June 2012 - Haskell-Cafe

Re: [Haskell-cafe] ANN: Portackage - a hackage portal
by Andrew Seniuk 16 Jun '12

16 Jun '12

Hi ___, Thank you for your suggestion to provide JSON, which seconds Derek's earlier suggestion. I have anonymised your name since you wrote to me but didn't CC the list. :) As much as I wish the web interface was up to snuff, it is really too slow. While I continue to try to resolve that, there is something like a JSON feed available now. I'm new to these things and may be doing it wrong. Any advice would be appreciated, either in accelerating the web interface or in providing the JSON. I have no idea whether it parses or not. JSON Schema: http://www.fremissant.net/portackage/packages.schema.json.txt Hopefully compressed JSON data: http://www.fremissant.net/portackage/packages.php Raw JSON data in case the compressed doesn't work: http://www.fremissant.net/portackage/packages.json The hackage data updated as of today (June 14). For now this is only the packages which expose modules (i.e. not including binary packages). No particular reason, I just ran out of time this afternoon and thought I'd post what I've got. The fields present in the JSON are those available on the HTML table version. There are several dozen other fields which come up more or less infrequently in the Hackage packages, and they could all be included. The complete list of modules is included (rather than the summary versions used in the HTML table version). A next step would be for the server to incrementally update the JSON data based on the Hackage RSS feed. Kind Regards, Andrew On Wed, Jun 13, 2012 at 7:58 AM, ___ wrote: > Hi Andrew > > Portackage is a great site for browsing packages, but it's really > really slow for filtering per column. What about offering an > additional non-javascript version of the pages -- searching with plain > text in a browser works quite well. > > On 4/30/12, Andrew Seniuk <rasfar(a)gmail.com> wrote: >> Oops, the three links in summary are: >> >> module tree: http://fremissant.net/portackage/modules.php >> library packages: http://fremissant.net/portackage/portackage.php >> other packages: http://fremissant.net/portackage/binary.php >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > -- > ___

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Installing pandoc / json with ghc 6.12.1
by Rico Moorman 16 Jun '12

16 Jun '12

Hello, I recently tried to install the latest version of pandoc on my debian stable box (with ghc 6.12.1) using virthualenv as sandboxing mechanism. But instead of a normal installation, compilation of the json package (more specifically the Text.JSON.Parsec module) hangs and causes ghc to consume around 2GB of RAM (out of 3GB) and the system to swap and hang. For reference ... I just created a plain virthualenv (for sandboxing) and after that ran (tmp) foo@bar:/tmp$ cabal install json I also tried if this has something to do with version 0.5 (the latest version) only, to no avail. Are there other users of pandoc/json or related packages who encountered/solved this issue? Am I doing something wrong here? Best regards, Rico

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class instances for kinds with finite ty constrs, does this make sense?
by ex falso 15 Jun '12

15 Jun '12

Hi, i've been using DataKinds for a while and there seems to be a recurring annoyance that could be possibly eliminated with a language extension. It goes something like this: data Tuple (l :: [*]) where Unit :: Tuple '[] Comma :: a -> Tuple as -> Tuple (a ': as) data Proxy k = Proxy class TupleLength (l :: [*]) where tupleLength :: Proxy l -> Integer instance TupleLength '[] where tupleLength _ = 0 instance (TupleLength as) => TupleLength '(a : as) where tupleLength _ = 1 + tupleLength (Proxy :: Proxy as) so far so good. but now if we want to use TupleLength: printLength :: (TupleLength l) => Tuple l -> IO () printLength t = print $ tupleLength t we always have to put the class restriction (TupleLength l) there, even though all possible type constructors of [*] have a TupleLength instance defined! If we had a way to signal that all ty constrs are catered for then the compiler could check this, and deduce that l::[*] is always an instance. Well that's the hypothesis. Of course this would only make sense for kinds with finite no. of ty constructors, namely DataKinds-lifted types. Does this make sense? ex

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Re: [Haskell-cafe] What extension do I need to write "type Job = Map k a"?
by Arlen Cuss 15 Jun '12

15 Jun '12

(resending to café, turns out I wasn't subbed from this address.) Hi Magicloud, This is correct; because you've hidden the type-variables away by universally quantifying them, there's no more level of specificity you can get back *out* of them than just "some kind of Map" (Job = M.Map k b, where k ≠ k0, b ≠ b0). If you have a Job type which can store *any* kind of Map (forall k a. Job (Map k a)), then that means you could have a Job with a Map Int Bool, and a Job with a Map String (Float -> Float), and they'd both have the same type "Job". You can't do anything with the values within, because you're being too permissive about what a Job is. You may want "data Job k a = Job (Map k a)", *or* if you do actually use one kind of Map only, then why not "data Job = Job (Map Int String)" (substituting your real types for Int and String). In this case, you could also consider using newtype ("newtype Job = Job { getJob :: Map Int String }") to provide the guarantee that you're getting a Job (and not any Map Int String) without performance loss. Let me know if I've been more confusing than helpful; Arlen On Thursday, 14 June 2012 at 1:16 PM, Magicloud Magiclouds wrote: > Hi there, > Thanks for the reply. To be clear, all I want is to "avoid having to > type type variables all over the place". What should I do? My original > code with RankNTypes and ImpredicativeTypes does not work.... > > The "type Job = forall k a. M.Map k a" works now. But function uses > it does not. Compiler complains about "Couldn't match expected type > `Job' with actual type `M.Map k0 b0'". > > On Wed, Jun 13, 2012 at 9:15 PM, Daniel Peebles <pumpkingod(a)gmail.com (mailto:pumpkingod@gmail.com)> wrote: > That doesn't require existential quantification, but it'll need Rank-2 typesif you ever do anything with Job. Unfortunately, a universally quantifiedJob like what you wrote (or what Magicloud seems to want) is only inhabitedby the empty Map. > > > An existentially quantified Job, as you might get with > > > data Job = forall k a. Job (Map k a) > > > does let you wrap up any Map containing anything in it, but unfortunatelythe only thing you can do with that map afterwards is ask for "structural"properties about it, like whether it's empty or how many elements it has init. You could ask to enumerate the elements in it, but you wouldn't be ableto touch any of them because you wouldn't know what their types were. > > > So I'm not really sure how to interpret the question. Was the goal to have aheterogeneous Map, maybe? Or just to avoid having to type type variables allover the place? Both of those are possible but require a bit moresophistication with types. > > > -Dan > > > > > On Wed, Jun 13, 2012 at 7:32 AM, Ismael Figueroa Palet<ifigueroap(a)gmail.com (mailto:ifigueroap@gmail.com)> wrote: > > > > Do you want to hide the specific types of the job? Presumably to thendefine a type JobList = [Job] ?You can do that with the ExistentialQuantification extension. > > > > type Job = forall k a. Map k atype JobList = [Job] > > > > ??Note you can't unpack the types k a once you have hidden them. But thetypechecker can use it to ensure some static property.Also you could use unsafeCoerce to do some casts, but *only if you are*sure* that things will go OK*. > > > > > > > 2012/6/13 Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com (mailto:magicloud.magiclouds@gmail.com)> > > > > > Hi,I've forgotten this.This is OK:type Job k a = Map k aAnd this is OK:{-# LANGUAGE RankNTypes #-} -- or LiberalTypeSynonyms?type Job = forall a. forall k. Map k a > > > > > Then how to write it like this?type Job = Map k a--竹密岂妨流水过山高哪阻野云飞 > > > > > And for G+, please use magiclouds#gmail.com (http://gmail.com) > > > > > _______________________________________________Haskell-Cafe mailing listHaskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > > > > > > > > > > > > > > > > > > > --Ismael > > > > > > > _______________________________________________Haskell-Cafe mailing listHaskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > > > > -- > 竹密岂妨流水过 > 山高哪阻野云飞 > > And for G+, please use magiclouds#gmail.com (http://gmail.com) > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org (mailto:Haskell-Cafe@haskell.org) > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Current uses of Haskell in industry?
by Chris Smith 14 Jun '12

14 Jun '12

It turns out I'm filling in for a cancelled speaker at a local open source user group, and doing a two-part talk, first on Haskell and then Snap. For the Haskell part, I'd like a list of current places the language is used in industry. I recall a few from Reddit stories and messages here and other sources, but I wonder if anyone is keeping a list. -- Chris

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[Announce] AusHac 2012 - The Aussie Haskell Hackathon
by Alex Mason 14 Jun '12

14 Jun '12

We know many of you have been holding your breaths for this (hopefully none of you have passed out though!) for quite a while... but we’re pleased to announce that we’re finally getting around to announcing AusHac 2012! This year, we’re moving AusHac to Sydney proper: Atlassian has kindly offered the use of their offices next to Darling Harbour, so there’s more places to stay and eat than in previous years! It’s right across the harbour from Google’s headquarters where FP-Syd is hosted, and we’re starting on the Friday after next month’s FP-Syd. When: Friday 20th - Sunday 22nd of July Where: Atlassian 173/185 Sussex Street Sydney NSW 2000, Australia (02) 9262 1443 Map: http://goo.gl/maps/JoNg If you're looking for places to stay, check out AirBnB (http://www.airbnb.com) the prices are very reasonable (around $100/night within walking distance of Atlassian if you’re lucky). This year we’d like to see a few more talks on interesting things people are working on (Yes, the stuff you are working on IS interesting, and we want to hear about it!). If you’d like to give a talk, please let us know in the sign up sheet. Speaking of which, here it is: http://tinyurl.com/AusHac2012SignUp Please let us know if you’re considering coming along, so we can organise things like adequate power etc. We’d love for you to come along for the whole time or just a few hours, we’re happy for people to drop in when they want, on any day. The more people we have come along, the more awesome it will be. We hope to see you there! Cheers, Alex Mason and Ivan Miljenovic

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What extension do I need to write "type Job = Map k a"?
by Magicloud Magiclouds 14 Jun '12

14 Jun '12

Hi, I've forgotten this. This is OK: type Job k a = Map k a And this is OK: {-# LANGUAGE RankNTypes #-} -- or LiberalTypeSynonyms? type Job = forall a. forall k. Map k a Then how to write it like this? type Job = Map k a -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Haskell Weekly News: Issue 231
by Daniel Santa Cruz 14 Jun '12

14 Jun '12

Welcome to issue 231 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 03 to 09, 2012. Announcements Paolo Capriotti announced the patchlevel release of GHC 7.4.2. "This release contains a number of bugfixes relative to 7.4.1, so we recommend upgrading." [1] http://goo.gl/X3Z85 Quotes of the Week * roconnor: there's a right way, and a wrong way, and then there is the ekmett way. Top Reddit Stories * Haskell Platform 2012.2.0.0 released Domain: hackage.haskell.org, Score: 104, Comments: 33 On Reddit: [2] http://goo.gl/ZNa9h Original: [3] http://goo.gl/JjJg2 * Why free monads matter Domain: haskellforall.com, Score: 61, Comments: 47 On Reddit: [4] http://goo.gl/hfKlc Original: [5] http://goo.gl/JPpsp * 3 Flavors of MVar: Lock, Var, Barrier Domain: neilmitchell.blogspot.com, Score: 43, Comments: 20 On Reddit: [6] http://goo.gl/3eEkO Original: [7] http://goo.gl/ek7Qq * Introducing the HERMIT Equational Reasoning Framework Domain: ittc.ku.edu, Score: 36, Comments: 20 On Reddit: [8] http://goo.gl/UC0sF Original: [9] http://goo.gl/ifS7f * Complicating conduit? Domain: yesodweb.com, Score: 35, Comments: 45 On Reddit: [10] http://goo.gl/W4rfE Original: [11] http://goo.gl/rU0vK * Beautiful Code, Compelling Evidence: OpenGL, Cairo tutorial [PDF, 33 pages] Domain: renci.org, Score: 33, Comments: 2 On Reddit: [12] http://goo.gl/qKY8w Original: [13] http://goo.gl/r5SVj * Forklift - a pattern for performing monadic actions in a worker thread Domain: apfelmus.nfshost.com, Score: 30, Comments: 7 On Reddit: [14] http://goo.gl/B6brV Original: [15] http://goo.gl/mWVAv * Dependently typed programming with singletons (Eisenberg, Weirich)[PDF] Domain: cis.upenn.edu, Score: 29, Comments: 3 On Reddit: [16] http://goo.gl/y3fYv Original: [17] http://goo.gl/RqM5m * Continuation-based relative-time FRP Domain: paolocapriotti.com, Score: 22, Comments: 6 On Reddit: [18] http://goo.gl/CFSK7 Original: [19] http://goo.gl/cP0cJ * Explaining the Prompt monad with a clearer representation Domain: joeysmandatory.blogspot.com, Score: 21, Comments: 42 On Reddit: [20] http://goo.gl/ZZM93 Original: [21] http://goo.gl/Im7SF * Lazily Evaluate length of a list Domain: self.haskell, Score: 21, Comments: 29 On Reddit: [22] http://goo.gl/cKUOt Original: [23] http://goo.gl/cKUOt Top StackOverflow Questions * Killing a thread when MVar is garbage collected votes: 22, answers: 4 Read on SO: [24] http://goo.gl/cvoqw * How does one write efficient Dynamic Programming algorithms in Haskell? votes: 16, answers: 3 Read on SO: [25] http://goo.gl/mkMTp * How to apply a polymorphic function to a Dynamic value votes: 11, answers: 1 Read on SO: [26] http://goo.gl/3jIVr * Haskell Typeclass for Tuples votes: 11, answers: 3 Read on SO: [27] http://goo.gl/pvDah * Monadic expressions in conditionals - GHC compiles, cabal refuses votes: 10, answers: 2 Read on SO: [28] http://goo.gl/QXMXh * GHC Optimization: Collatz conjecture votes: 10, answers: 2 Read on SO: [29] http://goo.gl/AfiEC * Haskell: Lists vs Streams votes: 10, answers: 3 Read on SO: [30] http://goo.gl/qn1dE * Is my program Turing-complete? votes: 9, answers: 2 Read on SO: [31] http://goo.gl/PLf5q * How to find the optimal processing order? votes: 9, answers: 1 Read on SO: [32] http://goo.gl/P3XZ2 * How to do fast data deserialization in Haskell votes: 9, answers: 1 Read on SO: [33] http://goo.gl/Synm2 * What's so bad about OverlappingInstances? votes: 9, answers: 1 Read on SO: [34] http://goo.gl/M17Rp Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.glasgow.user/22181 2. http://hackage.haskell.org/platform/?2012.2.0.0 3. http://www.reddit.com/r/haskell/comments/uipvz/haskell_platform_2012200_rel… 4. http://www.haskellforall.com/2012/06/you-could-have-invented-free-monads.ht… 5. http://www.reddit.com/r/haskell/comments/utxq2/why_free_monads_matter/ 6. http://neilmitchell.blogspot.com/2012/06/flavours-of-mvar_04.html 7. http://www.reddit.com/r/haskell/comments/uk9wu/3_flavors_of_mvar_lock_var_b… 8. http://www.ittc.ku.edu/csdlblog/?p=105 9. http://www.reddit.com/r/haskell/comments/ujdet/introducing_the_hermit_equat… 10. http://www.yesodweb.com/blog/2012/06/complicating-conduit 11. http://www.reddit.com/r/haskell/comments/urc75/complicating_conduit/ 12. http://www.renci.org/wp-content/pub/tutorials/BeautifulCode.pdf 13. http://www.reddit.com/r/haskell/comments/uqc33/beautiful_code_compelling_ev… 14. http://apfelmus.nfshost.com/blog/2012/06/07-forklift.html 15. http://www.reddit.com/r/haskell/comments/uq44h/forklift_a_pattern_for_perfo… 16. http://www.cis.upenn.edu/~eir/papers/2012/singletons/paper.pdf 17. http://www.reddit.com/r/haskell/comments/uic9g/dependently_typed_programmin… 18. http://paolocapriotti.com/blog/2012/06/04/continuation-based-relative-time-… 19. http://www.reddit.com/r/haskell/comments/uklsq/continuationbased_relativeti… 20. http://joeysmandatory.blogspot.com/2012/06/explaining-prompt-monad-with-sim… 21. http://www.reddit.com/r/haskell/comments/unpaz/explaining_the_prompt_monad_… 22. http://www.reddit.com/r/haskell/comments/uomz4/lazily_evaluate_length_of_a_… 23. http://www.reddit.com/r/haskell/comments/uomz4/lazily_evaluate_length_of_a_… 24. http://stackoverflow.com/questions/10871303/killing-a-thread-when-mvar-is-g… 25. http://stackoverflow.com/questions/10906053/how-does-one-write-efficient-dy… 26. http://stackoverflow.com/questions/10889682/how-to-apply-a-polymorphic-func… 27. http://stackoverflow.com/questions/10899804/haskell-typeclass-for-tuples 28. http://stackoverflow.com/questions/10867300/monadic-expressions-in-conditio… 29. http://stackoverflow.com/questions/10876110/ghc-optimization-collatz-conjec… 30. http://stackoverflow.com/questions/10942450/haskell-lists-vs-streams 31. http://stackoverflow.com/questions/10878846/is-my-program-turing-complete 32. http://stackoverflow.com/questions/10879838/how-to-find-the-optimal-process… 33. http://stackoverflow.com/questions/10902405/how-to-do-fast-data-deserializa… 34. http://stackoverflow.com/questions/10942136/whats-so-bad-about-overlappingi…

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Perth Functional Programmers meetup group launched
by Todd Owen 14 Jun '12

14 Jun '12

We are pleased to announce that a functional programming user group has recently been formed in Perth, Australia. Being a small community, we aim to keep the group as inclusive as possible, and welcome new members from all levels of experience and language backgrounds. (That said, Haskellers currently account for about half our membership, followed by F#, and then a heap of other languages). As this is a global mailing list, I would also like to invite our colleagues from around Australia, and indeed anywhere in the world, to get in touch if you are ever passing through Perth. We're always happy to share a beer and talk about programming! For more information, visit: http://www.meetup.com/PerthFP/

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Warning when attempting to install cabal-dev on the latest HP
by Edward Amsden 13 Jun '12

13 Jun '12

http://hpaste.org/69885 The warning in that paste occurs when I attempt $ cabal update $ cabal install cabal-dev This is curious because it wants to install a previous version of mtl. I can't figure out why. There doesn't seem to be any dependency between cabal-dev and mtl. According to hackage, the dependencies for cabal-dev are: cabal-dev -> (base, Cabal) Cabal -> (base, filepath) filepath -> (base) base -> () The dependencies for mtl are (just to show no conflicts) mtl -> (base, transformers) transformers -> (base) base -> () And there's not a versioning conflict with base. Why is cabal insisting on backdating mtl then?

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