April 2015 - Haskell-Cafe

Re: [Haskell-cafe] Distributed and persistent events bus in Haskell
by timmy tofu 08 Apr '15

08 Apr '15

We're working on something with 0MQ, if you (or anyone else reading) go that route and want to compare notes, hit me off-list. On Apr 8, 2015 8:01 AM, <haskell-cafe-request(a)haskell.org> wrote: > Send Haskell-Cafe mailing list submissions to > haskell-cafe(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > or, via email, send a message with subject or body 'help' to > haskell-cafe-request(a)haskell.org > > You can reach the person managing the list at > haskell-cafe-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Haskell-Cafe digest..." > > > Today's Topics: > > 1. Distributed and persistent events bus in Haskell (Arnaud Bailly) > 2. Re: Distributed and persistent events bus in Haskell > (Kyle Marek-Spartz) > 3. Re: Distributed and persistent events bus in Haskell > (Arnaud Bailly) > 4. Re: MinGHC for GHC 7.10.1 available (Aaron Contorer) > 5. status of deb.haskell.org? (Jeremy) > 6. help w/ improving custom Stream for parsec (Maurizio Vitale) > 7. Anyone interested in taking over network-uri? (Johan Tibell) > 8. Re: Anyone interested in taking over network-uri? (Alexey Shmalko) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 7 Apr 2015 16:50:49 +0200 > From: Arnaud Bailly <arnaud.oqube(a)gmail.com> > To: Haskell Cafe <haskell-cafe(a)haskell.org> > Subject: [Haskell-cafe] Distributed and persistent events bus in > Haskell > Message-ID: > < > CAL4zPaqpKXdOibD3_HECt+0V0ZniH0kKpA_ESQ30P7g9Ly_5cg(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > Hello, > > I am implementing an application using event sourcing as primary storage > for data, which implies I need a way to durably and reliably store streams > of events on stable storage. I also need to be able to have an event > distribution system on top of that persistent storage so that components > can subscribe to stored events. > > So far I have implemented a simple store, e.g. a flat file, which reuses > the format of Apache Kafka (just in case...). Not very robust nor > sophisticated but can work for moderate loads. Now I am looking for the > event distribution part in the hope of being able to reuse some distributed > event bus system that might exist somewhere and not having to roll my own. > > I have had a look couple of months ago at Vaultaire, Marquise and friends, > but I am not sure they are really suited to my use case: They seem to be > geared toward very high workload and throughput, like log or huge data > streams analysis. > > Thanks for any pointer you might share, > -- > Arnaud Bailly > > twitter: abailly > skype: arnaud-bailly > linkedin: http://fr.linkedin.com/in/arnaudbailly/ >

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ANN: hledger 0.25
by Simon Michael 08 Apr '15

08 Apr '15

I'm pleased to announce hledger and hledger-web 0.25. This release brings GHC 7.10 compatibility, terminal width awareness, useful averages and totals columns, and a more robust hledger-web add form. Full release notes: http://hledger.org/release-notes#hledger-0.25 . Release contributors: Simon Michael, Julien Moutinho. hledger (http://hledger.org) is a command-line tool and haskell library for tracking financial transactions, which are stored in a human-readable plain text format. It can also read CSV or timelog files. It provides useful reports, and can also help you record new transactions interactively. Add-on commands include hledger-web (a web interface), hledger-irr (for calculating internal rate of return) and hledger-interest (for generating interest transactions). hledger is inspired by and largely compatible with Ledger, and can be used with some Ledger files. Installation: cabal update; cabal install hledger [hledger-web] or see http://hledger.org/download and http://hledger.org/installing <http://hledger.org/installing> for more options. Best! -Simon

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MinGHC for GHC 7.10.1 available
by Michael Snoyman 07 Apr '15

07 Apr '15

I've just uploaded a new release of MinGHC, including GHC 7.10.1 and cabal-install 1.22.2.0. This release can be downloaded from: https://s3.amazonaws.com/download.fpcomplete.com/minghc/minghc-7.10.1-i386.… In the process, I also needed to upload a cabal-install binary for Windows, which is available at: https://s3.amazonaws.com/download.fpcomplete.com/minghc/cabal-install-1.22.… I've tested this distribution, but only lightly. Feedback from others would be useful :)

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Distributed and persistent events bus in Haskell
by Arnaud Bailly 07 Apr '15

07 Apr '15

Hello, I am implementing an application using event sourcing as primary storage for data, which implies I need a way to durably and reliably store streams of events on stable storage. I also need to be able to have an event distribution system on top of that persistent storage so that components can subscribe to stored events. So far I have implemented a simple store, e.g. a flat file, which reuses the format of Apache Kafka (just in case...). Not very robust nor sophisticated but can work for moderate loads. Now I am looking for the event distribution part in the hope of being able to reuse some distributed event bus system that might exist somewhere and not having to roll my own. I have had a look couple of months ago at Vaultaire, Marquise and friends, but I am not sure they are really suited to my use case: They seem to be geared toward very high workload and throughput, like log or huge data streams analysis. Thanks for any pointer you might share, -- Arnaud Bailly twitter: abailly skype: arnaud-bailly linkedin: http://fr.linkedin.com/in/arnaudbailly/

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FFI Ptr Question
by Michael Jones 07 Apr '15

07 Apr '15

I am having trouble figuring out how to pass callbacks to C. Given the definition below it is not clear how to define a function and pass it. I tried: getNumberOfRows :: CInt getNumberOfRows = 1 table <- wxcGridTableCreate getNumberOfRows… but the compiler barks at me: src/MainGui.hs:577:32: Not in scope: data constructor ‘Ptr’ even through I do the imports: import Foreign import Foreign.C There is also a _obj as the first value of wxcGridTableCreate and the callbacks, and I am not sure how to make a this* for it. I would have throughout that wxcGridTableCreate would return a Ptr. The goal is to pass haskell functions into wxcGridTableCreate so the grid can call back to haskell. Can someone show an example of just this one function, how to wrap a Ptr around it, and what to do with _obj? DEFINITION wxcGridTableCreate :: Ptr a -> Ptr b -> Ptr c -> Ptr d -> Ptr e -> Ptr f -> Ptr g -> Ptr h -> Ptr i -> Ptr j -> Ptr k -> Ptr l -> Ptr m -> Ptr n -> Ptr o -> Ptr p -> Ptr q -> IO (WXCGridTable ()) wxcGridTableCreate _obj _EifGetNumberRows _EifGetNumberCols _EifGetValue _EifSetValue _EifIsEmptyCell _EifClear _EifInsertRows _EifAppendRows _EifDeleteRows _EifInsertCols _EifAppendCols _EifDeleteCols _EifSetRowLabelValue _EifSetColLabelValue _EifGetRowLabelValue _EifGetColLabelValue = withObjectResult $ wx_ELJGridTable_Create _obj _EifGetNumberRows _EifGetNumberCols _EifGetValue _EifSetValue _EifIsEmptyCell _EifClear _EifInsertRows _EifAppendRows _EifDeleteRows _EifInsertCols _EifAppendCols _EifDeleteCols _EifSetRowLabelValue _EifSetColLabelValue _EifGetRowLabelValue _EifGetColLabelValue foreign import ccall "ELJGridTable_Create" wx_ELJGridTable_Create :: Ptr a -> Ptr b -> Ptr c -> Ptr d -> Ptr e -> Ptr f -> Ptr g -> Ptr h -> Ptr i -> Ptr j -> Ptr k -> Ptr l -> Ptr m -> Ptr n -> Ptr o -> Ptr p -> Ptr q -> IO (Ptr (TWXCGridTable ())) typedef int _cdecl (*TGridGetInt)(void* _obj); typedef int _cdecl (*TGridIsEmpty)(void* _obj, int row, int col); typedef void* _cdecl (*TGridGetValue)(void* _obj, int row, int col); typedef void _cdecl (*TGridSetValue)(void* _obj, int row, int col, void* val); typedef void _cdecl (*TGridClear)(void* _obj); typedef int _cdecl (*TGridModify)(void* _obj, int pos, int num); typedef int _cdecl (*TGridMultiModify)(void* _obj, int num); typedef void _cdecl (*TGridSetLabel)(void* _obj, int idx, void* val); typedef void* _cdecl (*TGridGetLabel)(void* _obj, int idx); EWXWEXPORT(void*,ELJGridTable_Create)(void* self,void* _EifGetNumberRows,void* _EifGetNumberCols,void* _EifGetValue,void* _EifSetValue,void* _EifIsEmptyCell,void* _EifClear,void* _EifInsertRows,void* _EifAppendRows,void* _EifDeleteRows,void* _EifInsertCols,void* _EifAppendCols,void* _EifDeleteCols,void* _EifSetRowLabelValue,void* _EifSetColLabelValue,void* _EifGetRowLabelValue,void* _EifGetColLabelValue) { return (void*)new ELJGridTable (self,

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[haskell.org Google Summer of Code] Call for Mentors
by Edward Kmett 07 Apr '15

07 Apr '15

We have had a rather large pool of potential students apply for this year's Google Summer of Code, but, ultimately, Google won't let us ask for a slot unless we have a potential mentor assigned in advance. On top of that, one thing we try to do with each project is wherever possible, assign both a primary and a backup mentor, so the available mentoring pool is drawn a little thin. Many hands make for light work, though: If you've mentored or thought about mentoring in years past, I'd encourage you to sign up on google-melange for the Google Summer of Code at: https://www.google-melange.com/gsoc/homepage/google/gsoc2015 and request a connection to haskell.org as a Mentor. Once you've done this you can help us vote on proposals, and should something seem appropriate to you, you can flag yourself as available as a potential mentor or backup mentor for one (or more) of the projects. We have a couple of weeks left to rate proposals and request slots, but it'd be good to make as much progress as we can this week. If you have any questions, feel free to reach out to me, or to Shachaf Ben-Kiki or Gershom Bazerman who have been helping out with organizational issues this year. We also have a #haskell-gsoc channel on irc.freenode.net if you have questions about what is involved. Thank you for your time and consideration, -Edward Kmett

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Call for Haskell.org committee self-nominations
by Gershom B 06 Apr '15

06 Apr '15

Dear Haskellers, We have been overdue for some time in calling for a new round of nominations to the Haskell.org Committee. We have three members due for retirement -- Jason Dagit, Edward Kmett, and Brent Yorgey. The committee would like to thank them for their excellent service. To nominate yourself, please send an email to committee at haskell.org by 21 April 2015. The retiring members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. Being a member of the committee does not necessarily require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. Strong leadership, communication, and judgement are very important characteristics for committee members. The role is about setting policy, providing direction/guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with the Haskell.org funds (and donations). As overseers for policy regarding the open source side of Haskell, committee members must also be able to set aside personal or business related bias and make decisions with the good of the open source Haskell community in mind. More details about the committee's roles and responsibilities are on *https://wiki.haskell.org/Haskell.org_committee <https://wiki.haskell.org/Haskell.org_committee>* If you have any questions about the process, please feel free to e-mail us at committee at haskell.org or to contact one of us individually. Regards, Gershom

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How to implement a source-sink pattern
by Roland Lutz 06 Apr '15

06 Apr '15

Hi! I'm trying to implement a source-sink type of pattern where there are a number of sinks, each connected to exactly one source, and a number of sources, each connected to zero or more sinks. The program consists of some modules, each defining their own sources and sinks. To illustrate this, here's what this would look like in C: /* global.h */ struct source { char *state; /* some more fields */ } struct sink { struct source *source; char *state; /* some more fields */ } struct sink **get_sinks_for_source(struct source *source); /* module_a.c */ struct source a_source, another_source; struct sink foo, bar, baz; ... foo.source = &a_source; ... Since getting the list of sinks for a source is a common operation, I'd probably define some kind of reverse map which is updated when a sink is remapped to a new source. I tried to rebuild this in Haskell, but the result is ridiculously complicated. Since I can't use pointers, I had to define an ordinal type class to enumerate the sources and sinks and use this to look up the actual data from the world state. But then I couldn't define the Sink type properly as I can't do: data SinkInfo = Source a => SinkInfo { sinkSource :: a, sinkState :: String } I have to add the source type as a type attribute, leading to a world state with as many parameters as there are sinks. Also, I couldn't figure out how to implement a function like sinksForSource :: Source a => WorldState p q r -> a -> [Sink b => b] since the source types won't match the type of the lookup key, and the sinks can be from different modules. Now here is the actual code (comments indicate where I intend to split it into individual files): {- Global.hs -} data SourceInfo = SourceInfo { sourceState :: String } class Eq a => Source a where getSourceInfo :: WorldState p q r -> a -> SourceInfo data SinkInfo a = SinkInfo { sinkSource :: a, sinkState :: String } class Sink a where getSinkInfo :: Source x => WorldState x x x -> a -> SinkInfo x -- should allow different arguments to WorldState {- ModuleA.hs -} data ModuleASource = ASource | AnotherSource deriving (Eq) instance Source ModuleASource where getSourceInfo world ASource = aSource $ moduleAState world getSourceInfo world AnotherSource = anotherSource $ moduleAState world data ModuleASink = Foo | Bar | Baz instance Sink ModuleASink where getSinkInfo world Foo = foo $ moduleAState world getSinkInfo world Bar = bar $ moduleAState world getSinkInfo world Baz = baz $ moduleAState world data ModuleAState p q r = ModuleAState { aSource :: SourceInfo, anotherSource :: SourceInfo, foo :: SinkInfo p, bar :: SinkInfo q, baz :: SinkInfo r } sinksForSourceInModuleA :: Source x => ModuleAState x x x -> x -> [ModuleASink] -- should allow different arguments to ModuleAState and return Sink b => [b] sinksForSourceInModuleA (ModuleAState _ _ foo bar baz) source = (if sinkSource foo == source then [Foo] else []) ++ (if sinkSource bar == source then [Bar] else []) ++ (if sinkSource baz == source then [Baz] else []) {- Main.hs -} data WorldState p q r = WorldState { moduleAState :: ModuleAState p q r } initState :: WorldState ModuleASource ModuleASource ModuleASource initState = WorldState $ ModuleAState (SourceInfo "a source init state") (SourceInfo "another source init state") (SinkInfo ASource "foo init state") (SinkInfo ASource "bar init state") (SinkInfo AnotherSource "baz init state") remapBar :: WorldState p q r -> WorldState p ModuleASource r remapBar (WorldState a) = WorldState $ a { bar = SinkInfo AnotherSource (sinkState $ bar a) } sinksForSource :: Source x => WorldState x x x -> x -> [ModuleASink] -- should allow different arguments to ModuleAState and return Sink b => [b] sinksForSource (WorldState a) source = sinksForSourceInModuleA a source main :: IO () main = let before = initState after = remapBar before in do putStrLn $ "Number of sinks for another source before: " ++ (show $ length $ sinksForSource before AnotherSource) putStrLn $ "Number of sinks for another source after: " ++ (show $ length $ sinksForSource after AnotherSource) There are some problems with this code: * I couldn't figure out how to resolve the circular references which are created by splitting this into individual files. * Source and sink ordinals can't be mixed between modules, so I wouldn't be able to add a module B anyway. * Looking up the source/sink states by ordinal keys is kind of cumbersome but works. * To get the list of sinks connected to a given source, the whole world state has to be polled. I expect this to be grossly inefficient (unless Haskell does some magic here) but I'm not sure how to add a cache in a consistent way. I can't believe this should be so complicated in Haskell, so I guess I'm trying to do this in an un-Haskell-ish way, or maybe there's something obvious I haven't seen. I'd be happy about any suggestions. Roland

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Initializing dynamic libraries
by Henk-Jan van Tuyl 06 Apr '15

06 Apr '15

L.S., I am using C++ dynamic libraries via a dynamic library written in C. The first thing my Haskell software does, is calling a C function to initiate the C++ libraries; but the C++ libraries generate warnings about not being able to set certain things, before the Haskell program initializes everything. Is there some automatic call to the dynamic libraries done, before execution of the main function in the Haskell program? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Functional dependencies conflict
by Daniel Trstenjak 05 Apr '15

05 Apr '15

Hi, I'm getting the compile error: Gamgine/Image/PNG/Internal/Parser.hs:14:10: Functional dependencies conflict between instance declarations: instance Monad m => Stream LB.ByteString m Word8 -- Defined at Gamgine/Image/PNG/Internal/Parser.hs:14:10 instance Monad m => Stream LB.ByteString m Char -- Defined in ‘Text.Parsec.Prim’ The relevant stuff from the parsec 3.1.9 code[1] is: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleContexts, UndecidableInstances #-} ... import qualified Data.ByteString.Lazy.Char8 as CL import qualified Data.ByteString.Char8 as C ... class (Monad m) => Stream s m t | s -> t where uncons :: s -> m (Maybe (t,s)) instance (Monad m) => Stream CL.ByteString m Char where uncons = return . CL.uncons instance (Monad m) => Stream C.ByteString m Char where uncons = return . C.uncons And from my code[2] is: {-# LANGUAGE BangPatterns, FlexibleInstances, MultiParamTypeClasses, FlexibleContexts #-} ... import qualified Data.ByteString.Lazy as LB ... instance (Monad m) => Stream LB.ByteString m Word8 where uncons = return . LB.uncons As you can see, the instances are for different ByteString types, therefore I don't quite get where GHC sees here any conflicts. Greetings, Daniel [1] https://github.com/aslatter/parsec/blob/master/Text/Parsec/Prim.hs [2] https://github.com/dan-t/Gamgine/blob/master/Gamgine/Image/PNG/Internal/Par…

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