- Haskell-Cafe - Haskell.org

Debug tracing in Haskell
by Clinton Mead 30 Nov '16

30 Nov '16

I've been debugging some Haskell code, including a lot of work on Mutable Vectors, so naturally I've found `trace` useful, and in particular `traceM`, because it fits in nicely with monadic code. The problem is that unlike `assert`, both `trace` and `traceM` execute unconditionally, I can't toggle them with a compiler flag. I could however do something like this in every file I want to do tracing. {-# LANGUAGE CPP #-} #ifdef TRACE trace = Debug.Trace.trace traceM = Debug.Trace.traceM #else trace _ = id traceM _ = pure () #endif But already, that's a lot of boilerplate. I'd also like to trace based on debug levels. For example, at TRACE level 2, only print trace statements at level 1 or 2, but at TRACE level 4, print trace statements at level 1, 2, 3 and 4, providing more detail (but more noise). This makes the above code even more complex. This wouldn't be a problem if I could put the code in a separate package, but I can't, as then whether tracing is on or not depends on the compiler settings of the tracing package, not on the calling package, which defeats the purpose somewhat. I considered using implicit parameters to quietly pass whether I want tracing and at what level into the tracing module, but it seems to be that implicit parameters can't be defined at the top level. It gets even more complex. When debugging, I might want to print something of type 'a'. Obviously 'a' will need some sort of show method for this to work, but outside of debug mode I don't want to restrict my function's types to things which are showable. So I considered doing this: #ifdef TRACE type DebugShow a = Show a debugShow = show #else type DebugShow a = () debugShow _ = error "DEBUG SHOWING OUTSIDE OF DEBUG MODE" #endif And of course, if you're only using `debugShow` as part of an argument to `trace`, lazy evaluation will avoid `debugShow` ever being called when tracing is not enabled. But put all this together and you've got around a dozen lines of boilerplate just to do tracing, without even having tracing levels yet, that have to be put in every module you want to use tracing, but don't want a whole lot of debug data being spat out in a release compile. Also, adding any other tracing functions just makes this longer. The only approach I can think of so far is to whack this all in a template haskell module and add code that splices it all into the current module: e.g. #ifdef TRACE $(traceFunctions True) #else $(traceFunctions False) #endif Where `traceFunctions` is a template haskell function that dumps all the appropriate functions and type definitions mentioned above (and maybe more) at the top level of the calling module. So my questions are: 1. Is there a better way? And 2. Has this problem already been solved?

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complexity of functions in PriorityQ.hs
by Michael Litchard 30 Nov '16

30 Nov '16

I'm trying to figure out the complexity of the functions in Melissa O'Neill's Priority Queue library, Priority.Q.hs Below find definitions, and what I think the complexity is. I'd appreciate feedback, especially on the ones I have marked as guesses. data PriorityQ k v = Lf | Br {-# UNPACK #-} !k v !(PriorityQ k v) !(PriorityQ k v) deriving (Eq, Ord, Read, Show) O(log n) . This is a guess but alternating which side of the sub-tree is being traversed is my hint. insert :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v insert wk wv (Br vk vv t1 t2) | wk <= vk = Br wk wv (insert vk vv t2) t1 | otherwise = Br vk vv (insert wk wv t2) t1 insert wk wv Lf = Br wk wv Lf Lf I have no idea. Reasoning help here please. siftdown :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v -> PriorityQ k v siftdown wk wv Lf _ = Br wk wv Lf Lf siftdown wk wv (t @ (Br vk vv _ _)) Lf | wk <= vk = Br wk wv t Lf | otherwise = Br vk vv (Br wk wv Lf Lf) Lf siftdown wk wv (t1 @ (Br vk1 vv1 p1 q1)) (t2 @ (Br vk2 vv2 p2 q2)) | wk <= vk1 && wk <= vk2 = Br wk wv t1 t2 | vk1 <= vk2 = Br vk1 vv1 (siftdown wk wv p1 q1) t2 | otherwise = Br vk2 vv2 t1 (siftdown wk wv p2 q2) Whatever the cost of siftdown is. deleteMinAndInsert :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v deleteMinAndInsert wk wv Lf = error "Empty PriorityQ" deleteMinAndInsert wk wv (Br _ _ t1 t2) = siftdown wk wv t1 t2 This looks like O(log N) to me. leftrem :: PriorityQ k v -> (k, v, PriorityQ k v) leftrem (Br vk vv Lf Lf) = (vk, vv, Lf) leftrem (Br vk vv t1 t2) = (wk, wv, Br vk vv t2 t) where (wk, wv, t) = leftrem t1 leftrem _ = error "Empty heap!" The cost of siftdown, which is probably more expensive than leftrem. deleteMin :: Ord k => PriorityQ k v -> PriorityQ k v deleteMin (Br vk vv Lf _) = Lf deleteMin (Br vk vv t1 t2) = siftdown wk wv t2 t where (wk,wv,t) = leftrem t1 deleteMin _ = error "Empty heap!" Thanks for the feedback. I'm new to computational analysis and this is the most complicated code I have tried to tackle thus far. I have Okasaki's book, but am having trouble making best use of it.

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HWWG
by Ben Spencer 30 Nov '16

30 Nov '16

Hello all, Is there a way to get involved with the HWWG? Is there a slack channel or a mailing list? Best, Ben Spencer Sent from my iPhone

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[ANN] Sprinkles - zero-programming web dev platform
by Tobias Dammers 30 Nov '16

30 Nov '16

A little something I've been working on: https://sprinkles.tobiasdammers.nl/ You feed it a bunch of templates, content in files, databases, APIs, etc., define routes and map them onto data sources and templates using a YAML configuration file, and it'll serve that as a full blown website. It sits somewhere in between a classic CMS and a static site generator. The code is available on github: https://github.com/tdammers/sprinkles Both the sprinkles website and that for ginger (https://ginger.tobiasdammers.nl/) are built on Sprinkles (following some advice about dog food I read somewhere). Before I unleash this to a more general audience, I'd like to get some honest (harsh) criticism here, on both the code itself and the available materials (website, documentation). Feel free to roast me relentlessly. -- Tobias Dammers - tdammers(a)gmail.com

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Munich Haskell Meeting, 2016-11-30 @ 19:30
by Christian Neukirchen 28 Nov '16

28 Nov '16

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Wednesday, November 30 at Max-Emanuel-Brauerei at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-nov-2016/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Package takeover: indents
by Jasper Van der Jeugt 28 Nov '16

28 Nov '16

The indents [1] package provides a very simple way to create indentation-aware parsers with Parsec. Unfortunately, it does not compose very well (the user cannot use a custom monad anymore), and there are several bugs in it, for example this pretty bad one [2]. I have patches that has been running in production at Fugue [3] for over a year which fixes all of these issues, while keeping a fully compatible API. I have tried contacting the author (CC'ed) over a year ago (June 2015) but I haven't received a response. The latest package upload is from May, 2012. I would like to take over this package so I can share the fixes. [1]: https://hackage.haskell.org/package/indents [2]: http://lpaste.net/344393 [3]: https://fugue.co/ Regards, Jasper

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Searching for free copy of Bird paper
by Michael Litchard 28 Nov '16

28 Nov '16

I'm looking for a copy of the paper below. Can anyone point me to a free copy? Bird, R. S. (1984) Using circular programs to eliminate multiple traversals of data

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Creating a instance for (Storable Text)
by Cody Goodman 27 Nov '16

27 Nov '16

Any ideas on how I'd write this instance? I know for it to be storable it needs a size, couldn't I just get the length of the text? Has anyone did anything similar? I ran into a surprise when trying to use storable-record and derive-storable and tried using them with a record that contained a Text field. My end goal is having a vector of records that could contain Text, UTCTime, or other similarly complex types which I can write as an mmap to disk and then read back to the original format. Thanks!

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2nd Cfa TTT 2017 (Type Theory Based Tools)
by Andreas Abel 27 Nov '16

27 Nov '16

2ND CALL FOR ABSTRACTS TTT : Type Theory Based Tools ============================= Satellite workshop of POPL 2017, Paris France, January 15th [We have funding possibilities for students and young researchers, see below. Note the early deadline!] Overview ======== The aim of this workshop is to showcase modern tools based on type theory, whether designed for programming or for verification, whether academic projects or used in an industrial setting. It will provide a forum to highlight and discuss their common and their distinctive features, and the future directions of development of the tools. The program will consist of invited and contributed talks, and will encourage informal discussion. Abstracts will be displayed on the website of the workshop but there will be no proceedings. We solicit abstract submissions proposing demos, case studies, describing the impact of a theoretical result on practice, or any other aspect of the development and use of tools based on type theory. In particular, we welcome submissions about prototype implementations and promising work in progress, as soon as they have the potential of raising interesting discussions. This workshop is funded by the EUTypes COST project (https://eutypes.cs.ru.nl/) The program will include a plenary discussion on the role of the EUTypes project in the community and planning of activities for 2017. Invited Speakers ================ Robbert Krebbers, Delft University of Technology, Netherlands Aaron Tomb, Galois, US More speakers to be confirmed Registration ============ Registration information will be soon available at the main POPL 2017 website: http://conf.researchr.org/home/POPL-2017 Participant funding =================== The EUTypes COST project (https://eutypes.cs.ru.nl/) can fund students and young researchers from countries participating in the project to attend the workshop – check http://www.cost.eu/COST_Actions/ca/CA15123?parties to see if your country is listed. The application should include the following information: * Your name * Your institution * The name of your supervisor(s) * The url of your webpage if you have one * Your research topics or interests in 1 paragraph and should be sent to ttt2017(a)easychair.org as soon as possible and no later than November 20th. Contact ======= For any query about this workshop, please contact us at ttt2017(a)easychair.org Call for abstracts ================== Submissions for talks and demonstrations should be described in an extended abstract, between 1 and 2 pages in length. We suggest formatting the text using the two-column SIGPLAN LaTeX style (9pt font). Submission page: https://easychair.org/conferences/?conf=ttt2017 Important dates =============== Submission deadline: 30 November 2016 Notification: 15 December 2016 Workshop: 15 January 2017 Program Committee ================= Andreas Abel, Gothenburg University, Sweden Andrej Bauer, University of Ljubljana, Slovenia Assia Mahboubi, Inria Université Paris-Saclay, France Keiko Nakata, Germany -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www2.tcs.ifi.lmu.de/~abel/ _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post. -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www2.tcs.ifi.lmu.de/~abel/

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Haskell TLS and monad-control?
by Viktor Dukhovni 25 Nov '16

25 Nov '16

[ Knowledge of DANE entirely optional, if some of the below is Greek to you, just skip the bits that are unfamiliar. ] I have the beginning of a DANE verification module for hs-tls as an alternative to Data.X509.Validation. So far it just handles the easy case of checking just the leaf certificate against DANE-EE(3) certificate usage TLSA records, the rest is more complex, but follows naturally enough. Having verified the certificate a la DANE, I'd like to be able to return more detail to the caller than just success/failure. This part seems difficult to do cleanly. The TLS client state is encapsulated in a State monad which keeps track of the shared (SMTP) protocol state across a source/conduit/sink triple: type SmtpM = StateT ProtoState IO source :: Source SmtpM ByteString proto :: Conduit ByteString SmtpM ByteString sink :: Sink ByteString SmtpM () When the peer supports STARTTLS, I perform a TLS handshake, and make use of a TLS-enabled source/sink pair. This works well enough, but I also need to capture TLS-handshake metadata in the protocol state: * The peer's validated certificate chain. * The DNS name matched in the peer certificate. * Which TLSA record matched the peer's chain * ... A plausible interface is for the DANE version of the X509 verification code to expose one or more optional callbacks that will invoke a function of the caller's choice that will be passed the desired metadata. It would then be up to that function to squirrel this data away for later use. If this callback were to be invoked in the context of the application state monad, I'd just call "modify" in the callback and examine the results post-handshake as needed. However, life is not so simple. The TLS handshake is performed via Network.TLS.handshake, which internally calls the certificate verification code via: processCertificate :: ClientParams -> Context -> Handshake -> IO (RecvState IO) processCertificate cparams ctx (Certificates certs) = do -- run certificate recv hook ctxWithHooks ctx (\hooks -> hookRecvCertificates hooks $ certs) -- then run certificate validation usage <- catchException (wrapCertificateChecks <$> checkCert) rejectOnException case usage of CertificateUsageAccept -> return () CertificateUsageReject reason -> certificateRejected reason return $ RecvStateHandshake (processServerKeyExchange ctx) where shared = clientShared cparams checkCert = (onServerCertificate $ clientHooks cparams) (sharedCAStore shared) (sharedValidationCache shared) (clientServerIdentification cparams) certs processCertificate _ ctx p = processServerKeyExchange ctx p which lives in the base IO monad, and even if I pass in the current state to the `checkCert` hook, there is no opportunity to return the modified state into a context where "restoreM" can make appropriate updates in the caller. The best I can do is provide the hook with a suitable mutable object (likely an MVar). Interestingly enough, the outer Network.TLS.handshake function appears to be more flexible: handshake :: MonadIO m => Context -> m () Which makes possible calls of the form: res <- liftBaseWith $ \runInIO -> do Sys.timeout tmout $ Sys.tryIOError $ runInIO $ TLS.handshake ctx case res of Just x | Right st <- x -> restoreM st; ... success ... | Left e <- x -> ... I/O Error ... _ -> ... timeout ... which turn out futile, since `handshake` immediately switches to doing all the work in the IO monad, and so the underlying internals are not compatible with MonadControl. This prevents back-propagation of state changes via the various callbacks in TLS.ClientParams.clientHooks. -- | Handshake for a new TLS connection -- This is to be called at the beginning of a connection, and during renegotiation handshake :: MonadIO m => Context -> m () handshake ctx = liftIO $ handleException $ withRWLock ctx (ctxDoHandshake ctx $ ctx) where handleException f = catchException f $ \exception -> do let tlserror = maybe (Error_Misc $ show exception) id $ fromException exception setEstablished ctx False sendPacket ctx (errorToAlert tlserror) handshakeFailed tlserror So my question is whether it makes sense to rework the TLS modules to live in a more abstract monad (as in the handshake function) and only work in the base IO monad briefly, when performing actual I/O operations? Thus, perhaps instead: processCertificate :: MonadIO m => ClientParams -> Context -> Handshake -> m (RecvState m) ... Doing this throughout the TLS stack looks a lot of work, so the question is perhaps whether such an effort would be justified? Or is it too late to retrofit monad control over large existing code bases, with the monad control pattern mostly suitable just for de novo work? -- Viktor.

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Is it possible to get the selector functions when defining a generic class?
by chris＠kahn.pro 24 Nov '16

24 Nov '16

Hey all! I'm trying to understand what's going on in GHC.Generics and defining a generic class... I understand that there's a `Selector` class and `selName` function that can get the *name* of a selector, but is there a way to access the selector function itself? The documentation conveniently avoids examples involving records and is otherwise quite barren. So if I have a data type like... data Person = Person { name :: String , age :: Int } deriving Generic instance MyTypeClass Person I want my generic implementation of MyTypeClass to be able to access each selector function in the record, f :: Person -> String, g :: Person -> Int, etc. Chris

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Overriding CPP preprocessor on OSX
by Cody Goodman 24 Nov '16

24 Nov '16

I'm having some problems[0] with the derive-storable package on OSX I don't have on Ubuntu. I would like to use g++ rather than clang to preprocess CPP code. Is there a way to do this with cabal? Thanks! 0: https://github.com/mkloczko/derive-storable/issues/1

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Re: [Haskell-cafe] Is it possible to get the selector functions when defining a generic class?
by David Feuer 24 Nov '16

24 Nov '16

I don't know much about this database stuff, but I'm pretty sure you want to *build* the selector, rather than *extracting* it. You should use the Generics metadata to get the string for parsing/printing, but aside from that, you just have the products. On Nov 23, 2016 10:19 PM, "Chris Kahn" <chris(a)kahn.pro> wrote: Aaaand you'll get mine twice since I forgot to reply-all the first time :) Sure, so in postgresql-simple there are two classes for automatically generating functions that encode/decode database rows, `FromRow` and `ToRow`. In the Hasql library--another postgres library--the encoders and decoders must be written by hand for each user-defined type. I want to write a class that will automatically generate these. I successfully wrote a `FromRow` class that can generate Hasql's `Row` type, since it's basically identical to what's in postgresql-simple's `FromRow`. But in Hasql the encoder type, Params, is contravariant and encoders are defined like: personEncoder :: Params Person personEncoder = contramap name (value text) <> contramap age (value int) The `value text` part can be determined based on the type information, but it's also expecting a matching selector function. I'm at a total loss for how I could generate something like this. On 11/23/2016 10:06 PM, David Feuer wrote: > Sorry if anyone gets this twice; the first copy somehow went to a > non-existent Google Groups version of haskell-cafe. > > GHC.Generics doesn't offer any built-in support for such things. It > *looks* like there *might* be some support in packages built around > generics-sop. When you're working directly with GHC.Generics, the > notion of a record barely even makes sense. A record is seen as simply > a possibly-nested product. For example, ('a','b','c') will look > *approximately* like 'a' :*: ('b' :*: 'c'). You're generally not > "supposed" to care how large a record you may be dealing with, let > alone what field names it has. May I ask what you're actually trying > to do? Your specific request sounds peculiarly un-generic. > > On Wed, Nov 23, 2016 at 9:52 PM, <chris(a)kahn.pro> wrote: >> Hey all! >> >> I'm trying to understand what's going on in GHC.Generics and defining a >> generic class... I understand that there's a `Selector` class and `selName` >> function that can get the name of a selector, but is there a way to access >> the selector function itself? The documentation conveniently avoids examples >> involving records and is otherwise quite barren. >> >> So if I have a data type like... >> >> data Person = Person >> { name :: String >> , age :: Int >> } deriving Generic >> >> instance MyTypeClass Person >> >> I want my generic implementation of MyTypeClass to be able to access each >> selector function in the record, f :: Person -> String, g :: Person -> Int, >> etc. >> >> Chris >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post.

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ANN: Haskell Website Working Group
by Gershom B 24 Nov '16

24 Nov '16

The following text is reproduced from Neil Mitchell’s blog (I’m just the messanger) where it lives at: https://neilmitchell.blogspot.com/2016/11/the-haskellorg-website-working-gr… == The Haskell.org Website Working Group (HWWG) == Haskell represents both a language and a user community - and moreover a fantastic community full of friends, fun, and deep technical debate. Unfortunately, in recent times the community has started to fracture, e.g. Cabal vs Stack, haskell.org vs haskell-lang.org. These divisions have risen above technical disagreements and at some points turned personal. The solution, shepherded by Simon Peyton Jones, and agreed to by both members of the haskell.org committee and the maintainers of haskell-lang.org, is to form the Haskell Website Working Group (HWWG). The charter of the group is at the bottom of this post. The goal of the Haskell Website Working Group is to make sure the Haskell website caters to the needs of Haskell programmers, particularly beginners. In doing so we hope to either combine or differentiate haskell.org and haskell-lang.org, and give people clear recommendations of what "downloading Haskell" means. Concretely, we hope that either haskell-lang.org redirects to haskell.org, or that haskell-lang.org ends up being used for something very different from today. == The Haskell Website Working Group (HWWG) == - Scope and goals - * The HWWG is responsible for the design and content of the user-facing haskell.org web site, including tutorials, videos, news, resource, downloads, etc. * The HWWG is not responsible for: * The infrastructure of haskell.org * Toolchains, Hackage, compilers, etc * The HWWG focuses on serving users of Haskell, not suppliers of technology or libraries. * An explicit goal is to re-unite the haskell.org and haskell-lang.org web sites. - Expected mode of operation - * HWWG is not responsible for actually doing everything! The web site is on github. Anyone can make a pull request. The general expectation is that uncontroversial changes will be accepted and committed without much debate. * If there is disagreement about a proposed change, it's up to the HWWG to engage in (open) debate, and to seek input as widely as time allows, but finally to make a decision. - Membership - Initial membership comprises of: * Neil Mitchell (chair) * Nicolas Wu * Andrew Cowie * Vincent Hanquez * Ryan Trinkle * Chris Done It is expected the committee will change over time, but the mechanism has not yet been thought about. - Rules of engagement - * Recognising that honestly-held judgements may differ, we will be scrupulously polite both in public and in private. * Recognising that Haskell has many users, and that different users have different needs and tastes, we want haskell.org to be inclusive rather than exclusive, providing a variety of alternative resources (toolchains, tutorials, books, etc) clearly signposted with their intended audiences. * Ultimately the haskell.org committee owns the haskell.org URL, but it delegates authority for the design and content of the web site to the HWWG. In extremis, if the haskell.org committee believes that the HWWG is mismanaging the web site, it can revoke that delegation.

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debugging vector index out of range
by briand＠aracnet.com 23 Nov '16

23 Nov '16

Hi, I'm getting a vector index out of range. I'm trying to figure out how to get something to give the line number of the offender. However, attempting to use 'ghc -prof' gives me a problem because several of the libraries I'm using don't have profiled versions. Is there some way for me to debug this ? That is some way to debug it without inserting debug traces everywhere checking the vector indices ? Thanks.

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Intended behavior of -no-rtsopts-suggestions
by Michael Snoyman 23 Nov '16

23 Nov '16

I'm trying to compile an executable which will totally ignore RTS options flags (have the runtime completely ignore them). It seemed like a combo of -rtsopts=none and -no-rtsopts-suggestions[1] would achieve this. However, I can't figure out what the presence of that flag is doing. Can someone give me a pointer to what my expected behavior should be here, and even better how to achieve my goal of having the runtime ignore +RTS on the command line? $ cat Main.hs import System.Environment main = getArgs >>= print $ ghc -rtsopts=none -no-rtsopts-suggestions Main.hs -fforce-recomp && ./Main +RTS -s [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Main: RTS options are disabled. Link with -rtsopts to enable them. $ ghc -rtsopts=none Main.hs -fforce-recomp && ./Main +RTS -s [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Main: RTS options are disabled. Link with -rtsopts to enable them. [1] https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/phases.html…

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Bangalore Haskell User Group Meetups
by Harendra Kumar 22 Nov '16

22 Nov '16

This is for Haskellers in Bangalore, I apologize for wide distribution. We are having Haskell meetups in Bangalore on a regular basis now. If you are in Bangalore please join us by signing up at https://www.meetup.com/The-Bangalore-Haskell-User-Group/ . You can also join the gitter chat room at https://gitter.im/blrhaskell/Lobby . We have had a few lively and productive sessions in the recent past and hope to continue to do so in future. Looking forward to your participation. -harendra

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Haskell with ncurses
by Atrudyjane 21 Nov '16

21 Nov '16

As a beginning Haskeller working my way through the Haskell book, I'm looking ahead at different options for a user interface. The idea is to build various small tools for personal use. After reading around, it looks like web apps are preferred. Tried to use the Atom editor, and it just seemed a little slow on my machine. The interfaces don't need to be complex or "pretty", just clean looking. I would almost prefer something simple like ncurses. What are your opinions/experiences with using ncurses with Haskell? Or is that the wrong path altogether? Regards, Andrea Sent with [ProtonMail](https://protonmail.com) Secure Email.

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[cabal-install bootstrap failed] ghc-pkg: missing --global-package-db option, location of global package database unknown
by CRUMEYROLLE Pierre 21 Nov '16

21 Nov '16

hello i try to reinstall cabal in a customized directory but bootstrap launch fail because ghc-pkg list is failed thanks for help $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.10.3 $ ./bootstrap.sh --no-doc Using gcc for C compiler. If this is not what you want, set CC. Using /usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2 instead. Checking installed packages for ghc-7.10.3... ghc-pkg: missing --global-package-db option, location of global package database unknown Error during cabal-install bootstrap: running 'ghc-pkg list' failed $ ghc-pkg list ghc-pkg: missing --global-package-db option, location of global package database unknown

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Investigating Evaluation, Typing and Deduction -- WIP
by Rik Howard 21 Nov '16

21 Nov '16

Comments welcome. http://www.dcs.bbk.ac.uk/~rik/gallery/investigating-evaluation-typing-and-d…

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Am I misunderstanding something about `StableName` and `hashStableName`?
by raichoo 20 Nov '16

20 Nov '16

Hi, I've been playing around with `System.Mem.StableName` and `hashStableName` and stumbled across a behavior that seems to be rather weird. First of all the documentation of `hashStableName` says the following: The Int returned is not necessarily unique; several StableNames may map to the same Int (in practice however, the chances of this are small, so the result of hashStableName makes a good hash key). OK, sounds fine, let's put this to the test. So I wrote a little program. module Main where import Control.Monad import System.Mem.StableName import System.Environment main :: IO () main = do args <- getArgs res <- forM [0..10000] $ \i -> do x <- makeStableName i let h' = hashStableName x unless (null args) $ print h' return h' putStrLn "---------------------------" print $ minimum res print $ maximum res Running this program without an argument only prints aus the minimum and maximum hash values that where produced. So surprises here: $ ./Test --------------------------- 1 10000 Here is how things get a bit weird when I make the progam print out the hash values while they are being produced I get a completely different result. Not only that but I get a whole lot of hash collisions, since the hashes produced only seem to be within the range of 1 and 260. $ ./Test print [...] 128 127 126 125 124 123 122 121 120 119 118 117 116 115 114 --------------------------- 1 260 Am I missing something fundamental here, because this behavior seems to be rather confusing. Kind regards, raichoo

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Esqueleto maintainership
by Christopher Allen 20 Nov '16

20 Nov '16

Cross-posting my intent to take over maintainership of Esqueleto. Can read more here: https://github.com/haskell-infra/hackage-trustees/issues/84 This has gone on for awhile, but it's beginning to eat into maintenance of my work projects and it seems like there are others eager to contribute and help share the load where there wasn't 6 or 12 months ago. Cheers, Chris Allen

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difficulty writing GEq instance
by Anders Papitto 20 Nov '16

20 Nov '16

Hello! I'm having a lot of trouble writing a Data.GADT.Compare.GEq instance - can anyone help me fill in the blank? For context - I'm generating Tag types automatically for using with DSum, and I need a GEq instance. That's part of an attempt to add efficient rendering of sum types to Reflex/Reflex-Dom (https://github.com/anderspapitto/reflex-sumtype- render/blob/master/src/ReflexHelpers.hs). I put this question on stackoverflow as well a day ago ( http://stackoverflow.com/questions/40698207/how-can-i- write-this-geq-instance). Here's the code (it's a full, standalone file - you can copy it into Foo.hs and run ghc to see the full error I'm facing). The error I get is that when I try to recursively call geq on the unwrapped x and y, I can't because ghc considers them to have different types - Quux a and Quux b. However, the whole point of why I'm trying to call geq is to see if a and b are the same, so I'm pretty confused. Note that I'm making use of the generics-sop library, which is where NP and NS and I come from. {-# LANGUAGE GADTs #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE RankNTypes #-} module Foo where import Data.GADT.Compare import Generics.SOP import qualified GHC.Generics as GHC data Quux i xs where Quux :: Quux (NP I xs) xs newtype GTag t i = GTag { unTag :: NS (Quux i) (Code t) } instance GEq (GTag t) where -- I don't know how to do this geq (GTag (S x)) (GTag (S y)) = let _ = x `geq` y in undefined

2 2

How to profile haskell web applications?
by Moritz Angermann 19 Nov '16

19 Nov '16

Hi, I’m looking to (perf, heap) profile a haskell web application. With ekg and the ekg wai middleware I can get some data on the rts, gc, and latency. What’s the approach towards performance (-p) and heap (-h) profiling for web applications? Cheers, moritz

1 0

GHC plugin: custom wired-in package, module, and type class
by J. Stutterheim 18 Nov '16

18 Nov '16

Hi all, I'm looking for a way to add my own wired-in package/module/type class for use in a core-to-core plugin. My first challenge is to identify the type class that I want to find. I have the following code: myPkgUnitId :: UnitId myPkgUnitId = fsToUnitId (fsLit "mypackage") mkMyModule :: FastString -> Module mkMyModule m = mkModule myPkgUnitId (mkModuleNameFS m) myModule :: Module myModule = mkMyModule (fsLit "My.Module") myClassName :: Name myClassName = clsQual myModule (fsLit "MyClass") myClassKey myClassKey :: Unique -- How do I get this? Here the missing piece is the myClassKey. Looking at how this is done for, e.g., the Monad class: monadClassKey :: Unique monadClassKey = mkPreludeClassUnique 8 we see that it has a hard-coded unique identifier. Is there a way to do the same for my own type class? Or to somehow find the Unique key dynamically? The second challenge is more or less the same in that I want to identify a member of MyClass: myFunMName :: Name myFunMName = varQual myModule (fsLit "myFun") myFunMClassOpKey myFunMClassOpKey :: Unique -- How do I get this? Again, the identifier is hard-coded for, e.g., ">>=". Any insights will be much appreciated. Best regards, Jurriën

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4.4.2 Fixity Declarations - and what happens when they are missing
by Johannes Waldmann 18 Nov '16

18 Nov '16

Yeah, I know, it's bikeshedding, some 30 years after the fact, but - How on Earth was this allowed to happen? "Any operator lacking a fixity declaration is assumed to be infixl 9" https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-820061 If I don't specify associativity or precedence for an operator, then the parser should simply reject any expression whose meaning would depend on such declarations. So I'd have to write parentheses at each (ambiguous) use site - or write a fixity declaration. Both would serve to document the code. The current situation feels like "the type of identifiers without type declaration is int". Short of repairing the standard - I'd welcome GHC to print a huge warning for each definition and use (!) of "operator without fixity decl". - J. PS: to show that I did at least some research * there's no mention of this in 4.2 "Infix operators" in http://research.microsoft.com/~simonpj/papers/history-of-haskell/history.pdf * Scala also has default rules (6.12.3) "The precedence of an infix operator is determined by the operator’s first character." "Operators ending in a colon ‘ : ’ are right-associative. All other operators are left-associative." and why do I care at all? This: https://github.com/jwaldmann/haskell-obdd/issues/1

1 0

Preventing dereferencing an STRef after taking an action
by Timotej Tomandl 18 Nov '16

18 Nov '16

Hi, is it possible to prevent an STRef passed to one ST action from being passed further on to another ST action? I am trying to prevent these usages: invalid :: STRef s a -> STRef s b -> ST s () invalid x y=action1 x y >>=action2 x y invalid1 :: STRef s a -> STRef s b -> ST s () invalid1 x y=action1 x y >>=action2 y while still being able to do this: valid :: STRef s a -> STRef s b -> ST s () valid x y=action1 x y >>=action2 x where action1 will modify x and y inplace, but in the end only x will hold the useful information. In other words I am trying to make it impossible for any action after action1 to have a same argument as action1's second argument. timo

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Re: [Haskell-cafe] commenting out a line of code
by Bernhard Herzog 17 Nov '16

17 Nov '16

On 15.11.2016, David Turner wrote: > The outer do block should be desugared first, and the `putStrLn "done"` has > the same indent as the `if` so it gets a semicolon, in that block, meaning > it looks like this: > > main = do > { if True > then do > putStrLn "A" > else do > -- putStrLn "B" > ; putStrLn "done" > } That's what one would expect. Except that GHC treats the 'putStrLn "done"' as part of the do-block in the else branch by default. This is due to the NondecreasingIndentation extension which is enabled by default (see https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/bugs.html#c…) With the option -XNoNondecreasingIndentation (or -XHaskell2010) GHC does produce an error message because of the empty do-block. Bernhard

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[ANN] Ginger: Jinja-like HTML templates for Haskell
by Tobias Dammers 17 Nov '16

17 Nov '16

Dear Cafe, I've been working on this for a while now, and figured now is as good a time as any to announce it properly, so here it is. https://ginger.tobiasdammers.nl/ Ginger is a Haskell implementation of Jinja (http://jinja.pocoo.org/) the best-of-breed in the "DOM-agnostic HTML template engines for Python" arena (or at least I think it is). The reason I built this is because upon researching HTML templating solutions for Haskell, I noticed that almost all of them are typesafe compile-time solutions (either EDSLs or something TH/QQ), which is awesome but doesn't fit my use cases. I believe I am not alone here. Code is on github: https://github.com/tdammers/ginger And also on bitbucket: https://bitbucket.org/tdammers/ginger And of course Ginger is available from hackage: http://ginger.tobiasdammers.nl/ Any kind of feedback is very welcome. Thanks for your time! - Tobias -- Tobias Dammers - tdammers(a)gmail.com

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CFP: Dutch Functional Programming Day 2017
by J. Stutterheim 17 Nov '16

17 Nov '16

Call for Participation: Dear Friends of Functional Programming, The next Dutch FP Day will be held on Friday, January 6 2017 at the Radboud University in Nijmegen. You are invited to some or all of: participate, give a presentation, join the dinner. For further information, please see the website. https://wiki.clean.cs.ru.nl/NL-FP_dag_2017 Feel free to forward this invitation to colleagues and students who might be interested. Best regards on behalf of Markus Klinik

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commenting out a line of code
by Johannes Waldmann 15 Nov '16

15 Nov '16

Dear Cafe - Sometimes, a comment can change the meaning of a program. No? Well, but "commenting out" a line of code certainly does. So, can you guess what this program will output? main = do if True then do putStrLn "A" else do -- putStrLn "B" putStrLn "done" I nominate this for inclusion in a hypothetical Haskell equivalent of http://www.javapuzzlers.com/ - J.

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.haddock files
by Patrick Pelletier 15 Nov '16

15 Nov '16

Where can I find documentation about .haddock files? What is in them? Is it possible to generate a .haddock file from source code and then later produce HTML documentation from the .haddock file? Or is that not what it's for? The documentation for Haddock doesn't even seem to mention .haddock files, unless I'm missing something. I tried subscribing to the Haddock mailing list so I could post this question there, but at http://projects.haskell.org/cgi-bin/mailman/subscribe/haddock I got the message: "We're sorry, we hit a bug! Mailman experienced a very low level failure and could not even generate a useful traceback for you. Please report this to the Mailman administrator at this site." Anybody know what's going on with that? Thanks, --Patrick

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bizarre behaviour
by Lawrence Wickert 14 Nov '16

14 Nov '16

Hi, Ok, I am just learning Haskell. So, every day I open a text editor, a pdf file of a Haskell book and a terminal to run ghci. I resize the windows and go on my merry way. Yesterday I decided to install this script https://raw.githubusercontent.com/hotice/webupd8/master/session with the object to save my session windows and restore them the next day. Worked fine although I had to authorize some keyring popup box before it would work. Now I opened Google Chrome and an iHeart radio link. At this point the following appeared at the ghci prompt: Prelude> Vector smash protection is enabled. ghci continues to run fine. If I close and then re-open the iHeart link the message reappears. I am using Ububtu 16.04LTS and I don't use drugs or alcohol. Any ideas as to what is going on? Larry

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Re: [Haskell-cafe] Named arguments to functions?
by Sylvain Henry 14 Nov '16

14 Nov '16

Hi Saurabh, Implicit parameters already look like named arguments: > :set -XImplicitParams > let f = ?x + ?y > let { ?y = 5; ?x = 10 } in f 15 > let g = f where { ?x = 5; ?y = 10 } > g 15 Perhaps we could extend TypeApplications to support the following more natural syntax: > f @(?y = 5) @(?x = 10) which would be syntaxic sugar for the code above. Sylvain On 13/11/2016 14:38, Saurabh Nanda wrote: > Thank you for your reply. > > If you are interested in something like this, you may want to > start a discussion of the design on one of the other lists, or > perhaps write up a proposal for GHC. > > > While I would like the general area of records getting better in > Haskell, I don't think I have the necessary technical chops to lead a > GHC proposal around this. Do you know any proposal/extension in GHC > that is already trying to solve: (a) records, and/or (b) named arguments? > > The proposals currently under discussion are on the `github` URL. > > > Possible to update the prime.haskell.org <http://prime.haskell.org> > website to reflect the current state of affairs? > > -- Saurabh. > > > On 10/11/2016 22:59, Iavor Diatchki wrote: >> Hello, >> >> I would say that this is a too complex of a change to discuss for >> Haskell Prime without implementation or prior experience. If you >> are interested in something like this, you may want to start a >> discussion of the design on one of the other lists, or perhaps write >> up a proposal for GHC. >> >> The proposals currently under discussion are on the `github` URL. >> >> Cheers, >> -Iavor >> >> >> >> >> >> On Thu, Nov 10, 2016 at 2:16 AM, Saurabh Nanda >> <saurabhnanda(a)gmail.com <mailto:saurabhnanda@gmail.com>> wrote: >> >> Hi Everyone, >> >> I was going through >> https://prime.haskell.org/wiki/Process#Proposals >> <https://prime.haskell.org/wiki/Process#Proposals> and noticed >> the following: >> >> > Ideally the language change should be implemented already, so >> that experience gained with the implementation can inform the >> discussion >> >> Is this a hard requirement? I would like to see the ability to >> pass named arguments to functions somehow (most probably through >> anonymous records). I'm unaware of any existing language >> extension that solves this problem. Does this render the >> discussion out-of-bounds from a Haskell Prime perspective? Has >> this already been proposed and considered? >> >> Also, going by what is given on the Wiki, I'm confused about >> where to check the current status of proposals that are being >> considered. Which is the primary link: >> >> * >> https://prime.haskell.org/query?status=new&status=assigned&status=reopened&… >> <https://prime.haskell.org/query?status=new&status=assigned&status=reopened&…> >> * Or, https://github.com/haskell/rfcs >> <https://github.com/haskell/rfcs> >> >> -- Saurabh. >> >> _______________________________________________ >> Haskell-prime mailing list >> Haskell-prime(a)haskell.org <mailto:Haskell-prime@haskell.org> >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime >> <http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-prime> >> >>

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Re: [Haskell-cafe] megaparsec 5.1.2 - missing doc links on hackage
by Mark Karpov 13 Nov '16

13 Nov '16

Hey Charles, I uploaded the docs manually. If you run into this problem again remember that there is usually a version of the docs available on Stackage as well: https://www.stackage.org/nightly-2016-11-13/package/megaparsec-5.1.2 Best, Mark Charles Martin writes: > Just drawing your attention to the missing documentation links for the > current version: > https://hackage.haskell.org/package/megaparsec-5.1.2 > > Cheers - Charles

1 0

Cross compilation
by Jan von Löwenstein 12 Nov '16

12 Nov '16

Hi, I have a Mac OS X workstation and want to compile a cli for Linux (and potentially Windows) in addition to OS X. I could not find anything but https://ghc.haskell.org/trac/ghc/wiki/CrossCompilation. This has an explanation how I can build a cross compiling GHC for a specific combination of build and target system/architecture. Stack has an open issue regarding cross compilation. Is that really all there is out there? Regards Jan

3 4

TH GadtC parameter
by Paolino 12 Nov '16

12 Nov '16

Hello, I couldn't find the evidence for the first parameter of GadtC in Con data type to be a [Name] in place of a simpler Name. https://hackage.haskell.org/package/template-haskell-2.11.0.0/docs/Language… How should I match a normal gadt constructor syntax with that? Thank you Paolino

2 1

GHC API: type class instance check
by J. Stutterheim 11 Nov '16

11 Nov '16

Hi all, I'm writing a core-to-core GHC plugin and have two questions: - Given some t :: Type, how can I check whether there exists an instance of some type class C for t? - How can I check whether some type class C exists, and if so, how can I get a value c (presumably :: Class) of this class? Thanks! - Jurriën

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Announcing Req -- HTTP client library
by Mark Karpov 11 Nov '16

11 Nov '16

Hi, I would like to announce a new HTTP client library called Req, or rather a new API to tried-and-true http-client. The Haddock is available here: https://www.stackage.org/haddock/nightly-2016-11-11/req-0.1.0/Network-HTTP-… It happens to be a complete tutorial on its own, so you can just read through it and it will be clear what it's all about and what kind of API is proposed. Hackage: https://hackage.haskell.org/package/req GitHub: https://github.com/mrkkrp/req Best, Mark

1 0

Getting Haddock docs onto Hackage (using Stack)
by de Buitléir, Amy 11 Nov '16

11 Nov '16

A package I uploaded to Hackage a week ago still has no docs. (It says "Docs pending" and "Build status unknown".) What is the best way to get Haddock documentation for a package onto Hackage if you are using Stack? Once upon a time, it used to be built automatically after the package was uploaded, and if it wasn't, you could find out the reason why by checking the build logs. When that stopped working, I used a script called hackage-docs.sh that I got from somewhere. But now I've migrated from Cabal to Stack, and the old script no longer works. I can modify it to work with Stack, but is there a better way? Should I instead use the "delete documentation and trigger rebuild" button on Hackage? Here's the package in question: http://hackage.haskell.org/package/grid-7.8.7

5 6

Atomically modify two MVars
by Lian Hung Hon 10 Nov '16

10 Nov '16

Dear cafe, Is it possible to atomically modify two MVars? (I know it can be done with TVars) Regards, Hon

3 3

ghc installation on redhat7
by CRUMEYROLLE Pierre 10 Nov '16

10 Nov '16

Hello y want to install from source ghc 7.10 on red hat 7 y have installed by rpm method ghc 7.6.3 on my host because for generate ghc it's necessary to have pre install a ghc compiler. y have also perl version 5.16.3 installed. y have downloaded on https://www.haskell.org/ghc/download_ghc_7_10_1 the source distribution ghc-7.10.1-src.tar.bz2 after tar xvf ghc-7.10.1-src.tar.bz2 y launch ./configure --prefix=$HOME y have this error "Could not find module `System.Directory'" my trace below, thanks for explain $ ./configure --prefix=$HOME checking for gfind... no checking for find... /usr/bin/find checking for sort... /usr/bin/sort checking for GHC Git commit id... given ca00def1d7093d6b5b2a937ddfc8a01c152038eb checking for ghc... /usr/bin/ghc checking version of ghc... 7.6.3 checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... x86_64-unknown-linux-gnu Build platform inferred as: x86_64-unknown-linux Host platform inferred as: x86_64-unknown-linux Target platform inferred as: x86_64-unknown-linux GHC build : x86_64-unknown-linux GHC host : x86_64-unknown-linux GHC target : x86_64-unknown-linux configure: Building in-tree ghc-pwd utils/ghc-pwd/Main.hs:4:8: Could not find module `System.Directory' Use -v to see a list of the files searched for. configure: error: Building ghc-pwd failed best regards

2 2

What does intero add to GHCi?
by Yuji Yamamoto 10 Nov '16

10 Nov '16

Hello, Haskellers! I've recently seen many tools for Haskell IDE backend such as ghc-mod, hsdev, haskell-ide-engine, ide-backend, and intero. In addition, I found GHCi added commands to support IDEs: :complete, :type-at, etc. Then, I heard intero is a fork of GHCi. Actually intero's commands look same with GHCi's. So, what does intero add to GHCi? Thanks in advance. -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep Facebook: http://www.facebook.com/igrep Google+: https://plus.google.com/u/0/+YujiYamamoto_igrep

1 0

Asynchronous computations and error handling in the free monad
by Damian Nadales 09 Nov '16

09 Nov '16

Yesterday I posted the following question to SO: http://stackoverflow.com/questions/40495304/abstract-result-types-in-free-m… As discussion with Benjamin ensued, in which he stated that it is better to be explicit about the fact that we want parts of our code to run asynchronously. I am not so sure. I also have a similar question about how would you deal with fatal errors that could happen at the interpreter. I guess this could be modeled in the free monad by having a command returning a `Maybe` or `Either` type. However we're adding details about the interpreters in our DSL, since we are stating that certain command could cause fatal errors. So my question is what approach do you or would you use?

2 1

Avoid sharing
by Michael Roth 08 Nov '16

08 Nov '16

Hello! A short question, given: data Seed = ... data Value = ... someGenerator :: Seed -> [Value] createTwo :: Seed -> ([Value], [Value]) createTwo s = (as, bs) where as = someGenerator s bs = drop 1000000000 (someGenerator s) Is it guaranteed that 'someGenerator s' is created twice and not shared between 'as' and 'bs'? Is this by language design? Are there any GHC options that change the behaviour? Thank you, Michael

5 7

TIOBE Index: Is Haskell finally going to hit the top 20?
by Henk-Jan van Tuyl 08 Nov '16

08 Nov '16

L.S., I know the TIOBE index[0] is not very scientific, but it is nice to see Haskell moving up in the TIOBE Index. From tiobe.com: " TIOBE Index for November 2016 November Headline: Is Haskell finally going to hit the top 20? Some people say that Haskell is the most mature purely functional programming language available nowadays. It has quite a long history, dating back from 1990 and its community is growing, although slowly. This month Haskell is only 0.255% away from the top 20 at position 23. Let's see what happens the next few months. " Haskell ranked 34th last month. Regards, Henk-Jan van Tuyl [0] http://www.tiobe.com/tiobe-index/ -- 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://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

3 2

Strange space leak
by Tom Ellis 07 Nov '16

07 Nov '16

Can anyone explain why this leaks space: main :: IO () main = let loop () = return () *> loop () in loop () whilst this doesn't: main :: IO () main = let loop = return () *> loop in loop I'm stumped.

3 3

FFT, via homogeneous functions?
by David Banas 07 Nov '16

07 Nov '16

Hi all, I’m trying to implement the fast Fourier transform (FFT), using the homogeneous functions described in Sec. 4.2 of *Arrows and Computation* by Ross Paterson: -- FFT implementation -- Note my reversal of Ross' even/odd convention, below. -- I did this, to remain consistent with the explanation, above, which came from a pre-existing work. fft :: RealFloat b => Hom (Complex b) (Complex b) fft = id :&: proc (e, o) -> do e' <- fft -< e o' <- fft >>> twiddle -< o unriffle -< f (e', o') where f (x, y) = (x + y, x - y) twiddle :: RealFloat b => Hom (Complex b) (Complex b) twiddle = id :&: twiddle' 1 twiddle' :: RealFloat b => Int -> Hom (Pair (Complex b)) (Pair (Complex b)) twiddle' n = (id *** (* phi)) :&: (((twiddle' n') *** ((twiddle' n') >>> (arr ((* phi) `prod` (* phi))))) >>> unriffle) where phi = cis (-pi / (fromIntegral (2 ^ n))) n' = n + 1 I’m getting this output, which means that, while my *twiddle* function is working for all tree depths tested, my *fft* function fails for tree depths greater than 3: Tree depth: 0 Testing twiddles...True Testing FFT...True Tree depth: 1 Testing twiddles...True Testing FFT...True Tree depth: 2 Testing twiddles...True Testing FFT...True Tree depth: 3 Testing twiddles...True Testing FFT...True Tree depth: 4 Testing twiddles...True Testing FFT...False Tree depth: 5 Testing twiddles...True Testing FFT...False And I’m wondering if anyone sees the flaw in my code. More here: https://htmlpreview.github.io/?https://github.com/capn-freako/Haskell_Misc/… <https://htmlpreview.github.io/?https://github.com/capn-freako/Haskell_Misc/…> Thanks! -db

1 0

Guidance on symbol error
by Christopher Howard 07 Nov '16

07 Nov '16

Hi list. I'm trying to build gruff on Debian Jessie with GHC 7.6.3. I'm building gruff-0.3.3 because gruff-0.3.4 dependencies seem to be too new for Jessie. I was able to build all the dependencies of gruff-0.3.3 (after installing a ton of -dev packages). But when cabal tries to build the gruff package itself, I get this error: /usr/bin/ld: dist/build/gruff/gruff-tmp/src/compute.o: undefined reference to symbol '_ZdaPv@@GLIBCXX_3.4' //usr/lib/x86_64-linux-gnu/libstdc++.so.6: error adding symbols: DSO missing from command line It's been quite a while since I did any C++ programming... could somebody clarify for me what this means? Thank you. -- https://qlfiles.net My PGP public key ID is 0x340EA95A (pgp.mit.edu)

2 1

ResourceT "unmasked" during allocation?
by Viktor Dukhovni 07 Nov '16

07 Nov '16

I am looking at using ResourceT to manage the acquisiition and deallocation of network sockets. In comparing resourceT's `allocate` with `bracket` I was somewhat perplexed to find that `allocate` in Control.Monad.Trans.Resource seems to not mask exceptions during allocation (see "No mask" comment I inserted): allocateRIO :: IO a -> (a -> IO ()) -> ResourceT IO (ReleaseKey, a) allocateRIO acquire rel = ResourceT $ \istate -> liftIO $ E.mask $ \restore -> do a <- restore acquire -- ^^^ No mask ^^^ key <- register' istate $ rel a return (key, a) allocate :: MonadResource m => IO a -- ^ allocate -> (a -> IO ()) -- ^ free resource -> m (ReleaseKey, a) allocate a = liftResourceT . allocateRIO a By contrast in Exception.Base we see that `bracket` masks the initialization action: bracket before after thing = mask $ \restore -> do a <- before -- ^^^ masked! ^^^ r <- restore (thing a) `onException` after a _ <- after a return r This seems to suggest that ResourceT is more exposes to resource leaks via asynchronous exceptions. Say if a file descriptor is a allocated, but an exception interrupts "allocate" before it can register the resource. Perhaps I am missing something, or there are good reasons for the difference in approach. Can anyone shed some light on the reason why ResourceT differs (seemingly) from bracket in this regard. -- Viktor.

2 1