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A yet another question about subtyping and heterogeneous collections
by Dmitry Vyal 22 Oct '12

22 Oct '12

Hello list! I've been experimenting with emulating subtyping and heterogeneous collections in Haskell. I need this to parse a binary representation of objects of a class hierarchy in C++ program. So far I implemented upcasting using a chain of type classes and now I'm playing with heterogeneous lists. For future purposes It would be ideal to be able to have something like these functions: upcast_list :: [LikeC] -> [LikeA] downcast_list :: [LikeA] -> [LikeC] First one only replaces the existential wrapper leaving the actual value intact, and the second one also filters the list, passing the elements with specific enough type. I can implement this particular functions, but what's about a more general one? Something like cast_list :: [a] -> [b], where a and b are existential types from one hierarchy. Something like LikeA and LikeC in my example. Is my approach feasible? Is there a better one? Am I missing something obvious? Any relevant advices are welcome. The example code follows: {-# LANGUAGE FlexibleInstances, UndecidableInstances, OverlappingInstances, ExistentialQuantification, DeriveDataTypeable #-} import Data.Typeable import Data.Maybe data A = A {a_x :: Int} deriving (Show, Typeable) data B = B {b_x :: Int, b_a :: A} deriving (Show, Typeable) data C = C {c_z :: Int, c_b :: B} deriving (Show, Typeable) data D = D {d_w :: Int, d_c :: C, d_a :: A} deriving (Show, Typeable) class ALike x where toA :: x -> A class BLike x where toB :: x -> B class CLike x where toC :: x -> C class DLike x where toD :: x -> D instance ALike A where toA = id instance BLike B where toB = id instance CLike C where toC = id instance DLike D where toD = id instance ALike B where toA = b_a instance BLike C where toB = c_b instance CLike D where toC = d_c instance (BLike x) => (ALike x) where toA = (toA :: B -> A) . toB instance CLike x => BLike x where toB = toB . toC a1 = A 1 b1 = B 2 (A 2) c1 = C 3 b1 d1 = D 4 c1 (A 10) print_a :: ALike x => x -> String print_a v = "A = " ++ show (a_x $ toA v) sum_a :: (ALike x, ALike y) => x -> y -> String sum_a v1 v2 = "A1 = " ++ show (a_x $ toA v1) ++ " A2 = " ++ show (a_x $ toA v2) data LikeA = forall a. (ALike a, Typeable a) => LikeA a instance ALike LikeA where toA (LikeA x) = toA x get_mono :: Typeable b => [LikeA] -> [b] get_mono = catMaybes . map ((\(LikeA x) -> cast x)) data LikeC = forall c. (CLike c, Typeable c) => LikeC c instance CLike LikeC where toC (LikeC x) = toC x lst_a = [LikeA a1, LikeA b1, LikeA c1, LikeA d1] lst_c = [LikeC c1, LikeC d1]

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Re: [Haskell-cafe] Blocking IO & FIFOs
by Jason Dusek 22 Oct '12

22 Oct '12

2012/10/20 Asten, W.G.G. van (Wilfried, Student M-CSC) <w.g.g.vanasten(a)student.utwente.nl>: > Would you be happy with a solution like this: > > - First create two handles to two files in the tmp directory > - Then use StdStream's UseHandle to redirect std_err and std_out > (using CreatePipe for std_in) to these files > - Then write your query to the Handle for std_in > - waitForProcess > - Collect std_out and std_err from the temporary files. > > If that is not satisfactory you may want to check out conduit-process > (http://hackage.haskell.org/packages/archive/process-conduit/0.5.0.2/doc/htm…) > that also does some interaction with a process and interleaves std_in > and std_out. It should not be to hard to combine std_err into that > concept. I also faced this same problem and in one case solved it by > using a temporary file to hold my content which was also deleted > afterwards (This immediately prevented the content building up in > memory). For my application, it's important to be able to run multiple queries against the same Bash session. Waiting for Bash to shut down is thus not a viable way to finalize the response. Perhaps I can spawn two cats with their outputs connected to the FIFOs and wait for them to terminate. -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B

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Oxford Haskell Users Group
by Nicolas Wu 22 Oct '12

22 Oct '12

Dear all, After a long pause over the summer, the Oxford Haskell Users Group will be meeting this week, at 8pm on Friday 26th October, at the Mitre pub on High Street, Oxford [1]. The group meets in an informal setting with no real agenda: typically we turn up, have a few pints, and enjoy a good chat about what we've been doing with Haskell. If you'd like to have more details about our meetings, then join our mailing list [2], where our meetings are usually announced. The format of OxHUG will be changing slightly: we'll be meeting once a month rather than fortnightly, and rotating the day of the week. The full details of the meetings can be found on the OxHUG calendar [3], though at the moment only this week's meeting is planned. Nick [1]: http://goo.gl/maps/QEN3o [2]: https://groups.google.com/group/oxhug [3]: https://www.google.com/calendar/embed?src=pjc31qnpq48rvvncq72utqtang%40grou…

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Solving integer equations in Haskell
by Justin Paston-Cooper 22 Oct '12

22 Oct '12

Hello, Can anyone suggest a library written in Haskell which can solve equations of the form xM(transpose(x)) = y, where x should be an integer vector, M is an integer matrix and y is an integer? I'm aware that Mathematica can do this, but I would like something written in Haskell. I haven't been sure of what exact keywords I should be searching for, hence my asking here. Thanks, Justin

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ANN: hledger 0.19
by Simon Michael 21 Oct '12

21 Oct '12

I'm pleased to announce the release of hledger 0.19! This release has just two visible changes: a much faster balance command, and support for the latest GHC and libs. (A hledger-web update will follow.) hledger is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. In addition to reporting, it can also help you record new transactions, or convert CSV data from your bank. Add-on packages include hledger-web, a web interface. hledger is inspired by and compatible with John Wiegley's Ledger. For more, see http://hledger.org . Install it: cabal update; cabal install hledger . For installation help, see http://hledger.org/MANUAL.html#installing . Or, sponsor a ready-to-run binary for your platform: http://hledger.org/DOWNLOAD.html . Release notes: hledger, hledger-lib: support GHC 7.6 and latest cmdargs, haskeline, split balance report no longer has an O(n^2) slowdown with large numbers of accounts, and is generally more speedy. Benchmark on a 2010 macbook: +-------------------------------------------++--------------+--------------+--------+ | || hledger-0.18 | hledger-0.19 | ledger | +===========================================++==============+==============+========+ | -f data/100x100x10.journal balance || 0.21 | 0.07 | 0.09 | | -f data/1000x1000x10.journal balance || 10.13 | 0.47 | 0.62 | | -f data/1000x10000x10.journal balance || 40.67 | 0.67 | 1.01 | | -f data/10000x1000x10.journal balance || 15.01 | 3.22 | 2.36 | | -f data/10000x1000x10.journal balance aa || 4.77 | 4.40 | 2.33 | +-------------------------------------------++--------------+--------------+--------+ build version is set with CPP instead of cabal-file-th Release contributors: Simon Michael, Sergei Trofimovich Release stats: 106 days, 21 commits, 1 end-user fix, 0 end-user features since last release Project stats: 222 unit & functional tests, 7740 lines of code including hledger-web, 21 committers

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serialize an unknown type
by Corentin Dupont 21 Oct '12

21 Oct '12

Hi the list! I have a simple question, how can I serialize/deserialize a structure like this: data InputChoice c deriving Typeable data Event a where InputChoice :: (Eq c, Show c) => [c] -> c -> Event (InputChoice c) (...) I'd like that the values of type "c" get serialized to a String... That's the easy part, but for deserializing, oops! Cheers, Corentin

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How do I specify language literals in hsparql?
by Andrew Pennebaker 21 Oct '12

21 Oct '12

If the raw SPARQL is rdfs:label "D (programming language)"@en, what would the hsparql <http://hackage.haskell.org/package/hsparql> syntax be? The docs<https://github.com/robstewart57/hsparql/blob/master/tests/DBPedia.hs> don't include any language literal examples. -- Cheers, Andrew Pennebaker www.yellosoft.us

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Blocking IO & FIFOs
by Jason Dusek 20 Oct '12

20 Oct '12

Hi all, I am developing a coroutine-like interface to Bash. http://hpaste.org/76523 The idea is that one can send multiple "queries" to an interpreter and then collect the results of each query. How do we know when Bash is done with each query? Waiting for "no more output" seems ambiguous; so the way CoBash works is: * Each query gets a tmp dir with two named pipes in it. * The query is wrapped in redirections to the pipes. * The pipes are removed when the query completes. This does work, sort of: +Prelude> :load CoBash.hs [1 of 1] Compiling CoBash ( CoBash.hs, interpreted ) Ok, modules loaded: CoBash. *CoBash> tuple@(i,o,e,p) <- start e :: Handle i :: Handle o :: Handle p :: ProcessHandle tuple :: (Handle, Handle, Handle, ProcessHandle) *CoBash> query tuple "for n in {1..4}; do sleep 1; echo $n; done" ("1\n2\n3\n4\n","") it :: (ByteString, ByteString) I say sort of because it is quite brittle. Many commands do not return at all, for example: *CoBash> query tuple "uname -a" The way I retrieve the output from the FIFOs seems dangerous: (,) <$> Bytes.hGetContents oh <*> Bytes.hGetContents eh Surely, the FIFO for STDERR can not be read from until the FIFO for STDOUT is finished; but if there is a great deal of error output then the process will fill the FIFO's buffer and get stuck. If we switch the order of the reads, the "for n in ..." example above blocks: (,) <$> Bytes.hGetContents eh <*> Bytes.hGetContents oh I have tried a few different ways to read from the two handles concurrently; for example, by giving each thread an MVar to put the contents in, or by using hGetNonBlocking on a list of handles in a loop. Using the latter method, I never get EOF; it just collects empty strings forever. For comparison's sake, the expect behaviour with FIFOs is: In the first terminal: :; mkfifo fifo :; cat > fifo a b c d ^D :; In the second terminal: :; cat < fifo a b c d :; Here I "open" the FIFO for reading with < while opening it for writing with >. As long as the writer writes, the reader reads; when the writer closes the write end of the pipe, the reader receives EOF. Trying to duplicate the read behaviour in Haskell, using hGetContents from GHCi while using cat to write to the FIFO, doesn't work; which seems a little bogus. There have a been a few past threads about FIFOs and their troublesome interaction with Haskell's async-by-default IO style. To switch to System.Posix for IO -- and deal with Ptr Word8, in order to handle binary data -- seems like an awful step down. -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B {-# LANGUAGE OverloadedStrings , ScopedTypeVariables , ParallelListComp , TupleSections #-} module CoBash where import Control.Applicative import Control.Concurrent import Control.Concurrent.MVar import Control.Exception import Control.Monad import Data.Bits import Data.ByteString.Char8 (ByteString) import qualified Data.ByteString.Char8 as Bytes import Data.Maybe import Data.Monoid import qualified GHC.IO.Handle.FD import System.IO import System.IO.Error import System.Process import System.Posix.ByteString import System.IO.Temp import qualified Text.ShellEscape as Esc start :: IO (Handle, Handle, Handle, ProcessHandle) start = runInteractiveProcess "bash" [] Nothing (Just []) query :: (Handle, Handle, Handle, ProcessHandle) -> ByteString -> IO (ByteString, ByteString) query (i, _, _, _) query = withFIFOs query' where query' ofo efo = do Bytes.hPut i cmd hFlush i [oh, eh] <- mapM openFIFO [ofo, efo] (,) <$> Bytes.hGetContents oh <*> Bytes.hGetContents eh -- Works. -- (,) <$> Bytes.hGetContents eh <*> Bytes.hGetContents oh -- Blocks. where cmd = Bytes.unlines ["{", query, "} 1>" <> ofo <> " 2>" <> efo] shutdown :: (Handle, Handle, Handle, ProcessHandle) -> IO () shutdown (i, _, _, p) = () <$ hClose i <* waitForProcess p openFIFO path = GHC.IO.Handle.FD.openFileBlocking (Bytes.unpack path) ReadMode -- | Run an IO action with two FIFOs in scope, which will removed after it -- completes. withFIFOs :: (RawFilePath -> RawFilePath -> IO a) -> IO a withFIFOs m = withSystemTempDirectory "cobash." m' where m' = (uncurry m =<<) . mk . Bytes.pack mk d = (o, e) <$ (createNamedPipe o mode >> createNamedPipe e mode) where (o, e) = (d <> "/o", d <> "/e") mode = ownerReadMode .|. ownerWriteMode .|. namedPipeMode

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Monad.Reader #21 call for copy
by Edward Z. Yang 20 Oct '12

20 Oct '12

Call for Copy: The Monad.Reader - Issue 21 -------------------------------------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! The submission deadline for Issue 21 will be: **Tuesday, January 1** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (ezyang(a)mit.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

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Getting PID of a child process
by Jason Dusek 19 Oct '12

19 Oct '12

Hi All, Using `System.Process.runInteractiveProcess', I can start a process and get a handle to it: runInteractiveProcess :: FilePath -> [String] -> Maybe FilePath -> Maybe [(String, String)] -> IO (Handle, Handle, Handle, ProcessHandle) For diagnostic purposes, I'd like to print the PID of the process attached to this handle -- how best to do that? -- Jason Dusek pgp // solidsnack // C1EBC57DC55144F35460C8DF1FD4C6C1FED18A2B

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