- Haskell-Cafe - Haskell.org

Status of Acid-State and IxSet
by JP Moresmau 30 May '15

30 May '15

Hello, for a project I decided to use Acid-State and IxSet, and was pretty happy with them. I enjoyed working with Haskell structures for storing and querying data, and my data could fit in memory. However, now I've moved to GHC 7.10, and for example syb-with-class, a dependency of ixset, does not build with template-haskell 2.10. So I'm wondering, are these projects still alive and kicking? Are if I want some memory based storage and indexing, should I look at some other projects (maybe VCache http://hackage.haskell.org/package/vcache) Or is it just bad timing and GHC 7.10 compatible versions of these libraries are in the works? Thanks! -- JP Moresmau http://jpmoresmau.blogspot.com/

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recursive datakinds
by Nicholls, Mark 30 May '15

30 May '15

Hello, If I were trying to do something like creating types like.... > {-# LANGUAGE DataKinds #-} > {-# LANGUAGE ExplicitForAll #-} > {-# LANGUAGE FlexibleContexts #-} > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE GADTs #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE PolyKinds #-} > {-# LANGUAGE StandaloneDeriving #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE TypeOperators #-} > {-# LANGUAGE UndecidableInstances #-} > {-# LANGUAGE ScopedTypeVariables #-} > data MyList a where > MyEnd :: MyList '[] > MyCons :: a -> MyList b -> MyList (a ': b) > mysimplelist :: MyList '[Integer] > mysimplelist = MyCons 1 (MyEnd) what about > myrecursivelist = MyCons 1 myrecursivelist Cafe2.lhs:23:30: Occurs check: cannot construct the infinite type: b ~ a : b Expected type: MyList b Actual type: MyList (a : b) Relevant bindings include myrecursivelist :: MyList (a : b) (bound at Cafe2.lhs:23:3) In the second argument of 'MyCons', namely 'myrecursivelist' In the expression: MyCons 1 myrecursivelist can I "fix" this somehow? I obviously want the type of myrecursivelist to be MyList '[Integer, Integer, Integer, Integer, Integer, Integer, Integer, Integer,.....] Something like a completely nonsensical... MyList (Fix (Integer ': )) P.S. Yes, I barely understand "fix" as a function, let alone a data type. 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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Haskell Weekly News
by Kim-Ee Yeoh 30 May '15

30 May '15

*Top Picks:* - Tony Day, Brisbane-based investment strategist and high-frequency-trading hacker, rides 100% idiomatic Haskell <http://tonyday567.github.io/blog/mvc-todo/> into the Single Page (web)-App <http://en.wikipedia.org/wiki/Single-page_application> space. How? He spins the GHCJS transpiler on Gabriel Gonzalez's Model-View-Controller library to obtain his own TodoMVC benchmark demo <http://tonyday567.github.io/static/index-auto.html>. Per GHCJS, the production spans multi-megabytes of javascript. Not to be missed: auto-run, i.e. click the QuickCheck-powered checkbox labeled "Let haskell do the work." Github repo <https://github.com/tonyday567/mvc-todo>. Along the way, he discovers how crippled Javascript is without sum types. As noted on HN <https://news.ycombinator.com/item?id=9591868#up_9601842>, "Typos and missing cases represent a very large set of trivial bugs." He believes superior FP features such as sum types makes it "much harder for haskell to avoid success." Woe is us. /r/haskell <http://www.reddit.com/r/haskell/comments/36y9jc/haskell_as_an_mvc_framework/> - Michael Walker, a Ph.D. student at York, reveals <http://www.reddit.com/r/haskell/comments/37dv8y/what_side_projects_have_you…> his personal book-collection management web-app <http://www.barrucadu.co.uk/bookdb/> that runs on top of persistent <https://hackage.haskell.org/package/persistent>, WAI <https://hackage.haskell.org/package/wai>, and web-routes <https://hackage.haskell.org/package/web-routes>. Public domain. <https://github.com/barrucadu/bookdb> - David Christiansen announces on /r/haskell <http://www.reddit.com/r/haskell/comments/36az3n/idris_0918_released_with_fa…> Idris 0.9.18 <http://www.idris-lang.org/idris-0-9-18-released/> with fancier records. Top comment says Idris is more type-friendly than Haskell despite the "esoteric academics behind dependent type theory." Why? Because Idris, helpfully offers suggestions that turn ill- into well-typed code. Haskell doesn't. - Remember JP Moresmau dropping EclipseFP <http://haskell.1045720.n5.nabble.com/Haskell-Weekly-News-tp5809360.html>? Can Leksah take its place? Hamish Mackenzie announces a 7.10-ready Leksah 0.15.0 <https://github.com/leksah/leksah/wiki/Leksah-0.15.0> on /r/haskell <http://www.reddit.com/r/haskell/comments/37emp2/leksah_0150/>. Top new feature? Support for GHCJS, which excites Phil Freeman of PureScript fame. Here's a 2min video clip on How to make a ghcjs-dom application in Leksah <https://www.youtube.com/watch?v=zQnExdDL63c>. - Roman Cheplyaka writes a short 'n sweet tutorial <https://ro-che.info/articles/2015-05-28-force-list> on how a list may be variously forced. He summarizes the similarities and differences in a table that goes from the shallowestly evaluated seq () to forceSpine to forceElements to the deepest rnf. /r/haskell <http://www.reddit.com/r/haskell/comments/37ky62/how_to_force_a_list/> - Justin Leitgeb, Rails developer and Co-Founder / CTO of Stack Builders, a Haskell-enabled software consultancy, deprioritizes learning Clojure, Go, Erlang, and Scala in favor of Agda, Coq, Idris, Elm, and Liquid Haskell <http://www.stackbuilders.com/news/another-personal-programming-language-roa…>. He will invest a couple of weeks at the Oregon PL Summer School starting June 15 <https://www.cs.uoregon.edu/research/summerschool/summer15/>. - Tony Morris announces a three-day Haskell-based Intro to FP course in Melbourne, July 21-23 this year. Pitched at beginners, it features learning by coding. Free; application deadline: July 10 <https://groups.google.com/forum/#!topic/nicta-fp/Dnq-oh4nJbg>. - By popular request <http://haskell.1045720.n5.nabble.com/Workshop-on-Type-Inference-and-Automat…>, František Farka and his team recorded all of a workshop on type inference in Dundee, Scotland <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/> held a couple of weeks ago. Highly-viewed talks <https://www.youtube.com/user/qmbevents/videos> include Edwin Brady <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/#edwin> on implementing dependent types in Idris and Conor McBride <http://staff.computing.dundee.ac.uk/frantisekfarka/tiap/#conor> on "Type Inference Needs Revolution." /r/haskell <http://www.reddit.com/r/haskell/comments/36dfcx/workshop_on_type_inference_…> - Michael Hicks at UMD, PC chair of POPL 2012, writes a PL research apologia <http://www.pl-enthusiast.net/2015/05/27/what-is-pl-research-and-how-is-it-u…> cum pitch for new grad students. An informal poll he did shows PL Ph.D.s get good jobs. He explains that "The ethos of PL research is to not just find solutions to important problems, but to find the *best expression of those solutions.*" As ethos specimens, he gives three: probabilistic programming, incremental computation a.k.a. self-adaptive computation, and authenticated data structures (see LambdaAuth <http://amiller.github.io/lambda-auth/>). HN-worthy <https://news.ycombinator.com/item?id=9614178>. - Jan Stolarek gives a glimpse <http://lambda.jstolarek.com/2015/05/injective-type-families-for-haskell/> of GHC's new injective type families feature, which is joint research with SPJ and Richard Eisenberg. It brings Haskell one step closer to having a notion of type functions as opposed to mere 'constructors.' The feature is useful in type-level hacking. It allows the arguments of a type family to be inferred solely by result type. But Lennart Augustsson on /r/haskell <http://www.reddit.com/r/haskell/comments/37bvh0/injective_type_families_for…> gives a trivial example where even when a type family isn't injective, its argument can still be inferred knowing a particular result type. He regrets that GHC doesn't do this. - Joe Nelson uploads the video and a summary <http://begriffs.com/posts/2015-05-24-safe-haskell.html> of Kristen Kozak introducing the Safe Haskell extension to the SF Bay Area Haskell Users Group <http://www.meetup.com/Bay-Area-Haskell-Users-Group/events/220312001/> on May 12. Slides here <http://wordroute.com/grayjay/safe-haskell.pdf>. Redditor beerdude26 <http://www.reddit.com/r/haskell/comments/37a1cq/using_the_safe_haskell_lang…> adds the link to and tidies up the Safe Haskell wiki <https://wiki.haskell.org/Safe_Haskell> entry. - Out of frustration with existing Windows GUI FFIs, Luka Horvat <http://lukahorvat.github.io/programming/2015/05/24/haskellforms/> creates bindings for WinForms <https://github.com/LukaHorvat/HaskellForms>, Microsoft's .NET menus-and-widgets library. He binds to F# instead of lower-level C++/C# because it's easier. /r/haskell <http://www.reddit.com/r/haskell/comments/37464x/haskellforms_winforms_bindi…> - Gabriel Gonzalez <http://www.haskellforall.com/2015/05/the-internet-of-code.html> demoes another piece of Morte, what he describes as his "pandoc for programming languages." He prototypes "distributing typed code over the internet where the unit of compilation is individual expressions." /r/haskell <http://www.reddit.com/r/haskell/comments/36d12v/haskell_for_all_the_interne…> - Taylor Fausak forsakes TagSoup <https://hackage.haskell.org/package/tagsoup> and scrapes websites <http://taylor.fausak.me/2015/05/21/scraping-websites-with-haskell/> the hard way using xml-conduit. He concludes that it's "tougher than doing the same thing in scripting languages, but hopefully easier than [his blog readers] expected." - Stuart Popejoy writes a monad tutorial <http://slpopejoy.github.io/posts/Effectful01.html>. A redditor finds it an "entertaining explanation of monads." Another asks the OOP-ish question: are monads "basically wrappers that contain 'impure' data and actions with some common functions?" See /r/haskell <http://www.reddit.com/r/haskell/comments/374c3o/effectful_haskell_io_monads…> *Announcement:* Semen Trygubenko has stepped down from publishing his edition of HWN. Losing him means there will be no issue next week. *Quotes of the Week:* - UnoOuzo <https://twitter.com/UnoOuzo/status/601658244201848832>: "The type of my love is parametrically polymorphic. It is unbounded." Looking into ways to cite this in my thesis. - Conor McBride <http://www.reddit.com/r/haskell/comments/36y9jc/haskell_as_an_mvc_framework…>: Algebra is a posh way of saying "construction kit". - Michael Hicks <http://www.pl-enthusiast.net/2015/05/27/what-is-pl-research-and-how-is-it-u…>: The ethos of PL research is to not just find solutions to important problems, but to find the *best expression of those solutions. * - There is a fountain of youth: it is your mind, your talents, the creativity you bring to your life and the lives of people you love. When you learn to tap this source, you will truly have defeated age. -- Sophia Loren -- Kim-Ee

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-fdefer-more-errors
by Tom Ellis 29 May '15

29 May '15

Deferring type errors until runtime is a feature which probably helps a lot with the adoption of Haskell. Newcomers can get the encouragement of a mostly-working program even if there are type errors in a few places. I can write foo = 1 + 1 bar = head True main = print foo and still run the program or load it in GHCi. Does anyone think it would also be beneficial to have "-fdefer-name-errors"? If I write foo = 1 + 1 bar = baz main = print foo but baz doesn't exist it would still be nice to let the program run or be loaded in GHCi. This could be achieved by replacing all missing variables by "error 'baz was not defined'" or similar. If a suitable parser could be written it might even be possible to defer syntax errors! This kind of thing would make the experience with Haskell gentler for newcomers, but also more pleasant for veterans! Tom

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Re: [Haskell-cafe] Any recommendations on how to interact with the running state of a program?
by Anatoly Yakovenko 29 May '15

29 May '15

How would the program get updated values? via an IORef? This thing is running until the user decides the output looks good Anatoly On Thu, May 28, 2015 at 12:16 PM Carlos López-Camey <c.lopez(a)kmels.net> wrote: > Cloud Haskell + Event streaming via some Protocol (like json on > websockets) ? > > > > 2015-05-25 15:14 GMT-06:00 Anatoly Yakovenko <aeyakovenko(a)gmail.com>: > >> I am playing around with some deep learning algorithms and I need a way >> to periodically read the intermediate result and change some of the system >> variables. >> >> Any ideas on how to best approach this problem? I was thinking of >> something basic, like having the program periodically read and write from >> some files. >> >> Thanks, >> Anatoly >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> >> >

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Haskell wiki: user account
by Julien Cretel 29 May '15

29 May '15

Hi there, I'd like to get a user account for the Haskell wiki. Desired username: Jubobs Cheers -- Julien Cretel PhD student University College Cork, Ireland Email: j.cretel(a)umail.ucc.ie

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Anyone using Tidal/emacs like emacs-live/overtone?
by Cody Goodman 29 May '15

29 May '15

Does anyone have a working setup where you enter Haskell code from the tidal library and use a keybinding to execute that statement (or multiple)? Basically wanting to try livecoding Tidal code like all the overtone videos: https://vimeo.com/22798433

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help with glfw seg-fault
by briand＠aracnet.com 29 May '15

29 May '15

Hi, GLFW.swapBuffers w is the culprit. Not sure where to go from there. That's a pretty low level failure. Any suggestions. Brian

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Type inference doesn't always calculate the most generic type?
by Christophe Delord 29 May '15

29 May '15

Hello, I'm new here. Sorry if this question has already been asked. I have noticed something strange about type inference. Prelude> :t abs abs :: Num a => a -> a Prelude> let mapabs = map abs Prelude> :t mapabs mapabs :: [Integer] -> [Integer] Prelude> :t (map abs) (map abs) :: Num b => [b] -> [b] I think that "mapabs" and "map abs" should have the same generic type. In a program I can force the type like this: mapabs = map abs mapabs' :: (Num a) => [a] -> [a] mapabs' = map abs but mapabs still have the type [Integer] -> [Integer] Are there any reason for this? Regards, Christophe Delord.

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(Why) must this function remain nameless?
by Clinton Mead 29 May '15

29 May '15

The following code (somewhat contrived) code compiles on GHC without issue, and towards the end repeatedly uses the lambda function "(\fn (D x) -> f fn x)". But when I simply try to give it a name, like so: h = (\fn (D x) -> f fn x) I then get a compile error. How can I name this function, or must it remain forever nameless? ( Ideone link: http://ideone.com/mtuYnK ) --- {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE NoMonomorphismRestriction #-} import GHC.Exts (Constraint) type family Parent a class C a b where f :: b -> a -> String instance (C (Parent a) b) => C a b where f _ _ = f (undefined :: b) (undefined :: Parent a) data A1 = A1 data A2 = A2 data A3 = A3 data A4 = A4 type instance Parent A2 = A1 type instance Parent A3 = A2 type instance Parent A4 = A3 data F1 = F1 data F3 = F3 instance C A1 F1 where f _ _ = "F1" instance C A3 F3 where f _ _ = "F3" type family Constraints t a :: Constraint data D t = forall a. (Constraints t a) => D a type instance Constraints A1 a = (C a F1) type instance Constraints A2 a = (C a F1, C a F3) type instance Constraints A3 a = (C a F1, C a F3) type instance Constraints A4 a = (C a F1, C a F3) main = do putStrLn (f F1 A1) putStrLn (f F1 A2) putStrLn (f F1 A3) putStrLn (f F1 A4) putStrLn (f F3 A3) putStrLn (f F3 A4) putStrLn $ (\fn (D x) -> f fn x) F1 ((D A1) :: D A1) putStrLn $ (\fn (D x) -> f fn x) F1 ((D A2) :: D A1) putStrLn $ (\fn (D x) -> f fn x) F3 ((D A3) :: D A2) -- h = (\fn (D x) -> f fn x)

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