- Haskell-Cafe - Haskell.org

Exploring Programming Language Theory
by Danny Gratzer 10 Dec '12

10 Dec '12

Hello, Sorry in advance for the soft question: Recently I have been studying more about how a lazy functional language is designed and compiled and have been reading Peyton-Jones's book "implementing functional languages: an introduction"

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Re: [Haskell-cafe] education or experience?
by Doug McIlroy 10 Dec '12

10 Dec '12

> Yes... CS academics delivers less than it could/should; > and whatever this delivery is, its asymptotically sub-linear. > Some of it is to do with the not-quick-enough takeup of FP in academia, > though there are obviously many other factors as well. > http://blog.languager.org/2011/02/cs-education-is-fat-and-weak-1.html > and sequel is about this: how we are not getting over the quirks of the > past history of CS in present day teaching. Here too suggestions for > modifications/ change of emphasis are appreciated. Rusi's cogent blog post includes a list of techniques/concepts that the "unconverted" could profitably pick up from the FP community. In fact the FP community came late to some of these, just as programming languages at large came late to garbage collection. Lazy evaluation--at the heart of spreadsheets since the beginning. Pattern matching--native to string processing (e.g. COMIT, SNOBOL). Appeared nearly in its present form in COGENT (1965). Booleans as first class*--surely this is a joke. Algol 60 had them. Matlab exploits them heavily (though represented as doubles). Data orientation--COBOL fostered this outlook; see Michael Jackson. As long as Lisp ruled, FP lagged on data types. FP also deserves credit for infinite data structures (though the special case of stream processing dates way back). Doug McIlroy * It's amusing to note that "real" Booleans--the ones that Boole used--were integers. For Boole, or(a,b) = a + b - a*b.

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Re: [Haskell-cafe] education or experience?
by Doug McIlroy 10 Dec '12

10 Dec '12

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How can I avoid buffered reads?
by Leon Smith 10 Dec '12

10 Dec '12

I have some code that reads (infrequently) small amounts of data from /dev/urandom, and because this is pretty infrequent, I simply open the handle and close it every time I need some random bytes. The problem is that I recently discovered that, thanks to buffering within GHC, I was actually reading 8096 bytes when I only need 16 bytes, and thus wasting entropy. Moreover calling hSetBuffering handle NoBuffering did not change this behavior. I'm not sure if this behavior is a bug or a feature, but in any case it's unacceptable for dealing with /dev/urandom. Probably the simplest way to fix this is to write a little C helper function that will read from /dev/urandom for me, so that I have precise control over the system calls involved. But I'm curious if GHC can manage this use case correctly; I've just started digging into the GHC.IO code myself. Best, Leon {-# LANGUAGE BangPatterns, ViewPatterns #-} import Control.Applicativeimport Data.Bitsimport Data.Word(Word64)import qualified Data.ByteString as Simport qualified Data.ByteString.Lazy as Limport Data.ByteString.Internal (c2w)import qualified System.IO as IOimport qualified Data.Binary.Get as Get showHex :: Word64 -> S.ByteStringshowHex n = s where (!s,_) = S.unfoldrN 16 f n f n = Just (char (n `shiftR` 60), n `shiftL` 4) char (fromIntegral -> i) | i < 10 = (c2w '0' - 0) + i | otherwise = (c2w 'a' - 10) + i twoRandomWord64s :: IO (Word64,Word64)twoRandomWord64s = IO.withBinaryFile "/dev/urandom" IO.ReadMode $ \handle -> do IO.hSetBuffering handle IO.NoBuffering Get.runGet ((,) <$> Get.getWord64host <*> Get.getWord64host) <$> L.hGet handle 16 main = do (x,y) <- twoRandomWord64s S.hPutStrLn IO.stdout (S.append (showHex x) (showHex y)) {- Relevant part of strace: open("/dev/urandom", O_RDONLY|O_NOCTTY|O_NONBLOCK) = 3 fstat(3, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 9), ...}) = 0 ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff367e528) = -1 EINVAL (Invalid argument) ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7ffff367e528) = -1 EINVAL (Invalid argument) read(3, "N\304\4\367/\26c\"\3218\237f\214yKg~i\310\r\262\"\224H\340y\n\376V?\265\344"..., 8096) = 8096 close(3) = 0 -}

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education or experience?
by Christopher Howard 09 Dec '12

09 Dec '12

I'm at something of a crossroads, and I'm hoping to get a bit of free career advice. I really enjoy programming with Haskell (and a few other exotic languages), and was hoping I could eventually make a living in that sort of field. Not rich and famous, necessarily, just enough to get by comfortably. I'm trying to decide, however; should I go back to school, finish my B.S. and pursue a Masters in CompSci? Or would the time (and money) be better spent aggressively pursuing volunteer work for companies, hoping to eventually get the experience and contacts that lead to a paying job? To be honest, I don't really want to go back to school, because I learn a lot faster (and more economically) on my own. However, I'm not sure which path is the fastest, and safest, approach to an actual paycheck. (Also, I'm something of a die-hard FOSS purist, if that affects the discussion at all.) -- frigidcode.com

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Help optimize fannkuch program
by Branimir Maksimovic 09 Dec '12

09 Dec '12

Well, playing with Haskell I have literally trasnlated my c++ program http://shootout.alioth.debian.org/u64q/program.php?test=fannkuchredux&lang=… got decent performance but not that good in comparisonwith c++ On my machine Haskell runs 52 secs while c++ 30 secs.(There is Haskell entry that is fastest but unfortunately does not runs on test machine is on par with c++http://shootout.alioth.debian.org/u64q/program.php?test=fannkuchredux&la… is something which I have missing since programsare identical.Aa with previous entries you gurus here helped a lot in both helpand learning experience.I simply love Haskell ;)I plan to contribute this program as it is much faster than current runningentry http://shootout.alioth.debian.org/u64q/program.php?test=fannkuchredux&lang=… if it is multithreaded and my is not. This is program: {-# LANGUAGE CPP, BangPatterns #-}{- The Computer Language Benchmarks Game http://shootout.alioth.debian.org/ contributed by Branimir Maksimovic -} import System.Environmentimport Text.Printfimport Data.Bits import qualified Data.Vector.Unboxed.Mutable as VMimport qualified Data.Vector.Generic.Mutable as VGimport qualified Data.Vector.Unboxed as V main = do n <- getArgs >>= readIO.head (checksum,maxflips) <- fannkuch n printf "%d\nPfannkuchen(%d) = %d\n" checksum n maxflips fannkuch n = do !perm <- V.unsafeThaw $ V.fromList [1..n] !tperm <- VG.new n !cnt <- VG.replicate n 0 let loop :: Int -> Int -> Int -> IO(Int,Int) loop !c !m !pc = do !b <- next_permutation perm n cnt if b == False then return (c,m) else do VM.unsafeCopy tperm perm !flips <- count_flips tperm 0 loop (c + (if pc .&. 1 == 0 then flips else -flips)) (max m flips) (pc+1) r <- loop 0 0 1 return r next_permutation :: VM.IOVector Int -> Int -> VM.IOVector Int-> IO(Bool)next_permutation !perm !n !cnt = do !i <- loop 1 if(i >= n) then return False else do !v <- VM.unsafeRead cnt i VM.unsafeWrite cnt i (v+1) return True where loop :: Int -> IO(Int) loop !i | i < n = do !tmp <- VM.unsafeRead perm 0 let rotate :: Int -> IO() rotate !j = if j >= i then do VM.unsafeWrite perm i tmp return () else do !v <- VM.unsafeRead perm (j+1) VM.unsafeWrite perm j v rotate (j+1) rotate 0 !v <- VM.unsafeRead cnt i if v >= i then do VM.unsafeWrite cnt i 0 loop (i+1) else return i | otherwise = return i count_flips :: VM.IOVector Int -> Int -> IO(Int)count_flips !tperm !flips = do !f <- VM.unsafeRead tperm 0 if f == 1 then return flips else do VG.reverse $ VM.unsafeSlice 0 f tperm count_flips tperm (flips+1)

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Best approach to avoid dependency hells
by Ivan Perez 08 Dec '12

08 Dec '12

Hello everyone, I've spent the last couple of days fighting my way around a dependency hell with my own libraries and packages. If I install them package by package (by hand), I'm very likely to hit one of these conflicts that I'm talking about. A simple example of something that did happen: - Package A depends on bytestring, and is compatible with both 0.9.* and 0.10.* - Package B depends on ghc, which I installed using the package manager and which depends on bytestring 0.9.2 - Package B depends on package A. I do not want to add to the cabal files that B depends on bytestring 0.9.*, because it's not true, or that A depends on bytstring 0.9, because it's also not true. I would like to the best approach to 1) avoid these situations and 2) fix them when they happen. Regarding (2), Is there any way to automatically rebuild all the broken packages? Regarding (1), I thought about using cabal-dev, but I think I'll just have the same problem, won't I? After all, I started from a clean fresh ghc installation and I just installed what I needed to make my program run. I also thought about adding process, bytestring and other global packages to the list of constraints in ~/.cabal/config. Would this be a good approach, or will it lead to bigger headaches in the future (for instance, in the form of programs that just cannot be installed at all)? Also, is there any way to detect potential dependency hells without having to install a package? (Some tool that will analyse the dependency information (statically) and determine more strict or incorrect upper and lower bounds for the dependencies of a package.) Cheers, Ivan

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Safe Haskell safe-inferred status
by Omari Norman 08 Dec '12

08 Dec '12

Is there a way to determine whether a module has been marked safe by GHC for purposes of Safe Haskell? The GHC 7.4 docs say that if I compile a module and I don't use the -XSafe or -XTrustworthy flag, GHC will automatically figure out whether the module would have compiled with -XSafe and, if so, marks the module as safe. Where does GHC record this determination? It doesn't seem to show up in ghc-pkg dump (there you can see trusted packages, but I am looking for safe-inferred status for modules.) ghc --show-iface seems to come up dry too. I know Haddock shows this information but sometimes it says "Safe Haskell: None". I have no idea what this means so I was wondering what GHC itself says. Thanks. --Omari

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Understanding GHC's instance inference.
by Takayuki Muranushi 08 Dec '12

08 Dec '12

Dear everyone, I have a code https://github.com/nushio3/practice/blob/master/instance-inference/zipf-11-… that produces a type-error when I remove a type signature. https://github.com/nushio3/practice/blob/master/instance-inference/zipf-11.… The strange point is the following error message from ghc 7.6.1 (abbreviated; the full error text is included as the comment of the code): No instance for (PType (Cons V.Vector (V.Vector Double) (Cons V.Vector (V.Vector Char) (Cons V.Vector (V.Vector Int) (Nil V.Vector)))) ((Double -> Char -> Int -> String) -> a0)) arising from a use of `forZN' The type variable `a0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there is a potential instance available: instance [overlap ok] (Zip v, Reduce v f0 vaS r (Nil v)) => PType (Cons v (v i) vaS) ((i -> f0) -> v r) ghc presents only one potential instance, and it seems to me that by setting v = V.Vector i = Double f0 = Char -> Int -> String r = String etc, you can actually match the 'potential' instance. The fact that this code typechecks with an additional type signature (zipf-11-1.hs) shows that there is a way to satisfy the constraint. So the question: why the instance inference stops here, when there is only one potential instance and no ambiguity? Or is it? What are potential instances? I thought the cause of the error is that ghc cannot match (v r) with (a0), and so I tried to shrink the example before I post. However my guess was wrong. Tinyer example did typecheck: https://github.com/nushio3/practice/blob/master/instance-inference/copy-01.… https://github.com/nushio3/practice/blob/master/instance-inference/copy-02.… https://github.com/nushio3/practice/blob/master/instance-inference/copy-04.… If you have any sample codes that fails to typecheck and presents only one "potential instance" , that will be a great clue for me. Also, please recommend me any reading material for instance inference. (Haskell 98 deals with instance declaration syntax, and there are variety of text for type inference, but I could find few for instance inference.) Best, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Maintainer of fixpoint
by Petr P 07 Dec '12

07 Dec '12

Dear Haskellers, I've made some minor improvements of fixpoint package < http://hackage.haskell.org/package/fixpoint>. I tried to contact the original maintainer twice, but without success. The package hasn't been updated for 4 years. How should I proceed? Should I start a new fork of the package? Or is there a way how to take over an orphaned package? Thanks for help, Petr Pudlak

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yaml and aeson Was: Growing Haskell Platform
by Michael Snoyman 07 Dec '12

07 Dec '12

On Fri, Dec 7, 2012 at 10:54 AM, Roman Cheplyaka <roma(a)ro-che.info> wrote: > * Michael Snoyman <michael(a)snoyman.com> [2012-12-07 09:52:07+0200] > > Let me bring up one other package: yaml (written by me). I think it's a > > pretty good fit for the standard YAML packaging library, since it simply > > reuses the existing infrastructure from the aeson package (i.e. the > ToJSON > > and FromJSON typeclasses, and the Value datatype). But I'm actually a bit > > concerned about proposing it, based on some history. > > When I was looking at existing YAML libraries, I rejected yours when I > saw JSON types. (I did see the "do not let that confuse you, it's > intentional" warning, and no, it didn't help.) > > Besides giving the feeling that the API is a hack, this will inevitably > confuse readers of my code who are not familiar with the library and, > just based on the names, will assume that the code is working with JSON, > not YAML. > > If you mean the naming is a hack, I agree. But if we pretend for a moment that the word JSON was actually replaced with YAML everywhere, I think the abstraction is correct. The Value datatype represents a very sane subset of YAML that people usually want to deal with. And if they want to deal with all the glories of aliases and tags, we still have the Text.Libyaml interface which preserves all information. In other words, the hack is in the name, not the abstraction. > IIUC, "all of the existing tools for JSON processing" are mainly just > existing FromJSON instances. I am not sure how common this situation is, > but for that you could define a newtype whose FromYAML instance would > internally use the FromJSON instance of the underlying type. > > It's the two typeclasses and the Value datatype. Don't underestimate the importance of reusing infrastructure here. If we had our own types and classes in YAML, then everyone who wants to include serialization classes in their packages would have to write redundant instances for both packages. Most likely, that would just mean incomplete instances for both packages. It would also mean that we'd have to duplicate libraries like aeson-lens, and duplicate all bugfixes in built-in instances, Generics code, TH code, etc. As a side benefit, the current setup allows code to remain serialization-format-agnostic. For example, Persistent can read configuration from either JSON to YAML files. Since YAML is a superset of JSON, that's not such an amazing statement, but it does mean that users can avoid an extra yaml package dependency if they really are just sticking with JSON data. > Or perhaps the JSON-agnostic subset of aeson could be extracted into a > separate library, which both aeson and yaml would depend on. > > I'd be very much in favor of that. I'm not so irked by the "name hack" to have bothered bringing this up with Bryan, but if he's willing to make that change, I'd certainly be grateful. I actually think it would be more logical for the typeclasses to be called ToValue and FromValue anyway, since that more directly states what they do. > As for toYAML/toJSON, I guess most of the time they are different anyway — > otherwise it's defeating the purpose of YAML to be more human-readable > than JSON. > > I don't think that's true in practice. Most of the readability of YAML comes from the syntax, not the choice of actual serialization structure. But I could be mistaken. Michael

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schedule function
by Corentin Dupont 06 Dec '12

06 Dec '12

Hi all, I have a simple question: is there in the libraries a function that can schedule a variable list of events for me? For example of the type: type Schedule = [UTCTime, IO()] startSchedule :: TVar Schedule -> IO ThreadId The function startSchedule will have to execute my actions at the given times. Of course the TVar can be changed by another thread, so the schedule will have to be recomputed. Thanks! Corentin

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Generics & pattern matching
by Joerg Fritsch 06 Dec '12

06 Dec '12

I have the code below that I pluged from SO. My problem is that although I sort of understand how with rewrite True and rewrite False it is suppressed that always the first element of the tuple stays as-is; I cannot find any way to apply this to the last element also. In essence there are times when I would need to change all elements of the tuple but the first and at other times I would need to change all elements except the first and the last. I don't quite understand (yet) how I could do that. Is there any way to match for a specific element eg. the third from the right or so? --Joerg {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} import Data.Typeable import GHC.Generics rewrite_ :: (Generic a, Generic b, Rewriter (Rep a), Rewrite (Rep a) ~ Rep b) => a -> b rewrite_ = to . rewrite False . from class Rewriter f where type Rewrite f :: * -> * rewrite :: Bool -> f a -> (Rewrite f) a instance Rewriter f => Rewriter (M1 i c f) where type Rewrite (M1 i c f) = M1 i c (Rewrite f) rewrite x = M1 . rewrite x . unM1 instance Typeable c => Rewriter (K1 i c) where type Rewrite (K1 i c) = K1 i String rewrite False (K1 x) | Just val <- cast x = K1 val rewrite _ _ = K1 "NIL" instance (Rewriter a, Rewriter b) => Rewriter (a :*: b) where type Rewrite (a :*: b) = Rewrite a :*: Rewrite b rewrite x (a :*: b) = rewrite x a :*: rewrite True b y0 :: (String, Int, Double) y0 = ("something", 3, 4.788) y1 :: (String, String, String, (Int, Int)) y1 = ("something else", "Hello", "NIL", (4,6)) main :: IO () main = do print (rewrite_ y0 :: (String, String, String)) print (rewrite_ y1 :: (String, String, String, String))

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Are there REPA linear algebra routines? e.g. Eigenvalues?
by KC 06 Dec '12

06 Dec '12

:) -- -- Regards, KC

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The end of an era, and the dawn of a new one
by Simon Peyton-Jones 06 Dec '12

06 Dec '12

Friends You'll have seen Simon Marlow's recent announcement (included below): | Today I'm announcing that I'm leaving Microsoft Research. Simon Marlow and I have worked together on GHC for nearly two decades. During much of that time we have shared a cup of coffee every morning (one cup each, since you ask), to discuss what's going on in GHC-land. We had frequent detailed and illuminating debates about design alternatives. Simon is the absolute master of GHC's code generation, runtime system, garbage collector, support for parallel execution, and much else besides. His sheer programming skill in rooting out obscure and hard-to-reproduce runtime system bugs has always amazed me. He is more than just a great hacker, of course. He is a leader in our community, has a strong publication record, chaired the Haskell Prime committee for a year, and is sought after for tutorials about parallel programming. I owe Simon a great debt, as I think we all do, for the care and skill he has lavished on GHC. He's not going to disappear entirely, but he will be spending much less time on GHC than in the past. That is a change, but it's a change that was always going to happen sometime, and while change is sometimes uncomfortable, it can also open up new possibilities. GHC is over 20, and like my own children (who are actually all a bit younger than GHC), that's not a bad time to start to head out into the world. Over time, more and more people have started to help with the task of developing and maintaining GHC. We'll have to do more of that. These days GHC has a very large "surface area" (running on different platforms, dynamic linking, debugger, profiler, GHCi, libraries and Cabal, etc etc), and has grown far beyond what any single person or research group can manage. Lest you should wonder, I myself will certainly continue to work on GHC. But my job is to be a *researcher*, and I am not well equipped -- in temperament or expertise -- to deal with the myriad complexities of deployment, upgrade policy, portability, and the like. Well Typed (mostly in the guise of Ian Lynagh) will continue to handle some of these issues, but again there is too much for one person to do. So we need your help. Seriously. Rather than just thinking "GHC HQ will sort it out", I would welcome you saying "Shall I take responsibility for X?". Johan, Bryan, and others offering to be GHC Performance Tsars is a great recent example. But don't wait to be asked: just make an offer if you see a gap or an open ticket --- and I think there will be plenty of those. Particularly valuable are offers to take responsibility for a particular area (eg the LLVM code generator, or the FFI). I'm hoping that this sea change will prove to be quite empowering, with GHC becoming more and more a community project, more resilient with fewer single points of failure. I am going to miss that cup of coffee though. Thank you Simon! Facebook is lucky to have you. Onward and upward. Simon (PJ) | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell- | users-bounces(a)haskell.org] On Behalf Of Simon Marlow | Sent: 22 November 2012 11:27 | To: haskell; glasgow-haskell-users(a)haskell.org | Subject: Leaving Microsoft | | Today I'm announcing that I'm leaving Microsoft Research. | | My plan is to take a break to finish the book on Parallel and Concurrent | Haskell for O'Reilly, before taking up a position at Facebook in the UK | in March 2013. | | This is undoubtedly a big change, both for me and for the Haskell | community. I'll be stepping back from full-time GHC development and | research and heading into industry, hopefully to use Haskell. It's an | incredibly exciting opportunity for me, and one that I hope will | ultimately be a good thing for Haskell too. | | What does this mean for GHC? Obviously I'll have much less time to work | on GHC, but I do hope to find time to fix a few bugs and keep things | working smoothly. Simon Peyton Jones will still be leading the project, | and we'll still have support from Ian Lynagh, and of course the | community of regular contributors. Things are in a reasonably stable | state - there haven't been any major architectural changes in the RTS | lately, and while we have just completed the switchover to the new code | generator, I've been working over the past few weeks to squeeze out all | the bugs I can find, and I'll continue to do that over the coming months | up to the 7.8.1 release. | | In due course I hope that GHC can attract more of you talented hackers | to climb the learning curve and start working on the internals, in | particular the runtime and code generators, and I'll do my best to help | that happen. | | Cheers, | Simon

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Maintainer of hgettext
by Ivan Perez 06 Dec '12

06 Dec '12

Hello haskellers, A few months ago I sent an email to Vasyl Pasternak regarding a couple of bugs in hgettext [1], together with a small patch that fixes them. I never received an answer and I can see that the error persists. Does anyone know where he is? What would you recommend at this point? Cheers, Ivan. [1] http://hackage.haskell.org/package/hgettext

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Vector mysteries
by Fixie Fixie 06 Dec '12

06 Dec '12

Hi I am trying to implement Show for a storable-mutable-vector. Does anyone have a clue why the type-system is killing me :-) Code is below... Cheers Felix ---- {-# LANGUAGE ExistentialQuantification #-} import Control.Monad (liftM2) import qualified Data.Vector.Unboxed as V import qualified Data.Vector.Unboxed.Mutable as MV import Data.Vector.Storable.Mutable import GHC.Prim (RealWorld) import Control.Monad.Primitive import Control.Monad import qualified Data.Vector as VEC import qualified Data.Vector.Generic.Mutable as GM import Data.Int import Data.Typeable.Internal import Data.Primitive data Row = forall m s. (Storable s, MV.Unbox s, Prim s, PrimMonad m) => Row (MV.MVector (PrimState m) s) instance Show Row where show (Row row) = do let xx = MV.read row 0 "Done" main :: IO () main = do print "Done" I get this error: Could not deduce (PrimState m ~ PrimState m0) from the context (Storable s, MV.Unbox s, Prim s, PrimMonad m) bound by a pattern with constructor Row :: forall (m :: * -> *) s. (Storable s, MV.Unbox s, Prim s, PrimMonad m) => MV.MVector (PrimState m) s -> Row, in an equation for `show' NB: `PrimState' is a type function, and may not be injective Expected type: MV.MVector (PrimState m0) s Actual type: MV.MVector (PrimState m) s In the first argument of `MV.read', namely `row' In the expression: MV.read row 0

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sequential logic
by Christopher Howard 06 Dec '12

06 Dec '12

Hi. I was wondering what the various (and especially most simple) approaches one could take for working with (simulating or calculating) sequential logic in Haskell. By sequential logic, I mean like wikipedia describes, where a system is made up of logic gates, whose output is dependent not only on the logic operation of the gate, but on the previous state of the gate. (Like in electronics where the system can be driven by a clock signal or have memory.) Does one have to get into FRP for that sort of thing? Or use some kind of FSM framework? Or is there some kind of fancy state monad suitable for that? Or...? I'm no electronic or digital engineer, but for learning purposes I've been trying to see if I could build an (extremely simple) virtual processor or calculator in Haskell, using only the Bool type and a few of the boolean operators (and functions composed of the aforementioned), reproducing things like half adders and full adders as functions in the program. Of course, I quickly ran into the stateful aspect of the problem, in subjects like clock signal and flip flops. -- frigidcode.com

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Haskell Weekly News: Issue 253
by Daniel Santa Cruz 06 Dec '12

06 Dec '12

Welcome to issue 253 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of November 25 to December 01, 2012. Quotes of the Week * edwardk: this breaks my previous record of 6 unsafeCoerce's in a line * bos:Harpers gona harp. Top Reddit Stories * Pretty Output in GHCi (HOWTO in comments) Domain: i.imgur.com, Score: 82, Comments: 14 On Reddit: [1] http://goo.gl/2c7Ns Original: [2] http://goo.gl/uYZAs * Stable, Vetted Hackage: Call for Participation Domain: yesodweb.com, Score: 59, Comments: 24 On Reddit: [3] http://goo.gl/QrJEz Original: [4] http://goo.gl/1PlBK * Strict vs non-strict semantics: "Why can’t I just be a little lazy?" Domain: blog.ezyang.com, Score: 42, Comments: 20 On Reddit: [5] http://goo.gl/P2nSm Original: [6] http://goo.gl/WDhk6 * io-stream: simple and easy to use primitives for I/O using streams Domain: github.com, Score: 37, Comments: 10 On Reddit: [7] http://goo.gl/uDbGG Original: [8] http://goo.gl/sGqtY * Toward a Secret Sky: Building a Haskell web app with Snap: Snap quickstart guide Domain: unpythonic.blogspot.ca, Score: 36, Comments: 15 On Reddit: [9] http://goo.gl/6UdY8 Original: [10] http://goo.gl/Y6wli * Hackage: unittyped-0.1, an extendable library for type-safe computations including units Domain: hackage.haskell.org, Score: 34, Comments: 23 On Reddit: [11] http://goo.gl/f7b1M Original: [12] http://goo.gl/jmZCi * Easy JSON parsing in Haskell with Aeson Domain: blog.raynes.me, Score: 34, Comments: 18 On Reddit: [13] http://goo.gl/JERS0 Original: [14] http://goo.gl/Ccfbm * 1st São Paulo Haskell Meeting Domain: plus.google.com, Score: 29, Comments: 4 On Reddit: [15] http://goo.gl/MKVPb Original: [16] http://goo.gl/yaLUi * Conal Elliott: "Parallel speculative addition via memoization" Domain: conal.net, Score: 26, Comments: 11 On Reddit: [17] http://goo.gl/EQZxj Original: [18] http://goo.gl/wg0KD * Elm: Making the Web Functional [video/slides] Domain: infoq.com, Score: 26, Comments: 1 On Reddit: [19] http://goo.gl/0qAJM Original: [20] http://goo.gl/bldR0 * From traced monoidal to compact closed: geometry of interaction explained with (OCaml) code Domain: semantic-domain.blogspot.fr, Score: 18, Comments: 22 On Reddit: [21] http://goo.gl/auq1p Original: [22] http://goo.gl/4XvRw * Static Analysis with Applicatives Domain: gergo.erdi.hu, Score: 17, Comments: 13 On Reddit: [23] http://goo.gl/sNLm9 Original: [24] http://goo.gl/Lvxpn * 24 Pull Requests - Does Anyone Have Project Suggestions? Domain: self.haskell, Score: 16, Comments: 3 On Reddit: [25] http://goo.gl/U2bsY Original: [26] http://goo.gl/U2bsY * what if haskell-platform was a hackage metapackage? Domain: self.haskell, Score: 16, Comments: 7 On Reddit: [27] http://goo.gl/SPIlO Original: [28] http://goo.gl/SPIlO * Does anyone know if more videos are going to be posted to Haskell Live TV? Domain: youtube.com, Score: 15, Comments: 2 On Reddit: [29] http://goo.gl/jJxWb Original: [30] http://goo.gl/NVk3D * Data-driven hacks for DB querying — is this evil? Domain: hpaste.org, Score: 13, Comments: 22 On Reddit: [31] http://goo.gl/0J5eJ Original: [32] http://goo.gl/gfRLS * Why does this result not appear in this search in Hoogle? Domain: self.haskell, Score: 12, Comments: 10 On Reddit: [33] http://goo.gl/DGTu2 Original: [34] http://goo.gl/DGTu2 Top StackOverflow Questions * Is putStrLn thread-safe? votes: 12, answers: 1 Read on SO: [35] http://goo.gl/rjRyN * How do you prove that a function is unique for its type? votes: 11, answers: 1 Read on SO: [36] http://goo.gl/odOSy * Laziness/strictness between data and newtype votes: 10, answers: 2 Read on SO: [37] http://goo.gl/Dkqv8 * Why do we use folds to encode datatypes as functions? votes: 10, answers: 1 Read on SO: [38] http://goo.gl/RRVuE * Haskell repa - how to reduce array and return index? votes: 7, answers: 2 Read on SO: [39] http://goo.gl/HpXhR * How much is applicative really about applying, rather than “combining”? votes: 7, answers: 1 Read on SO: [40] http://goo.gl/zq6j3 * Why is that not lazy votes: 7, answers: 2 Read on SO: [41] http://goo.gl/F8H0m * Biapplicative and Bimonad? votes: 6, answers: 1 Read on SO: [42] http://goo.gl/7C01J * reactive-banana-wx `sink` does not generate an event enabling post-sink processing votes: 6, answers: 1 Read on SO: [43] http://goo.gl/pXdTu * Expand type synonyms, type families with GHCi votes: 6, answers: 1 Read on SO: [44] http://goo.gl/G6Sks * Converting expression to pointfree style (Haskell) votes: 6, answers: 1 Read on SO: [45] http://goo.gl/36gLO * ADTs vs. Typeclasses — Canonical use votes: 6, answers: 1 Read on SO: [46] http://goo.gl/ofpbX * Haskell: Monitor a file without polling (à la inotify in linux) votes: 6, answers: 3 Read on SO: [47] http://goo.gl/JGzI0 Until next time, Daniel Santa Cruz References 1. http://i.imgur.com/t6hOx.png 2. http://www.reddit.com/r/haskell/comments/144biy/pretty_output_in_ghci_howto… 3. http://www.yesodweb.com/blog/2012/11/stable-vetted-hackage 4. http://www.reddit.com/r/haskell/comments/141hww/stable_vetted_hackage_call_… 5. http://blog.ezyang.com/2012/11/why-cant-i-just-be-a-little-lazy/ 6. http://www.reddit.com/r/haskell/comments/13tt44/strict_vs_nonstrict_semanti… 7. https://github.com/snapframework/io-streams 8. http://www.reddit.com/r/haskell/comments/13z9i4/iostream_simple_and_easy_to… 9. http://unpythonic.blogspot.ca/2012/11/building-haskell-web-app-with-snap-sn… 10. http://www.reddit.com/r/haskell/comments/13sb85/toward_a_secret_sky_buildin… 11. http://hackage.haskell.org/package/unittyped-0.1 12. http://www.reddit.com/r/haskell/comments/13t795/hackage_unittyped01_an_exte… 13. http://blog.raynes.me/blog/2012/11/27/easy-json-parsing-in-haskell-with-aes… 14. http://www.reddit.com/r/haskell/comments/141ch4/easy_json_parsing_in_haskel… 15. https://plus.google.com/events/cng3rcv1tjl84g2juddk1i36icg 16. http://www.reddit.com/r/haskell/comments/13vctv/1st_s%C3%A3o_paulo_haskell_… 17. http://conal.net/blog/posts/parallel-speculative-addition-via-memoization 18. http://www.reddit.com/r/haskell/comments/13yenz/conal_elliott_parallel_spec… 19. http://www.infoq.com/presentations/Elm 20. http://www.reddit.com/r/haskell/comments/1423uc/elm_making_the_web_function… 21. http://semantic-domain.blogspot.fr/2012/11/in-this-post-ill-show-how-to-tur… 22. http://www.reddit.com/r/haskell/comments/13zzxl/from_traced_monoidal_to_com… 23. http://gergo.erdi.hu/blog/2012-12-01-static_analysis_with_applicatives 24. http://www.reddit.com/r/haskell/comments/143wpd/static_analysis_with_applic… 25. http://www.reddit.com/r/haskell/comments/143c0e/24_pull_requests_does_anyon… 26. http://www.reddit.com/r/haskell/comments/143c0e/24_pull_requests_does_anyon… 27. http://www.reddit.com/r/haskell/comments/1445jh/what_if_haskellplatform_was… 28. http://www.reddit.com/r/haskell/comments/1445jh/what_if_haskellplatform_was… 29. http://www.youtube.com/user/HaskellLiveTV 30. http://www.reddit.com/r/haskell/comments/13x0z9/does_anyone_know_if_more_vi… 31. http://hpaste.org/78401 32. http://www.reddit.com/r/haskell/comments/13xi63/datadriven_hacks_for_db_que… 33. http://www.reddit.com/r/haskell/comments/13r9ii/why_does_this_result_not_ap… 34. http://www.reddit.com/r/haskell/comments/13r9ii/why_does_this_result_not_ap… 35. http://stackoverflow.com/questions/13549706/is-putstrln-thread-safe 36. http://stackoverflow.com/questions/13575583/how-do-you-prove-that-a-functio… 37. http://stackoverflow.com/questions/13566673/laziness-strictness-between-dat… 38. http://stackoverflow.com/questions/13575894/why-do-we-use-folds-to-encode-d… 39. http://stackoverflow.com/questions/13570354/haskell-repa-how-to-reduce-arra… 40. http://stackoverflow.com/questions/13625905/how-much-is-applicative-really-… 41. http://stackoverflow.com/questions/13663430/why-is-that-not-lazy 42. http://stackoverflow.com/questions/13556314/biapplicative-and-bimonad 43. http://stackoverflow.com/questions/13580121/reactive-banana-wx-sink-does-no… 44. http://stackoverflow.com/questions/13595773/expand-type-synonyms-type-famil… 45. http://stackoverflow.com/questions/13596335/converting-expression-to-pointf… 46. http://stackoverflow.com/questions/13610164/adts-vs-typeclasses-canonical-u… 47. http://stackoverflow.com/questions/13642619/haskell-monitor-a-file-without-…

1 0

QuickCheck shrink
by warrensomebody＠gmail.com 05 Dec '12

05 Dec '12

Can someone point me at some documentation or examples of how to write and use a QuickCheck shrink function. I can't seem to find anything online. Thanks, Warren

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Re: [Haskell-cafe] The end of an era, and the dawn of a new one
by Johan Tibell 05 Dec '12

05 Dec '12

On Wed, Dec 5, 2012 at 12:37 PM, David Terei <davidterei(a)gmail.com> wrote: > I have always considered the LLVM code generator my responsibility and > will continue to do so. I don't seem to find the time to make > improvements to it but make sure to keep it bug free and working with > the latest LLVM releases. So if others want are interested in working > on it then there is plenty of room for that but I'll continue to > maintain it and be responsible for it at the least. I will maintain the I/O manager as per usual, probably together with Bryan and Andreas (but I cannot speak for them of course).

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Design of a DSL in Haskell
by Joerg Fritsch 05 Dec '12

05 Dec '12

This is probably a very basic question. I am working on a DSL that eventuyally would allow me to say: import language.cwmwl main = runCWMWL $ do eval ("isFib::", 1000, ?BOOL) I have just started to work on the interpreter-function runCWMWL and I wonder whether it is possible to escape to real Haskell somehow (and how?) either inside ot outside the do-block. I thought of providing a defautl-wrapper for some required prelude functions (such as print) inside my interpreter but I wonder if there are more elegant ways to co-loacate a DSL and Haskell without falling back to being a normal library only. --Joerg

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RFC: Changes to Travis CI's Haskell support
by Simon Hengel 05 Dec '12

05 Dec '12

Hi, currently the default to test Haskell projects on Travis CI [1] is: install: - cabal install --enable-tests script: - cabal test The issue with this is that it runs the test-suite twice, which is a waste of resources and delays build reports. This was an oversight on my part, when I adapted Travis's Haskell support for cabal-install-0.14.0. I think the right thing to do is: install: - cabal install --only-dependencies --enable-tests script: - cabal configure --enable-tests && cabal build && cabal test Please let me know if you think there are better ways to do it, or if you see any issues. Cheers, Simon [1] https://travis-ci.org/

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confirming order of kinds and rank of types
by dude 05 Dec '12

05 Dec '12

I hope you can help me confirm my understanding of the order of kinds and rank of types as reported in [1,2]. I have been validating the mapping functions in Section 2 against the GHC 7.4.2 type checker by explicitly annotating the universal quantifiers as described in [1]. 1. Just to baseline my understanding, given the following data type data U a = UC (forall a. a) -- :k U, U :: * -> * -- order of kind = 1 -- :t UC, UC :: (forall a1. a1) -> U a -- rank of type = 2 am I correct that the order of the kind is 1 and the rank of the type constructor is 2? See page 3 of [1] for counting the order and also page 3 of [2] for counting rank. 2. Assuming the answer to #1 above is yes, am I also correct that the order of Fix is 2 and the rank of In is also 2 and the annotation to In does not affect its rank? newtype Fix f = In (forall f. f (Fix f)) -- :k Fix, Fix :: (* -> *) -> * -- order of kind = 2 -- :t In, In :: (forall (f1 :: * -> *). f1 (Fix f1)) -> Fix f -- rank of type still = 2 3. Are the explicit annotations I added to the following fixpoint operator correct? Hinze indicates this is a third order kind and it seems that although the annotation on HIn has a higher order, the rank of the type hasn't changed, correct? newtype HFix h a = HIn (forall h. (forall a. h (HFix h) a)) -- :k HFix, HFix :: ((* -> *) -> * -> *) -> * -> * -- per hinze order of kind = 3 -- :t HIn, HIn :: (forall (h1 :: (* -> *) -> * -> *) a1. h1 (HFix h1) a1) -> HFix h a -- still rank of type = 2 because there's one quantifier to the left of the function arrow. If all that all checks out, I have a few follow up questions: 4. Hinze reports that mapHFix is Rank3, but the following type checks with the Rank2Type pragma and if I remove the explicit universal quantifiers in HIn. Put the quantifiers back in an it does not type check. mapHFix maph mapa (HIn v) = HIn (maph (mapHFix maph) mapa v) 5. Also, the inferred signature for mapHFix differs from the one stated by Hinze. It's this ... mapHFix :: ((t2 -> HFix t t1 -> HFix h a) -> t2 -> t (HFix t) t1 -> h (HFix h) a) -> t2 -> HFix t t1 -> HFix h a What is the rank of this type signature? The type variables are not in the same order as Hinze's and I was able to just wrap a (forall t2 t t1 h a.) around the whole thing and it type checked. Why are they different? 6. What is the correct type signature for mapHFix? The correct type signature should support changing the pragma from Rank2Types to RankNTypes, then today's 7.4.2 type checker will succeed where it could not in 1999 when the paper was written, correct? Thanks so much for helping me confirm my understanding of these issues! -- dude 1. Polytypic Values Posess Polykinded Types. (Hinze, 1999) 2. A Direct Algorithm for Type Inference in the Rank 2 Fragment of the Second Order Lambda Calculus. (Kfoury, 1993)

1 0

Naive matrix multiplication with Accelerate
by Clark Gaebel 05 Dec '12

05 Dec '12

Hello cafe, I've recently started learning about cuda and hetrogenous programming, and have been using accelerate [1] to help me out. Right now, I'm running into trouble in that I can't call parallel code from sequential code. Turns out GPUs aren't exactly like Repa =P. Here's what I have so far: import qualified Data.Array.Accelerate as A import Data.Array.Accelerate ( (:.)(..) , Acc , Vector , Scalar , Elt , fold , slice , constant , Array , Z(..), DIM1, DIM2 , fromList , All(..) , generate , lift, unlift , shape ) import Data.Array.Accelerate.Interpreter ( run ) dotP :: (Num a, Elt a) => Acc (Vector a) -> Acc (Vector a) -> Acc (Scalar a) dotP xs ys = fold (+) 0 $ A.zipWith (*) xs ys type Matrix a = Array DIM2 a getRow :: Elt a => Int -> Acc (Matrix a) -> Acc (Vector a) getRow n mat = slice mat . constant $ Z :. n :. All -- Naive matrix multiplication: -- -- index (i, j) is equal to the ith row of 'a' `dot` the jth row of 'b' matMul :: A.Acc (Matrix Double) -> A.Acc (Matrix Double) -> A.Acc (Matrix Double) matMul a b' = A.generate (constant $ Z :. nrows :. ncols) $ \ix -> let (Z :. i :. j) = unlift ix in getRow i a `dotP` getRow j b where b = A.transpose b' -- I assume row indexing is faster than column indexing... (Z :. nrows :. _ ) = unlift $ shape a (Z :. _ :. ncols) = unlift $ shape b This, of course, gives me errors right now because I'm calling getRow and dotP from within the generation function, which expects Exp[ression]s, not Acc[elerated computation]s. So maybe I need to replace that line with an inner for loop? Is there an easy way to do that with Accelerate? Thanks for your help, - Clark [1] http://hackage.haskell.org/package/accelerate

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Is hackage.haskell.org down?
by Ivan Perez 04 Dec '12

04 Dec '12

Hi haskellers, I've been having problems to access hackage.haskell.org for the past 2-4 hours. Is everything ok? Cheers, Ivan

3 2

PLMW: Mentoring at POPL. Second Call for Participation
by Gareth Smith 04 Dec '12

04 Dec '12

Students! You have until Sunday to apply for funding to come to both POPL and PLMW! CALL FOR PARTICIPATION SIGPLAN Programming Languages Mentoring Workshop, Rome Tuesday January 22, 2013 Co-located with POPL 2013 PLMW web page: http://www.doc.ic.ac.uk/~gds/PLMW/index.html After the resounding success of the first Programming Languages Mentoring Workshop at POPL 2012, we proudly announce the 2nd SIGPLAN Programming Languages Mentoring Workshop (PLMW), co-located with POPL 2013 and organised by Nate Foster, Philippa Gardner, Alan Schmitt, Gareth Smith, Peter Thieman and Tobias Wrigstad. The purpose of this mentoring workshop is to encourage graduate students and senior undergraduate students to pursue careers in programming language research. This workshop will provide technical sessions on cutting-edge research in programming languages, and mentoring sessions on how to prepare for a research career. We will bring together leaders in programming language research from academia and industry to give talks on their research areas. The workshop will engage students in a process of imagining how they might contribute to our research community. We especially encourage women and underrepresented minority students to attend PLMW. Since PLMW will be in Rome this year, we particularly look forward to seeing Eastern European students at the workshop. This workshop is part of the activities surrounding POPL, the Symposium on Principles of Programming Languages, and takes place the day before the main conference. One goal of the workshop is to make the POPL conference more accessible to newcomers. We hope that participants will stay through the entire conference, and will also attend the POPL tutorials on Monday 21st January which are free to PLMW registered attendees. Through the generous donation of our sponsors, we are able to provide scholarships to fund student participation. These scholarships will cover reasonable expenses (airfare, hotel and registration fees) for attendance at both the workshop and the POPL conference. Students attending this year will get one year free student membership of SIGPLAN The workshop registration is open to all. Students with alternative sources of funding are welcome. APPLICATION for PLMW scholarship: The scholarship application can be accessed from the workshop web site (http://www.doc.ic.ac.uk/~gds/PLMW/index.html) The deadline for full consideration of funding is 9th December, 2012. Selected participants will be notified from Friday 14th December, and will need to register for the workshop by December 24th. SPONSORS: Google Imperial College London Jane Street Monoidics NSF Resource Reasoning SIGPLAN vmware

1 0

Is it possible to have constant-space JSON decoding?
by Iustin Pop 04 Dec '12

04 Dec '12

Hi, I'm trying to parse a rather simple but big JSON message, but it turns out that memory consumption is a problem, and I'm not sure what the actual cause is. Let's say we have a simple JSON message: an array of 5 million numbers. I would like to parse this in constant space, such that if I only need the last element, overall memory usage is low (yes, unrealistic use, but please bear with me for a moment). Using aeson, I thought the following program will be nicely-behaved: > import Data.Aeson > import qualified Data.Attoparsec.ByteString.Lazy as AL > import qualified Data.ByteString.Lazy as L > > main = do > r <- L.readFile "numbers" > case AL.parse json r :: AL.Result Value of > AL.Fail _ context errs -> do > print context > print errs > AL.Done _ d -> case fromJSON d::Result [Value] of > Error x -> putStrLn x > Success d -> print $ last d However, this uses (according to +RTS -s) ~1150 GB of memory. I've tried switching from json to json', but while that uses slightly less memory (~1020 MB) it clearly can't be fully lazy, since it forces conversion to actual types. Looking at the implementation of "FromJSON [a]", it seems we could optimise the code by not forcing to a list. New (partial) version does: > AL.Done _ d -> case d of > Array v -> print $ V.last v And this indeed reduces the memory, when using json', to about ~700MB. Better, but still a lot. It seems that the Array constructor holds a vector, and this results in too much strictness? Looking at the memory profiles (with "json" and "Array"), things are quite interesting - lots of VOID, very small USE, all generated from Data.Aeson.Parser.Internal:array. Using -hd, we have a reasonable equal split between various attoparsec combinators, Data.Aeson.Parser.Internal epressions, etc. So, am I doing something wrong, or is it simply not feasible to get constant-space JSON decoding? Using the 'json' library instead of 'aeson' is no better, since that wants the input as a String which consumes even more memory (and dies, when compiled with -O2, with out of stack even for 64MB stack). thanks, iustin

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Re: [Haskell-cafe] Design of a DSL in Haskell
by Brent Yorgey 03 Dec '12

03 Dec '12

(Sorry, forgot to reply to the list initially; see conversation below.) On Mon, Dec 03, 2012 at 03:49:00PM +0100, Joerg Fritsch wrote: > Brent, > > I believe that inside the do-block (that basically calls my > interpreter) I cannot call any other Haskell function that are not > recognized by my parser and interpreter. This seems to just require some sort of "escape mechanism" for embedding arbitrary Haskell code into your language. For example a primitive embed :: a -> CWMWL a (assuming CWMWL is the name of your monad). Whether this makes sense, how to implement embed, etc. depends entirely on your language and interpreter. However, as you imply below, this may or may not be possible depending on the type a. In that case I suggest making embed a type class method. Something like class Embeddable a where embed :: a -> CWMWL a I still get the feeling, though, that I have not really understood your question. > I am also trying to learn how I could preserve state from one line > of code of my DSL to the next. I understand that inside the > interpreter one would use a combination of the state monad and the > reader monad, but could not find any non trivial example. Yes, you can use the state monad to preserve state from one line to the next. I am not sure what you mean by using a combination of state and reader monads. There is nothing magical about the combination. You would use state + reader simply if you had some mutable state as well as some read-only configuration to thread through your interpreter. xmonad is certainly a nontrivial example but perhaps it is a bit *too* nontrivial. If I think of any other good examples I'll let you know. -Brent > > > On Dec 3, 2012, at 1:23 PM, Brent Yorgey wrote: > > > On Sun, Dec 02, 2012 at 03:01:46PM +0100, Joerg Fritsch wrote: > >> This is probably a very basic question. > >> > >> I am working on a DSL that eventuyally would allow me to say: > >> > >> import language.cwmwl > >> main = runCWMWL $ do > >> eval ("isFib::", 1000, ?BOOL) > >> > >> I have just started to work on the interpreter-function runCWMWL and I wonder whether it is possible to escape to real Haskell somehow (and how?) either inside ot outside the do-block. > > > > I don't think I understand the question. The above already *is* real > > Haskell. What is there to escape? > > > >> I thought of providing a defautl-wrapper for some required prelude > >> functions (such as print) inside my interpreter but I wonder if > >> there are more elegant ways to co-loacate a DSL and Haskell without > >> falling back to being a normal library only. > > > > I don't understand this sentence either. Can you explain what you are > > trying to do in more detail? > > > > -Brent > >

2 2

Can not use ST monad with polymorphic function
by Dmitry Kulagin 03 Dec '12

03 Dec '12

Hi Cafe, I try to implement some sort of monadic fold, where traversing is polymorphic over monad type. The problem is that the code below does not compile. It works with any monad except for ST. I suspect that monomorphism is at work here, but it is unclear for me how to change the code to make it work with ST. fold :: Monad m => (Int -> m ()) -> m () fold f = mapM_ f [0..20] selectFold :: Monad m => String -> IO ((Int -> m ()) -> m ()) selectFold method = do -- in real program I'd like to choose between -- different fold methods, based on some IO context return fold useFold :: Monad m => ((Int -> m ()) -> m ()) -> m () useFold fold' = fold' f where f _ = return () -- some trivial iterator main = do fold'' <- selectFold "some-method-id" print $ runST $ useFold fold'' Thank you! Dmitry

4 5

To my boss: The code is cool, but it is about 100 times slower than the old one...
by Fixie Fixie 03 Dec '12

03 Dec '12

Hi all haskellers I every now and then get the feeling that doing my job code in Haskell would be a good idea. I have tried a couple of times, but each time I seem to run into performance problems - I do lots of heavy computing. The problem seems to be connected to lazy loading, which makes my programs so slow that I really can not show them to anyone. I have tried all tricks in the books, like !, seq, non-lazy datatypes... I was poking around to see if this had changed, then I ran into this forum post: http://stackoverflow.com/questions/9409634/is-indexing-of-data-vector-unbox… The last solution was a haskell program which was in the 3x range to C, which I think is ok. This was in the days of ghc 7.0 I then tried compile the programs myself (ghc 7.4.1), but found that now the C program now was more that 100x faster. The ghc code was compiled with both O2 and O3, giving only small differences on my 64-bit Linux box. So it seems something has changed - and even small examples are still not safe when it comes to the lazy-monster. It reminds me of some code I read a couple of years ago where one of the Simons actually fired off a new thread, to make sure a variable was realized. A sad thing, since I am More that willing to go for Haskell if proves to be usable. If anyone can see what is wrong with the code (there are two haskell versions on the page, I have tried the last and fastest one) it would also be interesting. What is your experience, dear haskellers? To me it seems this beautiful language is useless without a better lazy/eager-analyzer. Cheers, Felix

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Is anyone working on a sparse matrix library in Haskell?
by Mark Flamer 03 Dec '12

03 Dec '12

I am looking to continue to learn Haskell while working on something that might eventually be useful to others and get posted on Hackage. I have written quite a bit of Haskell code now, some useful and a lot just throw away for learning. In the past others have expressed interest in having a native Haskell sparse matrix and linear algebra library available(not just bindings to a C lib). This in combination with FEM is one of my interests. So my questions, is anyone currently working on a project like this? Does it seem like a good project/addition to the community? I'm also interested if anyone has any other project idea's, maybe even to collaborate on. Thanks -- View this message in context: http://haskell.1045720.n5.nabble.com/Is-anyone-working-on-a-sparse-matrix-l… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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[ANN] xmobar 0.16 released
by Jose A. Ortega Ruiz 03 Dec '12

03 Dec '12

I've just released xmobar 0.16, the lightweight text-based system monitor written in Haskell. See http://projects.haskell.org/xmobar, and full release notes below. As always, thanks a bunch to the many contributors, who keep helping me improving xmobar and making it more and more useful. The continuous flow of patches and suggestions is one the nicest things about hacking on and maitaining xmobar. This is a somewhat risky release, because some users are reporting problems with GHC 7.6.1 and xmonad (see links below), but there are so many new features and fixes, and happy users of the unstable version, that i thought a release was in order. Please, do not hesitate to complain about problems you might encounter: i'll do my best to fix them in subsequent releases. Thanks! ## Version 0.16 (Dec 3, 2012) _New features_ - New monitor `AutoMPD`, which uses asynchronous events to display MPD status (thanks to Ben Boeckel). - New monitor `BufferedPipeReader` displaying data from multiple pipes (thanks to Jochen Keil). - New monitor `DynNetwork`, which detects the active interface automatically, by Reto Hablützel - New monitor, `Locks`, displaying the status of lock keys, by Patrick Chilton. - Extension for DBUS signal handling (Jochen Keil) - Hide/Reveal: one can send signals to xmobar and make it (un)hide itself (Jochen again). - `PipeReader`'s default text is now configurable, by Reto Hablützel. - Dependencies updated to latest mtl and libmpd (thanks to Sergei Trofimovich). - Dependencies on the deprecated dbus-core removed in favour of dbus 0.10 (thanks to Jochen Keil). - MPris2 now includes genre and composer among its fields. _Bug fixes_ - `DiskIO` now can report overall activity in all partitions of a device which is not mounted itself (e.g., sda when sda1, sda3, etc. are the mounted partitions). Thanks to John Soros. See [github #73]. - `DiskU`, the disk usage monitor, works again correctly on Linux, instead of randomly crashing every now and then, and reporting wrong used size. - When using antialiased fonts, we were causing a memory leak in the X server by repeatedly allocating colors that, apparently, the server doesn't know how to get rid of (even when told so!). We're caching them now and X server memory doesn't grow. - Compilation errors and warnings with GHC 7.6 removed (thanks to Raghavendra D Prabhu for his reports in [github #71]). _Known problems_ Some users have reported problems with xmobar compiled with GHC 7.6 in ArchLinux: see [github #78] and pointers therein. Please, send reports of any problems or successes in that regard so that we can fix any remaining issues. Thanks! [github #71]: https://github.com/jaor/xmobar/issues/71 [github #73]: https://github.com/jaor/xmobar/issues/73 [github #78]: https://github.com/jaor/xmobar/issues/78

1 0

How to incrementally update list
by Branimir Maksimovic 03 Dec '12

03 Dec '12

Problem is following short program:list = [1,2,3,4,5] advance l = map (\x -> x+1) l run 0 s = srun n s = run (n-1) $ advance s main = do let s = run 50000000 list putStrLn $ show s I want to incrementally update list lot of times, but don't knowhow to do this.Since Haskell does not have loops I have to use recursion,but problem is that recursive calls keep previous/state parameterleading to excessive stack.and memory usage.I don't know how to tell Haskell not to keep previousstate rather to release so memory consumption becomesmanagable. Is there some solution to this problem as I think it is rathercommon?

7 11

Is there any movement/need in new 'base' package and co?
by Евгений Пермяков 02 Dec '12

02 Dec '12

It is clear, that current 'core' Haskell packages bear some weight of legacy and back-compatibility, and accumulates garbage/problems over time. Some of the garbage can be dropped and some was here for so long, that it can't be dropped without major buzz. For example, we have *Monad* type-class, that is hardwired into Haskell syntax. However, it allows arbitrary fail by definition, and not all monads should provide this option. Still, it is almost impossible to move the method into separate interface, as every *Monad* instance will have to be rewritten. Lack of standard packed text/binary string type is also a problem. It is not so problem on itself, as problem of support: different io packages, parser tools and so on have to choose what string type to use, which lead to fragmentation of library field. Hierarchy of numeric classes is also often questioned. More points of problems may be found if one will study Hackage long enough. So, the questions arise: - When problems will ruin the language? - When and which actions are needed to avoid this? - If major rewrite will be initiated, what problems it should target? - And how to make the transition easier? ====

2 1

Fwd: Conduit and pipelined protocol processing using a threadpool
by Alexander V Vershilov 02 Dec '12

02 Dec '12

Forwarding to maillist: Hi, Nicolas. Just my 2 cents. You can try to 'break' your server side application in two parts, reader and writer then you can have: main = do ch1 <- ... -- inbound channel of type (message ,outchannel) replicateM n $ forkIO $ sourceTBMChan ch1 $$ loop1 -- ^^ you can move this code into myApp if you need per client workers runTCPServer config $ \ad -> do ch2 <- ... -- outbound channel {- now will start 2 threads for reader and writer) bracket (forkIO $ sourceTBMChan ch2 $= appSource ad) (\t -> threadDestroy t >> atomically (closeTBMChan ch2)) (const $ appSource ad $$ loop2 ch2) where loop1 = do mm <- await case mm of Just (m, out) -> process m >>= atomically (writeTBMChan out) >> loop1 Nothing -> return () {- using conduit-stm in loop2 will fail as it will close channel at the end of the processing -} loop2 ch = await >>= \x -> when (isJust x) (writeTBMChan ch (fromJust x, ch) >> loop2 ch) I have not checked this code, but the same approach works 2 projects. -- Alexander Vershilov On 28 November 2012 09:21, Nicolas Trangez <nicolas(a)incubaid.com> wrote: > On Wed, 2012-11-28 at 09:17 +0200, Michael Snoyman wrote: >> >> >> >> On Tue, Nov 27, 2012 at 7:25 PM, Nicolas Trangez >> <nicolas(a)incubaid.com> wrote: >> Michael, >> >> On Tue, 2012-11-27 at 17:14 +0200, Michael Snoyman wrote: >> > I think the stm-conduit package[1] may be helpful for this >> use case. >> > Each time you get a new command, you can fork a thread and >> give it the >> > TBMChan to write to, and you can use sourceTBMChan to get a >> source to >> > send to the client. >> >> >> That's +- what I had in mind. I did find stm-conduit before >> and did try >> to get the thing working using it, but these attempts failed. >> >> I attached an example which might clarify what I intend to do. >> I'm aware >> it contains several potential bugs (leaking threads etc), but >> that's >> beside the question ;-) >> >> If only I could figure out what to put on the 3 lines of >> comment I left >> in there... >> >> Thanks for your help, >> >> Nicolas >> >> >> >> The issue is that you're trying to put everything into a single >> Conduit, which forces reading and writing to occur in a single thread >> of execution. Since you want your writing to be triggered by a >> separate event (data being available on the Chan), you're running into >> limitations. >> >> >> The reason network-conduit provides a Source for the incoming data and >> a Sink for outgoing data is specifically to address your use case. You >> want to take the data from the Source and put it into the Chan in one >> thread, and take the data from the other Chan and put it into the Sink >> in a separate thread. Something like: >> >> >> myApp appdata = do >> chan1 <- ... >> chan2 <- ... >> replicateM_ 5 $ forkIO $ worker chan1 chan2 >> forkIO $ appSource appdata $$ sinkTBMChan chan1 >> sourceTBMChan chan2 $$ appSink appdata >> >> >> You'll also want to make sure to close chan1 and chan2 to make sure >> that your threads stop running. > > Thanks, I +- figured that out last night. Only thing left is managing > the handshake before forking off the workers, if possible (otherwise > things become very messy IMHO), but ResumableSources etc might be of use > here. > > Maybe there's a library somewhere in here... > > Thanks a bunch, > > Nicolas > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe -- С уважением, Вершилов Александр ( mail-to: alexander.vershilov(a)gmail.com ) -- С уважением, Вершилов Александр ( mail-to: alexander.vershilov(a)gmail.com )

1 0

debugging memory corruption
by Evan Laforge 02 Dec '12

02 Dec '12

Ever since upgrading to 7.6.1 I regularly get panics like this: seq: internal error: evacuate: strange closure type -1958168540 (GHC version 7.6.1 for x86_64_apple_darwin) Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug I've seen some variations, but basically I think it just means someone is corrupting memory and it goes unnoticed until the GC trips over it. This happens infrequently (maybe once in 15m, very roughly, it's extremely inconsistent) in non-optimized code, and frequently (maybe once in 5m) in optimized code. This only happens during interactive use, not during testing or profiling. I had a problem like this once before, and it took a very long time to track down. And in fact, I never really tracked it down, I just got a test that could semi-reliably reproduce it, and then by trial and error discovered that if I changed the alignment of a particular Storable instance from 1 to 4, the problem stopped happening (and 1 should have been correct, it was a struct of chars). Not exactly a satisfying solution, and now I'm thinking all I did was make ghc 6 stop manifesting it, and with 7 it's back again. I'm most suspicious of the FFI usage since it's easy to corrupt memory in C++ and even easier to write a bogus Storable instance that does the same, but I really have no idea what or where. My first thought was to start cutting out portions (especially FFI-using portions) to try to isolate it, but it's slow going because it can sometimes take quite a while to the bug to come out again. My second thought was that I need a more automated way to reproduce it, but it's nontrivial because it only comes up when I'm using the interactive GUI parts, which are also incidentally a big chunk of FFI. And even if I do get a repro, as I did before, it just means I can more quickly poke randomly at things hoping they change it, but even if I can get it to stop happening it doesn't mean I understood it, or even really fixed it. This is also the kind of bug (well, it was last time), which is highly dependent on the code, so add one print and it stops happening. I have a sort of complicated scheme where I pass a wrapped haskell function callback along with a wrapped freeHaskellFunPtr to free the last one, along with itself, maybe it's something to do with that. Anyone out there with ideas or advice on how to track down this kind of bug? My next thought is to try to automate the GUI parts, or maybe just the FFI part, so I can write a program to randomly fuss with it until it crashes. I've also tried valgrind, but it doesn't report anything suspicious. But it also doesn't seem to work on FFI Storable corruption, I've tried intentionally inserting a bad poke and valgrind still won't report it. Thanks in advance for any insight! Actually, there's a whole other discussion which has been nagging at me for a while, though another thread would be more appropriate. But in short it's that it feels like hsc2hs is just too low level, and too error-prone. It's tempting to use because it comes with ghc, but it seems bad to tell people "haskell is a safe language, but as soon as you want to talk to C you're writing totally unchecked pokes and peeks". Maybe I should go evaluate the alternatives like c2hs, or maybe safety features can added to hsc2hs. Wouldn't it be nice to have ghc come with a high level and safe FFI language?

2 2

Why is boxed mutable array so slow?
by Branimir Maksimovic 02 Dec '12

02 Dec '12

I have made benchmark test inspired by http://lemire.me/blog/archives/2012/07/23/is-cc-worth-it/ What surprised me is that unboxed array is much faster than boxed array.Actually boxed array performance is on par with standard Haskell listwhich is very slow.All in all even unboxed array is about 10 times slower than Java version.I don't understand why is even unboxed array so slow.But! unboxed array consumes least amount of RAM.(warning, program consumes more than 3gb of ram) bmaxa@maxa:~/examples$ time ./Cumulboxed arraylast 262486571 seconds 4.972unboxed arraylast 262486571 seconds 0.776listlast 262486571 seconds 6.812 real 0m13.086suser 0m11.996ssys 0m1.080s -------------------------------------------------------------------------{-# LANGUAGE CPP, BangPatterns #-}import System.CPUTimeimport Text.Printfimport Data.Array.IOimport Data.Array.Baseimport Data.Intimport Control.DeepSeqimport System.Mem main :: IO()main = do (newArray_ (0,n'-1) :: IO(A)) >>= test "boxed array" performGC (newArray_ (0,n'-1) :: IO(B)) >>= test "unboxed array" performGC begin <- getCPUTime printf "list\nlast %d" $ last $ force $ take n' $ sum' data' end <- getCPUTime let diff = (fromIntegral (end - begin)) / (10^12) printf " seconds %.3f\n" (diff::Double) test s a = do putStrLn s begin <- getCPUTime init' a partial_sum a end <- getCPUTime let diff = (fromIntegral (end - begin)) / (10^12) last <- readArray a (n'-1) printf "last %d seconds %.3f\n" last (diff::Double) n' :: Intn' = 50 * 1000 * 1000 type A = IOArray Int Int32type B = IOUArray Int Int32 init' a = do (_,n) <- getBounds a init a 0 n where init a k n | k > n = return () | otherwise = do let !x = fromIntegral $ k + k `div` 3 unsafeWrite a k x init a (k+1) n partial_sum a = do (_,n) <- getBounds a k <- unsafeRead a 0 ps a 1 n k where ps a i n s | i > n = return () | otherwise = do k <- unsafeRead a i let !l = fromIntegral $ s + k unsafeWrite a i l ps a (i+1) n l data' :: [Int32]data' = [k + k `div` 3 | k <- [0..] ] sum' = scanl1 (+)

4 7

what is the purpose of GHC.Exts.lazy?
by jared simpson 01 Dec '12

01 Dec '12

docs say: The call '(lazy e)' means the same as e, but lazy<http://hackage.haskell.org/packages/archive/base/latest/doc/html/GHC-Exts.h…> has a magical strictness property: it is lazy in its first argument, even though its semantics is strict. why do i want to use magic during strictness analysis?

1 0

Simple GUI Form
by Rune Harder Bak 01 Dec '12

01 Dec '12

Hi I have some input parameters data Input = ... that I need the user to enter in a gui pop-up. (windows people...) The rest of the app is not gui (or perhaps progress could be displayed in a log-window) What is the easiest way to make such a GUI form? It need to compile for both Linux and Windows, so I though WxWidgets was ideal, and I got wx[1] < 0.90 to install (using wxWidgets2.8) on both windows and linux. Now I just need to create the form, but how do you do that? Any clues or links to examples? I have never used wxwidgets on any platform or done any other form of GUI before for that matters. (apart from some Visual Basic ten years ago, and html). I tried looking at wx examples, but I couldn't find this simple use case explained anywhere. I installed wx in the first place because WxGeneric[2] seemed exactly what I needed, but I can't get it to compile using ghc7.4.2 from haskell-platform. Anybody got that working or have some other simple method? Help much appreciated! -Rune [1] http://hackage.haskell.org/package/wx-0.13.2.3 [2] http://hackage.haskell.org/package/WxGeneric

3 9

computation over containers, greatly simplified notation.
by Takayuki Muranushi 01 Dec '12

01 Dec '12

Dear all, I came up with an idea to greatly simplify some kinds of array computations. It should work well with many kinds of arrays. Is this new? https://gist.github.com/4162375 These few days, I've been trying to rewrite a hydrodynamic simulation code that used Data.Vector (~250 lines), to Repa [1] . It seemed promising, but soon I realized that I needed to use Repa.map and Repa.zipWith almost everywhere. I need careful thinking to transform every lines (that used vector's indexing) to Repa's point-free stile. Is there any better ways? Then I realized that I was the author of Paraiso [2], a DSL for stencil computation. One of its feature is to write down array computation just as if it were scalar computation. Basically what I need is ZipList-like Applicative instances for vectors and Repa arrays. Why not they support ZipVector? Because 'pure' of zipList was an infinite list and you can't do infinite vectors. Then I came up with this idea. https://gist.github.com/4162375 the wrapper W does several things: it represents the 'pure,' homogeneous array in a space-efficient manner, and also serves as a newtype-wrapper of Num (and possibly Fractional, Floating...) instances. Questions are: is this technology new? or promising? doomed? It seems to me like a free-Applicative, like the free-Monad theory. Are they related? The function 'backend' helps to mix in the non-zip-like computations. How can we remove the 'undefined' in the 'backend?' Some of Repa computations are Monads. W needs to be a monad transformer to incooperate this. Also I'm grateful to past cafe discussion on existing Zippable implementations [3][4] . [1] hackage.haskell.org/package/repa [2] http://hackage.haskell.org/package/Paraiso [3] http://www.haskell.org/pipermail/haskell-cafe/2009-July/064403.html [4] http://hackage.haskell.org/packages/archive/category-extras/latest/doc/html… -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

2 4

Are storable-mutable-vectors in two dimensions possible in Haskell?
by Fixie Fixie 01 Dec '12

01 Dec '12

Hello haskellers I am struggling with this package: Data.Vector.Storable.Mutable I am creating vectors like this: MV.new 1000 :: IO (V.MVector (PrimState IO) Int) Now I would like access to this vectors in linear time, like I could have done in C using pointers. The problem is that I do not know which datastructure to keep my vectors in. If I put them in a list, the access is very slow. Even a binary tree is very slow compared to a C-version. I have tried to make an array og vector of storable-mutable vectors, but have not manged to accomplish this - even though there must be a haskell way :-) Any clues? Cheers, Felix

2 2

Why Kleisli composition is not in the Monad signature?
by damodar kulkarni 01 Dec '12

01 Dec '12

Hi, The Monad class makes us define bind (>>=) and unit (return) for our monads. Why the Kleisli composition (>=>) or (<=<) is not made a part of Monad class instead of bind (>>=)? Is there any historical reason behind this? The bind (>>=) is not as elegant as (>=>), at least as I find it. Am I missing something? - Damodar

17 32

Can a GC delay TCP connection formation?
by Jeff Shaw 01 Dec '12

01 Dec '12

Hello, I've run into an issue that makes me think that when the GHC GC runs while a Snap or Warp HTTP server is serving connections, the GC prevents or delays TCP connections from forming. My application requires that TCP connections form within a few tens of milliseconds. I'm wondering if anyone else has run into this issue, and if there are some GC flags that could help. I've tried a few, such as -H and -c, and haven't found anything to help. I'm using GHC 7.4.1. Thanks, Jeff

11 17

ANN: zeromq3-conduit, Conduit bindings for zeromq3-haskell
by Nicolas Trangez 30 Nov '12

30 Nov '12

Hi, I published zeromq3-conduit, a small library which facilitates using zeromq3-haskell, a binding for ZeroMQ 3.x, in a Conduit-based application. The 'examples' folder in the source repository contains ports of the zeromq3-haskell examples. The library also contains a module which might ease using plain zeromq3-haskell: it lifts some of the IO actions into MonadIO and provides a ReaderT which carries a Context around, so sockets can be created without passing this around explicitly. Package is on Hackage [1] and Github [2]. Comments/feedback/pull requests/... welcome! Nicolas [1] http://hackage.haskell.org/package/zeromq3-conduit [2] https://github.com/NicolasT/zeromq3-conduit/

1 0

ANN: base-compat
by Simon Hengel 30 Nov '12

30 Nov '12

Hi, I just released the first version of a package that "provide new additions to base for older versions of base" [2]. So far the following is covered: readMaybe readEither lookupEnv getExecutablePath Source is on GitHub [2]; patches welcome ;) Cheers, Simon [1] http://hackage.haskell.org/package/base-compat [2] https://github.com/sol/base-compat

2 2

Category Theory and Haskell
by Gytis Žilinskas 30 Nov '12

30 Nov '12

Greetings, I'm only taking my very first steps learning Haskell, but I believe that this mailing list might be appropriate for my question. How difficult would it be to study category theory and simultaneously come up with Haskell examples of various results that it presents? I believe some background is in order. I am a second year maths student (well actually I'm in-between 2nd and 3rd, but let's not delve into details). I have also done some 2nd year CS courses so I have some background in that too. My general interests seem to lie somewhere between maths and compsci (I guess theoretical cs is the word for it), but it doesn't look like there is a good way to study the subject at an undergrad level. Most CompSci undergrad courses seem to concentrate too much on the trade of being a programmer and only present the basics of the theoretical results. While maths departments seem to be reluctant to make room in the undergrad curriculum for a huge subject that, after all, is maths. Recently, whenever I try reading up on something that looks like an exciting topic to me, I hit my head against category theory. That seems to suggest, that I should check it out, as it might be a fun subject to study, or at least provide valuable insights elsewhere. I've tried looking for some resources online. Most of them are outright scary and clearly have prerequisites that I lack. Some a little bit more accessible. Overall, various similar questions on different stackexchanges and even some old discussions on this mailing list, imply that to study category theory one has to know many examples from other advanced fields, in order to see the patterns that category theory describes. Now I obviously lack that. Some basic group theory, linear algebra, real/complex analysis and metric spaces is pretty much all I've got. Moreover, most sources say that without the knowledge of examples - category theory is an extremely dry subject to study. Therefore, my preliminary plan is the following: Read: * Barry Mazur - When is one thing equal to some other thing? * Conceptual mathematics : a first introduction to categories / F. William Lawvere, Stephen H. Schanuel. (People say that it is rather lightweight, but at this point my goal is to see the general idea of category theory and get some taste of it) Then, in the 2nd semester, I have an option of taking a self-study module, and if I don't get bored by cats by that time - it would be great to invest some more time in it (or rather, I'll do anything to avoid applied maths *shudder*). However, I'll need to present my case before the department staff and this, basically, is why I'm here. Right now, it looks like the main argument against me, would be that I can't appreciate the subject since I lack the fundamentals in other areas of maths. So I was hoping, I could convince them that everything I lack example wise - I'll find in Haskell. Okay, "everything" might be too strong of a statement, but I don't really know what I'm talking about so it's a bit difficult to calibrate. Could you comment whether Haskell would serve me well for this purpose? I've also seen these lecture notes: http://www.haskell.org/haskellwiki/User:Michiexile/MATH198. But I'm not sure if they won't be deemed too comp sci targeted. Therefore, I'm putting more hope into "The Joy of Cats" which, at least according to the first few pages, doesn't assume any prior knowledge that I don't have. So that's that, I would appreciate any advice you could give. Gytis

4 3

Recursive timezone-loading function
by David Thomas 29 Nov '12

29 Nov '12

https://github.com/dlthomas/tzcache A small bit of code, but seems likely to be useful enough that I figured I should share. I've a few notes/questions: 1) Does this already exist somewhere I missed? 2) It seems silly to make this its own library - any suggestions where it could be added? 3) Is the traverse-a-directory-and-populate-a-map pattern one worth abstracting? If so, where should that go? 4) Presently, it's a static cache entirely pre-loaded. This seems fine, as it's not a terribly huge amount of data, but it's worth noting. 5) Any comments on the code generally? Improvements? Complaints? Thanks, - David

3 2

delete http://www.haskell.org/haskellwiki/Haskell_IDE
by Roman Beslik 29 Nov '12

29 Nov '12

Hi. There is more verbose page http://www.haskell.org/haskellwiki/IDEs . I registered on http://www.haskell.org/haskellwiki/ , but have not found the "Delete Page" command, wiki software help pages, or feedback channel, so I'm writing here.

4 5

SOX - play simple
by Gary Klindt 29 Nov '12

29 Nov '12

Dear Cafe, after installing the Sox library (cabal install sox) I wanted to let run a minimal example from http://hackage.haskell.org/packages/archive/sox/0.2.2.2/doc/html/Sound-Sox-… module Main where import Sound.Sox.Play import Sound.Sox.Signal.List --import Sound.Sox.Option.Format import Data.Int main = do simple Sound.Sox.Signal.List.put Option.none 11025 (iterate (1000+) (0::Data.Int.Int16)) in this version, I get the error: Not in scope: `Option.none' So, I imported Sound.Sox.Option.Format, because there is a none with the right type. I also changed Option.none to none. Then the program compiles and I get the runtime error: fd:4: hClose: resource vanished (Broken pipe) What is wrong here? I appreciate your help! Best regards, Gary

2 1