- Haskell-Cafe - Haskell.org

Towards a better time library (announcing tz)
by Mihaly Barasz 08 Apr '14

08 Apr '14

I would like to propose reforming the 'time' [1] library. Initially, I was just planning to announce my new 'tz' [2] library, but realized that I have a more important agenda. Namely: Haskell needs a better time library! Let me summarize what are ‒ in my view ‒ the biggest deficiencies of 'time': 1. Inefficient data structures and implementations. 2. Ad-hoc API which is hard to remember and frustrating to work with. 3. Conceptually wrong representations and/or missing concepts. The wonderful thyme [3] package (by Liyang HU) improves a lot on #1 by choosing better data structures and careful implementations and on #2 by lensifying the API. But, it was the #3 that caused me the most frustration lately; most importantly the time zone handling. There is a TimeZone data type in 'time', but it is a misnomer, it basically represents a fixed time difference (with a label and a DST flag). 'time' basically adapts the broken approach from libc: you can work with one time zone at a time, which is defined globally for your program (via the TZ environment variable). So, the transformation between UTCTime and LocalTime which should have been a pure function can only be done in IO now. Like this: do tz <- getTimeZone ut return $ utcToLocalTime tz ut Oh, and just to hammer down on the point #1 from the list above. This code runs in about 6100 ns on my machine. The drop-in replacement from tz: utcToLocalTimeTZ [4] (which is actually pure) runs in 2300 ns. While this is a significant improvement, it's easy to miss the point where the bulk of the inefficiency comes from the data structures. In my main project we represent times as Int64 (raw nanoseconds since UNIX epoch; and similar representation for zoned times). And to convert those to and from different time zones we need 40 ns. That's right, a 150 _times_ improvement! (There are many other interesting benchmark results that I could mention. An exciting bottom line: we can actually beat the libc in many use-cases!) The 'tz' package is still very much in flux. I will try to solidify the API soon, but until then it should be considered more of a proof of concept. There is some missing functionality, for example. On the other hand, there are the 'timezone-series' [5] and 'timezone-olson' [6] packages that together provide about the same functionality as 'tz' (minus the efficiency), and I'd like to explore if we could remove some of the overlap. But, all kind of suggestions and requests are welcome! More importantly, I'd like to hear the opinions of the community about the general issue of a better time library! Do we need one? How should we proceed about it? I think, Haskell could potentially have one of the best time libraries, but the current de-facto standard is mediocre at best. Unfortunately, designing a good time library is very far from trivial, as many existing examples demonstrate. And I definitely don't know enough for it. (I understand time zone info files, now that I wrote tz, but that's just a tiny fraction of what's needed.) So, if you think you can contribute to the design (have important use-cases in mind, know good examples of API, have some experience working with dates and time, etc. etc.) ‒ speak up! Mihaly Footnotes: [1] http://hackage.haskell.org/package/time [2] http://hackage.haskell.org/package/tz [3] http://hackage.haskell.org/package/thyme [4] http://hackage.haskell.org/package/tz-0.0.0.1/docs/Data-Time-Zones.html#v:u… [5] http://hackage.haskell.org/package/timezone-series [6] http://hackage.haskell.org/package/timezone-olson

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Haskell + emacs + Ghc-mod 4.0.1
by Arnaud Bailly 08 Apr '14

08 Apr '14

Hello, I upgrade my emacs configuration with latest ghc-mod and all of sudden nothing works (again). When I try to load a .hs file from a project I got the first line highlighted with the error "cannot satisfy -package spec" and I cannot got type, info or doc from that file. Also I do not have anymore syntax errors highlighting I used to see. Any help greatly appreciated. I would also appreciate any pointer explaining how to setup and maintain a proper Haskell dev environment. Haskell is not the language I use at day work but I have been using it for years and every so often my emacs configuration is broken and I have a hard time getting it back to sane state. Thanks in advance Arnaud Here is my emacs configuration: ;; haskell coding (require 'haskell-mode) (require 'haskell-cabal) (autoload 'ghc-init "ghc" nil t) (add-hook 'haskell-mode-hook (lambda () (ghc-init))) (eval-after-load "haskell-mode" '(progn (define-key haskell-mode-map (kbd "C-,") 'haskell-move-nested-left) (define-key haskell-mode-map (kbd "C-.") 'haskell-move-nested-right) (define-key haskell-mode-map (kbd "C-c v c") 'haskell-cabal-visit-file) (define-key haskell-mode-map (kbd "C-c C-c") 'haskell-compile) (define-key haskell-mode-map (kbd "C-x C-d") nil) (setq haskell-font-lock-symbols t))) (add-hook 'haskell-mode-hook 'turn-on-haskell-decl-scan) (add-hook 'haskell-mode-hook 'turn-on-haskell-indentation) (eval-after-load "which-func" '(add-to-list 'which-func-modes 'haskell-mode)) (eval-after-load "haskell-cabal" '(define-key haskell-cabal-mode-map (kbd "C-c C-c") 'haskell-compile)) Here is the output of ls -l ~/Library/Haskell/bin: lrwxr-xr-x 1 arnaud staff 45 Dec 27 22:36 HsColour -> ../ghc-7.6.3/lib/hscolour-1.20.3/bin/HsColour lrwxr-xr-x 1 arnaud staff 50 Dec 31 07:11 aeson-pretty -> ../ghc-7.6.3/lib/aeson-pretty-0.7/bin/aeson-pretty lrwxr-xr-x 1 arnaud staff 52 Dec 31 07:11 biblio2yaml -> ../ghc-7.6.3/lib/pandoc-citeproc-0.2/bin/biblio2yaml lrwxr-xr-x 1 arnaud staff 49 Jan 10 16:25 cabal -> ../ghc-7.6.3/lib/cabal-install-1.18.0.2/bin/cabal lrwxr-xr-x 1 arnaud staff 46 Jan 15 13:51 cabal-dev -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/cabal-dev lrwxr-xr-x 1 arnaud staff 39 Dec 27 22:36 cpphs -> ../ghc-7.6.3/lib/cpphs-1.17.1/bin/cpphs lrwxr-xr-x 1 arnaud staff 43 Dec 28 14:17 doctest -> ../ghc-7.6.3/lib/doctest-0.9.10/bin/doctest lrwxr-xr-x 1 arnaud staff 55 Jan 15 13:51 fake-ghc-cabal-dev -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/fake-ghc-cabal-dev lrwxr-xr-x 1 arnaud staff 62 Jan 26 11:15 ghc-events -> ../../../projects/data-scientist/.cabal-sandbox/bin/ghc-events lrwxr-xr-x 1 arnaud staff 42 Apr 3 15:04 ghc-mod -> ../ghc-7.6.3/lib/ghc-mod-4.0.1/bin/ghc-mod lrwxr-xr-x 1 arnaud staff 43 Apr 3 13:35 ghc-modi -> ../ghc-7.6.3/lib/ghc-mod-4.0.1/bin/ghc-modi lrwxr-xr-x 1 arnaud staff 55 Jan 15 13:51 ghc-pkg-6_8-compat -> ../ghc-7.6.3/lib/cabal-dev-0.9.2/bin/ghc-pkg-6_8-compat lrwxr-xr-x 1 arnaud staff 47 Dec 31 07:11 hakyll-init -> ../ghc-7.6.3/lib/hakyll-4.4.2.0/bin/hakyll-init lrwxr-xr-x 1 arnaud staff 45 Mar 31 13:57 hasktags -> ../ghc-7.6.3/lib/hasktags-0.68.7/bin/hasktags lrwxr-xr-x 1 arnaud staff 36 Mar 17 06:09 hell -> ../ghc-7.6.3/lib/hell-0.0.1/bin/hell lrwxr-xr-x 1 arnaud staff 39 Dec 27 22:36 hlint -> ../ghc-7.6.3/lib/hlint-1.8.55/bin/hlint lrwxr-xr-x 1 arnaud staff 47 Apr 3 15:07 hspec-discover -> ../ghc-7.6.3/lib/hspec-1.9.1/bin/hspec-discover lrwxr-xr-x 1 arnaud staff 58 Dec 31 07:11 make-pandoc-man-pages -> ../ghc-7.6.3/lib/pandoc-1.12.2.1/bin/make-pandoc-man-pages lrwxr-xr-x 1 arnaud staff 62 Jan 15 04:25 operational-TicTacToe -> ../ghc-7.6.3/lib/operational-0.2.2.1/bin/operational-TicTacToe lrwxr-xr-x 1 arnaud staff 43 Jan 15 05:14 pandoc -> ../ghc-7.6.3/lib/pandoc-1.12.2.1/bin/pandoc lrwxr-xr-x 1 arnaud staff 56 Dec 31 07:11 pandoc-citeproc -> ../ghc-7.6.3/lib/pandoc-citeproc-0.2/bin/pandoc-citeproc lrwxr-xr-x 1 arnaud staff 44 Dec 27 19:34 pdf2line -> ../ghc-7.6.3/lib/pdf2line-0.0.1/bin/pdf2line lrwxr-xr-x 1 arnaud staff 43 Dec 27 19:34 pdfdump -> ../ghc-7.6.3/lib/pdf2line-0.0.1/bin/pdfdump lrwxr-xr-x 1 arnaud staff 42 Jan 15 05:26 word2vec -> ../ghc-7.6.3/lib/word2vec-0.1/bin/word2vec lrwxr-xr-x 1 arnaud staff 43 Dec 31 07:11 yaml2json -> ../ghc-7.6.3/lib/yaml-0.8.5.2/bin/yaml2json lrwxr-xr-x 1 arnaud staff 34 Mar 13 08:58 yate -> ../ghc-7.6.3/lib/yate-0.1/bin/yate -- Arnaud Bailly FoldLabs Associate: http://foldlabs.com

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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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