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Question about typeclass & constraints
by jean-christophe mincke 08 Apr '14

08 Apr '14

Hello Consider the following data, class and instance: class PP m where create :: a -> m a data A a = A a instance PP A where create a = A a And then: class CB a where fb :: a -> Int data B m a = B (m a) If I try to define an instance of PP for B with the following implementation: instance (PP m) => PP (B m) where create a = let _ = fb a in B (create a) GHC issues the (expected) error: Could not deduce (CB a) arising from a use of 'fb' from the context (PP m). So the workaround I am using is to define a new class PP' where the type a is made more explicit. That allows the constraint (CB a) to be stated in an instance declaration. class PP' m a where create' :: a -> m a instance (PP m) => PP' m a where create' = create instance (PP m, CB a) => PP' (B m) a where create' a = let _ = fb a in B (create a) So, my question is: is there a way to achieve the same results without having to create the new class PP'? Thank you for your help J-C

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ANN: Melbourne Haskell Users Group
by Lyndon Maydwell 08 Apr '14

08 Apr '14

Hi Haskellers, A few people have been meeting up in the Melbourne CBD to talk about Haskell for the past couple of months. This month I've asked Bernie Pope (who organised the Melbourne Functional Programming Union) to give a talk about two libraries he has written in Haskell that compile Python code to Haskell, and Python Byte-Code. I thought I'd take the opportunity to announce the this meetup and the group in general to the wider Haskell community in case there is anyone in Melbourne who would be interested in coming along. The talk will be on Thursday, April 24. Details can be found at: http://www.meetup.com/Melbourne-Haskell-Users-Group/events/174401572/ Hope to see you there! - Lyndon

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Can I write this function without doing the action on a different line?
by Matt Joiner 07 Apr '14

07 Apr '14

1. packageBase <- do 2. curDir <- getCurrentDirectory 3. return . last . splitDirectories . normalise . joinPath $ curDir:maybeToList package

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Fwd: c2hs: E.g. #include <iostream> OK in *.cpp's, but not in *.h's
by Nick Rudnick 07 Apr '14

07 Apr '14

---------- Forwarded message ---------- From: Nick Rudnick <nick.rudnick(a)gmail.com> Date: 2014-04-07 15:28 GMT+02:00 Subject: Re: [Haskell-cafe] c2hs: E.g. #include <iostream> OK in *.cpp's, but not in *.h's To: Ian Ross <ian(a)skybluetrades.net> Hi Ian, thanks for clearing up :-) So you mean the trick of hsqml primarily is (A) dummy typedefs to char, e.g.: typedef char HsQMLStringHandle; (B) 'extern' sigantures, e.g.: extern void hsqml_init_string(HsQMLStringHandle*); THIS WORKS... Occasionally, one might use two headers, e.g. a *.hpp as actual C++ header neither listed in the *.cabal nor the *.chs, where instead it would be represented by a *.h, as described above. No more...?? Thanks and cheers, Nick 2014-04-07 14:04 GMT+02:00 Ian Ross <ian(a)skybluetrades.net>: Hi Nick, > > As far as I can tell from a quick look at hsqml, the stuff in Setup.hs > isn't related to C2HS. What's more relevant is that hsqml defines a C > library wrapper around the C++ library it uses -- C2HS is only used on that > C wrapper, not on the C++ library itself. In general, C2HS doesn't support > C++ at all, because it needs to be able to parse the header files that it > includes and we use the C parser from the languagge-c package to do that. > Any C++ constructs will just cause breakage. And your "old school" FFI > definition doesn't look in any header files, which is why it has no trouble. > > Basically, if you want to do something like what hsqml does, just take a > look in the cbits directory there, in particular at hsqml.h which is the > header that defines the C wrapper around the C++ library. This hsqml.h > header is the one that's included in the C2HS files. If you want to wrap a > C++ library directly, C2HS probably isn't the tool for the job. I know > there was some work during the last GSOC on a C++ FFI wrapper, but I don't > know if anything concrete came of it. > > Cheers, > > Ian. > > > > On 6 April 2014 20:22, Nick Rudnick <nick.rudnick(a)gmail.com> wrote: > >> Dear all, >> >> maybe this is a piece of c2hs behaviour I missed to understand, but it >> appears happening with C++ style includes deep in the header call hierarchy >> – in case of a such backend C++ library, if one wishes to refer to by *.h's >> in c2hs (e.g. for access to types of it), one might have a problem. >> >> - 8< cSample.h --------------------------------------------- >> #include <iostream> ///////// here, with c2hs, leading to build abort >> #ifdef __cplusplus >> extern "C" { >> #endif >> void sampleFunction(); >> #ifdef __cplusplus >> } >> #endif >> - 8< cSample.cpp --------------------------------------------- >> #include "cSample.h" >> #include <iostream> ///////// here no problem >> >> void sampleFunction(){ >> std::cout << "OK" << std::endl; >> } >> - 8< ---------------------------------------------------------------- >> >> While 'old school' FFI (foreign import ccall) handles this effortlessly, >> c2hs uses to abort with an exception like >> C/cSample.h:3:20: fatal error: iostream: No such file or directory >> compilation terminated. >> >> I see that hsqml gets around this, but apparently by tweaking Setup.hs in >> a way not quite trivial – would this be necessary in any case, or are there >> simpler alternatives? >> >> Please excuse if I oversaw something, but I am afraid I didn't see this >> interesting issue covered elsewhere. >> >> Thank you a lot in advance & cheers, Nick >> >> >> >> >> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > > > -- > Ian Ross Tel: +43(0)6804451378 ian(a)skybluetrades.net > www.skybluetrades.net >

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Domain Events in haskell
by Rouan van Dalen 07 Apr '14

07 Apr '14

Hi Cafe, I am trying to write a very simple implementation of an event publisher pattern but I am stuck and do not know how to do this in Haskell. I have the following code: ======================== {-# LANGUAGE RankNTypes, NamedFieldPuns #-} module Domain.DomainEventPublisher where import Control.Monad (forM_) import HsFu.Data.DateTime import Domain.Client data DomainEvent = ClientChangeAgeDomainEvent data DomainEventContext = DomainEventContext { domainEventContext_event :: DomainEvent , domainEventContext_occurredOn :: DateTime } deriving (Show) data DomainEventPublisher = DomainEventPublisher { domainEventPublisher_subscribers :: [DomainEventContext -> IO ()] } mkEventPublisher :: DomainEventPublisher mkEventPublisher = DomainEventPublisher [] subscribe :: DomainEventPublisher -> (DomainEventContext -> IO ()) -> DomainEventPublisher subscribe publisher eventHandler = DomainEventPublisher { domainEventPublisher_subscribers = eventHandler : (domainEventPublisher_subscribers publisher) } publish :: DomainEventPublisher -> DomainEventContext -> IO () publish DomainEventPublisher{ domainEventPublisher_subscribers } event = forM_ domainEventPublisher_subscribers ($ event) ======================== My problem is that the publish method returns IO (), which means that events can only be published from the IO monad, but I would like events to be 'publish-able' from pure code. I can live with event handlers (passed into the subscribe function) being in the IO monad. Is there a better way to implement this pattern in Haskell? I have been racking my brain on this for a while now and cannot seem to come up with a good implementation. Regards --Rouan.

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c2hs: E.g. #include <iostream> OK in *.cpp's, but not in *.h's
by Nick Rudnick 07 Apr '14

07 Apr '14

Dear all, maybe this is a piece of c2hs behaviour I missed to understand, but it appears happening with C++ style includes deep in the header call hierarchy – in case of a such backend C++ library, if one wishes to refer to by *.h's in c2hs (e.g. for access to types of it), one might have a problem. - 8< cSample.h --------------------------------------------- #include <iostream> ///////// here, with c2hs, leading to build abort #ifdef __cplusplus extern "C" { #endif void sampleFunction(); #ifdef __cplusplus } #endif - 8< cSample.cpp --------------------------------------------- #include "cSample.h" #include <iostream> ///////// here no problem void sampleFunction(){ std::cout << "OK" << std::endl; } - 8< ---------------------------------------------------------------- While 'old school' FFI (foreign import ccall) handles this effortlessly, c2hs uses to abort with an exception like C/cSample.h:3:20: fatal error: iostream: No such file or directory compilation terminated. I see that hsqml gets around this, but apparently by tweaking Setup.hs in a way not quite trivial – would this be necessary in any case, or are there simpler alternatives? Please excuse if I oversaw something, but I am afraid I didn't see this interesting issue covered elsewhere. Thank you a lot in advance & cheers, Nick

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Functional Software Engineering Posts
by Khushil Dep (Sphonic) 07 Apr '14

07 Apr '14

Symphonic Solutions Ltd (www.sphonic.com) is a disruptive technology company that leverages functional paradigms to enable engineered innovation. In real terms, we build high performance, low latency platforms that integrate to a plethora of third party API’s and apply intelligent manipulation and analysis to the data via a DSL. We help turn ‘Big Data’ into ‘Useful Information’. We also don’t use Hadoop - which is nice. We’re looking for software engineers who understand the functional domain extremely well. Ideally you will be a polyglot software engineer with skills and at least twelve months (commercial) experience in Scala (with with Akka) || Clojure || Haskell || NodeJS. We run all of these in production so it’s important you understand the difference between experimentation and production quality code. You should also be able to at least read one of either Python, Bash or C. We don’t allow Ruby anywhere on our platform so you should be comfortable about that. We’re looking for people who want to live the functional dream, not people who think it’s a ‘nice to have’ on their C.V please. You should understand the sympathetic orchestration of human, software, operating system and hardware. You should also be able to translate complex functional requirements into simple, well composed programs which work in concert to produce the desired result. Experience in working within a micro-services architecture will be looked on very favourably. We are flexible on working locations, including home working, with a London office based near Edgware Road station. During the early phases of engineering design some time in the office will be a necessity however and you should be comfortable about that. You should be able to work independently and within a wider team as well as manage your time such that your responsibilities in each sprint are delivered to a high quality and in a timely manner. You’ll be expected and empowered to make software design choices specific to your responsibilities in concert with the platform engineering team. You should be happy to work within an agile project management framework which supports Kanban. You should be familiar with working with JIRA, Confluence and Git. You will be expected to produce documentation around your sprint responsibilities which deliver a high level of knowledge retention and transfer. Your programs will be run on SmartOS platforms and so you should be able to build technology agnostic programs. In simple terms you should be happy not to rely on any operating system specific functionality in order to write your programs. You will be trained in technologies such as DTrace to better aid your diagnostic efforts. You will be provided with local build environments based off Vagrant which you should be happy to manage yourself though training and support from engineering will be available at all times. If this interests you, in the first instance, please answer the following questions : Question 1 Explain the relationship between the human and the machine. Describe the relationship in as much detail as you feel comfortable with. Question 2 Explain tail recursion to a 5 year old. Question 3 Explain how you would manage state in a functional manner to a 5 year old. We expect that you have public GitHub/Bitbucket repo’s sharing your work and/or research with functional languages and programs so please also send your GitHub/Bitbucket handle with your answers.Please send your answers and whatever you have of a C.V. to jobs(a)sphonic.com. --- Khushil Dep Head Of Engineering Symphonic Solutions Ltd khushildep(a)sphonic.com 07799908654 07415372737

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ANN: Budapest Hackathon 2014
by Csaba Hruska 07 Apr '14

07 Apr '14

Dear Haskellers, We're organizing the first ever Haskell Hackathon in Budapest, Hungary. When: Saturday 31 May 2014 - Sunday 1 June 2014 This will be a great opportunity to meet up with fellow Haskellers, and show off your cool hobby projects in one of the most vibrant cities in Europe. There's also room for working on existing tools, for example you can team up with Luite Stegeman at the event to hack on GHC-JS. Details about the when, where and how: http://www.haskell.org/haskellwiki/BudapestHackathon2014 See you all there!

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Second CFP: Higher-order, Typed, Inferred, Strict: ML Family Workshop
by oleg＠okmij.org 07 Apr '14

07 Apr '14

Higher-order, Typed, Inferred, Strict: ACM SIGPLAN ML Family Workshop Thursday September 4, 2014, Gothenburg, Sweden (immediately following ICFP and preceding OCaml Users and Developers Workshop) Call For Papers http://okmij.org/ftp/ML/ML14.html News: Post-proceedings will be published in EPTCS ML is a very large family of programming languages that includes Standard ML, OCaml, F#, SML#, Manticore, MetaOCaml, JoCaml, Alice ML, Dependent ML, Flow Caml, and many others. All ML languages, beside the great deal of syntax, share several fundamental traits. They are all higher-order, strict, mostly pure, and typed, with algebraic and other data types. Their type systems inherit from Hindley-Milner. The development of these languages has inspired a significant amount of computer science research and influenced a number of programming languages, including Haskell, Scala and Clojure, as well as Rust, ATS and many others. ML workshops have been held in affiliation with ICFP continuously since 2005. This workshop specifically aims to recognize the entire extended ML family and to provide the forum to present and discuss common issues, both practical (compilation techniques, implementations of concurrency and parallelism, programming for the Web) and theoretical (fancy types, module systems, metaprogramming). The scope of the workshop includes all aspects of the design, semantics, theory, application, implementation, and teaching of the members of the ML family. We also encourage presentations from related languages (such as Scala, Rust, Nemerle, ATS, etc.), to exchange experience of further developing ML ideas. The ML family workshop will be held in close coordination with the OCaml Users and Developers Workshop. Format Since 2010, the ML workshop has adopted an informal model. Presentations are selected from submitted abstracts. There are no published proceedings, so any contributions may be submitted for publication elsewhere. We hope that this format encourages the presentation of exciting (if unpolished) research and deliver a lively workshop atmosphere. Each presentation should take 20-25 minutes, except demos, which should take 10-15 minutes. The exact time will be decided based on the number of accepted submissions. The presentations will likely be recorded. Post-conference proceedings The post-proceedings of selected papers from the ML Family and the OCaml Users and Developers workshops will be published in the Electronic Proceedings in Theoretical Computer Science (EPTCS). The Program Committee shall invite interested authors of selected presentations to expand their abstract for inclusion in the proceedings. The submissions are to be reviewed according to the EPTCS standards. Coordination with the OCaml Users and Developers Workshop The OCaml workshop is seen as more practical and is dedicated in significant part to the OCaml community building and the evolution of the OCaml system. In contrast, the ML family workshop is not focused on any language in particular, is more research oriented, and deals with general issues of the ML-style programming and type systems. Yet there is an overlap, which we are keen to explore in various ways. The authors who feel their submission fits both workshops are encouraged to mention it at submission time or contact the Program Chairs. Scope We acknowledge the whole breadth of the ML family and aim to include languages that are closely related (although not by blood), such as Rust, ATS, Scala, Typed Clojure. Those languages have implemented and investigated run-time and type system choices that may be worth considering for OCaml, F# and other ML languages. We also hope that the exposure to the state of the art ML might favorably influence those related languages. Specifically, we seek research presentations on topics including but not limited to * Design: concurrency, distribution and mobility, programming for the web and embedded systems, handling semi-structured data, facilitating interactive programming, higher forms of polymorphism, generic programming, objects * Implementation: compilation techniques, interpreters, type checkers, partial evaluators, runtime systems, garbage collectors, etc. * Type systems: fancy types, inference, effects, overloading, modules, contracts, specifications and assertions, dynamic typing, error reporting, etc. * Applications: case studies, experience reports, pearls, etc. * Environments: libraries, tools, editors, debuggers, cross-language interoperability, functional data structures, etc. * Education: ML and ML-like languages in college or high-school, in general or computer science curriculum. Four kinds of submissions will be accepted: Informed Positions, Research Presentations, Experience Reports and Demos. * Informed Positions: A justified argument for or against a language feature. The argument must be substantiated, either theoretically (e.g., by a demonstration of (un)soundness, an inference algorithm, a complexity analysis), empirically or by a substantial experience. Personal experience is accepted as justification so long as it is extensive and illustrated with concrete examples. * Research Presentations: Research presentations should describe new ideas, experimental results, or significant advances in ML-related projects. We especially encourage presentations that describe work in progress, that outline a future research agenda, or that encourage lively discussion. These presentations should be structured in a way which can be, at least in part, of interest to (advanced) users. * Experience Reports: Users are invited to submit Experience Reports about their use of ML and related languages. These presentations do not need to contain original research but they should tell an interesting story to researchers or other advanced users, such as an innovative or unexpected use of advanced features or a description of the challenges they are facing or attempting to solve. * Demos: Live demonstrations or short tutorials should show new developments, interesting prototypes, or work in progress, in the form of tools, libraries, or applications built on or related to ML. (You will need to provide all the hardware and software required for your demo; the workshop organizers are only able to provide a projector.) Important dates Monday May 19 (any time zone): Abstract submission Monday June 30: Author notification Thursday September 4, 2014: ML Family Workshop Submission Submissions should be at most two pages, in PDF format, and printable on US Letter or A4 sized paper. A submission should have a synopsis (2-3 lines) and a body between 1 and 2 pages, in one- or two-column layout. The synopsis should be suitable for inclusion in the workshop program. Submissions must be uploaded to the workshop submission website before the submission deadline (Monday May 19, 2014). For any question concerning the scope of the workshop or the submission process, please contact the program chair. Program Committee Kenichi Asai Ochanomizu University, Japan Matthew Fluet Rochester Institute of Technology, USA Jacques Garrigue Nagoya University, Japan Dave Herman Mozilla, USA Stefan Holdermans Vector Fabrics, Netherlands Oleg Kiselyov (Chair) Monterey, CA, USA Keiko Nakata Tallinn University of Technology, Estonia Didier Remy INRIA Paris-Rocquencourt, France Zhong Shao Yale University, USA Hongwei Xi Boston University, USA

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New Functional Programming Job Opportunities
by Functional Jobs 07 Apr '14

07 Apr '14

Here are some functional programming job opportunities that were posted recently: Server Game Developer at Quark Games http://functionaljobs.com/jobs/8701-server-game-developer-at-quark-games Infrastructure Engineer at Zeta Project Germany GmbH http://functionaljobs.com/jobs/8699-infrastructure-engineer-at-zeta-project… Cheers, Sean Murphy FunctionalJobs.com

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