But another viable route might well be to use the GHC API, which > means that the result wouldn't be part of GHC at all, just a client of the > API. That would make it much easier to distribute upgrades etc, just as a > Cabal package. > > Simon > -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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ANN: package Boolean: Generalized booleans
by Conal Elliott 30 Jun '09

30 Jun '09

I just uploaded a new package [1] for generalized booleans, which provides type classes with generalizations of boolean values & operations, if-then-else, Eq and Ord. These values & types come up for me with every new deep DSEL, and I think they do for others as well. The design space has some tricky trade-offs, and I'm not positive I've found the optimum yet. Users & comments are very welcome. Please direct discussion to the haskell-cafe list (rather than haskell list). [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Boolean - Conal

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Re: [Haskell-cafe] Monoid wants a (++) equivalent
by Bryan O'Sullivan 30 Jun '09

30 Jun '09

On Tue, Jun 30, 2009 at 9:50 AM, David Leimbach <leimy2k(a)gmail.com> wrote: > I actually worry that this will make people think, more incorrectly, that > Monoids are about appending stuff only. > I think that adding a graphical operator as a synonym for mappend would actually help to address that, since the magic word "append" would no longer be nearly as common in source code, and that textual name certainly is (unhelpfully) suggestive of a specific semantics.

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Re: [Haskell-cafe] Making a strict (non-lazy) GCL Interpreter
by Hector Guilarte 30 Jun '09

30 Jun '09

Thanks Ketil, I'll try to give that a try, but as I said beforen, I don't know anything about monads, and since this project is for next monday and I have many other projects for the University for next week too I can't learn them right now... What I need to do is randomly select an element from a List... Any Ideas? Also, I had another question in my original e-mail... Any ideas about that? ------Original Message------ From: Ketil Malde To: Hector Guilarte Cc: haskell-cafe(a)haskell.org Subject: Re: [Haskell-cafe] Making a strict (non-lazy) GCL Interpreter Sent: Jun 30, 2009 05:00 Hector Guilarte <hectorg87(a)gmail.com> writes: > I need to randomly select ONE of the valid conditions and execute it's > instruction. I know there is a Random Monad, but it returns an IO > Int, No, this is not right. Values in the Random monad are computations that rely on randomness, but they can produce values of arbitrary type. (The 'a' in 'Random a', no?) > Is there anyway I can do some Random that doesn't involve IO? or any other > solution? Some options are: 1. Use the IO monad 2. Use the Random monad 3. Pass around a RandomGen explicitly 4. Generate an infinite stream of random values, and pass that around I think option 2 is the nicest, but option 4 may work if the use of randomness is limited. No. 3 does the same as 2 (I presume, I never looked) but with a lot more noise in your code, and no. 1 erases the separation between real IO, and computations -- which IME includes a lot of them -- that happen to depend on randomness (but require no other IO). -k -- If I haven't seen further, it is by standing in the footprints of giants

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Reflections on the ICFP 2009 programming contest
by Justin Bailey 30 Jun '09

30 Jun '09

Anyone have thoughts to share? I'd love to read others' experiences but there isn't much coming up with searches or on redditt ... I was happiest with the VM I implemented. Sadly, I wasn't able to solve any of the scenarios, but my VM ran damn fast. That didn't seem to matter so much this year as it did two years ago - I think I am still scarred by Endo's DNA .. Anyways, for those who care, the heart of my VM implementation was a monadic fold over the program, with a mutable array representing the machine's memory, all inside ''runSTUArray.'' I used a simple data type to represent the machine: data Machine = Machine { instrs :: [OpCode] , dataMem :: UArray Int Val , outputs :: UArray Int Val , status :: !Int , inputs :: IntMap Val } where ''dataMem'' holds the data memory area. To execute a program, I folded over the ''instrs'' list and updated memory as I went: exec :: Machine -> Input -> Machine exec m@(Machine { instrs = instrs }) inps = let m' = m { inputs = readInputs inps } newMem = runSTUArray $ do mem' <- thaw (dataMem m) foldM (step m') mem' (zip instrs [0..]) I considered ''unsafeThaw'' in the above, but performance didn't hurt me much and it seemed safer. Since I was recording all output I was worried that would break referential integrity. In any case, when the fold was finished I just assigned ''newMem'' to the new machine and moved to the next iteration. ''step'' takes the mutable array and updates it while the program executes. I won't show the whole body, as it just a separate definition for each opcode, just the type: step :: Machine -> STUArray s Int Val -> (OpCode, Addr) -> ST s (STUArray s Int Val) The only downside to this approach was the machine had three mutuable states: memory, output, and status. runSTUArray only wants to return a single array so inside step I had to hold all values in one array. Not a big deal but a little ugly. I kept a diary of my experience over the weekend at http://icfp2009.codeslower.com. I started out with such optimism ... Stil, I'm glad to have done it. Lots of fun! My thanks to the organizers!

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[help] Creating the simplest possible adhoc network
by lysgaard 30 Jun '09

30 Jun '09

Hi. I'm in the quest of exploring adhoc networks together with Haskell. Both are quite new to me. I found the http://hackage.haskell.org/package/adhoc-network-1.0.3 adhoc package in Hackage, it's made for the p2p chat client http://hackage.haskell.org/package/Barracuda Barracuda . I've tried to understand the barracuda client, but since my understanding of Haskell is not the best (yet!) the package becomes to massive for me to understand. I wondered if someone would be kind enough to help me build the simplest possible adhoc network peer using the http://hackage.haskell.org/package/adhoc-network-1.0.3 adhoc library What i want to make is just command line proof of concept code for the adhoc library. But since the documentation is rather sparse (German pdfs) I'm having real problems understanding how to achieve this. Have a nice summer! -Morten -- View this message in context: http://www.nabble.com/-help--Creating-the-simplest-possible-adhoc-network-t… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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6.12.1 planning
by Simon Marlow 30 Jun '09

30 Jun '09

Hi Folks, As usual, we're planning a major release of GHC around September. Here's our list of the main items currently scheduled for 6.12.1, and their status. If you have the time and inclination to help with any of these, please get involved! * Parallel performance. 6.12.1 will ship with the improvements to parallel performance described in our ICFP 2009 paper. Still to do: overhaul the +RTS GC settings, tune for good performance by default. * Parallel profiling: the new RTS tracing features will be included, and we hope to have a release of ThreadScope to coincide with GHC 6.12.1. ThreadScope is written using gtk2hs, and could benefit from someone with expertise in producing polished gtk2hs apps - if you can lend a hand, contact Satnam Singh <satnams(a)microsoft.com>. * Unicode I/O: the new Unicode I/O library is in, and will ship with 6.12.1. Still to do: decide on the public API for changing encodings and newline conversion. * Shared libraries: we intend to ship with shared library support on at least x86/Linux and x86-64/Linux. There are various tasks remaining to do here - Duncan, can we have a summary? * Data Parallel Haskell. Manuel, can you comment on the state of play? What can we expect in time for 6.12.1? * Plugin support in GHC. The patches are not yet in GHC, and as far as I know are awaiting review - Simon, can you say more? * The new backend code generator. At the moment, it seems unlikely that GHC 6.12.1 will ship with the new code generator enabled by default, although it may well be available for testing. Meanwhile, work on it continues. The smaller items are all embodied in tickets, here are the tickets currently in the 6.12.1 milestone (135): http://hackage.haskell.org/trac/ghc/query?status=new&status=assigned&status… and on the 6.12 branch (251): http://hackage.haskell.org/trac/ghc/query?status=new&status=assigned&status… I estimate there are 2 man years of work here - needless to say, we aren't going to fix all these tickets :) As usual, if you want to vote for something, add your email to the CC field of the ticket. Several of these tickets would make good tasks for a fledgling GHC hacker. e.g. http://hackage.haskell.org/trac/ghc/ticket/2362 (allow full import syntax in GHCi) has a lot of support, and is a nice self-contained task (but not a small one). Even if you're not a GHC hacker you can still help, e.g. by helping to narrow down the cause of a bug, or verifying a bug on your platform. Let me remind people that GHC HEAD has the new build system, and it's actually rather pleasant to work with. Even if you have no idea what you're doing, you can always say 'make' at the top level and the build system will figure out what needs doing (ok, so that's what build systems are supposed to do, but GHC's has never quite managed it until now!). Cheers, Simon

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