- Haskell-Cafe - Haskell.org

Let's teach GHC idiom brackets!
by Oliver Charles 19 Feb '15

19 Feb '15

Hi friends, Last night I was having a good natter with Chris Done, and the topic of idiom brackets came up. Specifically, Chris' hindent and structured-haskell-mode tools have both recently become aware of the quasiquoter in applicative-quoters <http://hackage.haskell.org/package/applicative-quoters> - allowing one to write ordinary Haskell code but have it formatted/edited as if it wasn't even inside a quasiquoter. However, it's a shame that Chris has to now be aware of this specific quasiquoter just to get his tools to work properly. Furthermore, it's a shame that *everyone* has to do the same whenever they want to manipulate Haskell code. That said, idiom brackets - at least to me - appears to be a very desirable syntax extension. A recent desire for this came to me while working with reactive-banana. As almost everything "interesting" is even an Event or a Behavior, there tends to be a lot of assembling programs with applicative syntax. Another obvious example is when using parser combinators. Therefore, I'd like to propose that we teach GHC about this desugaring, properly. Now, how we do that, and in fact, what I'm even proposing... that's all up for discussion! I spoke briefly with David Christiansen and a few others in the Idris IRC channel last night about their particular variation of idiom brackets, and it seems people are generally happy with that. Copying that almost verbatim is probably the first proposal I'd put forward. This is the first time I've proposed an extension to GHC, so I'll need some guidance to make this happen - both in terms of guiding the discussion through to a proposal, but ultimately implementing it. Perhaps the proposal will be ready for ZuriHac, and some GHC devs could help me work on it there. Anyway, that's enough rambling from me. I would love to hear your thoughts - do you think idiom brackets are a good idea? Do you have a particular tricky case that you think they need to support? I'm all ears. -- Ollie

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Very large data structures in the heap
by Dr. Olaf Klinke 19 Feb '15

19 Feb '15

I have a function that builds a (cyclic) data structure for searching text. The resulting object can be serialized and re-built. Real-world problems would, however, result in a tree with about 10^9 nodes. I take it that standard data type declarations are not very memory-efficient, so my (fully evaluated) search tree would not fit into memory as a heap object. Its serialised form should be of tractable size, e.g. by storing an array of labels and the tree structure as a bit array of parentheses. I'd like to separate serialization (IO) from traversal of the stucture (pure), but how can one avoid the entire heap object from being built? Can lazyness be exploited? Olaf

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ghc and libgmp compiled separately
by briand＠aracnet.com 19 Feb '15

19 Feb '15

i downloaded the centos65 binary of ghc only to find there was some sort of problem with libgmp on the system i am using. No problem i think, I'll just go grab the latest 4.x gmp compile it and use the --with-gmp flags to point ghc to my compiled version. didn't work. Don't have access to the machine from home, so i can't relay the precise error message (forgot to email it to myself), but it seems that the --with-gmp flags should have solved my problem (gmp compiled cleanly without any problems and put all the files in the right place). any obvious trickery involved when attempting something like this ? Thanks, Brian

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Testing polymorphic properties with QuickCheck
by Stuart Hungerford 19 Feb '15

19 Feb '15

Hi, I'd like to use QuickCheck to test polymorphic properties and I'm wondering if there's any community consenus on how to do this while avoiding creating large amounts of boilerplate test code. As an example, suppose I have this simple simplistic Monoid typeclass: class Monoid a where zero :: a (|+|) :: a -> a -> a And suppose I make these types Monoid instances: instance Monoid Integer where zero = 0 (|+|) = (+) instance Monoid Rational where zero = 0 (|+|) = (+) instance (Monoid a, Monoid b) => Monoid (a, b) where zero = (zero, zero) (|+|) (x, y) (u, v) = (x |+| u, y |+| v) Using QuickCheck and the Tasty test framework I'd like to test the Monoid laws across the relevant types (I'm only showing one part of one law here) as QuickCheck properties: prop_left_add_id :: (Eq a, Monoid a) => a -> Bool prop_left_add_id x = zero |+| x == x monoid_suite :: TestTree monoid_suite = testGroup "monoid" [ testProperty "left additive identity (Integer)" (prop_left_add_id :: Integer -> Bool), testProperty "left additive identity (Rational)" (prop_left_add_id :: Rational -> Bool), testProperty "left additive identity ((Integer, Integer))" (prop_left_add_id :: (Integer, Integer) -> Bool), -- ...] And this is where I start creating a lot of essentially boilerplate code. I'm wondering if there's any consensus on the way to go about this kind of testing? Researching this issue I've found ways of combining various GHC extensions and the type system to (I think) write the properties more at the type level. There are QuickCheck and Tasty modules that aim to find all properties in a module and test them, but polymorphic properties are automatically defaulted to Integer arguments. I'm also tempted by Template Haskell to write what's essentially code generation templates. Is there a better way to go about this? Thanks, Stu

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 138, Issue 30
by Stuart Hungerford 19 Feb '15

19 Feb '15

On Thu, Feb 19, 2015 at 12:13 PM, <haskell-cafe-request(a)haskell.org> wrote: > From: Tom Ellis <tom-lists-haskell-cafe-2013(a)jaguarpaw.co.uk> > To: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Testing polymorphic properties with > QuickCheck > I'm not quite sure what you're asking specifically, but maybe this will > help: > > {-# LANGUAGE ScopedTypeVariables #-} > > monoid_suite :: forall m. (Eq m, Monoid m) => String -> Proxy m -> TestTree > monoid_suite typename _ = testGroup "monoid" [ > testProperty ("left additive identity (" ++ typename ++ ")") > (prop_left_add_id :: m -> Bool), > > ... <other general monoid properties here> > ] > > monoid_suite_integer :: TestTree > monoid_suite_integer = monoid_suite "Integer" (Proxy :: Integer) Please excuse my ignorance as a relatively new Haskell user but is there anything special about that "Proxy m" type? Thanks, Stu

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How to force James Cook (author of dependent-map) evaluation?
by Alexey Uimanov 19 Feb '15

19 Feb '15

Hello people. There is a pull request https://github.com/mokus0/dependent-map/pull/2 which I need to be on hackage. I have sent a email to mokus(a)deepbondi.net but nothing happened. Does anyone can contact with James and tell him about my problem?

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Re: [Haskell-cafe] quickcheck for compiler testing
by Doug McIlroy 18 Feb '15

18 Feb '15

>> I'm ... wondering whether people use quickcheck at all for >> compiler testing. > it's worth giving a try, but keep in mind that you might still > need to use unit tests If your compiler is for a widely used language, you might consider a hybrid approach due to Bill McKeeman: generate random code and check it by automated crowd-sourcing. The crowd is a jury of other compilers, not necessarily even for the same hardware. The check is to compare compilability, compile time, run time and run results against those of the jury. McKeeman used a generative grammar whose productions were adjustably weighted to influence the characteristics of the generated code. Over the course of several months of overnight runs torturing a production C compiler, he turned up something like one anomaly per day, excluding jury errors and extra reports from a common root cause. (One amusing find was that the compiler took Omega(INT_MAX) time to evaluate the constant expression 1<<INT_MAX.) Doug

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cryptography in haskell
by Julian Ospald 18 Feb '15

18 Feb '15

Hi, I've been wondering about the state of cryptography in haskell. Not so much in the sense of "what libraries are out there?", but rather about the question what crpyto and IT security people think about ideas like rewriting something as OpenSSL in haskell. I know it can be technically done, but are there any remarks in this area that are important for practical security? For example, some people think that it can be dangerous to implement something like this in high-level languages (e.g. java which was vulnerable to timing attacks). Of course I think haskell can do a lot for us to make things safer: * type safety * referential transparency * explicit knowledge about side-effects and which kind ... But that doesn't tell me if it introduces new pitfalls/attack-vectors for practical cryptography implementations. -- Regards, Julian Ospald

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quickcheck for compiler testing
by Maurizio Vitale 17 Feb '15

17 Feb '15

I'm starting to work on my first real haskell program (I've only RWH exercises under my belt) and wondering whether people use quickcheck at all for compiler testing. I've seen uses of quickcheck for testing parsers, but I'm interested in generating well-formed programs (e.g. random programs with all declarations in reasonable random places). This could be used to test passes other than parsing (or even parsing, for languages that needs to distinguish identifiers, like the 'typedef' problem in C/C++). The only thing I can think of, is to use quickcheck for randomly generating statements, go over them and figure out free variables (incl. functions) and generate declarations in random places for them. But I'm not sure how this would play with test size reduction and doesn't look like a nice solution anyhow. Any idea or pointers to examples? or should I give up on quickcheck for this and just do direct testing? Thanks, Maurizio

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Haskell Weekly News
by Kim-Ee Yeoh 17 Feb '15

17 Feb '15

Welcome. Here are this week's picks: - Scott Turner <http://ghc.haskell.org/trac/ghc/ticket/8539#comment:25> proffered a solution in getting exponentiation of complex numbers right, which includes an at-a-glance table to see if everything's sane. - GHC is unsound! <https://mail.haskell.org/pipermail/ghc-devs/2015-January/008189.html> It has a type safety leak. Iavor Diatchki <https://mail.haskell.org/pipermail/ghc-devs/2015-February/008269.html> is hard at work plugging the hole. - Richard Eisenberg <https://mail.haskell.org/pipermail/ghc-devs/2015-February/008271.html> discovered that Debug.Trace.traceStack does a better job debugging GHC than pprTrace. - The curtain is pulled back on the new face of www.haskell.org. Chris Done designed it, Gershom Bazerman announced it, and unsung heroes migrated it and kept it running under the flash crowd onslaught. The raves include: - This is far more welcoming and user-friendly, especially to those who might be curious about Haskell. [link] <https://news.ycombinator.com/item?id=9052129#up_9052355> - The website redesign is great from a new programmer's perspective. I just started learning Haskell a week ago, and I wish this was the first thing I saw. Looks wonderful. Cheers! [link] <http://www.reddit.com/r/haskell/comments/2vy1lw/new_haskell_homepage_is_liv…> - This new, awesome site is using the framework Yesod, which is kinda like the Rails of the Haskell world, only it can do rad stuff like prevent XSS and 404s at *compile *time. Yes, you read that right. [link] <https://news.ycombinator.com/item?id=9052129#up_9052279> - Repo of the week: Samuel Gélineau's hawk <https://github.com/gelisam/hawk>, a haskell command-line text-processor similar to unix awk. Quotes of the week: - "I have made several forays into the impredicative swamp, and barely made it back to the shore alive." - Simon Peyton Jones - "Web programming is like a mature RPG: everything's extremely complicated and everyone's overpowered." - Max Goldstein Tweets: - In Buenos Aires, Marcote Torres tweets <https://twitter.com/marcote_torres/status/565644584060198914> about the first ever Haskell meetup. - And "In Soviet Russia, #haskell learns a you" as @aberls <https://twitter.com/aberls/status/565669752853778432> reminds us. Got a letter to the editor? Hit reply. -- Kim-Ee

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Re: [Haskell-cafe] Refactor unqualified imports to qualified ones automatically?
by Jeffrey Brown 17 Feb '15

17 Feb '15

Thank you, Henning. I found a description [1] of what the -ddump-minimal-imports flag you suggest does ("Dump a minimal set of imports"), and I don't know what that means. I tried running it anyway, but I can't get the code to compile via GHC (or make, or cabal install, though it seems to want to be built via at least one of those). It was abandoned long ago, and I'm not convinced its network of dependencies is something I can disentangle. [1] https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/flag-refere… On Sun, Feb 15, 2015 at 1:20 PM, Henning Thielemann < lemming(a)henning-thielemann.de> wrote: > > Maybe the GHC flag > > -ddump-minimal-imports > > helps? >

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HaskBoard?
by Mike Meyer 16 Feb '15

16 Feb '15

I asked about running GHCI on something like the Arduino due, and was told that there wasn't enough CPU on it for that. How about getting it running on something like a PyBoard? http://www.adafruit.com/products/2390

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New Functional Programming Job Opportunities
by Functional Jobs 16 Feb '15

16 Feb '15

Here are some functional programming job opportunities that were posted recently: Senior Developer (Big Data Tech Stack) at Recruit IT Group Ltd http://functionaljobs.com/jobs/8783-senior-developer-big-data-tech-stack-at… Senior Development Engineer at Lookingglass Cyber Solutions http://functionaljobs.com/jobs/8782-senior-development-engineer-at-lookingg… Cheers, Sean Murphy FunctionalJobs.com

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Is there a cross platform CA certificate bundle solution for HsOpenSSL?
by Marios Titas 16 Feb '15

16 Feb '15

If I want to use HsOpenSSL for a tls client application that verifies the server certificate I have to manually specify a CA certificate bundle containing the trusted roots. For example, in a linux system, I would do the following mkTlsContext :: IO Context mkTlsContext = do ctx <- context contextSetVerificationMode ctx (VerifyPeer True False Nothing) contextSetCADirectory ctx "/etc/ssl/certs" return ctx The problem is that the above solution only works for linux. Is there a cross-platform way to find a reasonable CA bundle and use it with HsOpenSSL? Note that the tls package has x509-system [1] that does exactly that. So I am basically asking if anybody has written something similar for HsOpenSSL. [1] https://hackage.haskell.org/package/x509-system

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Cancelled: Re: Typed TH
by Gautier DI FOLCO 16 Feb '15

16 Feb '15

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Événement annulé: Re: Typed TH - Lun. 16 Févr. 2015 09:00 - 10:00 (gautier.difolco@gmail.com)
by gautier.difolco＠gmail.com 16 Feb '15

16 Feb '15

Cet événement a été annulé et supprimé de votre agenda. Titre : Re: [Haskell-cafe] Typed TH Date : Lun. 16 Févr. 2015 09:00 - 10:00 Paris Agenda : gautier.difolco(a)gmail.com Participants : * Gautier DI FOLCO- organisateur * haskell-cafe * ben.franksen(a)online.de Invitation de Google Agenda : https://www.google.com/calendar/ Vous recevez ce message à l'adresse haskell-cafe(a)haskell.org, car vous participez à cet événement. Pour ne plus recevoir de notifications pour cet événement à l'avenir, refusez cet événement. Vous avez également la possibilité de créer un compte Google sur la page https://www.google.com/calendar/ et de définir vous-même les paramètres de notification pour l'intégralité de votre agenda.

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Invitation: Re: Typed TH - Lun. 16 Févr. 2015 09:00 - 10:00 (Gautier DI FOLCO)
by Gautier DI FOLCO 16 Feb '15

16 Feb '15

Vous avez été invité à l'événement suivant. Titre : Re: [Haskell-cafe] Typed TH Date : Lun. 16 Févr. 2015 09:00 - 10:00 Paris Agenda : Gautier DI FOLCO Participants : * Gautier DI FOLCO- organisateur * haskell-cafe * ben.franksen(a)online.de Détails de l'événement : https://www.google.com/calendar/event?action=VIEW&eid=XzcxaTNhZTFtY2RqM2diY… Invitation de Google Agenda : https://www.google.com/calendar/ Vous recevez ce message à l'adresse haskell-cafe(a)haskell.org, car vous participez à cet événement. Pour ne plus recevoir de notifications pour cet événement à l'avenir, refusez cet événement. Vous avez également la possibilité de créer un compte Google sur la page https://www.google.com/calendar/ et de définir vous-même les paramètres de notification pour l'intégralité de votre agenda.

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Typed TH
by Christopher Done 16 Feb '15

16 Feb '15

http://lpaste.net/116791 If someone feels like spending time exploring this and putting it in a library I will be interested in trying it. I saw some old work to patch GHC to do this including typed quotation of expressions, which doesn't seem to have arrived. Relatedly I made a trivial Lisp with untyped and typed quotation https://gist.github.com/chrisdone/516489f4f27846712225 It doesn't seem to be a much-explored area in the Lisp world, Haskell is treading new waters in some ways.

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gloss-examples install fails
by Gregory Guthrie 15 Feb '15

15 Feb '15

I installed gloss, and then the binary for llvm (http://llvm.org/builds/ ) But still gloss-examples fail, because it requires gloss-raster, which fails (below). I see some discussion of this from last year (https://mail.haskell.org/pipermail/haskell-cafe/2014-March/113001.html) Do I need to do some path manipulations to get this to work? I see llc & opt as mentioned in that note installed under mingw, but not under the llvm installed site. ------------------------------------------------------------------------ <C:\Windows\System32>cabal install gloss-raster Resolving dependencies... Configuring gloss-raster-1.9.2.1... Building gloss-raster-1.9.2.1... Failed to install gloss-raster-1.9.2.1 Build log ( C:\Users\guthrie\AppData\Roaming\cabal\logs\gloss-raster-1.9.2.1.log ): Building gloss-raster-1.9.2.1... Preprocessing library gloss-raster-1.9.2.1... [1 of 2] Compiling Graphics.Gloss.Raster.Array ( Graphics\Gloss\Raster\Array.hs, dist\build\Graphics\Gloss\Raster\Array.o ) <no location info>: Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM cabal: Error: some packages failed to install: gloss-raster-1.9.2.1 failed during the building phase. The exception was: ExitFailure 1 Running with -v, it adds: <no location info>: Warning: Couldn't figure out LLVM version! Make sure you have installed LLVM 8.3 found on system at: e:\Plang\Haskell Platform\bin\ghc.exe Using ghc-pkg version 7.8.3 found on system at: e:\Plang\Haskell Platform\bin\ghc-pkg.exe No ghcjs found No ghcjs-pkg found No greencard found -------------------------------------------

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Exception safe resource management with dynamic regions, call for review
by Yuras Shumovich 15 Feb '15

15 Feb '15

Hello, I'm asking for code and design review for `io-region` library. It is already on Hackage for convenience and easy access to haddock: https://hackage.haskell.org/package/io-region Manual exception handling in Haskell almost never is a good idea, except probably trivial cases where `bracket` is enough. A number of special libraries where created to help with exception handling. `io-regions` is one of them. Initially it was an attempt to solve the double-throw issue, but I found it unsolvable in library without changes in language itself. So now it is an attempt to design easy to use API with a bit different set of trade-offs. The library is based on idea of regions that own resources. The resources are released when the region is closed. Early releasing and ownership transferring are supported. Types are *not* used to prevent use-after-free errors. The API is not monadic, and it is the biggest difference from other libraries. Regions are first-class citizens, so you can manipulate them, pass as arguments to functions, send to other thread, store in data types. Such flexibility has its cost -- a bit more manual management. The implementation is based on STM and is thread safe, that allows e.g. atomic ownership transferring between threads. The nearest competitor I think is `resourset` package, and I still recommend it by default. Though I convinced myself that `io-region` is good enough, and I'm using it in productions (for two weeks already :) ) Thanks, Yuras

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Refactor unqualified imports to qualified ones automatically?
by Jeffrey Brown 15 Feb '15

15 Feb '15

Hi, In a previous post regarding overlapping images in wx [1], Stephen Tetley suggested I study Blobs [2]. I want to find its wx logic, but as it stands I can't distinguish the wx commands from the others, because wx is too large a library for me to know all the names in it, and Blobs imports most of its modules unqualified. Is there some automatic way of changing an unqualified import to a qualified import, so that every function from the module of interest would be prefixed to indicate where it comes from? If not, is there some other way around the problem? Thanks, Jeff [1] https://mail.haskell.org/pipermail/haskell-cafe/2015-January/117669.html [2] https://hackage.haskell.org/package/Blobs-0.3/src/

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ANN: New Haskell.org Homepage Now Live
by Gershom B 15 Feb '15

15 Feb '15

I’m pleased to announce that http://www.haskell.org has received its first significant design update since 2010! More significantly, for the first time since 2006 it is not a wiki, but an actual language homepage. More significantly still, for the first time ever, the Haskell homepage now runs on a fully Haskell-powered stack. I suppose some people might also be interested in the fact that it looks quite nice, and has significantly cleaned up the information it presents, so as to provide a more focused experience for newcomers looking to explore the language. The page is now includes the collective work of over 230 commits by 17 contributors. First and foremost among those is Chris Done, who conceived the vision, wrote the bulk of the code, and executed the design. Also thanks to our many admins, including Austin Seipp, Ricky Elrod, et al., who managed the deployment, and have been working behind the scenes to continue to modernize and update our creaking infrastructure. Onto a few key things to remember: * The wiki is still around at https://wiki.haskell.org and it is still a fantastic resource. It is now speedy again, and we have an account creation process. By all means, jump in, and help curate and cultivate its wealth of knowledge. * The new site is all on github: https://github.com/haskell-infra/hl — there you will also find an issue tracker as well. We are sure there are many gaffes, gaps, and oddities that remain to be patched up in the site, especially with regards to its content. Tickets, pull requests, and comments are encouraged. * The official home for our mailinglists is now http://mail.haskell.org — hopefully this will cause no problems or confusion, and the redirects should be in place properly. Please let us know if there are any issues. * We think we covered everything, but there is the possibility that some item, service, or redirect broke in the course of this transition. Please let us know if this is the case. * If you have been responsible for some subdirectory under the www.haskell.org banner, then you will find that you do not yet have an account on the new server. Please contact us and we’ll get you set up to continue to maintain that portion of the site. * As always, you can reach the haskell ops and admin team at admin(a)haskell.org, or on freenode irc at #haskell-infrastructure. Regards, Gershom (and also the Haskell.org Committee and Haskell.org Admin Team)

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Re: [Haskell-cafe] [Haskell] ANN: New Haskell.org Homepage Now Live
by Daniil Frumin 15 Feb '15

15 Feb '15

Dear Haskell.org team thank you for your great work! But I was wondering this as well, why there is no mention of the Haskell platform in the Downloads section? I am also a little bit concerned about the "News" section: arguably, the news about GHC and Haskell platform releases are more important than newest stackoverflow questions or reddit posts. The newsfeed on Python.org, for example, is almost exclusively dedicated to releases and real-life events (which is also located on the front page). On Sun, Feb 15, 2015 at 1:31 PM, David Sabel <sabel(a)ki.informatik.uni-frankfurt.de> wrote: > I wonder why the downloads section doesn't link to the Haskell platform. > What's the reason for it? > > David > > Am 15.02.2015 um 08:50 schrieb Gershom B: >> >> I’m pleased to announce that http://www.haskell.org has received its first >> significant design update since 2010! More significantly, for the first time >> since 2006 it is not a wiki, but an actual language homepage. More >> significantly still, for the first time ever, the Haskell homepage now runs >> on a fully Haskell-powered stack. I suppose some people might also be >> interested in the fact that it looks quite nice, and has significantly >> cleaned up the information it presents, so as to provide a more focused >> experience for newcomers looking to explore the language. >> >> The page is now includes the collective work of over 230 commits by 17 >> contributors. First and foremost among those is Chris Done, who conceived >> the vision, wrote the bulk of the code, and executed the design. Also thanks >> to our many admins, including Austin Seipp, Ricky Elrod, et al., who managed >> the deployment, and have been working behind the scenes to continue to >> modernize and update our creaking infrastructure. >> >> Onto a few key things to remember: >> >> * The wiki is still around at https://wiki.haskell.org and it is still >> a fantastic resource. It is now speedy again, and we have an account >> creation process. By all means, jump in, and help curate and cultivate its >> wealth of knowledge. >> * The new site is all on github: https://github.com/haskell-infra/hl — >> there you will also find an issue tracker as well. We are sure there are >> many gaffes, gaps, and oddities that remain to be patched up in the site, >> especially with regards to its content. Tickets, pull requests, and comments >> are encouraged. >> * The official home for our mailinglists is now http://mail.haskell.org >> — hopefully this will cause no problems or confusion, and the redirects >> should be in place properly. Please let us know if there are any issues. >> * We think we covered everything, but there is the possibility that >> some item, service, or redirect broke in the course of this transition. >> Please let us know if this is the case. >> * If you have been responsible for some subdirectory under the >> www.haskell.org banner, then you will find that you do not yet have an >> account on the new server. Please contact us and we’ll get you set up to >> continue to maintain that portion of the site. >> * As always, you can reach the haskell ops and admin team at >> admin(a)haskell.org, or on freenode irc at #haskell-infrastructure. >> >> Regards, >> Gershom >> (and also the Haskell.org Committee and Haskell.org Admin Team) >> >> _______________________________________________ >> Haskell mailing list >> Haskell(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell -- Sincerely yours, -- Daniil

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Code review. API Guidance.
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 15 Feb '15

15 Feb '15

Hello everyone. I have an application (a calculator repl [1]) and I'm looking to convert it into an package that provides both library and executable. I'm looking for suggestions for what might be a good API for such a library. I'm not looking for an extended code review, just the placement of functions in modules, and which functions to export. The current source tree looks like this. src/ $ tree . . ├── Calculator │ ├── Color.hs -- Colored output │ ├── Evaluator │ │ ├── Base.hs -- Common evaluation functions │ │ ├── Cmd.hs -- Evaluate special commands │ │ ├── Expr.hs -- Expression evaluation │ │ ├── Func.hs -- Convert specification for a function into a mathematical function │ │ └── Statement.hs -- Evaluate either expression or command │ ├── Help.hs -- Help message │ ├── Parser │ │ ├── Base.hs -- Common parsers, number, id etc. │ │ ├── Cmd.hs │ │ ├── Expr.hs │ │ └── Statement.hs │ └── Prim -- Primitives │ ├── Bindings.hs -- Variable and Function Bindings │ ├── Cmd.hs │ ├── Definitions.hs -- Basic definitions, pre-defined variables and functions etc. │ ├── Expr.hs │ ├── Function.hs -- Tried to formalize the notion of a function of multiple arguments without currying. │ └── Statement.hs ├── Main.hs └── Model └── Arithmetic.hs -- A calculator model built using Text.ParserCombinators.Parsec.Expr (for model testing) I'm trying to have a library because I want to use a lot of the functionality provided by calculator in plot-lab [2], which is a plotting application. I want to have a stable API, because I don't want to irresponsibly keep changing it according to my personal needs. [1] : https://github.com/sumitsahrawat/calculator [2] : https://github.com/sumitsahrawat/plot-lab -- Regards Sumit Sahrawat

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ANN: ZuriHac 2015
by Gleb Peregud 14 Feb '15

14 Feb '15

We am glad to announce ZuriHac 2015, a annual three-day Haskell hackathon in Zurich. This year it's sponsored by Google. When: May 29th - May 31st Where: Google office, Zurich, Switzerland ZuriHac is an international Haskell hackathon: a grassroots, collaborative coding festival with a simple focus, to build and improve Haskell libraries, tools, and infrastructure. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools, or even start new ones! I am happy to announce that Edward Kmett will give a talk. We are still in progress of nailing down the details, they will be available at the wiki page: http://www.haskell.org/haskellwiki/ZuriHac2015 Registration is not yet open, we will send an additional announcement email when it's open. We look forward to seeing you there! -- The organisers of ZuriHac 2015 Gleb Peregud Simon Meier Johan Tibell Alexander Bernauer

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Changes to Haskell Weekly News: In with the Old, In with the New
by Kim-Ee Yeoh 14 Feb '15

14 Feb '15

Dear Reader, A few days ago Daniel Santa Cruz stepped down from the helm of Haskell Weekly News. With others that read HWN regularly, I thank him for his long service and wish him and his family godspeed. Did you know John Goerzen started HWN in August 2, 2005? Now a week from today, February 21, 2015, Simon and Simon shall close your window of opportunity to opine on drastic changes to the prelude. By sheer coincidence, the very first issue of HWN links to a thread on Updating the Haskell Standard. John himself started the thread. Today the talk is about the Foldable and Traversable type classes. Back then it was about Overlapping and Undecidable instances, the FFI, and a Hierarchical namespace. But these are all features we take for granted today. What sweeping changes took hold over the last ten years. There was no hackage. Only in September 2005 did Isaac Jones and David Himmelstrup announce beta-testing of a cabal prototype. Imagine life without hackage. Today there are 7664 packages all ready to go. So it is with this ladened awareness that I have volunteered and been accepted to the open vacancy at HWN. I seek to alleviate a sorely felt void in the Haskell diaspora by elucidating the newsworthy. The first decision I made might catch you by surprise, so let me explain. You see, Semen Trygubenko has also stepped up to contribute to HWN by continuing the leaderboard format pioneered by Daniel. I proposed and he agreed to alternate weeks between HWN Leaderboard and HWN Light. I'm excited by this dual format. Lower frequency means longer wavelength, which also means looking at events from a wider angle. At the same time the dual format affords continuity from past editions that a readership has grown accustomed to. Even though some readers will be mortified by the loss in uniformity, I trust you won't be among them. I take full responsibility for any untowardness caused by this to-ing and fro-ing, including innocent kittens mysteriously vanishing into the night. The Light edition will come out this Tuesday; the Leaderboard edition next. Thank you for reading Haskell Weekly News. >From the desk of Poobah of HWN, Kim-Ee Yeoh

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Rationale for two separate map functions in ClassyPrelude
by Clinton Mead 14 Feb '15

14 Feb '15

ClassyPrelude has two map functions, namely: 1. "map" 2. "omap" "map" works on any Functor. However, things like "Text" are not functors as they aren't generic containers. As can be seen in the following code: module Main where import Prelude () import ClassyPrelude import qualified Data.Text as T import Data.Char as C main = do let l = [1,2,3] :: [Int] let t = (T.pack "Hello") let m = Just 5 print $ map (*2) l print $ map (*2) m print $ omap C.toUpper t return () Notice one has to use "omap" to deal with the Text. The thing is, I found it trivially easy to get "map" to work for both calls. Here's the code: {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} module Main where import Prelude hiding (map) import qualified Data.Text as T import Data.Char as C import Control.Monad (Functor) class CanMap a b where type Element a :: * type Container a b :: * map :: (Element a -> b) -> a -> Container a b instance (Functor f) => CanMap (f a) b where type Element (f a) = a type Container (f a) b = f b map = fmap instance CanMap T.Text Char where type Element T.Text = Char type Container T.Text Char = T.Text map = T.map main = do let l = [1,2,3] :: [Int] let m = Just 5 let t = (T.pack "Hello") print $ map (*2) l print $ map (*2) m print $ map C.toUpper t return () All that's required is to add instances to CanMap for any monomorphic containers. ClassyPrelude already does this anyway with "omap" in the Data.MonoTraversable module. I suspect however there's a good reason I'm missing about why there should be two separate map functions to deal with these alternate situations, but I'm wondering what that is.

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Why doesn't TH expose its parser?
by Christopher Done 13 Feb '15

13 Feb '15

What's the reason that there isn't a parseExp :: String -> Q Exp, parseDecl, etc.? It's a pretty common use case to write some syntactic extension that merely extends regular Haskell. We end up with the haskell-src-meta package which doesn't support full GHC Haskell and doesn't know how to parse infix applications. It doesn't give a strong story for quasi-quotes to be useful for extensions to Haskell's syntax itself, as opposed to just mini-DSLs. Here's a case in point: idiom brackets. You'd like to implement them like this: [i|foo bar mu|] → foo <$> bar <*> mu You want it to be full Haskell inside, so foo could easilly be a case or a let, etc., so a trivial way to do that would be to just parse the string "foo bar mu" with parseExp and then do the above simple transformation. Job done. In practice, we can't do this. Our best option is HSE, but it's not ideal. Another way to do this is: $(idiom [|foo bar mu|]) And because [|…|] produces a Q Exp, you can just go ahead and do the rewrite on the resulting Exp. Yay, we now have full GHC Haskell inside the idiom bracket, but we've lost the syntactic convenience. Given that [|…|] already does the String → Q Exp step, why not allow us to re-use this functionality in quasi quotes? Ciao!

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Complex Event Processing in Haskell
by Tom Ellis 13 Feb '15

13 Feb '15

I'm looking for examples of production-capable Complex Event Processing systems in Haskell along the lines of Esper: http://esper.codehaus.org/ I know of Nettle (and McNettle and Procera) from Yale, and it claims to have excellent performance. http://haskell.cs.yale.edu/nettle/ Is there anything else? Is there anything in another functional language I should look at? I know FRP systems in general deal with processing events, but I am looking specifically for things similar to Esper. Tom

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ICFP 2015: Final Call for Papers
by David Van Horn 13 Feb '15

13 Feb '15

===================================================================== 20th ACM SIGPLAN International Conference on Functional Programming ICFP 2015 Vancouver, Canada, August 31 - September 2, 2015 http://www.icfpconference.org/icfp2015 ===================================================================== Important Dates ~~~~~~~~~~~~~~~ Submissions due: Friday, February 27 2015, 23:59 UTC-11 https://icfp15.hotcrp.com/ Author response: Tuesday, April 21, 2015 through Thursday, 23 April, 2015 Notification: Friday, May 1, 2015 Final copy due: Friday, June 12, 2015 Scope ~~~~~ ICFP 2015 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Topics of interest include (but are not limited to): * Language Design: concurrency, parallelism, and distribution; modules; components and composition; metaprogramming; type systems; interoperability; domain-specific languages; and relations to imperative, object-oriented, or logic programming. * Implementation: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; garbage collection and memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources. * Software-Development Techniques: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling. * Foundations: formal semantics; lambda calculus; rewriting; type theory; monads; continuations; control; state; effects; program verification; dependent types. * Analysis and Transformation: control-flow; data-flow; abstract interpretation; partial evaluation; program calculation. * Applications: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia and 3D graphics programming; scripting; system administration; security. * Education: teaching introductory programming; parallel programming; mathematical proof; algebra. * Functional Pearls: elegant, instructive, and fun essays on functional programming. * Experience Reports: short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working. If you are concerned about the appropriateness of some topic, do not hesitate to contact the program chair. Abbreviated instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * By Friday, 27 February 2015, 23:59 UTC-11, submit a full paper of at most 12 pages (6 pages for an Experience Report) in standard ACM conference format, including bibliography, figures, and appendices. The deadlines will be strictly enforced and papers exceeding the page limits will be summarily rejected. * Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. * Each submission must adhere to SIGPLAN's republication policy, as explained on the web at http://www.sigplan.org/Resources/Policies/Republication * Authors of resubmitted (but previously rejected) papers have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the program chair will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no reviewer will read the annotated copies of the previous reviews. Overall, a submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are on the conference web site. Proceedings will be published by ACM Press. Authors of accepted submissions will have a choice of one of three ways to manage their publication rights. These choices are described at http://authors.acm.org/main.html Presentations will be videotaped and released online if the presenter consents. The proceedings will be freely available for download from the ACM Digital Library from one week before the start of the conference until two weeks after the conference. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. Papers must adhere to the standard ACM conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LaTeX is available at http://www.sigplan.org/Resources/Author/ Submission: Submissions will be accepted on the web using a link that will be posted at https://icfp15.hotcrp.com/ Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Author response: Authors will have a 72-hour period, starting at 0:00 UTC on Tuesday, 21 April 2015, to read reviews and respond to them. ACM Author-Izer is a unique service that enables ACM authors to generate and post links on either their home page or institutional repository for visitors to download the definitive version of their articles from the ACM Digital Library at no charge. Downloads through Author-Izer links are captured in official ACM statistics, improving the accuracy of usage and impact measurements. Consistently linking the definitive version of ACM article should reduce user confusion over article versioning. After your article has been published and assigned to your ACM Author Profile page, please visit http://www.acm.org/publications/acm-author-izer-service to learn how to create your links for free downloads from the ACM DL. Publication date: The official publication date of accepted papers is the date the proceedings are made available in the ACM Digital Library. This date may be up to two weeks prior to the first day of the conference. The official publication date affects the deadline for any patent filings related to published work. General Chair: Kathleen Fisher Tufts University (USA) Program Chair: John Reppy University of Chicago (USA) Program Committee: Amal Ahmed Northeastern University (USA) Jean-Philippe Bernardy Chalmers University of Technology (Sweden) Matthias Blume Google (USA) William Byrd University of Utah (USA) Andy Gill University of Kansas (USA) Neal Glew Google (USA) Fritz Henglein University of Copenhagen (Denmark) Gabriele Keller University of New South Wales and NICTA (Australia) Andrew Kennedy Microsoft Research Cambridge (UK) Neelakantan Krishnaswami Birmingham University (UK) Daan Leijen Microsoft Research Redmond (USA) Keiko Nakata FireEye Dresden (Germany) Mike Rainey INRIA Rocquencourt (France) Andreas Rossberg Google (Germany) Manuel Serrano INRIA Sophia Antipolis (France) Simon Thompson University of Kent (UK) David Van Horn University of Maryland (USA) Stephanie Weirich University of Pennsylvania (USA)

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[TFP 2015] 2nd call for papers
by Peter Achten 13 Feb '15

13 Feb '15

----------------------------- S E C O N D C A L L F O R P A P E R S ----------------------------- ======== TFP 2015 =========== 16th Symposium on Trends in Functional Programming June 3-5, 2015 Inria Sophia Antipolis, France http://tfp2015.inria.fr/ The symposium on Trends in Functional Programming (TFP) is an international forum for researchers with interests in all aspects of functional programming, taking a broad view of current and future trends in the area. It aspires to be a lively environment for presenting the latest research results, and other contributions (see below). Authors of draft papers will be invited to submit revised papers based on the feedback receive at the symposium. A post-symposium refereeing process will then select a subset of these articles for formal publication. The selected revised papers will be published as a Springer Lecture Notes in Computer Science (www.springer.com/lncs) volume. TFP 2015 will be the main event of a pair of functional programming events. TFP 2015 will be accompanied by the International Workshop on Trends in Functional Programming in Education (TFPIE), which will take place on June 2nd. The TFP symposium is the heir of the successful series of Scottish Functional Programming Workshops. Previous TFP symposia were held in * Edinburgh (Scotland) in 2003; * Munich (Germany) in 2004; * Tallinn (Estonia) in 2005; * Nottingham (UK) in 2006; * New York (USA) in 2007; * Nijmegen (The Netherlands) in 2008; * Komarno (Slovakia) in 2009; * Oklahoma (USA) in 2010; * Madrid (Spain) in 2011; * St. Andrews (UK) in 2012; * Provo (Utah, USA) in 2013; * and in Soesterberg (The Netherlands) in 2014. For further general information about TFP please see the TFP homepage. (http://www.tifp.org/) == INVITED SPEAKER == TFP is pleased to announce a talk by the following invited speaker: * Laurence Rideau is a researcher at INRIA and is interested in the semantics of programming languages , the formal methods, and the verification tools for programs and mathematical proofs. She participated in the beginnings of the Compcert project (certified compiler), and is part of the Component Mathematical team in the MSR-INRIA joint laboratory, who performed the formalization of the Feit-Thompson theorem successfully. Thirty years ago, computers barged in mathematics with the famous proof of the Four Color Theorem. Initially limited to simple calculation, their role is now expanding to the reasoning whose complexity is beyond the capabilities of most humans, as the proof of the classification of finite simple groups. We present our large collaborative adventure around the formalization of the Feit-Thompson theorem (http://en.wikipedia.org/wiki/Feit%E2%80%93Thompson_theorem) that is a first step to the classification of finite groups and that uses a palette of methods and techniques that range from formal logic to software (and mathematics) engineering. == SCOPE == The symposium recognizes that new trends may arise through various routes. As part of the Symposium's focus on trends we therefore identify the following five article categories. High-quality articles are solicited in any of these categories: Research Articles: leading-edge, previously unpublished research work Position Articles: on what new trends should or should not be Project Articles: descriptions of recently started new projects Evaluation Articles: what lessons can be drawn from a finished project Overview Articles: summarizing work with respect to a trendy subject Articles must be original and not simultaneously submitted for publication to any other forum. They may consider any aspect of functional programming: theoretical, implementation-oriented, or experience-oriented. Applications of functional programming techniques to other languages are also within the scope of the symposium. Topics suitable for the symposium include: Functional programming and multicore/manycore computing Functional programming in the cloud High performance functional computing Extra-functional (behavioural) properties of functional programs Dependently typed functional programming Validation and verification of functional programs Debugging and profiling for functional languages Functional programming in different application areas: security, mobility, telecommunications applications, embedded systems, global computing, grids, etc. Interoperability with imperative programming languages Novel memory management techniques Program analysis and transformation techniques Empirical performance studies Abstract/virtual machines and compilers for functional languages (Embedded) domain specific languages New implementation strategies Any new emerging trend in the functional programming area If you are in doubt on whether your article is within the scope of TFP, please contact the TFP 2015 program chair, Manuel Serrano. == BEST PAPER AWARDS == To reward excellent contributions, TFP awards a prize for the best paper accepted for the formal proceedings. TFP traditionally pays special attention to research students, acknowledging that students are almost by definition part of new subject trends. A student paper is one for which the authors state that the paper is mainly the work of students, the students are listed as first authors, and a student would present the paper. A prize for the best student paper is awarded each year. In both cases, it is the PC of TFP that awards the prize. In case the best paper happens to be a student paper, that paper will then receive both prizes. == SPONSORS == TFP is financially supported by <to be announced> == PAPER SUBMISSIONS == Acceptance of articles for presentation at the symposium is based on a lightweight peer review process of extended abstracts (4 to 10 pages in length) or full papers (20 pages). The submission must clearly indicate which category it belongs to: research, position, project, evaluation, or overview paper. It should also indicate which authors are research students, and whether the main author(s) are students. A draft paper for which ALL authors are students will receive additional feedback by one of the PC members shortly after the symposium has taken place. We use EasyChair for the refereeing process. Papers must be submitted at: https://easychair.org/conferences/?conf=tfp2015 Papers must be written in English, and written using the LNCS style. For more information about formatting please consult the Springer LNCS web site: http://www.springer.com/computer/lncs?SGWID=0-164-6-793341-0 == IMPORTANT DATES == Submission of draft papers: March 17, 2015 Notification: March 24, 2015 Registration: April 7, 2015 TFP Symposium: June 3-5, 2015 Student papers feedback: June 9, 2015 Submission for formal review: July 1, 2015 Notification of acceptance: September 8, 2015 Camera ready paper: October 8, 2015 == PROGRAM COMMITTEE == Janis Voigtländer University of Bonn, DE Scott Owens University of Kent, UK Neil Sculthorpe Swansea University, UK Colin Runciman University of York, UK Manuel Serrano Inria (PC chair), FR Rinus Plasmeijer University of Nijmegen, NL Tomas Petricek University of Cambridge, UK Marco T. Morazan Seton Hall University, USA Wolfgang De Meuter Vrije Universiteit Brussel, BE Michel Mauny Ensta ParisTech, FR Sam Lindley The University of Edinburgh, UK Daan Leijen Microsoft, USA Jurriaan Hage Utrecht University, NL Andy Gill University of Kansas, USA Thomas Gazagnaire University of Cambrige, UK Lars-Ake Fredlund Universidad Politécnica de Madrid, ES Jean-Christophe Filliatre Université Paris Sud Orsay, FR Marc Feeley Université de Montréal, CA Olaf Chitil University of Kent, UK Edwin Brady University of St Andrews, UK

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example
by Закиров Марат 13 Feb '15

13 Feb '15

Hi all, Algorithms on graphs in my opinion is something bad for parallel execution (especially such of them which have O(n) comlexity). So if I haskell give me an advantage in some graph application it will be great benchmark. Recently I tried to think about well-known by compiler people - so called "topologic-sort" or reverse post order numeration, but I gave up... There are just nothing to execute in parallel. Currently I am searching for graph (compiler) algorithm example written in haskell with persistent data structures which can be executed in parallel. And in the same time this example written in imperative style must not have good parallel execution form. Summarizing: I want to find example of data_persistence&functional style killer graph (compiler) application. Do you know such an example? -- *Regards, Marat.* *С уважением Марат.*

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A Question about the IO type detection
by Zongzhe Yuan 13 Feb '15

13 Feb '15

Hi i meet a problem when i was using readLn to read a int from user my function is like that do (some thing else that is working) n <- readLn (some thing else) f n x1 x2 x3 …… f has the type of Int -> something else because the type in this function is fixed, if the type of n is not Int, it will have exception However i want to detect whether the user input is valid or not and i have this function import Data.Typeable isInteger :: Typeable a => a -> Bool isInteger n = (typeOf n) == (typeOf (1::Int)) and add to do (some thing else that is working) n <- readLn if isInteger n then (some thing else) f n x1 x2 x3 …… else (something else) but it doesn’t work because of the reason “type has been inferred and fixed” How could i solve this problem? Zongzhe Yuan This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Anonymous FFI calls
by Francesco Mazzoli 12 Feb '15

12 Feb '15

Hi, I am in a situation where it would be very useful to call C functions without an explicit FFI import. For example, I'd like to be able to do (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 instead of declaring the foreign import explicitely at the top level. Is there a way to do this or to achieve similar results in some other way? If not, I imagine it would be easy to implement such a facility in GHC, given that the code implementing calling to C functions must already be present to implement "proper" FFI imports. I think such an addition would be useful in many cases. Thanks, Francesco

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GHC Hackaton (GHC internals) slides
by Maurizio Vitale 12 Feb '15

12 Feb '15

Anybody knows if the slides (or higher quality video) for the GHC Hackaton are available? I'm not sure which year, this video has been posted in 2012: https://www.youtube.com/watch?v=lw7kbUvAmK4&list=PLBkRCigjPwyeCSD_DFxpd246Y… Thanks! Maurizio

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Re: [Haskell-cafe] Anonymous FFI calls
by Michael Sloan 12 Feb '15

12 Feb '15

It works, cool! So at least this ought to address Francesco's use-case. Is there anything wrong with my code? It definitely looks like a GHC bug. Yes, I've successfully generated instance declarations from the top-level. I may as well describe the usecase: pretty printing a value of any type in GHCI. While this feature list isn't the main point here, it'd be very cool to do things like: * Look up the value of mutable references such as IORefs * Print out functions as just their type * Be able to evaluate particular subtrees of the pretty print output (like present [1]) * Select a subportion of the output and get a variable out of it Usage would look like "$(pretty 'x)". This would reify the type of x and transitively generate `Pretty` instances for things, by using my th-reify-many package [2]. It then generates an expression which invokes `pretty` on `x`, using these new instances. This instance generation can do clever things like check if a type has a "Data" instance, and use it if available. In this way, we can construct a pretty printer for ghci which will work for any type. Certainly, this is still possible without instance generation, but it'd be quite clever to not need to re-generate the pretty printing code for every invocation. -Michael [1] https://github.com/chrisdone/present [2] https://github.com/mgsloan/th-reify-many On Wed, Feb 11, 2015 at 6:51 PM, Geoffrey Mainland <mainland(a)cs.drexel.edu> wrote: > Hi Michael, > > Try this out: > > https://gist.github.com/mainland/f18cf3827c4b0b64b835 > > Have you had success generating instance declarations from top-level > splices? > > Geoff > > On 02/11/2015 07:41 PM, Michael Sloan wrote: >> Hi! >> >> I don't mean to be accusatory with the whole documentation thing - I >> totally understand that most folks have a lot going on! In this >> particular case, I got very excited when I initially saw addTopDecls, >> because it says it allows you to "Add additional top-level >> declarations". After implementing something to use it, I got rather >> disappointed as it couldn't add instances. >> >> So, when I say "not implemented", it's just that the docs say it >> allows you to add additional top-level decls, when you can really only >> add functions, variable bindings, and foreign import decls. >> >> I didn't realize that not re-exporting these functions is due to their >> experimental status. That makes sense! I'm glad it was a conscious >> decision. I just tried writing a demo of addTopDecls, but >> unfortunately using it to add a function and referencing this function >> causes a GHC internal error: >> https://gist.github.com/mgsloan/53d7fa50338c696e5c80 . I haven't >> tried it with a foreign import yet. >> >> -Michael >> >> On Wed, Feb 11, 2015 at 3:51 PM, Geoffrey Mainland >> <mainland(a)cs.drexel.edu> wrote: >>> Typed Template Haskell happened at the end of my tenure at MSR, and >>> there was a mad rush to get it in to the compiler in time for 7.8.3 at >>> the same time I was starting a new job. I'm afraid the documentation is >>> indeed wanting, but the alternative was no typed Template Haskell. >>> >>> One side-effect was that the functions you note as present only in >>> Language.Haskell.TH.Syntax were not thoroughly vetted, so we didn't >>> re-export them from Language.Haskell.TH. >>> >>> I am willing to help with documentation if people want to use this >>> functionality. Has anyone attempted to use addTopDecls? Michael, when >>> you say "it's not implemented," what do you mean? What, exactly, is not >>> implemented? >>> >>> Cheers, >>> Geoff >>> >>> On 2/11/15 6:32 PM, Michael Sloan wrote: >>>> I'd love for the implementation to be as powerful as the documentation >>>> suggests it is :D >>>> >>>> However, yes, in the meantime, fixing the documentation would be great! >>>> >>>> Also, I meant to say that addTopDecls is only exported by >>>> "Language.Haskell.TH.Syntax". While this is a digression, there are a >>>> few other handy functions that are oddly left out of >>>> "Language.Haskell.TH": addDependentFile, addModFinalizer, and possibly >>>> more. >>>> >>>> -Michael >>>> >>>> On Wed, Feb 11, 2015 at 3:25 PM, Simon Peyton Jones >>>> <simonpj(a)microsoft.com> wrote: >>>>> I would LOVE someone to improve the documentation for addTopDecls. Manuel Chakravarty and Geoff Mainland were responsible for the implementation. >>>>> >>>>> Simon >>>>> >>>>> | -----Original Message----- >>>>> | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Michael >>>>> | Sloan >>>>> | Sent: 11 February 2015 23:19 >>>>> | To: Francesco Mazzoli >>>>> | Cc: ghc-devs(a)haskell.org; haskell >>>>> | Subject: Re: [Haskell-cafe] Anonymous FFI calls >>>>> | >>>>> | It seems like addTopDecls[1] will able to help here. Unfortunately, >>>>> | the function is not well documented and not very discoverable because >>>>> | it's only exported by Language.Haskell.TH. >>>>> | >>>>> | The documentation doesn't mention that it can only be used to create >>>>> | new top level functions and FFI imports[2]. I think that adding FFI >>>>> | imports was the main motivation for implementing it. In the past >>>>> | I've wanted to generate instances via this function, but unfortunately >>>>> | it's not implemented.. >>>>> | >>>>> | Hope that helps! >>>>> | -Michael >>>>> | >>>>> | [1] http://hackage.haskell.org/package/template-haskell- >>>>> | 2.9.0.0/docs/Language-Haskell-TH-Syntax.html#v:addTopDecls >>>>> | >>>>> | [2] >>>>> | https://github.com/ghc/ghc/blob/1d982ba10f590828b78eba992e73315dee33f78a/ >>>>> | compiler/typecheck/TcSplice.hs#L818 >>>>> | >>>>> | On Wed, Feb 11, 2015 at 2:26 AM, Francesco Mazzoli <f(a)mazzo.li> wrote: >>>>> | > Hi, >>>>> | > >>>>> | > I am in a situation where it would be very useful to call C functions >>>>> | > without an explicit FFI import. For example, I'd like to be able to do >>>>> | > >>>>> | > (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 >>>>> | > >>>>> | > instead of declaring the foreign import explicitely at the top level. >>>>> | > >>>>> | > Is there a way to do this or to achieve similar results in some other >>>>> | > way? >>>>> | > >>>>> | > If not, I imagine it would be easy to implement such a facility in GHC, >>>>> | > given that the code implementing calling to C functions must already be >>>>> | > present to implement "proper" FFI imports. I think such an addition >>>>> | > would be useful in many cases. >>>>> | > >>>>> | > Thanks, >>>>> | > Francesco >>>>> | > _______________________________________________ >>>>> | > Haskell-Cafe mailing list >>>>> | > Haskell-Cafe(a)haskell.org >>>>> | > http://www.haskell.org/mailman/listinfo/haskell-cafe >>>>> | _______________________________________________ >>>>> | ghc-devs mailing list >>>>> | ghc-devs(a)haskell.org >>>>> | http://www.haskell.org/mailman/listinfo/ghc-devs >

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Re: [Haskell-cafe] Anonymous FFI calls
by Michael Sloan 12 Feb '15

12 Feb '15

Hi! I don't mean to be accusatory with the whole documentation thing - I totally understand that most folks have a lot going on! In this particular case, I got very excited when I initially saw addTopDecls, because it says it allows you to "Add additional top-level declarations". After implementing something to use it, I got rather disappointed as it couldn't add instances. So, when I say "not implemented", it's just that the docs say it allows you to add additional top-level decls, when you can really only add functions, variable bindings, and foreign import decls. I didn't realize that not re-exporting these functions is due to their experimental status. That makes sense! I'm glad it was a conscious decision. I just tried writing a demo of addTopDecls, but unfortunately using it to add a function and referencing this function causes a GHC internal error: https://gist.github.com/mgsloan/53d7fa50338c696e5c80 . I haven't tried it with a foreign import yet. -Michael On Wed, Feb 11, 2015 at 3:51 PM, Geoffrey Mainland <mainland(a)cs.drexel.edu> wrote: > Typed Template Haskell happened at the end of my tenure at MSR, and > there was a mad rush to get it in to the compiler in time for 7.8.3 at > the same time I was starting a new job. I'm afraid the documentation is > indeed wanting, but the alternative was no typed Template Haskell. > > One side-effect was that the functions you note as present only in > Language.Haskell.TH.Syntax were not thoroughly vetted, so we didn't > re-export them from Language.Haskell.TH. > > I am willing to help with documentation if people want to use this > functionality. Has anyone attempted to use addTopDecls? Michael, when > you say "it's not implemented," what do you mean? What, exactly, is not > implemented? > > Cheers, > Geoff > > On 2/11/15 6:32 PM, Michael Sloan wrote: >> I'd love for the implementation to be as powerful as the documentation >> suggests it is :D >> >> However, yes, in the meantime, fixing the documentation would be great! >> >> Also, I meant to say that addTopDecls is only exported by >> "Language.Haskell.TH.Syntax". While this is a digression, there are a >> few other handy functions that are oddly left out of >> "Language.Haskell.TH": addDependentFile, addModFinalizer, and possibly >> more. >> >> -Michael >> >> On Wed, Feb 11, 2015 at 3:25 PM, Simon Peyton Jones >> <simonpj(a)microsoft.com> wrote: >>> I would LOVE someone to improve the documentation for addTopDecls. Manuel Chakravarty and Geoff Mainland were responsible for the implementation. >>> >>> Simon >>> >>> | -----Original Message----- >>> | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Michael >>> | Sloan >>> | Sent: 11 February 2015 23:19 >>> | To: Francesco Mazzoli >>> | Cc: ghc-devs(a)haskell.org; haskell >>> | Subject: Re: [Haskell-cafe] Anonymous FFI calls >>> | >>> | It seems like addTopDecls[1] will able to help here. Unfortunately, >>> | the function is not well documented and not very discoverable because >>> | it's only exported by Language.Haskell.TH. >>> | >>> | The documentation doesn't mention that it can only be used to create >>> | new top level functions and FFI imports[2]. I think that adding FFI >>> | imports was the main motivation for implementing it. In the past >>> | I've wanted to generate instances via this function, but unfortunately >>> | it's not implemented.. >>> | >>> | Hope that helps! >>> | -Michael >>> | >>> | [1] http://hackage.haskell.org/package/template-haskell- >>> | 2.9.0.0/docs/Language-Haskell-TH-Syntax.html#v:addTopDecls >>> | >>> | [2] >>> | https://github.com/ghc/ghc/blob/1d982ba10f590828b78eba992e73315dee33f78a/ >>> | compiler/typecheck/TcSplice.hs#L818 >>> | >>> | On Wed, Feb 11, 2015 at 2:26 AM, Francesco Mazzoli <f(a)mazzo.li> wrote: >>> | > Hi, >>> | > >>> | > I am in a situation where it would be very useful to call C functions >>> | > without an explicit FFI import. For example, I'd like to be able to do >>> | > >>> | > (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 >>> | > >>> | > instead of declaring the foreign import explicitely at the top level. >>> | > >>> | > Is there a way to do this or to achieve similar results in some other >>> | > way? >>> | > >>> | > If not, I imagine it would be easy to implement such a facility in GHC, >>> | > given that the code implementing calling to C functions must already be >>> | > present to implement "proper" FFI imports. I think such an addition >>> | > would be useful in many cases. >>> | > >>> | > Thanks, >>> | > Francesco >>> | > _______________________________________________ >>> | > Haskell-Cafe mailing list >>> | > Haskell-Cafe(a)haskell.org >>> | > http://www.haskell.org/mailman/listinfo/haskell-cafe >>> | _______________________________________________ >>> | ghc-devs mailing list >>> | ghc-devs(a)haskell.org >>> | http://www.haskell.org/mailman/listinfo/ghc-devs >

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Applying for Haskell opportunities at Facebook
by Zongzhe Yuan 11 Feb '15

11 Feb '15

Hi I am a student in the University of Nottingham in year 2 and I am quite interested in Haskell. May I apply for the summer internship in this summer in London? I have learnt Haskell during the module Introduction to Functional Programming last Year and have the highest mark among the student from Ningbo campus. I could solve countdown problem and sudoku right now. I know Monad and know the basic idea about that. This semester I take the module Advanced Functional Programming taught by professor Graham Hutton. By the end of this semester, i will be good at monad using and learn more about the efficiency about the programming. Apart from Haskell, I also good at algorithm and Java programming. I deal the entire image processing procedure algorithm with a candy detect application. https://github.com/ZongzheYuan/ImageProcessing.git Here is my code and i am improving it continuously. I really looking forward to working at FaceBook and especially with Haskell. If i could have this opportunity, it will become my pleasure. Greeting Zongzhe Yuan 11/2/2015 This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Applying for Haskell opportunities at Facebook
by Zongzhe Yuan 11 Feb '15

11 Feb '15

Hi I am a student in the University of Nottingham in year 2 and I am quite interested in Haskell. May I apply for the summer internship in this summer in London? I have learnt Haskell during the module Introduction to Functional Programming last Year and have the highest mark among the student from Ningbo campus. I could solve countdown problem and sudoku right now. I know Monad and know the basic idea about that. This semester I take the module Advanced Functional Programming taught by professor Graham Hutton. By the end of this semester, i will be good at monad using and learn more about the efficiency about the programming. Apart from Haskell, I also good at algorithm and Java programming. I deal the entire image processing procedure algorithm with a candy detect application. https://github.com/ZongzheYuan/ImageProcessing.git Here is my code and i am improving it continuously. I really looking forward to working at FaceBook and especially with Haskell. If i could have this opportunity, it will become my pleasure. Greeting Zongzhe Yuan 11/2/2015 This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Applying for Haskell opportunities at Facebook
by Zongzhe Yuan 11 Feb '15

11 Feb '15

Hi I am a student in the University of Nottingham in year 2 and I am quite interested in Haskell. May I apply for the summer internship in this summer in London? I have learnt Haskell during the module Introduction to Functional Programming last Year and have the highest mark among the student from Ningbo campus. I could solve countdown problem and sudoku right now. I know Monad and know the basic idea about that. This semester I take the module Advanced Functional Programming taught by professor Graham Hutton. By the end of this semester, i will be good at monad using and learn more about the efficiency about the programming. Apart from Haskell, I also good at algorithm and Java programming. I deal the entire image processing procedure algorithm with a candy detect application. https://github.com/ZongzheYuan/ImageProcessing.git Here is my code and i am improving it continuously. I really looking forward to working at FaceBook and especially with Haskell. If i could have this opportunity, it will become my pleasure. Greeting Zongzhe Yuan 11/2/2015 This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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svgcairo/ghc vis cabal install error..
by madhub 11 Feb '15

11 Feb '15

Hi !, I tried installing svgcairo ( for installing ghc vis ). I am getting following error. Could you please help me out install ghc vis. Thanks in advance Madhu — environment details. I am on mac yosomite with gcc provided by OS X. Following is the gcc —version 22:54:34:~$ gcc --version Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1 Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn) Target: x86_64-apple-darwin14.1.0 Thread model: posix export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/opt/X11/lib/pkgconfig export LDFLAGS=-L/usr/local/lib export CFLAGS=-I/usr/local/include export LIBS=-L/usr/local/lib svgcairo error =============== Configuring svgcairo-0.13.0.1... Building svgcairo-0.13.0.1... Preprocessing library svgcairo-0.13.0.1... gtk2hsC2hs: Error in C header file. /usr/include/sys/qos.h:124: (column 34) [FATAL] >>> Syntax error! The symbol `__attribute__' does not fit here. cabal: Error: some packages failed to install: svgcairo-0.13.0.1 failed during the building phase. The exception was: ExitFailure 1

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Re: [Haskell-cafe] [Haskell] Rank-N types with (.) composition
by Tyson Whitehead 11 Feb '15

11 Feb '15

On 11/02/15 04:39 AM, Simon Peyton Jones wrote: > ($) has its own *typing rule*; it does not have a special type. It's very ad hoc, but ($) is used so much to decrease parens that (e1 $ e2) is almost special syntax! I do like the idea of being able to end a function application with a lambda, let, if, case, or do without having to use $ though. Would something like the following grammar rules be problematic for parsing? infixexp→ lexp qop infixexp | - infixexp | [fexp] lexp (*) | aexp (*) lexp → \ apat1 … apatn -> exp | let decls in exp | if exp [;] then exp [;] else exp | case exp of { alts } | do { stmts } (*) fexp → [fexp] aexp aexp → qvar | gcon | literal | ( exp ) | ( exp1 , … , expk ) | [ exp1 , … , expk ] | [ exp1 [, exp2] .. [exp3] ] | [ exp | qual1 , … , qualn ] | ( infixexp qop ) | ( qop⟨-⟩ infixexp ) | qcon { fbind1 , … , fbindn } | aexp⟨qcon⟩ { fbind1 , … , fbindn } https://www.haskell.org/onlinereport/haskell2010/haskellch3.html were I've added (*) to where I have added or removed lines (basically I've moved function application from lexp to infixexp in order to allow the last exp in a function application to be an lexp). > At the moment the *only* robust way to pass a polymorphic function to a polymorphic function (here, you are passing Wrap to (.)) is to wrap it in a newtype, much as Wrap does. Good to know. Thanks! -Tyson

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Regex-applicative and Data.Text
by Konstantine Rybnikov 11 Feb '15

11 Feb '15

I just tried some regex-applicative and it's amazing! Very nice library, thanks Roman! However, I can't figure out the best way to work with Data.Text.Text instead of String. The token would be Text, I guess, but then it breaks in composition, since type of `few anySym` would now return `[Text]`, not `Text`. Am I understanding this correctly that intention is to in issue #8? [0] Or is there a clever way to work with them today? Example code: {-# LANGUAGE OverloadedStrings #-} import qualified Data.Text as T import Data.Text (Text) import Text.Regex.Applicative main = do let input = "foo:\n--- blablabla\ttheend" let r1 = sym "foo:\n" *> sym "--- " *> few anySym <* sym "\t" <* few anySym :: RE Text Text putStrLn (show (input =~ r1)) Error is something like (this is an error for a bit different code, but should be very similar): Main.hs:14:40: Couldn't match type ‘[Text]’ with ‘Text’ Expected type: RE Text Text Actual type: RE Text [Text] In the second argument of ‘(*>)’, namely ‘few anySym’ In the first argument of ‘(<*)’, namely ‘few anySym *> sym "Actual stderr output differs from expected:" *> sym "--- " *> few anySym’ Thanks! [0]: https://github.com/feuerbach/regex-applicative/issues/8

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SmallCheck Depth
by Omari Norman 11 Feb '15

11 Feb '15

The original SmallCheck paper [0] gives a type data Name = P | Q | R and states that all values of type Name have depth 0. To me this suggests that all values of type Bool would have depth 0 as well, seeing as Bool would simply be data Bool = True | False Sure enough, if I load up ghci with SmallCheck 0.2.1, I get > series 0 :: [Bool] [True, False] But in QuickCheck 1.1.1 I get > list 0 series :: [Bool] [] Very puzzling to me is that it seems the definition of series for Bool has not changed in the new version. Both versions have something like series = cons0 True \/ cons0 False So what explains the different behavior? Thanks. --Omari [0] http://www.cs.york.ac.uk/fp/smallcheck/smallcheck.pdf

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Re: [Haskell-cafe] [Haskell] Rank-N types with (.) composition
by Henning Thielemann 11 Feb '15

11 Feb '15

On Wed, 11 Feb 2015, Simon Peyton Jones wrote: > ($) has its own *typing rule*; it does not have a special type. It's > very ad hoc, but ($) is used so much to decrease parens that (e1 $ e2) > is almost special syntax! I already wondered why runST $ do foo bar worked in some versions of GHC and not in others. I guess it is better to tell the programmer the truth, i.e. remove the special treatment of ($). If you save a programmer from wondering about why "runST $ do ... " does not work with the special typing rule for ($), the programmer will later wonder why (.) does not work or why his privately defined function application operator (e.g. for reversed order of argument and function) will not work. I think that I stay with "runST (do ...)" in order to be compatible with several GHC versions.

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Re: [Haskell-cafe] [Haskell] Rank-N types with (.) composition
by Henning Thielemann 11 Feb '15

11 Feb '15

On Tue, 10 Feb 2015, Tyson Whitehead wrote: > I came across something that seems a bit strange to me. Here is a > simplified version (the original was trying to move from a lens > ReifiedFold to a lens-action ReifiedMonadicFold) You are on Haskell(a)haskell.org here. Could you please move to haskell-cafe?

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stuck at the first part of chapter 1 of the CIS course
by Roelof Wobben 11 Feb '15

11 Feb '15

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Using Haskell types in a database
by Nikita Karetnikov 11 Feb '15

11 Feb '15

Suppose I need to represent a user in a database. I could make a table like this one (using Yesod's config/models syntax): User name Text email Text Maybe is_verified Bool default=false last_login UTCTime default=now() Instead, it would be better to do the following, so I couldn't swap name and email by accident or feed a wrong value to is_verified and last_login: User name UserName email UserEmail v Maybe last_login UserLastLogin where newtype UserName = UserName Text deriving (Show, Read) derivePersistField "UserName" newtype UserLastLogin = UserLastLogin UTCTime deriving (Show, Read) derivePersistField "UserLastLogin" data Verified data Unverified newtype UserEmail v = UserEmail Text deriving (Show, Read) derivePersistField "UserEmail" (So I could define functions like sendAnnouncement :: UserEmail Verified -> IO () sendVerification :: UserEmail Unverified -> IO ()) The Haskell part is not new and has been successfully used in the large (see http://code.haskell.org/~dons/talks/dons-google-2015-01-27.pdf, pp. 17--21), but I wonder whether it's reasonable in the database layer. The first table will be mapped to this in PostgreSQL: User name character varying not null email character varying is_verified boolean not null default false last_login timestamp with time zone not null default now() while the second will use character varying for everything. The only downsides of the latter approach I can think of: 1. You can no longer rely on the database to ensure (basic) type safety, so everything needs to be done in the Haskell layer. Otherwise, you will get runtime errors. 2. I haven't benchmarked this, but I assume that comparing, say, booleans should be faster than comparing varchars. So, is it worth it in practice? Do I need to worry about these issues? And are there any other problems?

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Injective type families for GHC
by Jan Stolarek 10 Feb '15

10 Feb '15

Haskellers, I am finishing work on adding injective type families to GHC. I know that in the past many people have asked for this feature. If you have use cases for injective type families I would appreciate if you could show them to me. My implementation has some restrictions and I want to see how severe these restrictions are from a practical point of view. Janek --- Politechnika Łódzka Treść tej wiadomości zawiera informacje przeznaczone tylko dla adresata. Jeżeli nie jesteście Państwo jej adresatem, bądź otrzymaliście ją przez pomyłkę prosimy o powiadomienie o tym nadawcy oraz trwałe jej usunięcie.

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Use of STM in business?
by Tom Ellis 10 Feb '15

10 Feb '15

>From associate of mine: "I am quite curious if anyone actually uses Haskell (esp STM) successfully in business." Of course I can anwser the general question about usage of Haskell in business, but I don't know much about STM specifically. Can anyone provide me with any references to where STM has been used in production in a business setting? Thanks, Tom

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