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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 28 Nov '16

28 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 28 Nov '16

28 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 25 Nov '16

25 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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