- Haskell-Cafe - Haskell.org

Transforming ASTs
by anakreon 15 May '15

15 May '15

This might be a duplicated message. The first time I posted it I had not subscribed to haskell cafe. Suppose the following data type for encoding Boolean expressions: data BExpr a = BTrue | BFalse | Id String | Not a | And a a | Or a a | BEq a a deriving (Functor) type Expr = Fix BExpr It is easy to produce a string representation of an expression or evaluate it: estr :: BExpr String -> String eval :: BExpr Bool -> Bool with the cata function from Data.Functor.Fixedpoint. Could you suggest a solution for transforming trees encoded as Exp into equivalent Expr (e.g Not Not a ~> a)? cata does not work since it expects a function f a -> a while a transformation would be f a -> f a.

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Can I specify the a in a phantom type to be limited to a sum type?
by Cody Goodman 15 May '15

15 May '15

How can I create Answers of type Gender, Race, or Age? These should be possible: λ> Answer Male λ> Answer White λ> Answer Black λ> Answer 28 Others such as using a string should not be possible: λ> Answer "a string" -- should throw type error {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE GADTs #-} {-# LANGUAGE StandaloneDeriving #-} module Tutorial where data Gender = Male | Female deriving (Show) data Race = White | Black deriving (Show) type Age = Int data Answer a where Answer :: Gender -> Answer Gender deriving instance Show (Answer w) -- λ> -- This is pretty useful and accepts Answers with the Gender -- λ> Answer Male -- Answer Male -- λ> :t Answer Male -- Answer Male :: Answer Gender -- λ> -- how do I also accept Race and Age? -- below I try to sort out how to use type families (unsuccessfully) before bed -- type family Answer :: * -- type instance Answer Gender = Answer Gender

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Fwd: Transforming ASTs
by Sergey Vinokurov 15 May '15

15 May '15

Hi anakreon, You should look at paramorphism and histomorphism recursion schemes. Paramorphism has type f (a, Fix f) -> a, and thus allows you to pattern match on the subtree that produced the result. But it's not enough to write simplification of (Not (Not x)) -> x because it'll have to access simplification result that is more than one level deep. But this recursion scheme would be helpful for simpler transformations like (And True x) -> x. As for original optimization of double negation I'd suggest to use histomorphism where you'll have access to all the expression subtrees annotated with intermediate results. E.g. {-# LANGUAGE DeriveFunctor #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE UndecidableInstances #-} data Cofree f a = a :< f (Cofree f a) deriving (Functor) strip :: (Functor f) => Cofree f a -> Fix f strip (_ :< x) = Fix $ fmap strip x newtype Fix f = Fix (f (Fix f)) deriving instance (Show (f (Fix f))) => Show (Fix f) unFix :: Fix f -> f (Fix f) unFix (Fix x) = x histo :: (Functor f) => (f (Cofree f a) -> a) -> Fix f -> a histo alg x = case histo' alg x of y :< _ -> y histo' :: (Functor f) => (f (Cofree f a) -> a) -> Fix f -> Cofree f a histo' alg = (\x -> alg x :< x) . fmap (histo' alg) . unFix simplify :: Fix BExpr -> Fix BExpr simplify = histo alg where alg :: BExpr (Cofree BExpr Expr) -> Expr alg (Not (_ :< (Not (x :< _)))) = x alg x = Fix $ fmap strip x main :: IO () main = do print $ simplify $ Fix (Not (Fix (Not (Fix (Not (Fix (And (Fix BTrue) (Fix BFalse)))))))) For more detailed exposition I'd suggest to look into https://github.com/willtim/recursion-schemes/raw/master/slides-final.pdf Sergey. On Fri, May 15, 2015 at 7:08 PM, anakreon <anakreonmejdi(a)gmail.com> wrote: > This might be a duplicated message. The first time I posted it I had not > subscribed to haskell cafe. > > Suppose the following data type for encoding Boolean expressions: > > data BExpr a = BTrue > | BFalse > | Id String > | Not a > | And a a > | Or a a > | BEq a a > deriving (Functor) > type Expr = Fix BExpr > > It is easy to produce a string representation of an expression or evaluate > it: > > estr :: BExpr String -> String > eval :: BExpr Bool -> Bool > > with the cata function from Data.Functor.Fixedpoint. > > Could you suggest a solution for transforming trees encoded as Exp into > equivalent Expr (e.g Not Not a ~> a)? > cata does not work since it expects a function f a -> a while a > transformation would be f a -> f a. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >

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Haskell Weekly News
by Kim-Ee Yeoh 15 May '15

15 May '15

*Top Picks:* - Is Servant <http://haskell-servant.github.io/>the most type-safe HTTP server library ever? Are the type signatures hard to read <https://news.ycombinator.com/item?id=9519782#up_9520750>? In addition to its utmost relevance as a web library, Servant is also an awesome case study in the type safety vs type readability trade-off spectrum, brought to you by Alp Mestanogullari and Julian Arni. HN <https://news.ycombinator.com/item?id=9519782> and /r/haskell <http://www.reddit.com/r/haskell/comments/35hiie/servant_04_released/> - Ozan Sener compiles Pandoc into JS via GHCJS and creates a web interface to it using the Reflex FRP library <https://github.com/ryantrinkle/reflex>. Markup.Rocks <https://github.com/osener/markup.rocks> is much loved on /r/haskell <http://www.reddit.com/r/haskell/comments/35ax22/ive_compiled_pandoc_with_gh…>. See also HN <https://news.ycombinator.com/item?id=9512293>. - Is Haskell a "Startup Secret Weapon"? Alexandr Kurilin reveals adoption challenges <https://github.com/commercialhaskell/commercialhaskell/blob/master/usage/fr…> at Front Row Education <https://www.crunchbase.com/organization/front-row>. Among them, slow build times. Also, "senior developers [that] get very frustrated when something wouldn't compile for hours and they couldn't find any help to move forward." Comments on HN <https://news.ycombinator.com/item?id=9519924> and /r/haskell <http://www.reddit.com/r/haskell/comments/35h8vh/how_front_row_education_use…> . - At Facebook, Bryan O'Sullivan debugs aeson's gigabyte space leak <http://www.serpentine.com/blog/2015/05/13/sometimes-the-old-ways-are-the-be…> on decoding a JSON megabyte of non-stop backslashes. Culprit? The streaming interface didn't match the use case. In place of streaming, Bryan now blasts bytes into a single big buffer, gaining 27x speed and 42x memory reduction. Comments on HN <https://news.ycombinator.com/item?id=9540389>, Proggit <http://www.reddit.com/r/programming/comments/35udjh/fixing_a_haskell_perfor…>, /r/haskell <http://www.reddit.com/r/haskell/comments/35udfy/fixing_an_aeson_performance…> . - Paul Chiusano's Unison programming platform <http://unisonweb.org/2015-05-07/about.html> hits the HN <https://news.ycombinator.com/item?id=9512955> and /r/haskell <http://www.reddit.com/r/haskell/comments/35cop9/unison_a_nextgeneration_pro…> headlines. Features include a browser-based UI that constrains edits to those that are well-typed. Also, DRY-ness up the wazoo: every type and term is uniquely identified by a hash a la Git. - Joey Hess reports <http://www.reddit.com/r/haskell/comments/35bw0b/at_last_debian_unstable_has…> that Debian unstable now has a working GHCi for ARM. The Template Haskell challenges have been surmounted. - Garrison Jensen blows the whistle <http://www.garrisonjensen.com/2015/05/13/haskell-programs-are-lies.html> on the impostor sieve on the front page of Haskell.org. In jest. A festive one-upmanship of fondly treasured code ensues on /r/haskell <http://www.reddit.com/r/haskell/comments/35vc31/the_real_way_to_generate_a_…>. And since bad publicity is better than no publicity, we owe kudos to Garrison. HN-worthy <https://news.ycombinator.com/item?id=9541478>. - Michael Snoyman decries use of ExceptT IO <https://www.fpcomplete.com/user/commercial/content/exceptions-best-practices> for exception handling because the user exception data type creates misleading expectations of comprehensiveness. The gotcha is that it doesn't cover IO exceptions! Furthermore, distinct exception types mean that the corresponding code can't compose. Instead? Use MonadThrow. /r/haskell <http://www.reddit.com/r/haskell/comments/35sk6w/best_practices_for_using_ex…> - JP Moresmau steps down as chief of EclipseFP <http://jpmoresmau.blogspot.com/2015/05/eclipsefp-end-of-life-from-me-at-lea…> and the companion Haskell packages BuildWrapper, ghc-pkg-lib, and scion-browser. Without anyone to take his place, the sun sets on EclipseFP. But the sun continues to shine on ide-backend (previously reported <http://haskell.1045720.n5.nabble.com/Haskell-Weekly-News-td5768107.html>), a GHC API wrapper akin to BuildWrapper. JP spitballs <http://haskell.1045720.n5.nabble.com/IDE-backend-Atom-editor-td5809240.html> on how he might move on to the Atom editor, jiggering it to use ide-backend-client. /r/haskell <http://www.reddit.com/r/haskell/comments/35y5v1/jp_moresmaus_blog_eclipsefp…> - Tatsuya Hirose translates Go By Example into Haskell. GBE <https://gobyexample.com/> comprises code samples annotated for an experienced programmer new to Go. For this first cut <http://lotz84.github.io/haskellbyexample/>, Tatsuya stays close to the original and creates Go-ish, imperative Haskell. Already good for Go-to-Haskell crossovers. Potentially excellent when done in idiomatic Haskell. /r/haskell <http://www.reddit.com/r/haskell/comments/3586e3/haskell_by_example/> - Tomas Petricek observes <http://tomasp.net/blog/2015/against-types/> the diversity of type theories and type systems and posits harm in any attempt at a single all-encompassing capture of the meaning of 'types'. What about unsound types? He doesn't offer a way out for those stuck with the appellation. Comments on HN <https://news.ycombinator.com/item?id=9545540> and /r/haskell <http://www.reddit.com/r/haskell/comments/35zzvu/against_the_definition_of_t…> . - John De Goes launches a crowdfund <https://www.indiegogo.com/projects/record-all-lambdaconf-2015-talks> to solicit $15,000 for video-recording 70 talks at LambdaConf 2015, which features many known Haskell programmers. /r/haskell <http://www.reddit.com/r/haskell/comments/35n14p/indiegogo_record_all_lambda…> - A redditor asks ex-Lispers what it's like moving to Haskell <http://www.reddit.com/r/haskell/comments/35bybo/lispers_of_haskell_whats_yo…>. "I miss Lisp parens" is a frequent answer. Also, Lisp has a better REPL experience. A biggie upside? The refactoring afforded by Haskell's type system. *Tip of the Week:* - When programming fractals, use the LLVM backend <https://ghc.haskell.org/trac/ghc/ticket/10400#comment:3> because it's "usually good at optimizing this kind of non-allocating code" as Reid Barton advises. -- Kim-Ee

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Equality Contstraint?
by Michael Jones 15 May '15

15 May '15

I am poking around in Arrow to understand it better. There is the following definition: second :: a b c -> a (d,b) (d,c) second f = arr swap >>> first f >>> arr swap where swap :: (x,y) -> (y,x) swap ~(x,y) = (y,x) Can someone explain what the ~ is? Searching the net resulted in frustration. I kind of think it is some kind of equality constraint but can’t find documentation. Perhaps if I knew what it was called I might succeed in finding something. If it is an equality constraint, please provide a reference if you have one. I did not find anything in a search including GHC, etc. Thanks

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trying to go from FFT type to Vector.Storable
by briand＠aracnet.com 15 May '15

15 May '15

Hi, I'm using the FFT package (the FFTW bindings), and for a complex FFT I get back : (FFTWReal r, Ix i, Shapable i) => CArray i (Complex r) -> CArray i (Complex r) at first I thought I could use elems to get to a list and then do Vector.Storable.fromList to get back to a Vector.Storable. Unfortunately r is FFTWReal so i need to map over the returned list and get a haskell Double back from that. FFTWReal maps to RealFloat which i found, but i haven't found anything that will take a RealFloat and give me a Double. Was hoping someone might know how to do that, or maybe have a little cleaner way to get from the FFT data type over to Vector. Thanks. Brian

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evaluate arbitrary code + mutable variables
by Jeffrey Brown 14 May '15

14 May '15

I wrote a graph-like data type, with some functions for adding statements and relationships between statements. (It's different from a graph in that there's no edge/vertex distinction; there are only statements, but a statement might refer to other statements.) Currently I'm using them in GHCI. Because it does not allow one to change a variable, I keep having to do things like this: let g0 = emptyDocument let s1 = newStatement ... let g1 = addStatement s1 g0 let s2 = newStatement ... let g2 = addStatement s2 g1 ... If I wrote a standalone application, I could use mutable variables, so I would not have to define a new object every time I want to modify an existing one. However I like being able to type in arbitrary code into GHCI. Can one have both of those at once? That is, could I either (1) use MVars from within GHCI, or (2) write a standalone app that lets the user evaluate arbitrary Haskell?

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Suggestion: "Sizable" super class for Storable
by jones.noamle＠gmail.com 14 May '15

14 May '15

Storable instances have a size, given by sizeOf. In many cases, we're not interested in peeking/poking data but only passing it opaquely via the FFI. A common use case is when the C API offers an "init" function such as: void mycontext_init(mycontext *context); For these cases it would be useful to know the size of "mycontext", so we could malloc it and pass a pointer to mycontext_init. Also, it allows Haskell-side code to decide how it wants to allocate the data, perhaps using some other (external) mechanism not related to the specific API that the FFI bindings are wrapping. c2hs would benefit by allowing users to use the '+' notation in function parameters (which generate malloc-and-pass style code), without having to guess the size of the structure. Instead, it could simply use the Sizable (TM) instance to get the size, and the user will define Sizable in any way they want (for example, using the {#sizeof#} macro, which is somewhat unreliable, or by hard-coding or manually entering the size or by some other method). Your thoughts are much appreciated!

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IDE-backend + Atom editor
by JP Moresmau 14 May '15

14 May '15

Hello, when the open sourcing of IDE-backend was announced ( https://www.fpcomplete.com/blog/2015/03/announce-ide-backend) one thing mentioned was Atom support, and indeed the home page for the client ( https://github.com/chrisdone/ide-backend-client) mentions Atom. But is there a project out there I can use that brings the two together? The ide-haskell (https://atom.io/packages/ide-haskell) Atom plugin still relies on ghc-mod as far as I can tell. I saw edsko tutorial on writing Atom plugin in Haskell ( http://edsko.net/2015/02/14/atom-haskell/) so it seem we are tentalizingly close to have Atom + Haskell implemented in Haskell, and I'd like to give it a go if there's anything available out there (and maybe help if needed). Thanks! JP -- JP Moresmau http://jpmoresmau.blogspot.com/

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CFP: Extended Deadline: Functional High-Performance Computing (held with ICFP)
by Geoffrey Mainland 14 May '15

14 May '15

====================================================================== CALL FOR PAPERS FHPC 2015 The 4th ACM SIGPLAN Workshop on Functional High-Performance Computing Vancouver, British Columbia, Canada, Canada September 3, 2015 https://sites.google.com/site/fhpcworkshops/ Co-located with the International Conference on Functional Programming (ICFP 2015) EXTENDED Submission Deadline: Friday, 22 May, 2015 (anywhere on earth) ====================================================================== The FHPC workshop aims at bringing together researchers exploring uses of functional (or more generally, declarative or high-level) programming technology in application domains where high performance is essential. The aim of the meeting is to enable sharing of results, experiences, and novel ideas about how high-level, declarative specifications of computationally challenging problems can serve as maintainable and portable code that approaches (or even exceeds) the performance of machine-oriented imperative implementations. All aspects of performance critical programming and parallel programming are in-scope for the workshop, irrespective of hardware target. This includes both traditional large-scale scientific computing (HPC), as well as work targeting single node systems with SMPs, GPUs, FPGAs, or embedded processors. It is becoming apparent that radically new and well founded methodologies for programming such systems are required to address their inherent complexity and to reconcile execution performance with programming productivity. Proceedings: ============ Accepted papers will be published by the ACM and will appear in the ACM Digital Library. * Submissions due: Friday, 22 May, 2015 (anywhere on earth) * Author notification: Friday, 26 June, 2015 * Final copy due: Sunday, 19 July, 2015 Submitted papers must be in portable document format (PDF), formatted according to the ACM SIGPLAN style guidelines (2 column, 9pt format). See http://www.sigplan.org/authorInformation.htm for more information and style files. Typical papers are expected to be 8 pages (but up to four additional pages are permitted). Contributions to FHPC 2015 should be submitted via Easychair, at the following URL: * https://www.easychair.org/conferences/?conf=fhpc15 The submission site is now open. The FHPC workshops adhere to the ACM SIGPLAN policies regarding programme committee contributions and republication. Any paper submitted must adhere to ACM SIGPLAN's republication policy. PC member submissions are welcome, but will be reviewed to a higher standard. http://www.sigplan.org/Resources/Policies/Review http://www.sigplan.org/Resources/Policies/Republication Travel Support: =============== Student attendees with accepted papers can apply for a SIGPLAN PAC grant to help cover travel expenses. PAC also offers other support, such as for child-care expenses during the meeting or for travel costs for companions of SIGPLAN members with physical disabilities, as well as for travel from locations outside of North America and Europe. For details on the PAC programme, see its web page (http://www.sigplan.org/PAC.htm) Programme Committee: ==================== Tiark Rompf (co-chair), Purdue University, USA Geoffrey Mainland (co-chair), Drexel University, USA Kevin Brown Stanford University, USA James Cheney University of Edinburgh, UK Albert Cohen INRIA, France David Duke University of Leeds, UK Yukiyoshi Kameyama University of Tsukuba, Japan Gabriele Keller University of New South Wales, Australia Paul H J Kelly Imperial College London, UK Trevor L. Mcdonell Indiana University, USA Greg Michaelson Heriot-Watt University, UK Cosmin E. Oancea University of Copenhagen, Denmark Markus Pueschel ETH Zurich, Switzerland Sukyoung Ryu KAIST, Korea Alexander Slesarenko Huawei, Russia Josef Svenningsson Chalmers University of Technology, Sweden

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all for Contributions - Haskell Communities and Activities Report, May 2015 edition (28th edition)
by Mihai Maruseac 13 May '15

13 May '15

Dear all, We would like to collect contributions for the 28th edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 17 May 2015 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format) ============================================================ This is the short story (one extra point to the story added from previous editions): * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * **NEW**: If you are working on a project that is looking for contributors, please write a short entry and submit it, mentioning that your are looking for contributors. The final report might have an index with such projects, provided we get enough such submissions. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac and Alejandro Serrano Mena FAQ: Q: What format should I write in? A: The required format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2015/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2015/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg or .png format, then, PNG being preferred for simplicity. Q: Should I send files in .zip archives or similar? A: No, plain file attachements are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use HCAR to ask for more contributors to your project, it is a good way to gain visibility and traction. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you don't know, the thing to do is not to get scared, but to learn." -- Atlas Shrugged.

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Job openings for Haskell, O'Caml programmers at Swedish spin-off startup
by Johan Glimming 13 May '15

13 May '15

Hi Haskell people, and beyond, Summary: Openings for software developers <https://goo.gl/vdu1d4> highly trained in functional programming, six developer or more entangled in our "teams", three with considerable Haskell, O'Caml experience, talents, entrepreneurial mind-set, and three for Scala, spear-edge Functor Consulting branch, with out-standing industrial track-records, as the new constructive programming paradigm is gradually unleashed to the software industry from Functor Group AB, Sweden. Keywords: type theory, dependent type, new programming paradigm, static analysis with dependent types, DDD-like development with solid Scandinavian-style Martin-Löf type theory operational semantics in the VM above LLVM, Haskell, O’Caml, embedded systems is one submarket. Official call with the details here: https://goo.gl/vdu1d4 <https://goo.gl/vdu1d4> <https://goo.gl/vdu1d4> We will always work closely with academia and internships, MSc theses and research collaborations are always interesting though right now we are focused on delivering to customers and building up the business meaning larger research projects are not yet possible, a large-scale EU project planned but not for this year. You’re most welcome to apply via jobs(a)functor.se <mailto:jobs@functor.se>. See the call for details! Yours Sincerely, Johan Glimming — Dr Johan Glimming Chief Technical Officer and Co-Founder at Functor AB and Functor Group AB LinkedIn profile: www.linkedin.com/in/glimming <http://www.linkedin.com/in/glimming> Mobile: +46-76-7646000 (direct) Main: +46-8-55005500 Functor AB, Box 7070, 164 07 Kista, SWEDEN Web: www.functorgroup.com <http://www.functorgroup.com/> Twitter: @functors <https://www.twitter.com/functors/>

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Start GHCI with a static library
by aditya siram 13 May '15

13 May '15

Hi all, I have a Haskell library I'd like to load up in GHCI along with a statically linked C library (libblah.a) . Is there any way to do this? My Googling only turns up instructions on loading a shared library (libblah.so). Thanks! -deech

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Is this functional reactive programming
by Clinton Mead 12 May '15

12 May '15

What I want to be able to do is something like this: do x <- newSTRef 2 y <- newSTRef 3 z <- letSTRef (x + y) r1 <- readSTRef z writeSTRef x 5 r2 <- readSTRef z return (r1, r2) This should return (6,15) The "letSTRef" is what's new. The value it returns can change based on the parts that make up it's function changing. I understand this syntax above isn't going to work (I'd have to use applicative at least I'd imagine) but my main question is that does something like this exist? Is it functional reactive programming or is it something else? I don't want to be reinventing the wheel if this type of idea is already implemented but I haven't recognised it.

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Call for participation: Applied Functional Programming (AFP) Summerschool 6-17 July 2015, Utrecht, Netherlands
by Atze Dijkstra 12 May '15

12 May '15

=========== AFP Summerschool 2015 =========== Applied Functional Programming (AFP) Summerschool July 6-17, 2015 Utrecht University, Department of Information and Computing Sciences Utrecht, The Netherlands Summerschool & registration website: http://www.utrechtsummerschool.nl/courses/science/applied-functional-progra… AFP website : http://www.cs.uu.nl/wiki/USCS contact : Uscs-afp(a)lists.science.uu.nl *** The 2015 edition of the Applied Functional Programming (AFP) Summerschool in Utrecht, Netherlands will be held from 6-17 July 2015. The summerschool teaches Haskell on both beginners and advanced levels via lectures and lab exercises. More info can be found via the references above, included here is a summary from the summerschool info: ``Typed functional programming in Haskell allows for the development of compact programs in minimal time and with maximal guarantees about robustness and correctness. The course introduces Haskell as well as its theoretical underpinnings such as typed lambda calculus, and Damas-Milner type inference. There is ample opportunity to put this all in practice during lab sessions. Typed functional programming languages allow for the development of robust, concise programs in a short amount of time. The key advantages are higher-order functions as an abstraction mechanism, and an advanced type system for safety and reusability. This course introduces Haskell, a state-of-the-art functional programming language, together with some of its theoretical background, such as typed lambda calculi, referential transparency, Damas-Milner type inference, type level programming, and functional design patterns. We will combine this with applications of functional programming, concentrating on topics such as language processing, building graphical user interfaces, networking, databases, and programming for the web. The goal of the course is not just to teach the programming language and underlying theory, but also to learn about the Haskell community and to get hands-on experience by doing lab exercises or a Haskell project of your own.'' *** This year is somewhat special in that the Tour de France starts in Utrecht the weekend before the start of the summerschool. It is an opportunity for enjoying the related festivities. It also implies that housing may be more difficult the weekend before the summerschool, something to keep in mind when you wait with registration until the latest. regards, - Atze - Atze Dijkstra, Department of Information and Computing Sciences. /|\ Utrecht University, PO Box 80089, 3508 TB Utrecht, Netherlands. / | \ Tel.: +31-30-2534118/1454 | WWW : http://www.cs.uu.nl/~atze . /--| \ Fax : +31-30-2513971 .... | Email: atze(a)uu.nl ............... / |___\ _______________________________________________ Haskell mailing list Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell

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Multiway tree vs rose tree
by Tara Athan 12 May '15

12 May '15

Hi - I am new to this list, and the only archive I could find is the Mailman archive, which is next to impossible to search. So this may be a topic that has been previously discussed, I don't know... The documentation at https://hackage.haskell.org/package/containers-0.3.0.0/docs/Data-Tree.html says that a rose tree is another name for multiway tree. But this is not true according to my understanding of the accepted definition in the CS literature (see e.g. http://doc.utwente.nl/66626/1/db-utwente-0000003528.pdf) Rose trees do not have labels at the nodes, only at the leaves (aka tips). There is another page https://wiki.haskell.org/Algebraic_data_type#Rose_tree that also uses this divergent definition. To further propagate this problem, the Wikipedia page https://en.wikipedia.org/wiki/Rose_tree uses these Haskell wiki pages as the definitive source. Tara

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3rd CFP FARM 2015 -- final reminder
by Anton Kholomiov 12 May '15

12 May '15

************************************************************ 3rd CFP FARM 2015 3rd ACM SIGPLAN International Workshop on Functional Art, Music, Modelling and Design Vancouver, Canada, 5 September, 2015 affiliated with ICFP 2015 Full Paper and Demo Abstract submission Deadline: 17 May ************************************************************ The ACM SIGPLAN International Workshop on Functional Art, Music, Modelling and Design (FARM) gathers together people who are harnessing functional techniques in the pursuit of creativity and expression. Functional Programming has emerged as a mainstream software development paradigm, and its artistic and creative use is booming. A growing number of software toolkits, frameworks and environments for art, music and design now employ functional programming languages and techniques. FARM is a forum for exploration and critical evaluation of these developments, for example to consider potential benefits of greater consistency, tersity, and closer mapping to a problem domain. FARM encourages submissions from across art, craft and design, including textiles, visual art, music, 3D sculpture, animation, GUIs, video games, 3D printing and architectural models, choreography, poetry, and even VLSI layouts, GPU configurations, or mechanical engineering designs. The language used need not be purely functional (“mostly functional” is fine), and may be manifested as a domain specific language or tool. Theoretical foundations, language design, implementation issues, and applications in industry or the arts are all within the scope of the workshop. Submissions are invited in two categories: * Full papers 5 to 12 pages using the ACM SIGPLAN template. FARM 2015 is an interdisciplinary conference, so a wide range of approaches are encouraged and we recognize that the appropriate length of a paper may vary considerably depending on the approach. However, all submissions must propose an original contribution to the FARM theme, cite relevant previous work, and apply appropriate research methods. * Demo abstracts Demo abstracts should describe the demonstration and its context, connecting it with the themes of FARM. A demo could be in the form of a short (10-20 minute) tutorial, presentation of work-in-progress, an exhibition of some work, or even a performance. Abstracts should be no longer than 2 pages, using the ACM SIGPLAN template and will be subject to a light-touch peer review. If you have any questions about what type of contributions that might be suitable, or anything else regarding submission or the workshop itself, please contact the organisers at: farm-2015(a)easychair.org KEY DATES: Full Paper and Demo Abstract submission Deadline: 17 May Author Notification: 26 June Camera Ready: 19 July Workshop: 5 September SUBMISSION All papers and demo abstracts must be in portable document format (PDF), using the ACM SIGPLAN style guidelines. The text should be in a 9-point font in two columns. The submission itself will be via EasyChair: https://easychair.org/conferences/?conf=farm2015 PUBLICATION Accepted papers will be included in the formal proceedings published by ACM Press and will also be made available through the the ACM Digital Library; see http://authors.acm.org/main.cfm for information on the options available to authors. Authors are encouraged to submit auxiliary material for publication along with their paper (source code, data, videos, images, etc.); authors retain all rights to the auxiliary material. WORKSHOP ORGANISATION Workshop Chair: Henrik Nilsson, University of Nottingham Program Chair: David Janin, University of Bordeaux Publicity Chair: Samuel Aaron, University of Cambridge Program Committee: Samuel Aaron, University of Cambridge Jean Bresson, IRCAM Paris David Broman, KTH and UC Berkeley David Janin (chair), University of Bordeaux Anton Kholomiov, Orffeus instrumental ensemble Moscow Alex Mclean, University of Leeds Carin Meier, Outpace Systems Henrik Nilsson, University of Nottingham Yann Orlarey, GRAME Lyon Donya Quick, Yale University Shigeki Sagayama, Meiji University Chung-chieh Shan, Indiana University Michael Sperber, Active Group GmbH Bodil Stokke, FutureAdLabs For further details, see the FARM website: http://functional-art.org

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ANN: codec: easy bidirectional serialization and first-class record construction
by Patrick Chilton 12 May '15

12 May '15

Tired of writing complementary parseJSON/toJSON, peek/poke or Binary get/put functions? codec <https://github.com/chpatrick/codec> provides easy bidirectional serialization of plain Haskell records in any Applicative context. All you need to do is provide a de/serializer for every record field in any order you like, and you get a de/serializer for the whole structure. The type system ensures that you provide every record exactly once. It also includes a library for general record construction in an Applicative context, of which creating codecs is just one application. *JSON:* data User = User { username :: Text , userEmail :: Text , userLanguages :: [ Text ] , userReferrer :: Maybe User } deriving Show genFields ''User userCodec :: JSONCodec User userCodec = obj "user object" $ User $>> f_username >-< "user" >>> f_userEmail >-< "email" >>> f_userLanguages >-< "languages" >>> f_userReferrer >-< opt "referrer" instance FromJSON User where parseJSON = parseVal userCodec instance ToJSON User where toJSON = produceVal userCodec *Bit fields:* ipv4Codec :: BinaryCodec IPv4 ipv4Codec = toBytes $ IPv4 $>> f_version >-< word8 4 >>> f_ihl >-< word8 4 >>> f_dscp >-< word8 6 >>> f_ecn >-< word8 2 >>> f_totalLength >-< word16be 16 >>> f_identification >-< word16be 16 >>> f_flags >-< word8 3 >>> f_fragmentOffset >-< word16be 13 >>> f_timeToLive >-< word8 8 >>> f_protocol >-< word8 8 >>> f_headerChecksum >-< word16be 16 >>> f_sourceIP >-< word32be 32 >>> f_destIP >-< word32be 32 instance Binary IPv4 where get = parse ipv4Codec put = produce ipv4Codec The same types and combinators can be used to make Storable or Binary instances, or used with pretty much any de/serialization library. The implementation only uses a little bit of Template Haskell to generate the f_... structures for each field and the rest is very basic in terms of extensions (no DataKinds or type families needed). Please take a look at the docs and the examples. I'd love to get some feedback before I put it on Hackage. :) All the best, Patrick

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using scoped type variables in proofs...
by Nicholls, Mark 11 May '15

11 May '15

Hello, I have written the below proof as an exercise. I want to explicitly annotate the proof with type variables.....but I cant get a line to work... > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE ExplicitForAll #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE GADTs #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE UndecidableInstances #-} > {-# LANGUAGE ScopedTypeVariables #-} > import Prelude hiding (head, tail, (++), (+), replicate) > import qualified Prelude as P > data Nat where > Z :: Nat > S :: Nat -> Nat > data SNat (a :: Nat) where > SZ :: SNat 'Z > SS :: SNat a -> SNat ('S a) here a nice plus > type family (n :: Nat) :+ (m :: Nat) :: Nat > type instance Z :+ m = m > type instance (S n) :+ m = S (n :+ m) lets try to do > data val1 :== val2 where > Refl :: val :== val If I prove its abelian then we get it... the "proof" works if we remove the type annotation but I want the annotation to convince myself I know whats going on. > theoremPlusAbelian :: SNat a -> SNat b -> (a :+ b) :== (b :+ a) > theoremPlusAbelian (SZ :: SNat a) (SZ :: SNat b) = > -- forall 0 and 0 > (Refl :: (a :+ b) :== (b :+ a)) > theoremPlusAbelian ((SS a) :: SNat a) (SZ :: SNat b) = > -- forall a + 1 and 0 > case theoremPlusAbelian (a :: (a ~ 'S a1) => SNat a1) (SZ :: SNat b) of > (Refl :: (a1 :+ b) :== (b :+ a1)) -> > (Refl :: (a :+ b) :== (b :+ a)) > theoremPlusAbelian (SZ :: SNat a) ((SS a) :: SNat b) = > -- forall 0 and a + 1 > case theoremPlusAbelian (SZ :: SNat a) (a :: (b ~ 'S b1) => SNat b1) of > (Refl :: (a :+ b1) :== (b1 :+ a)) -> > (Refl :: (a :+ b) :== (b :+ a)) > -- cant seem to prove this... > theoremPlusAbelian ((SS a) :: (SNat a)) ((SS b) :: (SNat b)) = > -- forall a+1 and b+1 > -- 1st prove (a + 1) + b = b + (a + 1)...from above > case theoremPlusAbelian ((SS a) :: (SNat a)) (b :: (b ~ 'S b1) => SNat b1) of THE COMMENTED LINE FAILS. Could not deduce ((b1 :+ 'S a1) ~ (a2 :+ 'S a1)) from the context (a ~ 'S a1) bound by a pattern with constructor SS :: forall (a :: Nat). SNat a -> SNat ('S a), in an equation for 'theoremPlusAbelian' at Cafe2.lhs:57:24-27 or from (b ~ 'S a2) bound by a pattern with constructor SS :: forall (a :: Nat). SNat a -> SNat ('S a), in an equation for 'theoremPlusAbelian' at Cafe2.lhs:57:45-48 NB: ':+' is a type function, and may not be injective The type variable 'b1' is ambiguous Expected type: (a :+ a2) :== (a2 :+ a) Actual type: (a :+ b1) :== (b1 :+ a) Relevant bindings include b :: SNat a2 (bound at Cafe2.lhs:57:48) a :: SNat a1 (bound at Cafe2.lhs:57:27) In the pattern: Refl :: (a :+ b1) :== (b1 :+ a) In a case alternative: (Refl :: (a :+ b1) :== (b1 :+ a)) -> case theoremPlusAbelian a (SS b) of { Refl -> case theoremPlusAbelian a b of { Refl -> Refl } } In the expression: case theoremPlusAbelian ((SS a) :: SNat a) (b :: b ~ S b1 => SNat b1) of { (Refl :: (a :+ b1) :== (b1 :+ a)) -> case theoremPlusAbelian a (SS b) of { Refl -> case theoremPlusAbelian a b of { Refl -> ... } } } > -- (Refl :: (a :+ b1) :== (b1 :+ a)) -> > Refl -> > -- now prove a + (b + 1) = (b + 1) + a...from above > case theoremPlusAbelian a (SS b) of > Refl -> > -- now prove a + b = b + a > case theoremPlusAbelian a b of > -- which seems to have proved a + b = b + a > Refl -> Refl CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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New Functional Programming Job Opportunities
by Functional Jobs 11 May '15

11 May '15

Here are some functional programming job opportunities that were posted recently: Haskell Web Engineer at Front Row Education http://functionaljobs.com/jobs/8823-haskell-web-engineer-at-front-row-educa… Sr. Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8822-sr-software-engineer-at-mcgraw-hill-edu… Cheers, Sean Murphy FunctionalJobs.com

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FFI nested structs - malloc - free
by Proclivis 11 May '15

11 May '15

FFI Gurus, I created a c2hs FFI of a nested C structure, where struct A has a pointer to a struct B. To do so, I used a malloc, but I am unsure if the memory will be freed when the resulting Ptr is freed. The example at this link uses the same technique, so it will serve as an example. https://github.com/ifesdjeen/haskell-ffi-tutorial/blob/master/src/Example.h… Line 48 and 51 do the malloc and assign the pointer in the struct, from inside a Storable poke implementation. But, there is no explicit free, nor a finalizer. Will the memory be freed when the Ptr of the Storable is freed? If it is, it implies that some magic keeps track of mallocs inside a poke, and creates finalizers. Or, this example leaks. If it leaks, how do I create a finalizer from inside a poke implementation? Mike

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First Call for Papers for IFL 2015
by publicityifl＠gmail.com 10 May '15

10 May '15

Hello, Please, find below the first call for papers for IFL 2015. Please forward these to anyone you think may be interested. Apologies for any duplicates you may receive. best regards, Jurriaan Hage Publicity Chair of IFL --- IFL 2015 - Call for papers 27th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES - IFL 2015 University of Koblenz-Landau, Koblenz, Germany In cooperation with ACM SIGPLAN September 14-16, 2015 http://ifl2015.wikidot.com/ Scope The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2015 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Peer-review Following the IFL tradition, IFL 2015 will use a post-symposium review process to produce the formal proceedings. All participants of IFL2015 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings do not count as publication for the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. Important dates August 10: Submission deadline draft papers August 12: Notification of acceptance for presentation August 14: Early registration deadline August 21: Late registration deadline September 7: Submission deadline for pre-symposium proceedings September 14-16: IFL Symposium December 1: Submission deadline for post-symposium proceedings January 15, 2016: Notification of acceptance for post-symposium proceedings March 1, 2016: Camera-ready version for post-symposium proceedings Submission details Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English. Papers must adhere to the standard ACM two columns conference format. For the pre-symposium proceedings we adopt a 'weak' page limit of 12 pages. For the post-symposium proceedings the page limit of 12 pages is firm. A suitable document template for LaTeX can be found at: http://www.acm.org/sigs/sigplan/authorInformation.htm Authors submit through EasyChair: https://easychair.org/conferences/?conf=ifl2015 Topics IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools in the context of functional programming. If you are not sure whether your work is appropriate for IFL 2015, please contact the PC chair at rlaemmel(a)acm.org. Topics of interest include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialization - run-time code generation - partial evaluation - (abstract) interpretation - metaprogramming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques - (industrial) applications Peter Landin Prize The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee Chair: Ralf Lämmel, University of Koblenz-Landau, Germany - Malgorzata Biernacka, University of Wroclaw, Poland - Laura M. Castro, University of A Coruña, Spain - Martin Erwig, Oregon State University, USA - Dan Ghica, University of Birmingham, UK - Andrew Gill, University of Kansas, USA - Stephan Herhut, Google, USA - Zhenjiang Hu, National Institute of Informatics (NII), Japan - Mauro Jaskelioff, CIFASIS/Universidad Nacional de Rosario, Argentina - Frédéric Jouault, ESEO, France - Oleg Kiselyov, Tohoku University, Japan - Lindsey Kuper, Indiana University, USA - Rita Loogen, Philipps-Universität Marburg, Germany - Akimasa Morihata, University of Tokyo, Japan - Atsushi Ohori, Tohoku University, Japan - Bruno C. D. S. Oliveira, The University of Hong Kong, Hong Kong - Frank Piessens, Katholieke Universiteit Leuven, Belgium - Norman Ramsey, Tufts University, USA - Matthew Roberts, Macquarie University, Australia - Manfred Schmidt-Schauss, Goethe-University Frankfurt, Germany - Simon Thompson, University of Kent, UK - Stephanie Weirich, University of Pennsylvania, USA - Steve Zdancewic, University of Pennsylvania , USA Venue The 27th IFL will be held in association with the Faculty of Computer Science, University of Koblenz-Landau, Campus Koblenz. Koblenz is well connected by train to several international airports. For instance, Koblenz can be reached from Frankfurt by high-speed train ICE within an hour. The modern Koblenz campus is close to the city center and can be reached by foot, bus, or cab. See the website for more information on the venue.

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Automatic liftIO or how to write this shorter?
by Gleb Popov 10 May '15

10 May '15

Hello haskell-cafe@ I'm writing a GUI app in Haskell and bindings to the widget toolkit i'm using in parallel. These bindings are very simple and all its functions have return type (IO something). So far so good, i wrote the following code to create an config window: createConfigUI root = do box <- Box.add root -- first field addToPackEnd box =<< do f <- Fr.add box setPartText f Nothing "E-mail" setPartContent f Nothing =<< do box <- Box.add box addToPackEnd box =<< do e <- Ent.add box Ent.singleLineSet e True -- Here onEvent "changed,user" e $ do reactOnUserInput e objectShow e return e objectShow box return box objectShow f return f -- next field addToPackEnd box =<< do ... Initially, i was quite satisfied with flipped bind use for creating UI elements and arranging them. Nested do scopes allow copypasting code without renaming variables and also provide some visual representation on widget hierarchy. But at some point i need to return some stuff from some inner do block into outmost. For example, at the line with comment "Here" i defined let setText t = entrySet e t and wanted to return it from whole createConfigUI action. Moreover, createConfigUI have much more fields, for each of them i want to do the same. My initial thought was to wrap everything with runWriter and just call tell setText wherever i want to gather all setter functions into a list, but i can't do this because i would need to put liftIO before every IO action all over the place. If only there was i way to turn an (IO a) into (MonadIO m => m a), it would be easy. Another solution is to make my bindings return (MonadIO m => m a). This would be equal effort of plugging liftIO's everywhere, but at least it would be hidden from user of bindings. I'd gone this way, but looked at gtk bindings first and found that (IO a) is used there. So, my questions are: 1. What would you recommend in my situation? Is it possible yield values from inner do blocks into outer without much hassle? 2. If there is nothing wrong with switching bindings from (IO a) to MonadIO typeclass, why not to do this for gtk, wxWidgets and nearly every FFI binding? Thanks in advance.

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[ANN] servant 0.4.0 released (+new website)
by Alp Mestanogullari 10 May '15

10 May '15

Hello everyone, We're happy to announce the releases of: - servant 0.4.0 - servant-server 0.4.0 - servant-client 0.4.0 - servant-jquery 0.4.0 - servant-docs 0.4.0 - servant-blaze 0.4.0 - servant-lucid 0.4.0 to Hackage. As well as a new website, same URL as before: http://haskell-servant.github.io/ -- which features a tutorial that's much more informative than the getting started we had for the previous version. The tutorial is available at http://haskell-servant.github.io/tutorial The highlights for this release are: - Multiple content-types support (with servant-blaze and servant-lucid offering a way to encode data for the HTML content type) - Handlers in other monads than `EitherT` - Response headers support - Safe links to endpoints - Saner types for aborting early in request handlers (the `Left` branch in `EitherT`) For more details about the new features, please read the release post: http://haskell-servant.github.io/posts/2015-05-10-servant-0.4-released.html Cheers -- Alp Mestanogullari

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How to estimate number of downloads of GHC or Haskell Platform?
by nr＠cs.tufts.edu 09 May '15

09 May '15

For a review, I'm trying to get a rough estimate of how many times GHC or the Haskell Platform have been downloaded. Although I'm sure I've seen this information graphed over time at workshops, my web-search skills are not turning up anything. I'd be quite happy to get an estimate to within a factor of ten. Can anyone suggest any sources, or a good way to estimate this number? Norman

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Using ClassyPrelude to handle either Text or ByteString
by Mike Meyer 08 May '15

08 May '15

Ok, I can't seem to figure out how to make this work, and since I'm using ClassyPrelude (from classy-prelude, latest version on hackage as of yesterday), I think cafe is a better place to ask than beginners. Sorry if this should have gone there. I want to read in a list of files, concatenate them, and then run the resulting data through a "process" function. So far, no problem. Given an opts record that contains the list of files and some control for the process function, this expression works: hPut stdout =<< process opts . concat <$> mapM (readFile . fpFromString) (files opts) (BTW, recommendations on cleaning that up gratefully accepted) However, I'd like to be able to read them all as either Text or ByteString, depending on options being passed to me. This doesn't work so well. If I give process a type of Text -> Text or ByteString->ByteString, everything works fine with no other changes. However, any attempt to make it polymorphic fails with ambiguous type warnings. I've tried a slew of things, and found a few that work. But none of them are what I'd call good, much less elegant. Is there a good way to do this?

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Re: [Haskell-cafe] RFC: "Native -XCPP" Proposal
by Herbert Valerio Riedel 08 May '15

08 May '15

Hello Christian, (I've re-CC'ed haskell-cafe, assuming this wasn't deliberate) On 2015-05-08 at 09:50:46 +0200, Christian Maeder wrote: > using cpphs is the right way to go! > > Rewriting it from scratch may be a good exercise but is (essentially) a > waste of time. > > However, always asking Malcolm to get source changes into cpphs would > be annoying. > > Therefore it would be great if the sources were just part of the ghc > sources (under git). > > Another "problem" might be the dependency "polyparse" that is currently > not part of the core libraries. A scheme was actually discussed privately to address this: We certainly don't want to expose cpphs/polyparse (and text!) as new packages in GHC's global pkg-db. Which we'd end up, if we handled cpphs as the other exposed boot libraries. Therefore we'd only use the few relevant modules from cpphs/polyparse as "other-modules" (i.e. internal hidden dependencies -- i.e. we wouldn't use cpphs/polyparse's .cabal files) compiled into GHC, but not exposed. We'd either create a new Git submodule to hold our "fork" of cpphs/polyparse, or just add it somewhere inside ghc.git > (I guess that replacing polyparse by something else would also be a nice > exercise.) > > So (for me) the only question is, if Malcolm is willing to transfer > control over cpphs to the haskell-community (or ghc head) - of course > with due acknowledgements! I don't think this will be necessary, as we don't need the cpphs-upstream to mirror each modifications immediately. The benefit of the scheme described above is that we'd be somewhat decoupled from cpphs' upstream, and can freely experiment in our "fork", and can sync up with Malcolm from time to time to merge improvements in both directions. -- hvr

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status of deb.haskell.org?
by Jeremy 08 May '15

08 May '15

The haskell status page says that deb.haskell.org has been down since mid-February. Are there any plans to bring it back up? -- View this message in context: http://haskell.1045720.n5.nabble.com/status-of-deb-haskell-org-tp5768430.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] RFC: "Native -XCPP" Proposal
by Howard B. Golden 08 May '15

08 May '15

At the risk of antagonizing some (most? all?) of you, how about... -XCPP stands for the native CPP -XGNUCPP stands for GNU's GCC CPP -XClangCPP stands for Clang's CPP -XCPPHS stands for CPPHS ... with the hope that TH is the future? Howard

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Efficient memory use for tens of thousands of connections
by Ben Bangert 08 May '15

08 May '15

I'm currently evaluating memory overhead for a variety of simple Websocket based echo server to get a gauge of minimal memory penalty per connection along with SSL overhead per connection. I've created two websocket servers in Haskell: https://github.com/bbangert/ssl-ram-testing/tree/master/Haskell One using Yesod + jaspers Websockets lib, the other with Yesod + Mishael Snoyman's native websocket lib. I was curious what options I might be missing that would ensure a more efficient use of memory, at the moment the non-SSL seems to use about 48kb per connection, while using Haskell's TLS via warp-tls introduces a huge amount of overhead (along with odd handshake issues). Here's what my results look like: No SSL Program started. Tester started. Tester finished connecting. Waiting: 360 Last Memory Increase: 2 WARNING: Program hasn't stopped increasing. Started with: 2633728 Ended with: 50896896 kB per connection: 47.13 SSL Program started. Tester started. yesod-customws-tls: HandshakeFailed (Error_Protocol ("bad record mac",True,BadRecordMac)) yesod-customws-tls: HandshakeFailed (Error_Protocol ("bad record mac",True,BadRecordMac)) yesod-customws-tls: HandshakeFailed (Error_Protocol ("bad record mac",True,BadRecordMac)) yesod-customws-tls: HandshakeFailed (Error_Protocol ("bad record mac",True,BadRecordMac)) yesod-customws-tls: HandshakeFailed (Error_Protocol ("bad record mac",True,BadRecordMac)) yesod-customws-tls: HandshakeFailed (Error_Protocol ("bad record mac",True,BadRecordMac)) yesod-customws-tls: HandshakeFailed (Error_Protocol ("bad record mac",True,BadRecordMac)) yesod-customws-tls: HandshakeFailed (Error_Protocol ("bad record mac",True,BadRecordMac)) WARNING: Failed to connect all clients Tester finished connecting. Waiting: 360 Last Memory Increase: 4 WARNING: Program hasn't stopped increasing. Started with: 3170304 Ended with: 454221824 kB per connection: 444.03 SSL Overhead per Conn: 396.90 I wait up to 6 minutes for memory to level out, it never does with or without SSL. Is this expected behavior? If not, what options am I missing? The ssl-ram-testing repo includes full directions on how to reproduce this and run this test. Cheers, Ben

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ANN: Important lambabot update (5.0.2.1 release)
by Bertram Felgenhauer 07 May '15

07 May '15

Hi, I've just released lambdabot-5.0.2.1 which plugs an embarrassing security hole in the @check command; if you are running lambdabot as an IRC bot, you should upgrade! Generally, lambdabot relies on SafeHaskell and not running user-supplied IO actions for safety. This is unlikely to be bullet-proof, so it's advisable to sandbox mueval. However, the @check command violated this basic principle, and allowed running arbitrary IO actions. This is now fixed by using the (new) QuickCheck-safe package that only uses unsafePerformIO for the specific purposes of catching exceptions and generating the initial seed for random number generation. Thanks to benzfr on Freenode for finding this! There are a few minor changes. Notably, we now ship compiler-specific versions of Pristine.hs so that lambdabot runs out of the box on both ghc-7.6.3 and ghc-7.8.3 (ghc-7.10.1 still needs some work.) and the dict plugin no longer supports looking up several words at once. Cheers, Bertram P.S. As I just realized, I forgot to update the Changelog that comes with lambdabot... will try to remember next time.

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Cannot run GHC-7.10.1 tests
by Jeremy Henty 07 May '15

07 May '15

I successfully built GHC-7.10.1 but I cannot run the tests (at least not when I follow the instructions at https://ghc.haskell.org/trac/ghc/wiki/Building/RunningTests) "make fast" fails like this: ===--- building phase 0 make -r --no-print-directory -f ghc.mk phase=0 phase_0_builds make[1]: Nothing to be done for `phase_0_builds'. ===--- building phase 1 make -r --no-print-directory -f ghc.mk phase=1 phase_1_builds make[1]: Nothing to be done for `phase_1_builds'. ===--- building final phase make -r --no-print-directory -f ghc.mk phase=final fast make[1]: *** No rule to make target `fast'. Stop. make: *** [fast] Error 2 "make testfast" fails in exactly the same way, except that the broken make target is "testfast" instead of fast. "make test" and "make fulltest" fail like this: make -C testsuite/tests CLEANUP=1 OUTPUT_SUMMARY=../../testsuite_summary.txt fast make[1]: Entering directory `/data/build.d/6.8/ghc-7.10.1/testsuite/tests' ../mk/boilerplate.mk:168: ../mk/ghcconfig_data_build.d_6.8_ghc-7.10.1_inplace_bin_ghc-stage2.mk: No such file or directory ../mk/ghc-config "/data/build.d/6.8/ghc-7.10.1/inplace/bin/ghc-stage2" >"../mk/ghcconfig_data_build.d_6.8_ghc-7.10.1_inplace_bin_ghc-stage2.mk"; if [ $? != 0 ]; then rm -f "../mk/ghcconfig_data_build.d_6.8_ghc-7.10.1_inplace_bin_ghc-stage2.mk"; exit 1; fi /bin/sh: ../mk/ghc-config: cannot execute binary file make[1]: *** [../mk/ghcconfig_data_build.d_6.8_ghc-7.10.1_inplace_bin_ghc-stage2.mk] Error 1 make[1]: Leaving directory `/data/build.d/6.8/ghc-7.10.1/testsuite/tests' make: *** [test] Error 2 Interestingly, for GHC-7.8.4 "make fast" and "make testfast" fail in exactly the same way but "make test" and "make fulltest" both work. So, how do I test GHC-7.10.1 ? Regards, Jeremy Henty

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Re: [Haskell-cafe] release timing
by Harry . 07 May '15

07 May '15

Simon PJ wrote ... > > | It would be nice to see #8224 in as well, but doesn't look like that's > | going to be fixed any time soon! Have GHC HQ ever considered asking > | Intel if they'd be willing to lend a hand? > > What makes you think that Intel would help with #8224? (It's about the IO manager.) The title is about the IO manager :-) The diagnosis ends with performance issues that appear tied to CPU design, something that Intel ought to know something about. > Also, GHC is an open source project. You don't have to wait for GHC HQ to do anything --- please go ahead and ask anyone you think could help! While it's not unheard of for a corporation to assist an open source project, particularly with issues specific to their products, the request would have a lot more credibility if it came from GHC HQ than a random GHC user. I also have no idea who in Intel should be contacted for such things. But in the spirit of your suggestion I'm cross-posting this to haskell-cafe to reach a wider audience. Does anyone out there know of a contact in Intel who might be able/willing to help with CPU performance issues?

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Re: [Haskell-cafe] RFC: "Native -XCPP" Proposal
by Johannes Waldmann 07 May '15

07 May '15

There is maybe another option: do what csharp does. Define a reduced "preprocessor" (for conditional compilation, but without macro expansion) as part of the language. (cf. https://msdn.microsoft.com/en-us/library/ed8yd1ha.aspx ) They must have have had good reasons for doing it this way. - J.W. PS: I was trying to add a proper reference (to the language spec) but this seems hard. It is Section 9.5 of ECMA-334. For viewing the newer specs, it seems you need to have MS-Word or VS installed. See also http://stackoverflow.com/questions/13467103/

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Re: [Haskell-cafe] minor confusion over kinds....
by Nicholls, Mark 06 May '15

06 May '15

lets try it the other way around > instance Foo' SNat1 where > type Bar' SNat1 = Z1 > bar' = SZ1 that works! (my language will be wrong here...but the jist is) So in some something of kind "*->*" is substitutable for a parameter of kind "k -> *" "Nat->*" is substitutable for a parameter of kind "k -> *" "*->*" is substitutable for a parameter of kind "* -> *" "Nat->*" is NOT substitutable for a parameter of kind "* -> *" Hmmmm So why would I ever want to define a type class of kind (* -> *) -> Constraint Instead of (k -> *) -> Constraint ? I think I don't completely understand what's going on. From: Nicholls, Mark Sent: 06 May 2015 1:02 PM To: haskell-cafe(a)haskell.org Subject: minor confusion over kinds.... And what "*" means as opposed to other things e.g. an explicit name "Nat" or a kind variable "k" > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE ExplicitForAll #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE GADTs #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE UndecidableInstances #-} what I want is a typeclass with instances of type level Naturals here's the class, the parameter is of kind (* -> *) i.e. a is some sort of wrapper that maps from types to values > class Foo (a :: * -> *) where > type Bar a > bar :: a (Bar a) lets ignore DataKinds define Zero and Succ as distinct types uninhabited...but of kind "*" and "k -> *" > data Z1 > data S1 nat SNat1 :: * -> * > data SNat1 a where > SZ1 :: SNat1 Z1 > SS1 :: SNat1 a -> SNat1 (S1 a) off we go... > instance Foo SNat1 where > type Bar SNat1 = Z1 > bar = SZ1 now lets do it the datakind route > data Nat where > Z :: Nat > S :: Nat -> Nat ahhh...SNat is of kind "Nat -> *" > data SNat a where > SZ :: SNat 'Z > SS :: SNat a -> SNat ('S a) so I cant make it an instance of the same type class "The first argument of 'Foo' should have kind '* -> *', but 'SNat' has kind 'Nat -> *' In the instance declaration for 'Foo SNat'" > --instance Foo SNat where > -- type Bar SNat = 'Z > -- bar = SZ unless I redefine my class (which seems a bit silly) > class Foo' (a :: k -> *) where > type Bar' a :: k > bar' :: a (Bar' a) now it works. > instance Foo' SNat where > type Bar' SNat = 'Z > bar' = SZ clearly named kinds (?) are somehow distinct from unnames ones? CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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Re: [Haskell-cafe] RFC: "Native -XCPP" Proposal
by Brandon Allbery 06 May '15

06 May '15

On Wed, May 6, 2015 at 11:36 AM, Stephen Paul Weber < singpolyma(a)singpolyma.net> wrote: > Yes. This is one of my favourite things in GHC-land -- that an existing, > good-enough, standardised, and widely-deployed solution was chosen over a > NiH reinvention of preprocessing I have to assume my irony detector is broken as well. Or maybe I should just assume that "all the world's Linux with gcc" is assumed to be forever true and forever reliable by "all right-thinking people" so let's just sweep the nonissue under the rug because it can oh so obviously never be a real issue.... Because I had to face this back a couple decades ago, when my employer ported an application written in a 4GL (database language) to SCO Unix. The 4GL assumed cpp was the ever reliable pcc one and broke very badly when SCO used one integrated into its lexer (making it even more tightly wedded to C syntax than clang's). Eventually we replaced its cpp with a wrapper that ran m4 and redid everything else in m4's syntax. Which is why I was always a bit worried about ghc relying on cpp, was unsurprised when clang caused issues, and am rather annoyed that there are people who believe that they can just ignore it because REAL users will always be on Linux with gcc and all them furriners using weird OSes like OS X and FreeBSD can safely be ignored with their not-the-One-True-OS-and-compiler platforms. Additional historical note that I assume True Believers will ignore as meaningless: X11 used to make the same assumption that cpp was always and forever guaranteed to be friendly to non-C and this still shows at times in things like xrdb resource databases. They did accept the inevitable and (mostly) stop abusing it that way, and are now moving away from imake which likewise assumes it's safe to use cpp on Makefiles. (And yes, I encounter the same inability to comprehend or accept change there.) -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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How to define a type-level append?
by Bas van Dijk 06 May '15

06 May '15

Hi, At the bottom I have a type-level programming related question, but first some preliminaries: > {-# LANGUAGE GADTs #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE DataKinds #-} > > import GHC.TypeLits I've implemented a protocol where a client can send a packet of orders to a server. An OrderPacket is either empty (represented by the "NoOrders" constructor) or consists of an Order and possibly more orders (represented by the: "order :> orders" constructor). An Order is defined by a type-level list of numeric addresses and a command. The semantics is that the server should address the command to the list of addresses. When the client sends a packet of orders, the server has to return a reply for each command for each address in order. I lifted this expectation into the type signature so that when the client sends an order the type-checker will complain if it tries to pattern match on the wrong reply. The above is formalized with the following GADT for a packet of orders (I left out some off-topic stuff): > data OrderPacket replies where > NoOrders :: OrderPacket '[] > (:>) :: !(Order addrs cmd) > -> !(OrderPacket replies) > -> OrderPacket > (RepliesFor addrs cmd :++: replies) > > infixr 5 :> > > data Order (addrs :: [Nat]) command where > Order :: (Command command) > => !command -> Order addrs command > > class Command command where > type Response command :: * > -- I left out some encoding/decoding related methods. > > type family RepliesFor (addrs :: [Nat]) > (cmd :: *) :: [*] where > RepliesFor '[] cmd = '[] > RepliesFor (addr ': addrs) cmd = > Reply addr cmd ': RepliesFor addrs cmd > > type family (xs :: [*]) :++: (ys :: [*]) :: [*] where > '[] :++: ys = ys > (x ': xs) :++: ys = x ': (xs :++: ys) > > infixr 5 :++: > > data Reply (addr :: Nat) command = > Response !(Maybe (Response command)) > -- I left out some other stuff that can be in a reply. > Now my problem, I would like to have the ability to append two OrderPackets. I would say the following type properly reflects what I want: > append :: OrderPacket replies1 > -> OrderPacket replies2 > -> OrderPacket (replies1 :++: replies2) How would I implement this? The following naive defintion > append NoOrders ys = ys > append (x :> xs) ys = x :> append xs ys gives the following type error: src/Test.lhs:77:25: Could not deduce ((RepliesFor addrs cmd :++: (replies :++: replies2)) ~ (replies1 :++: replies2)) from the context (replies1 ~ (RepliesFor addrs cmd :++: replies)) bound by a pattern with constructor :> :: forall (addrs :: [Nat]) cmd (replies :: [*]). Order addrs cmd -> OrderPacket replies -> OrderPacket (RepliesFor addrs cmd :++: replies), in an equation for ‘append’ at src/Test.lhs:77:11-17 NB: ‘:++:’ is a type function, and may not be injective Expected type: OrderPacket (replies1 :++: replies2) Actual type: OrderPacket (RepliesFor addrs cmd :++: (replies :++: replies2)) Relevant bindings include ys :: OrderPacket replies2 (bound at src/Test.lhs:77:20) xs :: OrderPacket replies (bound at src/Test.lhs:77:16) x :: Order addrs cmd (bound at src/Test.lhs:77:11) append :: OrderPacket replies1 -> OrderPacket replies2 -> OrderPacket (replies1 :++: replies2) (bound at src/Test.lhs:76:3) In the expression: x :> append xs ys In an equation for ‘append’: append (x :> xs) ys = x :> append xs ys I suspect I need to adapt the type signature of "append" to suite the definition. Cheers, Bas

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Re: [Haskell-cafe] RFC: "Native -XCPP" Proposal
by Herbert Valerio Riedel 06 May '15

06 May '15

On 2015-05-06 at 17:36:05 +0200, Stephen Paul Weber wrote: >>As you may be aware, GHC's `{-# LANGUAGE CPP #-}` language extension >>currently relies on the system's C-compiler bundled `cpp` program to >>provide a "traditional mode" c-preprocessor. > > Yes. This is one of my favourite things in GHC-land -- that an > existing, good-enough, standardised, and widely-deployed solution was > chosen over a NiH reinvention of preprocessing. This allows other > Haskell compilers to support CPP on basically any system (since cpp is > so standard) without much effort, or even if the compiler does not > support {-# LANGUAGE CPP -#} the user can easily run `cpp` over the > source files themselves before feeding the source into the compiler. > > Because it is a real `cpp` being used, the developmer must take care > to follow the CPP syntax in the file that will then be transformed > into Haskell by `cpp` in the same way that C, C++, and other > developers have to take extra care (especially around use of # and > end-of-line \) when using `cpp`, but this is the normal state of > affairs for a secondary preprocessor step. As a benefit, the source > code will be processable by standard `cpp` implementations available > for virtually every platform. > > In short, the current solution provides a very robust and portable way > to do pre-compile preprocessing, and I like it very much. The problem I have with that line of argument is that we're using the so-called 'traditional mode' of `cpp`[1] for which afaik there is no written down common specification different implementations commit to adhere to (well, that's because 'traditional mode' refers to some vague implementation-specific "pre-standard" cpp semantics). And how is the developer supposed to take care to follow the (traditional mode) CPP syntax, if he can't test it easily with all potentially used (traditional-mode) `cpp`s out there? This already has led to problems with Clang's cpp vs GCC's cpp. Moreover, I was under the impression that it's only a matter of time till `traditional mode` support may be dropped from C compiler toolchains. Otoh, we can't use an ISO C spec conforming c-preprocessor, as that would conflict even more heavily w/ Haskell's grammar to the point of being inpractical. [1]: https://gcc.gnu.org/onlinedocs/cpp/Traditional-Mode.html

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Re: [Haskell-cafe] RFC: "Native -XCPP" Proposal
by Brandon Allbery 06 May '15

06 May '15

On Wed, May 6, 2015 at 11:27 AM, Kosyrev Serge <_deepfire(a)feelingofgreen.ru> wrote: > Why *shouldn't* TH fill that role? What can be done about it? For one, it's difficult to make it available in cross compilers (granted, work is being done on this) and not available on some platforms (ARM has been a problem, dunno if it currently is). For another, I don't think you can currently control things like imports or LANGUAGE pragmas --- and as TH is currently constructed it's not clear that you could do so, or that you could do so in a way that is sane for users. This is not to say that I like cpp --- I'd like it a lot more if it weren't actually using a C preprocessor that is not actually under our control or guaranteed to be compatible with Haskell --- but it does provide a "meta" in a different dimension than TH does. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: [Haskell-cafe] RFC: "Native -XCPP" Proposal
by Brandon Allbery 06 May '15

06 May '15

On Wed, May 6, 2015 at 10:59 AM, Bardur Arantsson <spam(a)scientician.net> wrote: > (I'm not going to be doing any of the work, so this is just armchairing, > but this seems like an 80/20 solution would be warranted.) > Only if you're convinced it will remain 80/20 for the foreseeable future. I do not want to bet on Linux always being gcc (and dislike the One True Platform-ism that line of thought encourages). -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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minor confusion over kinds....
by Nicholls, Mark 06 May '15

06 May '15

And what "*" means as opposed to other things e.g. an explicit name "Nat" or a kind variable "k" > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE ExplicitForAll #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE GADTs #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE UndecidableInstances #-} what I want is a typeclass with instances of type level Naturals here's the class, the parameter is of kind (* -> *) i.e. a is some sort of wrapper that maps from types to values > class Foo (a :: * -> *) where > type Bar a > bar :: a (Bar a) lets ignore DataKinds define Zero and Succ as distinct types uninhabited...but of kind "*" and "k -> *" > data Z1 > data S1 nat SNat1 :: * -> * > data SNat1 a where > SZ1 :: SNat1 Z1 > SS1 :: SNat1 a -> SNat1 (S1 a) off we go... > instance Foo SNat1 where > type Bar SNat1 = Z1 > bar = SZ1 now lets do it the datakind route > data Nat where > Z :: Nat > S :: Nat -> Nat ahhh...SNat is of kind "Nat -> *" > data SNat a where > SZ :: SNat 'Z > SS :: SNat a -> SNat ('S a) so I cant make it an instance of the same type class "The first argument of 'Foo' should have kind '* -> *', but 'SNat' has kind 'Nat -> *' In the instance declaration for 'Foo SNat'" > --instance Foo SNat where > -- type Bar SNat = 'Z > -- bar = SZ unless I redefine my class (which seems a bit silly) > class Foo' (a :: k -> *) where > type Bar' a :: k > bar' :: a (Bar' a) now it works. > instance Foo' SNat where > type Bar' SNat = 'Z > bar' = SZ clearly named kinds (?) are somehow distinct from unnames ones? CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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Assistant Professorship in Nottingham
by Graham Hutton 06 May '15

06 May '15

Dear all, The School of Computer Science at the University of Nottingham is seeking to appoint a new Assistant Professor: http://www.nottingham.ac.uk/jobs/currentvacancies/ref/SCI41814 Applications in the area of the Functional Programming (FP) lab are encouraged. The FP lab is keen to receive applications from candidates with an excellent publication record, experience in combining theory with practice, and the ability to secure external funding to support their research. Further information about the FP lab is available from: http://fp.cs.nott.ac.uk The deadline for applications is Wednesday 15th July 2015. Best wishes, Graham -- Prof Graham Hutton Functional Programming Lab School of Computer Science University of Nottingham, UK http://www.cs.nott.ac.uk/~gmh 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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ANN: HList 0.4
by adam vogt 06 May '15

06 May '15

Hello List, I've uploaded HList-0.4 [1]. Some highlights from this release: * support for 7.6 <= GHC <= 7.10 * new modules HCurry and HSort * many new functions: see the changelog [2] * data HList becomes data family. Trade-offs are explained in the documentation [3] Regards, Adam [1] http://hackage.haskell.org/package/HList-0.4.0.0 [2] http://hackage.haskell.org/package/HList-0.4.0.0/changelog [3] http://hackage.haskell.org/package/HList-0.4.0.0/docs/Data-HList-HList.html…

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ANN: Yampa 0.9.7 and 0.10.0
by Ivan Perez 05 May '15

05 May '15

Dear all Two new versions of Yampa have just been made available on hackage. A short reminder for those who don't know: Yampa is a Functional Reactive Programming DSL structured around Signal Functions. It enforces a separation between effectful input/output and purely functional signal transformations. Arrow notation can be used to connect signal functions. Yampa has recently been used to write several games available on Github [2] and Google Play (for Android) [3, 4]. Regarding the new versions: - Version 0.9.7 is fully backwards compatible and includes minimal changes. - Version 0.10.0 is mostly, but not fully, backwards compatible and has been restructured in modules to make documentation easier. We expect this to help users find and understand the available combinators better. I have tried both versions with the game Haskanoid [2] and they work. If your project cannot be compiled or executed correctly with either version, please open an issue on our project page [1]. Yampa is being actively developed, and we are working on FRP extensions that we hope will be introduced in future versions. Code, documentation and research contributions are welcome. All the best Ivan [1] https://github.com/ivanperez-keera/Yampa [2] https://github.com/ivanperez-keera/haskanoid [3] https://play.google.com/store/apps/details?id=uk.co.keera.games.magiccookies [4] https://github.com/keera-studios/magic-cookies

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Re: [Haskell-cafe] Significant changes in -Wall for GHC 7.10
by Alexander Eyers-Taylor 05 May '15

05 May '15

Sorry I forgot to reply to the list. Here is my earlier email -------- Forwarded Message -------- Subject: Re: [Haskell-cafe] Significant changes in -Wall for GHC 7.10 Date: Tue, 05 May 2015 16:29:46 +0100 From: Alexander Eyers-Taylor <aeyerstaylor11(a)gmail.com> To: Omari Norman <omari(a)smileystation.com> Those seem to be preciously the flags enabled by -Wall. It seems when updating the documentation the negative was missed and it was updated to list the flags enabled by -Wall. I have submitted a ticket (#10386) to fix the bug. On 05/05/15 16:03, Omari Norman wrote: > CORRECTION: -fwarn-trustworthy-safe was not in -Wall in GHC 7.8, but > -fwarn-trustworthy-safe did not exist in GHC 7.8. > > On Tue, May 5, 2015 at 10:54 AM, Omari Norman <omari(a)smileystation.com > <mailto:omari@smileystation.com>> wrote: > > I noticed some significant changes to the flags included with > -Wall in GHC 7.10. Many flags that were previously included in > -Wall are now not included. These include: > > -fwarn-type-defaults > -fwarn-name-shadowing > -fwarn-missing-signatures > -fwarn-hi-shadowing > -fwarn-orphans > -fwarn-unused-do-bind > -fwarn-trustworthy-safe > > I can't find any mention in the release notes of this change in > behavior. > > I was wondering what the rationale is for these changes? Any link > to relevant discussion would be appreciated if it exists. > > Also, what opinion do people have? I previously kept my code > -Wall clean, but sometimes that would be a pain. For instance I > would munge local names so they wouldn't trigger > -fwarn-name-shadowing, and I would add signatures to integers so > they wouldn't trigger -fwarn-type-defaults. In fact, I only > noticed this change in behavior in 7.10 because I was considering > dumping -Wall, and when I looked at the 7.10 manual I saw that it > does not enable -fwarn-name-shadowing. (I still use 7.8 but > Google pulled the 7.10 manual.) Does this change indicate that > people were finding -Wall too onerous and, thus, not using it? > > I do remember reading complaints about -fwarn-unused-do-bind being > added to -Wall, but I previously felt dirty about name shadowing > though I wondered if munging names to avoid shadowing was actually > worse. > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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Significant changes in -Wall for GHC 7.10
by Omari Norman 05 May '15

05 May '15

I noticed some significant changes to the flags included with -Wall in GHC 7.10. Many flags that were previously included in -Wall are now not included. These include: -fwarn-type-defaults -fwarn-name-shadowing -fwarn-missing-signatures -fwarn-hi-shadowing -fwarn-orphans -fwarn-unused-do-bind -fwarn-trustworthy-safe I can't find any mention in the release notes of this change in behavior. I was wondering what the rationale is for these changes? Any link to relevant discussion would be appreciated if it exists. Also, what opinion do people have? I previously kept my code -Wall clean, but sometimes that would be a pain. For instance I would munge local names so they wouldn't trigger -fwarn-name-shadowing, and I would add signatures to integers so they wouldn't trigger -fwarn-type-defaults. In fact, I only noticed this change in behavior in 7.10 because I was considering dumping -Wall, and when I looked at the 7.10 manual I saw that it does not enable -fwarn-name-shadowing. (I still use 7.8 but Google pulled the 7.10 manual.) Does this change indicate that people were finding -Wall too onerous and, thus, not using it? I do remember reading complaints about -fwarn-unused-do-bind being added to -Wall, but I previously felt dirty about name shadowing though I wondered if munging names to avoid shadowing was actually worse.

2 4

finding things on haskell.org
by Doug McIlroy 05 May '15

05 May '15

The only search feature I'm aware of on the haskell website is the haskell wiki, which is a self-contained little corner. For example, the wiki says nothing about minGHC. To find out what the website has on the subject, I used a Google query: site:haskell.org minghc. Is there thought of a comprehensive site-search facility that can find anything, including stuff that hasn't been wiki-ized? Doug

2 1

Language.Perl? (static analysis of Perl code, in Haskell)
by Johannes Waldmann 05 May '15

05 May '15

I am investigating options for analyzing, and refactoring, legacy Perl code. Ideally, the tools would be in a high-level language like Haskell. I found [Pugs][1] and (related) [Language.Perl5][2] but both appear dead. Is there something for Perl that would provide what [hack][4] provides for PHP, and [typescript][5] for JS? (Perhaps I should write a Perl to PHP compiler.) Yes, I know that [Perl is not parseable][3]. - J.W. [1]: http://hackage.haskell.org/package/Pugs [2]: http://hackage.haskell.org/package/HsPerl5 [3]: http://www.perlmonks.org/?node_id=800599 [4]: http://hacklang.org/ [5]: http://www.typescriptlang.org/

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Re: [Haskell-cafe] Improvements to package hosting and security
by Mathieu Boespflug 05 May '15

05 May '15

This is a valid concern. One that I should have addressed explicitly in the proposal. Git is fairly well supported on Windows these days and installs easily. It could conceivably be included as part of MinGHC. There are many alternatives, but I doubt we'll need them: statically linking a C implementation (libgit2 or another), or a simple native implementation of the git protocol (the protocol is quite straightforward and is documented) and basic disk format. The same is true about GnuPG, via gpg4win, though note that under this proposal GnuPG wouldn't be a requirement for `cabal update` to work. Just an additional optional dependency which you'll want to have installed if you want to protect yourself from the attacks listed in the proposal. By the way, one side note about this Git proposal: it sides steps the discussion around how to add SSL support to cabal-install entirely. Since Git understands (among others) HTTPS natively, so we can outsource our support for that to Git. In any case SSL no longer becomes a necessity for protecting against MITM (the commit signing takes care of that), only a nice-to-have for privacy. On 28 April 2015 at 19:46, <radix(a)twistedmatrix.com> wrote: > To me, the elephant in the room is how the dependency on Git will be > handled. I'm not a Windows user, but how much more painful will it be to set > up a Haskell environment on Windows with a new dependency on Git? Will users > need to install it separately, or do you suggest embedding Git into the > relevant tools? Should the Haskell Platform bundle it? What about MinGHC? > Oh, and I guess the same question can be asked about GnuPG. > > I personally use a Mac and Homebrew, so it's pretty easy for me to install > those dependencies, and I'm sure the same is true on Linux. But also, not > everyone uses Homebrew (in fact, I'm sure most programmers on Macs don't use > it), so it's also worth considering whether the requisite tools should be > embedded in the "GHC for Mac OS X" distribution. > > On Linux this probably isn't an issue because pretty much everyone has a > decent dependency-tracking package manager. > > I don't know if you care personally about these issues, but I think any > proposal which introduces new dependencies to the core development > environment of Haskell should take it into consideration. Very few people > have Git and GPG already installed, and I think the new-user experience > should be considered, and I'm surprised nobody has mentioned it in this > entire thread (unless I missed it). > > -- radix (Christopher Armstrong) > > P.S. I'm very excited to see this work, including the emphasis on using the > well-researched TUF. Thanks to you and other people working on this. :) > > On Tuesday, April 28, 2015 at 5:07:56 AM UTC-5, Mathieu Boespflug wrote: >> >> Hi all, >> >> last week, I found some time to write up a very simple proposal that >> addresses the following goals simultaneously: >> >> - maintain a difficult to forge public audit log of Hackage updates; >> - make downloads from Hackage mirrors just as trustworthy as >> downloading from Hackage itself; >> - guarantee that `cabal update` is always pulling the freshest package >> index (called "snapshots" in the proposal), and detect when this might >> not be the case; >> - implement the first half of TUF (namely the index signing part >> discussed in Duncan's blog post, not the author package signing part) >> with fewer metadata files and in a way that reuses existing tooling; >> - get low-implementation-cost, straightforward and incremental `cabal >> update`. >> >> After a preliminary review from a few colleagues and friends in the >> community, here is the proposal, in the form of Commercial Haskell >> wiki page: >> >> >> https://github.com/commercialhaskell/commercialhaskell/wiki/Git-backed-Hack… >> >> The design constraints here are: >> >> - stay backwards compatible where the cost for doing so is low. >> - reuse existing tooling and mechanisms, especially when it comes to >> key management, snapshot identity, and distributing signatures. >> - Focus on the above 5 goals only, because they happen to all be >> solvable by changing a single piece of mechanism. But strive to reuse >> whatever mechanism others are proposing to solve other goals (e.g. >> certification of provenance using author package signing, as Chris >> Done has already proposed). >> >> To that effect, the tl;dr is that I'm proposing that we just use Git >> for maintaining the Hackage package index, that we use Git for >> synchronizing this locally, and that we use Git commit signatures for >> implementing the first half of TUF. The Git tooling currently assumes >> GnuPG keys for signatures, so I'm proposing that we use GnuPG keys for >> signing, and that we manage key revocation and any trust delegation >> between keys using GnuPG and its existing infrasture. >> >> I estimate the total effort necessary here to be the equivalent of 5-6 >> full time days overall. However, I have not pooled the necessary >> resources to carry that out yet. I'd like to get feedback first before >> going ahead with this, but in meantime, >> >> ** if there are any volunteers that would like to signal their intent >> to help with the implementation effort then please add your name at >> the bottom of the wiki page. ** >> >> Best, >> >> Mathieu >> >> On 18 April 2015 at 20:11, Michael Snoyman <mic...@snoyman.com> wrote: >> > >> > >> > On Sat, Apr 18, 2015 at 12:20 AM Bardur Arantsson >> > <sp...@scientician.net> >> > wrote: >> >> >> >> On 17-04-2015 10:17, Michael Snoyman wrote: >> >> > This is a great idea, thank you both for raising it. I was discussing >> >> > something similar with others in a text chat earlier this morning. >> >> > I've >> >> > gone ahead and put together a page to cover this discussion: >> >> > >> >> > >> >> > >> >> > https://github.com/commercialhaskell/commercialhaskell/blob/master/proposal… >> >> > >> >> > The document definitely needs more work, this is just meant to get >> >> > the >> >> > ball >> >> > rolling. As usual with the commercialhaskell repo, if anyone wants >> >> > edit >> >> > access, just request it on the issue tracker. Or most likely, send a >> >> > PR >> >> > and >> >> > you'll get a commit bit almost magically ;) >> >> >> >> Thank you. Just to make sure that I understand -- is this page only >> >> meant to cover the original "strawman proposal" at the start of this >> >> thread, or...? >> >> >> >> Maybe you intend for this to be extended in a detailed way under the >> >> "Long-term solutions" heading? >> >> >> >> I was imagining a wiki page which could perhaps start out by collecting >> >> all the currently identified possible threats in a table, and then all >> >> "participants" could perhaps fill in how their suggestion addresses >> >> those threats (or tell us why we shouldn't care about this particular >> >> threat). Of course other relevent non-threat considerations might be >> >> relevant to add to such a table, such as: how prevalent is the >> >> software/idea we're basing this on? does this have any prior >> >> implementation (e.g. the append-to-tar and expect that web servers will >> >> behave sanely thing)? etc. >> >> >> >> (I realize that I'm asking for a lot of work, but I think it's going to >> >> be necessary, at least if there's going to be consensus and not just a >> >> de-facto "winner".) >> >> >> >> >> > >> > Hi Bardur, >> > >> > >> > I don't think I have any different intention for this page than you've >> > identified. In fact, I thought that I had clearly said exactly what you >> > described when I said: >> > >> >> There are various ideas at play already. The bullets are not intended >> >> to >> >> be full representations of the proposals, but rather high level >> >> summaries. >> >> We should continue to expand this page with more details going forward. >> > >> > If this is unclear somehow, please tell me. But my intention absolutely >> > is >> > that many people can edit this page to add their ideas and we can flesh >> > out >> > a complete solution. >> > >> > Michael >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskel...@haskell.org >> > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> > >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskel...@haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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IFL 2015 -- call for papers
by Simon Thompson 05 May '15

05 May '15

IFL 2015 - Call for papers 27th SYMPOSIUM ON IMPLEMENTATION AND APPLICATION OF FUNCTIONAL LANGUAGES - IFL 2015 University of Koblenz-Landau, Koblenz, Germany In cooperation with ACM SIGPLAN September 14-16, 2015 http://ifl2015.wikidot.com/ Scope The goal of the IFL symposia is to bring together researchers actively engaged in the implementation and application of functional and function-based programming languages. IFL 2015 will be a venue for researchers to present and discuss new ideas and concepts, work in progress, and publication-ripe results related to the implementation and application of functional languages and function-based programming. Peer-review Following the IFL tradition, IFL 2015 will use a post-symposium review process to produce the formal proceedings. All participants of IFL2015 are invited to submit either a draft paper or an extended abstract describing work to be presented at the symposium. At no time may work submitted to IFL be simultaneously submitted to other venues; submissions must adhere to ACM SIGPLAN's republication policy: http://www.sigplan.org/Resources/Policies/Republication The submissions will be screened by the program committee chair to make sure they are within the scope of IFL, and will appear in the draft proceedings distributed at the symposium. Submissions appearing in the draft proceedings are not peer-reviewed publications. Hence, publications that appear only in the draft proceedings do not count as publication for the ACM SIGPLAN republication policy. After the symposium, authors will be given the opportunity to incorporate the feedback from discussions at the symposium and will be invited to submit a revised full article for the formal review process. From the revised submissions, the program committee will select papers for the formal proceedings considering their correctness, novelty, originality, relevance, significance, and clarity. Important dates August 10: Submission deadline draft papers August 12: Notification of acceptance for presentation August 14: Early registration deadline August 21: Late registration deadline September 7: Submission deadline for pre-symposium proceedings September 14-16: IFL Symposium December 1: Submission deadline for post-symposium proceedings January 15, 2016: Notification of acceptance for post-symposium proceedings March 1, 2016: Camera-ready version for post-symposium proceedings Submission details Prospective authors are encouraged to submit papers or extended abstracts to be published in the draft proceedings and to present them at the symposium. All contributions must be written in English. Papers must adhere to the standard ACM two columns conference format. For the pre-symposium proceedings we adopt a 'weak' page limit of 12 pages. For the post-symposium proceedings the page limit of 12 pages is firm. A suitable document template for LaTeX can be found at: http://www.acm.org/sigs/sigplan/authorInformation.htm Authors submit through EasyChair: https://easychair.org/conferences/?conf=ifl2015 Topics IFL welcomes submissions describing practical and theoretical work as well as submissions describing applications and tools in the context of functional programming. If you are not sure whether your work is appropriate for IFL 2015, please contact the PC chair at rlaemmel(a)acm.org. Topics of interest include, but are not limited to: - language concepts - type systems, type checking, type inferencing - compilation techniques - staged compilation - run-time function specialization - run-time code generation - partial evaluation - (abstract) interpretation - metaprogramming - generic programming - automatic program generation - array processing - concurrent/parallel programming - concurrent/parallel program execution - embedded systems - web applications - (embedded) domain specific languages - security - novel memory management techniques - run-time profiling performance measurements - debugging and tracing - virtual/abstract machine architectures - validation, verification of functional programs - tools and programming techniques - (industrial) applications Peter Landin Prize The Peter Landin Prize is awarded to the best paper presented at the symposium every year. The honored article is selected by the program committee based on the submissions received for the formal review process. The prize carries a cash award equivalent to 150 Euros. Programme committee Chair: Ralf Lämmel, University of Koblenz-Landau, Germany - Malgorzata Biernacka, University of Wroclaw, Poland - Laura M. Castro, University of A Coruña, Spain - Martin Erwig, Oregon State University, USA - Dan Ghica, University of Birmingham, UK - Andrew Gill, University of Kansas, USA - Stephan Herhut, Google, USA - Zhenjiang Hu, National Institute of Informatics (NII), Japan - Mauro Jaskelioff, CIFASIS/Universidad Nacional de Rosario, Argentina - Frédéric Jouault, ESEO, France - Oleg Kiselyov, Tohoku University, Japan - Lindsey Kuper, Indiana University, USA - Rita Loogen, Philipps-Universität Marburg, Germany - Akimasa Morihata, University of Tokyo, Japan - Atsushi Ohori, Tohoku University, Japan - Bruno C. D. S. Oliveira, The University of Hong Kong, Hong Kong - Frank Piessens, Katholieke Universiteit Leuven, Belgium - Norman Ramsey, Tufts University, USA - Matthew Roberts, Macquarie University, Australia - Manfred Schmidt-Schauss, Goethe-University Frankfurt, Germany - Simon Thompson, University of Kent, UK - Stephanie Weirich, University of Pennsylvania, USA - Steve Zdancewic, University of Pennsylvania , USA Venue The 27th IFL will be held in association with the Faculty of Computer Science, University of Koblenz-Landau, Campus Koblenz. Koblenz is well connected by train to several international airports. For instance, Koblenz can be reached from Frankfurt by high-speed train ICE within an hour. The modern Koblenz campus is close to the city center and can be reached by foot, bus, or cab. See the website for more information on the venue. Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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