- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Suppressing HLint on pattern match of files
by Graham Berks 16 Oct '13

16 Oct '13

Ah that sounds good, thanks will incorporate that. G On 11 October 2013 at 17:21:44, Adam Bergmark (adam(a)edea.se) wrote: You can also do this on a module basis with {-# OPTIONS -fno-warn-missing-signatures #-}. I prefer adding type signatures even for cases where I don't think it's important. GHC spits them out so almost no time spent, and someone might find it useful at some point. On Fri, Oct 11, 2013 at 5:53 PM, Dag Odenhall <dag.odenhall(a)gmail.com> wrote: Just add it next to -Wall wherever you‘re setting that, because I think you must be setting that yourself somewhere (it’s not on by default). On Thu, Oct 10, 2013 at 6:11 PM, Graham Berks <graham(a)fatlazycat.com> wrote: Ah good point :) Wonder if I can change it on cabal file somehow. Thanks On 10 October 2013 at 14:05:45, Dag Odenhall (dag.odenhall(a)gmail.com) wrote: Is that actually from HLint though? I think that comes from GHC with -Wall and can be disabled with -fno-warn-missing-signatures. On Thu, Oct 10, 2013 at 7:50 AM, Graham Berks <graham(a)fatlazycat.com> wrote: Hi, would like to disable 'Top-level binding with no type signature' In my test modules that are prefixed with Test. Is this possible ?? Thanks _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Announce: HDBI-1.3 and friends
by Dan 16 Oct '13

16 Oct '13

Thanks Alexey, Any plans for : 1 - Oracle support ? 3 - NoSql databases (Mongo, Cassandra, etc) support ? Cheers, Dan Date: Tue, 15 Oct 2013 23:55:16 +0600 From: Alexey Uimanov <s9gf4ult(a)gmail.com> To: Haskell Cafe <haskell-cafe(a)haskell.org> Subject: [Haskell-cafe] Announce: HDBI-1.3 and friends Message-ID: <CANtkurc-6sQ87Bji=O0OhEteZJFh=tD2yz8ckTLgYc7J2u_SoA(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" Hello haskellers! HDBI is the fork of HDBC but reworked. It supports SQlite3 and Postgresql for now. It also supports streaming with coduits. There is TH deriving mechanism to map database rows to Haskell structures and back. HDBI is trying to become simple but still powerfull and flexible database interface. It must be suitable to become the common RDBMS interface for higher level interfaces like persistent or haskelldb. The documentation is not very good, while I have no enough time to make some. In this version changed typeclass signatures of Connection and Statement. Now methods `run` and `execute` get any instance of `ToRow` and method `fetch` return an instance of `FromRow`. Note that [SqlValue] is also an instance of `FromRow` and `ToRow` typeclasses so you do not loose the control. Methods `fromRow` and `toRow` for [SqlValue] are just `id`. SQlite and Postgresql drivers are fixed as well as hdbi-conduit. There is also new helper functions, like `onei :: Integer -> [SqlValue]` which helps you to execute queries with one parameter or execute many queries consistinf of one parameter. Prelude Database.HDBI Database.HDBI.SQlite> :set -XScopedTypeVariables Prelude Database.HDBI Database.HDBI.SQlite> :set -XOverloadedStrings Prelude Database.HDBI Database.HDBI.SQlite> c <- connectSqlite3 ":memory:" Prelude Database.HDBI Database.HDBI.SQlite> runRaw c "create table test(val integer)" Prelude Database.HDBI Database.HDBI.SQlite> withTransaction c $ runMany c "insert into test(val) values (?)" $ map one [1..1000] <interactive>:7:76: Warning: Defaulting the following constraint(s) to type `Integer' ......... Prelude Database.HDBI Database.HDBI.SQlite> r :: (Maybe Integer) <- runFetchOne c "select sum(val) from test" () Prelude Database.HDBI Database.HDBI.SQlite> r Just 500500 Note here that the empty set is used as a parameter of query in `runFetchOne`. Empty set is an instance of `FromRow` and `ToRow` and return an empty list of [SqlValue]. Use empty list as a parameters is bad idea, because we could instantiate some another list of things, suppose the [Integer] as `FromRow` and `ToRow` instance. So it would lead to ambigous type because [SqlValue] is also a list instantiating `FromRow` and `ToRow`.

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haskell-platform problems on (latest) knoppix
by dhw＠thedance.net 16 Oct '13

16 Oct '13

Background: I'm an experienced Linuxer, but not experienced with the Debian package-management tools. I recently decided it was time to renew my acquaintance with Haskell, which began before the do-notation, with Hugs. Seeing the warnings on the ghc website, I decided to install a Linux distribution with a strongly checked package system. I chose Knoppix-7.2.0 i386 (the latest, dated mid-2013), which installed with few problems (now fixed). It claimed to be based on wheezy, the stable branch. Even though the version of haskell-platform in wheezy/stable was from mid-2012, I presumed it would be debugged by now - after all, that's what "stable" is supposed to mean. So I hooked it up to the net, and entered apt-get install haskell-platform After a few seconds, it said that there were broken packages - I think they were libglut-dev and libgl-dev - and that it couldn't fix them. Aptitude, which has a menu-item called 'fix broken packages', couldn't fix them, either. If I recall correctly, there were intrinsically unsatisfiable version constraints, i.e. >=V and <V for the same V. So my question is: have I somehow introduced this (I re-used a knoppix-data.img without ensuring it was empty) or is the problem upstream from me? (i.e does anyone else have this problem?) Looking at the open bug list for haskell- package, I can't tell whether it's a recognised bug. Subsidiary question: since I would prefer to spend time on Haskell rather then on the package system, would it be quicker (in days, not in milliseconds :-) to use nhc98 or Hugs98-Sep2006, obsolete though they are? All advice gratefully received, thanks, -David -------------------------------------------------------------------- mail2web.com Enhanced email for the mobile individual based on Microsoft® Exchange - http://link.mail2web.com/Personal/EnhancedEmail

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[ANNOUNCE] hzk - Haskell Zookeeper bindings
by diego souza 15 Oct '13

15 Oct '13

Dear Haskellers, A project that I'm working on depends on Zookeeper and as currently there is no bindings [1] for it I'm glad to announce the first release of a Haskell bindings for Apache Zookeeper. Any feedback will be highly appreciated. All zookeeper functions should be available but multi operations. I currently don't have any needs for them but if someone is interested in having them available let me know. Thanks! ~dsouza [1] Actually there is at least https://github.com/motus/haskell-zookeeper-client, but it is not available on hackage by the time I wrote this.

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Haskell Group Dresden?
by Daniel Trstenjak 15 Oct '13

15 Oct '13

Hi, is there any interest in having something like a Haskell Group in Dresden? Perhaps I could say something about the lens library, which would be more of a practical talk, how to define lenses and working with lenses, traversals, isos and prisms. Greetings, Daniel

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Proposal for a standard collection of algorithms/Data structures in haskell
by satvik chauhan 15 Oct '13

15 Oct '13

Hi cafe, Whenever you search for a library for doing some standard task on hackage there are lots of implementations available and some of them are poorly implemented without any thoughts given to design and speed. Most of them are left as it is without any further improvements/bug fixes. A limited collection of data-structures and associated algorithms are available in base. It would be nice for the community to maintain a standard collection of recommended implementations (both fast and well designed) of data structure and algorithms (similar to Boost for C++). This can again be maintained and moderated by the community. All the implementation in the collection can adhere to a standard design principle and idiomatic haskell. I would really love to hear your thoughts on this proposal. -Satvik

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Xcode 5 support for GHC iOS
by Luke Iannini 14 Oct '13

14 Oct '13

Hi all, Just wanted to let you know I've finished updating GHC iOS to support Xcode 5. The new scripts are at https://github.com/ghc-ios/ghc-ios-scripts/tree/xcode5 and you'll find updated instructions at http://ghc.haskell.org/trac/ghc/wiki/Building/CrossCompiling/iOS . Please let me know if you have any trouble! Cheers Luke

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No upgrade of GHC version in Ubuntu repository
by Vlatko Basic 14 Oct '13

14 Oct '13

Hi Caffe, deb packages of GHC (and Haskell Platform) in Ubuntu's (LTS Precise, 12.04) "universe" are not upgrading at all. GHC package is almost two years old, still on version 7.4.1, and GHC site still recommends to use Haskell Platform from repository. Even kernel is upgrading more often, and one would expect it is much more difficult. :-) What would be the reason for that? Is it abandoned? Best regards, vlatko

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Increasing memory use in stream computation
by Arie Peterson 14 Oct '13

14 Oct '13

(Sorry for the long email.) Summary: why does the attached program have non-constant memory use? ==== Introduction ==== I've written a program to do a big computation. Unfortunately, the computation takes a very long time (expectedly), and the memory use increases slowly (unexpectedly), until it fills up the entire memory and swap space of the computer (many gigabytes). The rough structure of the program is: • create a long (up to 20 million) list of objects; • compute a number for each of those objects; • compute the sum of the resulting list. I switched the intermediate data structure from a list to a Stream (from the stream-fusion package), hoping to fix the memory issue. It decreased both the memory use and the rate of its increase, but after a long time, the program still uses up all available memory. ==== A simple program After many hours of cutting down my program, I now have a small program (attached) that shows the same behaviour. It uses only said stream-fusion package, and vector. (I haven't yet tried to cut out the use of vector. I hope it is irrelevant, because all vectors are of fixed small size.) I compile the program with ghc-7.6.1 using > ghc --make -threaded -rtsopts -with-rtsopts="-M1G -K128M" -O2 -main-is > Test.main Test The rts options may not be strictly necessary: I added them at some point to allow the use of multiple cores, and to prevent the program from crashing the machine by using all available memory. When running the program, the resident memory quickly grows to about 3.5 MB (which I am fine with); this stays constant for a long time, but after about 7 minutes, it starts to grow further. The growth is slow, but I really would hope this program to run in constant memory. ==== The code ==== Note that I added an instance for Monad Stream, using concatMap. This is implicitly used in the definition of the big stream. The source of Data.Stream contains many alternative implementations of concat and concatMap, and alludes to the difficulty of making it fuse properly. Could it be that the fusion did not succeed in this case? Thanks for any help! Regards, Arie

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Re: [Haskell-cafe] Compiling arbitrary Haskell code
by Daniil Frumin 14 Oct '13

14 Oct '13

For those who are interested (and I already chatted with Chris on IRC), I've implemented a pastebin that is able to (among some other things) to run arbitrary Haskell code: http://paste.hskll.org/ I've also developed a 'restricted-workers' library for managing processes that should run in secured environment. I've described some of my endeavors in a blog post: http://parenz.wordpress.com/2013/07/15/interactive-diagrams-gsoc-progress-r… Bottom line: proper restrictions are hard, the necessary tools operate on a low level, there are some caveats too On Sat, Oct 12, 2013 at 12:30 AM, Christopher Done <chrisdone(a)gmail.com>wrote: > Is there a definitive list of things in GHC that are unsafe to > _compile_ if I were to take an arbitrary module and compile it? > > E.g. off the top of my head, things that might be dangerous: > > * TemplateHaskell/QuasiQuotes -- obviously > * Are rules safe? > * #includes — I presume there's some security risk with including any old > file? > * FFI -- speaks for itself > > I'm interested in the idea of compiling Haskell code on lpaste.org, > for core, rule firings, maybe even Th expansion, etc. When sandboxing > code that I'm running, it's really easy if I whitelist what code is > available (parsing with HSE, whitelisting imports, extensions). The > problem of infinite loops or too much allocation is fairly > straight-forwardly solved by similar techniques applied in mueval. > > SafeHaskell helps a lot here, but suppose that I want to also allow > TemplateHaskell, GeneralizedNewtypeDeriving and stuff like that, > because a lot of real code uses those. They only seem to be restricted > to prevent cheeky messing with APIs in ways the authors of the APIs > didn't want -- but that shouldn't necessarily be a security—in terms > of my system—problem, should it? Ideally I'd very strictly whitelist > which modules are allowed to be used (e.g. a version of TH that > doesn't have runIO), and extensions, and then compile any code that > uses them. > > I'd rather not have to setup a VM just to compile Haskell code safely. > I'm willing to put some time in to investigate it, but if there's > already previous work done for this, I'd appreciate any links. > > At the end of the day, there's always just supporting a subset of > Haskell using SafeHaskell. I'm just curious about the more general > case, for use-cases similar to my own. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Sincerely yours, -- Daniil

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Nested monadic monoids via Traversable?
by Hans Höglund 13 Oct '13

13 Oct '13

I have been experimenting with compositions of monads carrying associated monoids (i.e. Writer-style) and discovered the following pattern: ---------------------------------------------------------------------- {-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable, GeneralizedNewtypeDeriving #-} import Control.Monad import Control.Monad.Writer hiding ((<>)) import Data.Semigroup import Data.Foldable (Foldable) import Data.Traversable (Traversable) import qualified Data.Traversable as Traversable newtype Foo m a = Foo (Writer m a) deriving (Monad, MonadWriter m, Functor, Foldable, Traversable) newtype Bar m a = Bar { getBar :: [Foo m a] } deriving (Semigroup, Functor, Foldable, Traversable) instance Monoid m => Monad (Bar m) where return = Bar . return . return Bar ns >>= f = Bar $ ns >>= joinedSeq . fmap (getBar . f) where joinedSeq = fmap join . Traversable.sequence runFoo (Foo x) = runWriter x runBar (Bar xs) = fmap runFoo xs ---------------------------------------------------------------------- That is, given a type that is Monadic and Traversable, we can define a list of the same type as a monad, whose binding action "glues together" the nested Monoid values. A trivial example: ---------------------------------------------------------------------- -- annotate all elements in bar tells :: String -> Bar String a -> Bar String a tells a (Bar xs) = Bar $ fmap (tell a >>) xs -- a bar with no annotations x :: Bar String Int x = return 0 -- annotations compose with >>= y :: Bar String Int y = x <> tells "a" x >>= (tells "b" . return) -- and with join z :: Bar String Int z = join $ tells "d" $ return (tells "c" (return 0) <> return 1) -- runBar y ==> [(0,"b"),(0,"ab")] -- runBar z ==> [(0,"dc"),(1,"d")] ---------------------------------------------------------------------- However, I am concerned about the (Monad Bar) instance which seems ad-hoc to me, especially the use of sequence. Is there a more general pattern which uses a class other than Traversable? Any pointers would be much appreciated. Regards, Hans

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typeclass to select a list element
by Paolino 13 Oct '13

13 Oct '13

Hello, I'm trying to use a type class to select an element from a list. I would like to have a String "CC" as a value for l10'. {-# LANGUAGE MultiParamTypeClasses, GADTs,FlexibleInstances, DataKinds ,TypeFamilies, KindSignatures, FlexibleContexts, OverlappingInstances, StandaloneDeriving, UndecidableInstances #-} import Data.Nat import Data.Monoid data family X (n::Nat) :: * data L (n::Nat) where Q :: (Monoid (X n), Show (X n)) => L (Succ n) -> X n -> L n E :: Monoid (X n) => L n deriving instance Show (L n) data instance X n = String String instance Monoid (X n) where String x `mappend` String y = String $ x `mappend` y mempty = String "" deriving instance Show (X n) class Compose n n' where compose :: L n -> L n -> X n' instance Compose n n where compose (Q _ x) (Q _ y) = x `mappend` y compose _ _ = mempty instance Compose n n' where compose (Q x _) (Q y _) = compose x y compose _ _ = mempty l0 :: L Zero l0 = Q (Q E $ String "C") $ String "A" l0' :: L Zero l0' = Q (Q E $ String "C") $ String "B" l10' :: X (Succ Zero) l10' = compose l0 l0' l00' :: X Zero l00' = compose l0 l0' {- *Main> l00' String "AB" *Main> l10' String "" -} Thanks for help. paolino

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Haskellers in Nashville,TN
by heathmatlock 13 Oct '13

13 Oct '13

Anyone here from the Nashville, TN area? t would be nice to meet regularly with others to discuss and work with Haskell. Also, this group is now available: www.meetup.com/The-Haskell-Study-Group/ If you want to participate remotely, that works too, I'm just looking for others to meet with regularly. -- Heath Matlock +1 256 274 4225

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Haskell Weekly News: Issue 282
by Daniel Santa Cruz 11 Oct '13

11 Oct '13

Welcome to issue 282 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of September 22 to October 05, 2013. Quotes of the Week * monochrom: . o O ( error messages are like legalese. it contains complete information so that later lawyers can say "told you so" ) * BetaZiliani: "We escape from monads in the same way that we escape from a building on fire: we run." (#ICFP2013) * ReinH: ReinH's Lemma: There's always a connection to Yoneda's Lemma. * startling: it's sort of depressing when you spend a lot of time with traversals and things to get all your crucial algorithms down to like ten lines and then have like two hundred lines each for pretty-printing and for file I/O. * edwardk: you know that feeling when you wake up in the morning, and your teeth feel fuzzy, you don't know where you've been, but you strongly suspect in 9 months someone will be calling you for library support? * tac: I free the forgotten. * Anonymous: What do you call someone reading a book about category theory? A co-author! * Anonymous: A co-worker == generator of work tasks? * edwardkmett: At one point we had a type in lens where one of its arguments started taking parameters of the form i m a s t a b u. Upon reflection, we let it win the argument and decided not to implement the function. Top Reddit Stories * xkcd - Functional Programming Domain: xkcd.com, Score: 97, Comments: 58 On Reddit: [1] http://goo.gl/XSSTOR Original: [2] http://goo.gl/daH8om * Hackage 2 is up! Domain: hackage.haskell.org, Score: 96, Comments: 20 On Reddit: [3] http://goo.gl/hSyulq Original: [4] http://goo.gl/admcV * Programming - Why Haskell is Great - 10 minutes Domain: youtube.com, Score: 91, Comments: 18 On Reddit: [5] http://goo.gl/SoTDhR Original: [6] http://goo.gl/CwsaFq * GHC: now with -j flag for parallel builds Domain: ghc.haskell.org, Score: 88, Comments: 27 On Reddit: [7] http://goo.gl/tX0YZr Original: [8] http://goo.gl/8SepKH * I can finally say that I'm a Haskell developer (and I'm hiring)! Domain: self.haskell, Score: 85, Comments: 42 On Reddit: [9] http://goo.gl/fr2adZ Original: [10] http://goo.gl/fr2adZ * Hackage 2 switchover imminent Domain: haskell.org, Score: 80, Comments: 14 On Reddit: [11] http://goo.gl/5XyWkt Original: [12] http://goo.gl/Hf3XlL * The Haskell Cast #2 - Don Stewart on Real World Haskell Domain: haskellcast.com, Score: 72, Comments: 11 On Reddit: [13] http://goo.gl/JPukzq Original: [14] http://goo.gl/iOssHA * [ANN] Nomyx, the game where you can change the rules (0.3 beta) Domain: nomyx.net, Score: 57, Comments: 14 On Reddit: [15] http://goo.gl/Z3IFfL Original: [16] http://goo.gl/gmsH5e * The aftermath of my GSoC project Domain: self.haskell, Score: 55, Comments: 6 On Reddit: [17] http://goo.gl/2vAjMs Original: [18] http://goo.gl/2vAjMs * SPJ in need for compelling "lens" use-cases Domain: permalink.gmane.org, Score: 51, Comments: 57 On Reddit: [19] http://goo.gl/6VcDMh Original: [20] http://goo.gl/wEgQUi * Tidal cycles - loops and rhythms made with tweet sized Haskell Domain: twitter.com, Score: 50, Comments: 5 On Reddit: [21] http://goo.gl/zcQqrm Original: [22] http://goo.gl/10bSjH * Who'd have thought: I made a serial monitor with Threepenny-gui Domain: self.haskell, Score: 48, Comments: 14 On Reddit: [23] http://goo.gl/rFBlWl Original: [24] http://goo.gl/rFBlWl * Arrowised Material Shaders in Purely Functional Ray Tracing Domain: tcsavage.org, Score: 44, Comments: 2 On Reddit: [25] http://goo.gl/RHvpLq Original: [26] http://goo.gl/7cDmWq * Work at Erudify Domain: jaspervdj.be, Score: 42, Comments: 5 On Reddit: [27] http://goo.gl/mpFnRa Original: [28] http://goo.gl/gcSs3U * The Universal Properties of Map, Fold, and Filter Domain: jeremykun.com, Score: 41, Comments: 0 On Reddit: [29] http://goo.gl/CksT61 Original: [30] http://goo.gl/j7rSW6 * Conal Elliott : Searching for GUI and Graphics libraries in Haskell Domain: haskell.org, Score: 39, Comments: 29 On Reddit: [31] http://goo.gl/6krMxv Original: [32] http://goo.gl/EYG0af * Type level record syntax... would this make sense? Domain: self.haskell, Score: 39, Comments: 29 On Reddit: [33] http://goo.gl/ysjr3o Original: [34] http://goo.gl/ysjr3o * Haskell Resources List Domain: gist.github.com, Score: 38, Comments: 2 On Reddit: [35] http://goo.gl/vZ9yMH Original: [36] http://goo.gl/Oo75BT * [ANN] yi-0.7 Domain: haskell.org, Score: 38, Comments: 14 On Reddit: [37] http://goo.gl/FihoH9 Original: [38] http://goo.gl/pfx5r5 * Why PVP doesn't work Domain: ro-che.info, Score: 38, Comments: 106 On Reddit: [39] http://goo.gl/XdpZpG Original: [40] http://goo.gl/TkEBzt * Live blog of CUFP 2013 -- a prodigious and growing work of reporting by Anil Madhavapeddy Domain: syslog.cl.cam.ac.uk, Score: 35, Comments: 0 On Reddit: [41] http://goo.gl/JDJJAz Original: [42] http://goo.gl/SvsyBE * What Haskell tools do you use in every project? Domain: self.haskell, Score: 35, Comments: 22 On Reddit: [43] http://goo.gl/9z8Ylr Original: [44] http://goo.gl/9z8Ylr * GHC Core by example, episode 2: Evaluation Domain: alpmestan.com, Score: 35, Comments: 3 On Reddit: [45] http://goo.gl/i7UzV1 Original: [46] http://goo.gl/kuntPM * Oh my laziness! - An introduction to laziness, strictness and unpacking in GHC Haskell Domain: alpmestan.com, Score: 35, Comments: 16 On Reddit: [47] http://goo.gl/Gfwhd0 Original: [48] http://goo.gl/84omB7 * Job opportunity for junior web developer looking to get into Haskell Domain: self.haskell, Score: 35, Comments: 11 On Reddit: [49] http://goo.gl/FqFDff Original: [50] http://goo.gl/FqFDff * Short vlog style video "My Road to Haskell" by jekor Domain: youtube.com, Score: 33, Comments: 1 On Reddit: [51] http://goo.gl/sqjXcT Original: [52] http://goo.gl/GzjCs9 * Arguing against monad transformers for the right reasons Domain: blog.ezyang.com, Score: 32, Comments: 20 On Reddit: [53] http://goo.gl/b2tFy6 Original: [54] http://goo.gl/OfGzvr * Haskell Launchpad Interaction Domain: youtube.com, Score: 32, Comments: 18 On Reddit: [55] http://goo.gl/Qwnhvw Original: [56] http://goo.gl/VlyUqn Top StackOverflow Questions * Sets, Functors and Eq confusion votes: 36, answers: 3 Read on SO: [57] http://goo.gl/2TUC5X * writing a C# version of haskell infinite fibonacci series function votes: 12, answers: 5 Read on SO: [58] http://goo.gl/Nys1p7 * Enforced pattern order votes: 10, answers: 5 Read on SO: [59] http://goo.gl/4yPfrn * Haskell, multiplying Int and Float within a function votes: 10, answers: 4 Read on SO: [60] http://goo.gl/nVsl8Z * What requirements does non-strict semantics of Haskell have on the evaluation strategy? votes: 9, answers: 1 Read on SO: [61] http://goo.gl/wqWIVk * Random access on a huge file in haskell votes: 8, answers: 3 Read on SO: [62] http://goo.gl/gxes9Y * Not using a variable for <- votes: 8, answers: 4 Read on SO: [63] http://goo.gl/jZ9Eqk * A common pattern involving composition of functions (\a b -> f (g a) (g b)) votes: 8, answers: 2 Read on SO: [64] http://goo.gl/FVY6Zl * Haskell data :> constructor meaning votes: 8, answers: 1 Read on SO: [65] http://goo.gl/0ouMUi * In functional reactive programming, how do you share state between two parts of the application? votes: 7, answers: 1 Read on SO: [66] http://goo.gl/vY3Lsq Until next time, [67]+Daniel Santa Cruz References 1. http://xkcd.com/1270/ 2. http://www.reddit.com/r/haskell/comments/1n8gor/xkcd_functional_programming/ 3. http://hackage.haskell.org/ 4. http://www.reddit.com/r/haskell/comments/1n79sh/hackage_2_is_up/ 5. https://www.youtube.com/watch?v=RqvCNb7fKsg 6. http://www.reddit.com/r/haskell/comments/1nfbdm/programming_why_haskell_is_… 7. http://ghc.haskell.org/trac/ghc/ticket/910 8. http://www.reddit.com/r/haskell/comments/1mzfob/ghc_now_with_j_flag_for_par… 9. http://www.reddit.com/r/haskell/comments/1n8zmp/i_can_finally_say_that_im_a… 10. http://www.reddit.com/r/haskell/comments/1n8zmp/i_can_finally_say_that_im_a… 11. http://www.haskell.org/pipermail/haskell-cafe/2013-September/109007.html 12. http://www.reddit.com/r/haskell/comments/1n2rqu/hackage_2_switchover_immine… 13. http://www.haskellcast.com/episode/002-don-stewart-on-real-world-haskell/ 14. http://www.reddit.com/r/haskell/comments/1mymdf/the_haskell_cast_2_don_stew… 15. http://www.nomyx.net/ 16. http://www.reddit.com/r/haskell/comments/1nq468/ann_nomyx_the_game_where_yo… 17. http://www.reddit.com/r/haskell/comments/1n4ips/the_aftermath_of_my_gsoc_pr… 18. http://www.reddit.com/r/haskell/comments/1n4ips/the_aftermath_of_my_gsoc_pr… 19. http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/20400 20. http://www.reddit.com/r/haskell/comments/1nn61l/spj_in_need_for_compelling_… 21. http://twitter.com/tidalcycles/ 22. http://www.reddit.com/r/haskell/comments/1npjrq/tidal_cycles_loops_and_rhyt… 23. http://www.reddit.com/r/haskell/comments/1nidyp/whod_have_thought_i_made_a_… 24. http://www.reddit.com/r/haskell/comments/1nidyp/whod_have_thought_i_made_a_… 25. http://tcsavage.org/blog/2013/06/25/arrowised-materials-in-pfrt/ 26. http://www.reddit.com/r/haskell/comments/1nh0f9/arrowised_material_shaders_… 27. http://jaspervdj.be/posts/2013-09-29-erudify.html 28. http://www.reddit.com/r/haskell/comments/1ngbvi/work_at_erudify/ 29. http://jeremykun.com/2013/09/30/the-universal-properties-of-map-fold-and-fi… 30. http://www.reddit.com/r/haskell/comments/1nqmyg/the_universal_properties_of… 31. http://www.haskell.org/pipermail/haskell-cafe/2013-September/109022.html 32. http://www.reddit.com/r/haskell/comments/1n8c5i/conal_elliott_searching_for… 33. http://www.reddit.com/r/haskell/comments/1nf2ss/type_level_record_syntax_wo… 34. http://www.reddit.com/r/haskell/comments/1nf2ss/type_level_record_syntax_wo… 35. https://gist.github.com/leroux/6395804 36. http://www.reddit.com/r/haskell/comments/1n9z50/haskell_resources_list/ 37. http://www.haskell.org/pipermail/haskell-cafe/2013-October/109156.html 38. http://www.reddit.com/r/haskell/comments/1nn1vw/ann_yi07/ 39. http://ro-che.info/articles/2013-10-05-why-pvp-doesnt-work.html 40. http://www.reddit.com/r/haskell/comments/1ns193/why_pvp_doesnt_work/ 41. http://www.syslog.cl.cam.ac.uk/2013/09/22/liveblogging-cufp-2013/ 42. http://www.reddit.com/r/haskell/comments/1n10u8/live_blog_of_cufp_2013_a_pr… 43. http://www.reddit.com/r/haskell/comments/1nhntw/what_haskell_tools_do_you_u… 44. http://www.reddit.com/r/haskell/comments/1nhntw/what_haskell_tools_do_you_u… 45. http://alpmestan.com/2013/10/01/ghc-core-by-example-episode-2-evaluation/ 46. http://www.reddit.com/r/haskell/comments/1nktj7/ghc_core_by_example_episode… 47. http://alpmestan.com/2013/10/02/oh-my-laziness/ 48. http://www.reddit.com/r/haskell/comments/1nlxlo/oh_my_laziness_an_introduct… 49. http://www.reddit.com/r/haskell/comments/1npram/job_opportunity_for_junior_… 50. http://www.reddit.com/r/haskell/comments/1npram/job_opportunity_for_junior_… 51. http://www.youtube.com/watch?v=mpby_hz8lFc 52. http://www.reddit.com/r/haskell/comments/1n3f8s/short_vlog_style_video_my_r… 53. http://blog.ezyang.com/2013/09/if-youre-using-lift-youre-doing-it-wrong-pro… 54. http://www.reddit.com/r/haskell/comments/1n7mid/arguing_against_monad_trans… 55. http://www.youtube.com/watch?v=7lZ-DcidyMM 56. http://www.reddit.com/r/haskell/comments/1naslm/haskell_launchpad_interacti… 57. http://stackoverflow.com/questions/19177125/sets-functors-and-eq-confusion 58. http://stackoverflow.com/questions/19109152/writing-a-c-sharp-version-of-ha… 59. http://stackoverflow.com/questions/18989916/enforced-pattern-order 60. http://stackoverflow.com/questions/19019093/haskell-multiplying-int-and-flo… 61. http://stackoverflow.com/questions/18944370/what-requirements-does-non-stri… 62. http://stackoverflow.com/questions/18985353/random-access-on-a-huge-file-in… 63. http://stackoverflow.com/questions/19051668/not-using-a-variable-for 64. http://stackoverflow.com/questions/19064694/a-common-pattern-involving-comp… 65. http://stackoverflow.com/questions/19197339/haskell-data-constructor-meaning 66. http://stackoverflow.com/questions/19201901/in-functional-reactive-programm… 67. https://plus.google.com/105107667630152149014/about

1 0

Conditional lens
by Artyom Kazak 11 Oct '13

11 Oct '13

Hello! I am working with TypeReps, and while writing some functions I have noticed that I could use lenses to simplify them; however, I have stumbled upon some difficulties. First I’ll try to clarify which functions I want to write: * a function for converting TypeRep of, say, `Maybe x` to `[x]` (for all x). It requires checking if the TyCon is `Maybe` and replacing it with []-TyCon. If it wasn’t `Maybe`, I return Nothing. * a similar function for replacing `Char`s and `Lazy.Text`s to just `Text`. Again, if the TypeRep-to-be-replaced doesn’t satisfy my conditions, I return Nothing. These two functions (and some others, I suppose) can be written concisely with the help of one combinator. I don’t know how to write it as a composable Lens, so I’ll give it here as an ad-hoc Lens-modifying function instead: ifL :: (a -> Bool) -> Lens s t a b -> Lens s (Maybe t) (Maybe a) b ifL p l = lens getter setter where get s = getConst $ l Const s getter s = let a = get s in if p a then Just a else Nothing setter s b = let a = get s in if p a then Just (set l b s) else Nothing It works like this: > (0, 2) & ifL even fs .~ "hello" Just ("hello",2) > (1, 2) & ifL even fs .~ "hello" Nothing With `ifL`, my initial ugly changeTyCon :: TyCon -> TyCon -> TypeRep -> Maybe TypeRep changeTyCon tc tc' t | t^.tyCon == tc = Just $ t & tyCon .~ tc' | otherwise = Nothing boils down to changeTyCon tc tc' = ifL (== tc) tyCon .~ tc' Why did I call the initial version “ugly”? Well, because a) it manually handles `Maybe`s, and b) it has to perform both getting and setting (two passes). So, my questions are: 1. What would be the idiomatic way to write `ifL`? 2. How can I do something like `t ^. ifL (== tc) tyCon`? Currently it doesn’t work because view’s type has been simplified in lens-3.9. 3. Perhaps it would be better to represent `ifL` as a Traversal which simply ignores values that don’t match the condition? Then I could (?) use `failover` to do what I want. I searched for something filter-like in lens library, but haven’t found anything. 4. If I haven’t missed anything and it indeed can’t be done with bare lens, would `ifL` or something similar be welcome as an addition to the library? Thanks!

1 0

Using Quick Check generators for getting arbitrary value streams
by Luke Evans 10 Oct '13

10 Oct '13

I was hoping I could use Arbitrary instances to generate streams of values for test data. It looks like you're not 'supposed' to be trying this, other than for the specific purpose of then testing some properties on these streams within Quick Check itself. I'm looking for something like the sample' function in Quick Check, only to produce an infinite stream of values (rather than the 11 values that are clearly designed simply to give you a sense of the output of a generator). Am I out of luck, or is there yet some cunning way of doing this without hacking the Quick Check package to export the MkGen constructor? -- Luke

3 2

Suppressing HLint on pattern match of files
by Graham Berks 10 Oct '13

10 Oct '13

Hi, would like to disable 'Top-level binding with no type signature' In my test modules that are prefixed with Test. Is this possible ?? Thanks

2 2

hdbc-odbc not getting any data using mssql stored procedure
by grant 10 Oct '13

10 Oct '13

Hi, I am trying to call a stored procedure (exec gp_somestoredproc 123,22) using microsoft sql server 2008 R2 using hdbc-odbc. Here is the haskell code: import Database.HDBC import Database.HDBC.ODBC import Control.Monad import Data.Maybe import qualified Data.ByteString.Char8 as B test1 = do c <- connectODBC "Driver={SQL Server};Server=?;Database=?;uid=sa;pwd=?;" p <- prepare c "exec gp_somestoredproc 123,22" -- returns no data -- p <- prepare c "exec [sys].sp_datatype_info_100 0,@ODBCVer=4;exec gp_somestoredproc 123,22" -- all is good e <- execute p [] -- returns 6 (number of rows) putStrLn $ "execute " ++ show e r <- fetchAllRows' p putStrLn $ "fetchAllRows' " ++ show r The problem is that this code returns the number of rows correctly but doesn't return data nor are there any errors. However, I ran a perl program using perl dbi and got the data correctly. Here is the perl code: #!/usr/bin/perl use DBI; my $user = 'sa'; my $pw = '????'; my $dsn = '????'; my $dbh = DBI->connect("dbi:ODBC:$dsn", $user, $pw, {PrintError => 1, RaiseError => 1}); if (!$dbh) { print "error: connection: $DBI::err\n$DBI::errstr\n$DBI::state\n"; } my $type_info = $dbh->type_info(93); while(my($key, $value) = each(%$type_info)){ print "$key => $value\n"; }; my $sql = "exec gp_somestoredproc 123,22"; my $sth = $dbh->prepare($sql); my $r = $sth->execute; while (my ($db) = $sth->fetchrow_array) { print $db . "\n===\n"; } $dbh->disconnect if $dbh; I traced both versions and noticed that the perl dbi version first called exec [sys].sp_datatype_info_100 0,@ODBCVer=4 So I prefixed the stored proc call in haskell with "exec [sys].sp_datatype_info_100 0,@ODBCVer=4;" and it worked fine. In short: FAILS p <- prepare c "exec gp_somestoredproc 123,22" -- returns number of rows but no data WORKS p <- prepare c "exec [sys].sp_datatype_info_100 0,@ODBCVer=4;exec gp_somestoredproc 123,22" I have no idea why this works. sp_datatype_info_100 just dumps out the fields types ... More information: The stored procedure returns data with user defined field types. I have managed to do selects against tables with user defined field types without any problems using hdbc-odbc. I couldn't emulate this error on a local older version of mssql server (Microsoft SQL Server 2008 (SP1) - 10.0.2531.0 (X64) express) but the perl dbi prefixed the stored procedure call with "exec sp_datatype_info 0,@ODBCVer=3" I am running this against Microsoft SQL Server 2008 R2 (RTM) - 10.50.1797.0 (X64) I would appreciate any pointers you can give me. Thanks Grant

3 2

Call to Action - Fay FFI Bindings for AngularJS
by Adam Bergmark 10 Oct '13

10 Oct '13

I've talked to a lot of people that would like to be able to use Fay with AngularJS, this seems like a great idea! I don't use AngularJS personally, but I would be happy to help out with designing it and making code reviews. Has anyone started working on this already, and do you have any code/thoughts to share as a starting point? I created a stub repository under the faylang org on github: https://github.com/faylang/fay-angular I'll give anyone who wants to work on this push access, just send me an e-mail with your github account name, you can also create pull requests if you want a more in depth code review. Feel free to open up an issue if you have anything you would like to discuss. https://github.com/faylang/fay-angular/issues Regards, Adam

1 0

RankNTypes + ConstraintKinds to use Either as a "union"
by oleg＠okmij.org 10 Oct '13

10 Oct '13

Thiago Negri wrote: > Why type inference can't resolve this code? > {-# LANGUAGE RankNTypes, ConstraintKinds #-} > > bar :: (Num a, Num b) => (forall c. Num c => c -> c) ->Either a b ->Either a b > bar f (Left a) = Left (f a) > bar f (Right b) = Right (f b) > > bar' = bar (+ 2) -- This compiles ok > > foo :: (tc a, tc b) => (forall c. tc c => c -> c) -> Either a b -> Either a b > foo f (Left a) = Left (f a) > foo f (Right b) = Right (f b) > > foo' = foo (+ 2) -- This doesn't compile because foo' does not typecheck The type inference of the constraint fails because it is ambiguous. Observe that not only bar (+2) compiles OK, but also bar id. The function id :: c -> c has no constraints attached, but still fits for (forall c. Num c => c -> c). Let's look at the problematic foo'. What constraint would you think GHC should infer for tc? Num? Why not the composition of Num and Read, or Num and Show, or Num and any other set of constraints? Or perhaps Fractional (because Fractional implies Num)? For constraints, we get the implicit subtyping (a term well-typed with constraints C is also well-typed with any bigger constraint set C', or any constraint set C'' which implies C). Synonyms and superclass constraints break the principal types. So, inference is hopeless. We got to help the type inference and tell which constraint we want. For example, > newtype C ctx = C (forall c. ctx c => c -> c) > > foo :: (ctx a, ctx b) => C ctx -> (forall c. ctx c => c -> c) -> > Either a b -> Either a b > foo _ f (Left a) = Left (f a) > foo _ f (Right b) = Right (f b) > > foo' = foo (undefined :: C Num) (+2) Or, better > xyz :: (ctx a, ctx b) => C ctx -> Either a b -> Either a b > xyz (C f) (Left a) = Left (f a) > xyz (C f) (Right b) = Right (f b) > > xyz' = xyz ((C (+2)) :: C Num) > xyz'' = xyz ((C (+2)) :: C Fractional)

1 0

Interfacing real-time stocks data API
by Miro Karpis 10 Oct '13

10 Oct '13

Please, did/does anybody tried to interface with Haskell some real-time stocks data API? If yes, please which one? So far I came down to ActveTick,... thanks, m.

6 5

Case studies, step two
by Mike Meyer 10 Oct '13

10 Oct '13

I want to thank everyone who provided pointers for the last question about this. They were a big help. We're now trying to narrow things down a bit. If you have either converted part of a business project from a language like ruby or python to Haskell, or have a business project that integrates Haskell with such a language and are willing to take a short survey about it, could you get back to me? Thanks

1 0

Fwd: [Haskell-beginners] RankNTypes + ConstraintKinds to use Either as a "union"
by Thiago Negri 10 Oct '13

10 Oct '13

(from thread: http://www.haskell.org/pipermail/beginners/2013-October/012703.html) Why type inference can't resolve this code? > {-# LANGUAGE RankNTypes, ConstraintKinds #-} > > bar :: (Num a, Num b) => (forall c. Num c => c -> c) -> Either a b -> Either a b > bar f (Left a) = Left (f a) > bar f (Right b) = Right (f b) > > bar' = bar (+ 2) -- This compiles ok > > foo :: (tc a, tc b) => (forall c. tc c => c -> c) -> Either a b -> Either a b > foo f (Left a) = Left (f a) > foo f (Right b) = Right (f b) > > foo' = foo (+ 2) -- This doesn't compile because foo' does not typecheck > > -- Kim-Ee pointed out that this works: > type F tc a b = (tc a, tc b) => (forall c. tc c => c -> c) -> Either a b -> Either a b > foo' = (foo :: F Num a b) (+2)

1 0

FP relates events in NYC
by Dan Frumib 09 Oct '13

09 Oct '13

Hi folks, I will be visiting New York at the end of the October and I would like to catch some cool Haskell events if possible. I'll be staying in the city from 29th of October to the 10th of November and I would like to plan my trip ahead. I am looking for anything nerdy: talks, seminars, user groups, etc Thanks - Dan

1 0

Re: [Haskell-cafe] Interfacing real-time stocks data API
by Alexey Uimanov 09 Oct '13

09 Oct '13

Yes, the trading system, or code generator. It depends on what would be easier to implement. But firstly I need a simulator and history data crawler what I am working on. 2013/10/9 Miro Karpis <miroslav.karpis(a)gmail.com> > Hi Alexey, > thank you for response. You wrote that you are developing package for > testing trading systems. Are you planning to also build a trading system? > > Regards, > Miro > > > On Wed, Oct 9, 2013 at 10:28 AM, Alexey Uimanov <s9gf4ult(a)gmail.com>wrote: > >> I did not find such a library, but I am interested in Haskell trading >> automation too. >> >> BTW, I am developing package for testing trading systems, it can just >> download historical data from some free russian stock services for now. >> >> https://github.com/s9gf4ult/hadan >> >> If you interested we could join forces. >> >> >> 2013/10/9 Miro Karpis <miroslav.karpis(a)gmail.com> >> >>> Please, did/does anybody tried to interface with Haskell some real-time >>> stocks data API? If yes, please which one? So far I came down to >>> ActveTick,... >>> >>> thanks, >>> m. >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >>> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

1 0

ANN: hsqml-0.2.0.0
by Robin KAY 09 Oct '13

09 Oct '13

Dear All, I would like to announce version 0.2.0.0 of the HsQML graphics library. HsQML provides a Haskell binding to the Qt Quick framework. It allows you to write graphical applications where the front-end is written in Qt Quick's QML language (incorporating JavaScript) and the back-end is written in Haskell. The two layers are coupled together via a facility to define custom JavaScript objects through which QML code can call into Haskell and vice versa. HsQML requires an installation of Qt 4.7 or 4.8 (including QtDeclarative) present on your path. This release introduces several new features, including support for firing QML signals from Haskell code and MacOS support. A number of bugs have also been resolved. For more information, please see my web site [1] and the Hackage page [2]. The hsqml-morris demo application [3], which implements the game of Nine Men's Morris against an AI opponent, has also been updated to 0.2.0.0. It now uses QML signals to run the game's AI processing outside of the event loop so as to maintain a responsive UI. My goals for the next release are to work on writing a proper user manual and to migrate the library over to work with Qt 5.x. [1] http://www.gekkou.co.uk/software/hsqml [2] http://hackage.haskell.org/package/hsqml-0.2.0.0 [3] http://hackage.haskell.org/package/hsqml-morris-0.2.0.0 Regards, -- Robin KAY

1 0

ANN: E-book version of the Typeclassopedia
by Erlend Hamberg 08 Oct '13

08 Oct '13

While re-reading Brent Yorgey's Excellent Typeclassopedia I converted it to Pandoc Markdown in order to be able to create an EPUB version. Having a “real” e-book meant that I could comfortably read it on my e-book reader and highlight text and take notes while reading. I also fixed some minor issues while reading it. (These fixes were of course backported to the official Typeclassopedia version on the Haskell Wiki.) The EPUB file can be downloaded from Github: https://github.com/ehamberg/typeclassopedia-md/releases The Markdown source is also available in that repo and you can of course use Pandoc to convert the Markdown file to all the other output formats Pandoc supports. By using a program like Calibre, the EPUB file can be converted to other e-book formats such as the Kindle format. I hope people find this useful. :-) -- Erlend Hamberg ehamberg(a)gmail.com

5 4

POSA
by Kazu Yamamoto 08 Oct '13

08 Oct '13

Hi all, Michael Snoyman, Andreas Voellmy and I are invited to write an article about Warp to "The Performance of Open Source Applications": http://aosabook.org/en/index.html It is now open to the public including our article. http://aosabook.org/en/posa/warp.html Enjoy! --Kazu

1 0

A question regarding reading CPP definitions from a C header
by Ömer Sinan Ağacan 08 Oct '13

08 Oct '13

Hi all, Let's say I want to #include a C header file in my Haskell library just to read some macro definitions. The C header file also contains some C code. Is there a way to load only macro definitions and not C code in #include declarations in Haskell? What I'm trying to do is I'm linking my library against this C library but I want to support different versions of this C library, so I want to read it's version from one of it's header files. The problem is the header file contains some C code and makes my Haskell source code mixed with C source before compilation. Any suggestions would be appreciated, Thanks, --- Ömer Sinan Ağacan http://osa1.net

3 4

Lenses that work with Arrows
by Tom Ellis 07 Oct '13

07 Oct '13

Dear all, I introduce a very simple extension to the Lens datatype from Control.Lens that allows it to work with Arrows: https://gist.github.com/tomjaguarpaw/6865080 I would particularly like to discuss this with authors of Control.Lens to see if such an idea is suitable for inclusion in their library. I have also started a Reddit discussion here: http://www.reddit.com/r/haskell/comments/1nwetz/lenses_that_work_with_arrow… Tom

2 3

Lifting IO actions into Applicatives
by Michael Snoyman 07 Oct '13

07 Oct '13

I'm wondering if anyone's run into this problem before, and if there's a common solution. In Yesod, we have applicative forms (based originally on formlets). These forms are instances of Applicative, but not of Monad. Let's consider a situation where we want to get some user input to fill out a blog post datatype, which includes the current time: data Blog = Blog Title UTCTime Contents myBlogForm :: Form Blog myBlogForm = Blog <$> titleForm <*> something <*> contentsForm The question is: what goes in something? Its type has to be: something :: Form UTCTime Ideally, I'd call getCurrentTime. The question is: how do I lift that into a Form? Since Form is only an Applicative, not a Monad, I can't create a MonadIO instance. However, Form is in fact built on top of IO[1]. And it's possible to create a MonadTrans instance for Form, since it's entirely possible to lift actions from the underlying functor/monad into Form. So something can be written as: something = lift $ liftIO getCurrentTime This works, but is unintuitive. One solution would be to have an ApplicativeIO typeclass and then use liftIOA. My questions here are: 1. Has anyone else run into this issue? 2. Is there an existing solution out there? Michael [1] Full crazy definition is at: http://haddocks.fpcomplete.com/fp/7.4.2/20130922-179/yesod-form/Yesod-Form-…

15 22

Any precedent or plan for guaranteed-safe Eq and Ord instances?
by Ryan Newton 07 Oct '13

07 Oct '13

Hello all, Normally, we don't worry *too* much about incorrect instances of standard classes (Num, Eq, Ord) etc. They make the user's program wrong, but they don't compromise the type system. Unfortunately, with the LVish parallel programming library we do have a situation where incorrect instances of Eq and Ord can cause the "types to lie". In particular, something that claims to be a pure, non-IO type, can actually yield a different result on different runs, including throwing exceptions on some runs but not others. So what's the best way to lock down "SafeEq" and "SafeOrd" instances, taking control away from the user (at least with -XSafe is turned on)? We could derive our own SafeEq and SafeOrd instances based on GHC.Generics. BUT, that only helps if we can forbid the user from writing their own incorrect Generics instances when Safe Haskell is turned on. It looks like GHC.Generics is currently marked as "TrustWorthy": http://www.haskell.org/ghc/docs/7.4.1/html/libraries/ghc-prim-0.2.0.0/GHC-G… So.... could we get GHC.Generics marked as "Unsafe" and enable some more limited interface that is "Trustworthy"? (Allowing the user ONLY to do 'deriving Generic'). This would be similar to the new policy in GHC 7.8 of only allowing derived Typeable instances... -Ryan

11 22

Music update
by Mark Lentczner 07 Oct '13

07 Oct '13

Some might remember me asking about music packages a while back... An update: I ended up using Euterpea, which in turn uses both Codec.Midi and Sound.PortMidi. My working environment was to have my code loaded up in ghci, play MIDI into a software MIDI bus, and pipe that into MainStage 3 which ran the synths. The piece I was working on premiered last night at a concert in Wellington, NZ. The live recording will take a while, but you can hear a studio synth recording (from the above setup) here: https://soundcloud.com/mtnviewmark/plain-changes-2-all-synth-mix The code for the piece is all open source: https://github.com/mzero/PlainChanges2. In particular, there is a somewhat improved MIDI player<https://github.com/mzero/PlainChanges2/blob/master/src/Sound/MidiPlayer.hs> in there. Enjoy! - Mark P.S.: Yes, and now that that's done with, I can get on with the next Haskell Platform release!

3 3

[ANNOUNCE] Penny - double-entry accounting
by Omari Norman 07 Oct '13

07 Oct '13

I'm pleased to make the first public announcement of the availability of Penny, a double-entry command-line accounting system. Penny is inspired by Ledger, an excellent program in its own right. Ledger's websites and sales pitches are much better developed than those for Penny, so first take a look at Ledger's website to see if the concepts behind this program interest you: http://www.ledger-cli.org/ With that in mind, here is what distinguishes Penny from Ledger: * Smarter report formatting. When using Ledger I rarely bothered to reformat the reports because I could never remember how to use the cryptic formatting strings. Penny automatically formats reports to fit the width of your screen. You can easily add or remove information in your reports. * Use of color. Color is baked into Penny, making reports easier to read. It takes advantage of 256-color terminals where available. * Adherence to double-entry accounting principles. Ledger will allow you to enter unbalanced transactions. Penny never allows this. Even infinitesimally unbalanced transactions will render your entire ledger invalid. * Different handling of commodities. In part because of the strict adherence to double-entry accounting principles, Penny handles commodities (e.g. transactions involving multiple currencies, or currency and equities) differently than Ledger. Some might regard Penny's handling of commodities to be a pain, but I think it results in a system that is more predictable, has no corner cases, and is easy to test. * More orthogonal command-line interface. For example, in Ledger, you use one set of options to filter postings by date, and a different set of options to control the dates of postings that are shown in the register report. Penny uses identical options for these two purposes; what these options mean depends on where they appear on the command line. In addition, in Penny you can build filtering expressions of arbitrary complexity, using either infix or reverse polish notation. * Automated import of postings. You can import postings from your bank as long as it gives you OFX files. Many financial institutions supply OFX files, as it is the format used by Quicken and by the now-defunct Microsoft Money. Penny can also help automate a great deal of the process of reconciling your ledger with bank statements. This removes an enormous amount of the drudgery that is associated with maintaining a ledger. * Exposes a Haskell API. This is actually the primary reason I wrote Penny. I run several automated checks on my ledger. Doing this in Ledger was growing difficult because I had no standalone parser for the ledger files. With Penny I can easily write additional small programs to check up on the consistency of my ledger (for instance, to make sure I'm not using invalid accounts, or to make sure that a particular account has postings within a given time period.) The power of Haskell makes this much easier than it would be if I used a dynamically typed language like Haskell or, as I used to do with Ledger, a random assemblage of shell, Awk scripts, Haskell, and more. Another child of Ledger that is written in Haskell is hledger: http://hledger.org/ hledger is a great program; if you like it, please continue using it. In addition to what you can glean from the bullets above, here are some key ways Penny differs from hledger: * hledger is at least partially compatible with Ledger; Penny is incompatible with Ledger. There currently is no program to convert Ledger files to the format Penny uses (I once wrote such a program to convert my Ledger files to Penny; after that the program was of no use to me and the file format has since changed, rendering that code useless.) * hledger has a web interface. Penny does not and I doubt it ever will. * hledger has a robust community; Penny does not. * hledger supports more of Ledger's feature set with features such as timelog support, periodic reports, and balance assertions. Penny does not have these features. * hledger uses floating-point types to represent amounts: http://hackage.haskell.org/package/hledger-lib-0.21.3/docs/Hledger-Data-Typ… Penny uses only integers: http://hackage.haskell.org/package/penny-0.30.0.0/docs/Penny-Lincoln-Bits-Q… This page summarizes why that matters: http://stackoverflow.com/questions/3730019/why-not-use-double-or-float-to-r… * Penny is BSD3; hledger is GPL. And an entirely subjective comparison that may be biased: Penny is designed from the ground up to use Haskell's static type system and its module system to its maximum advantage, which makes Penny easier to use as a base for your own programs to check your ledger, and which reduces bugs. Penny has been in development for over a year now, and I use it to maintain my financial records. A set of QuickCheck tests, in conjunction with maximum exploitation of Haskell's static type system, help ensure the correctness of the code. However, as with all free software, Penny comes with no warranty. Use it at your own risk. Please report any bugs you find in the programs or the documentation to omari(a)smileystation.com. Penny is written only for UNIX-like operating systems. Currently it should build against Haskell Platform 2013.2.0.0. If it doesn't work on your Linux or Mac OS X system, please let me know. Get Penny using "cabal install penny" or look here: http://hackage.haskell.org/package/penny Website with manpages: http://massysett.github.io/penny/ Github: https://github.com/massysett/penny Enjoy. Omari

2 1

Converting MPTC+fundeps to type family / problem with polymorphic constant. Roles?
by Ryan Newton 07 Oct '13

07 Oct '13

The "abstract-par" class has used multi-parameter type classes with fundeps: http://hackage.haskell.org/package/abstract-par-0.3.1/docs/Control-Monad-Pa… And I'm trying to port it to use type families. But the following combination seems to be completely unusable for me right now: class ParFuture m => ParIVar m where new :: forall a . m (Future m a) You can see the error when you try loading this file at this line: https://github.com/iu-parfunc/lvars/blob/b60cafb941ea8f2f333ada9b5206cd2121… "Future" is a type-function from the superclass. I can't seem to use "new" in any way at any type without getting: Control/Par/Class.hs:161:9: Could not deduce (Future m a ~ Future m a0) from the context (ParIVar m, FutContents m a) bound by the type signature for newFull_ :: (ParIVar m, FutContents m a) => a -> m (IVar m a) at Control/Par/Class.hs:151:13-74 NB: `Future' is a type function, and may not be injective The type variable `a0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Expected type: m (IVar m a) Actual type: m (Future m a0) In the expression: (new :: m (IVar m a)) In a pattern binding: _ = (new :: m (IVar m a)) In an equation for `newFull_': newFull_ a = do { return undefined } where _ = (new :: m (IVar m a)) Failed, modules loaded: none. Ok, so it seems like the type of "new" may just be intrinsically unusable. There is no way, by constraining the output of the "new" function for the type system to infer what input to the Future type function was. (Type function not reversable / injective.) So am I just stuck? I don't want a dummy, unused argument just to communicate the type information to "new". Is this just a place where *MPTC + fundeps works but type familes do not? *Or is there some way that roles can save the day here? Do I need to *say that the 'a' parameter has a representation role*? Thanks, -Ryan

1 1

Poll & plea: State of GUI & graphics libraries in Haskell
by Conal Elliott 06 Oct '13

06 Oct '13

I'm polling to see whether there are will and expertise to reboot graphics and GUIs work in Haskell. I miss working on functional graphics and GUIs in Haskell, as I've been blocked for several years (eight?) due to the absence of low-level foundation libraries having the following properties: * cross-platform, * easily buildable, * GHCi-friendly, and * OpenGL-compatible. The last several times I tried Gtk2hs, I was unable to compile it on my Mac. Years ago when I was able to compile, the GUIs looked and interacted like a Linux app, which made them awkward and upleasant to use. wxHaskell (whose API and visual appearance I prefered) has for years been incompatible with GHCi, in that the second time I open a top-level window, the host process (GHCi) dies abruptly. Since my GUI & graphics programs are often one-liners, and I tend to experiment a lot, using a full compilation greatly thwarts my flow. For many years, I've thought that the situation would eventually improve, since I'm far from the only person who wants GUIs or graphics from Haskell. About three years ago, I built a modern replacement of my old Pan and Vertigo systems (optimized high-level functional graphics in 2D and 3D), generating screamingly fast GPU rendering code. I'd love to share it with the community, but I'm unable to use it even myself. Two questions: * Am I mistaken about the current status? I.e., is there a solution for Haskell GUI & graphics programming that satisfies the properties I'm looking for (cross-platform, easily buildable, GHCi-friendly, and OpenGL-compatible)? * Are there people willing and able to fix this situation? My own contributions would be to test and to share high-level composable and efficient GUI and graphics libraries on top of a working foundation. Looking forward to replies. Thanks, -- Conal

20 27

Haskellers in Minsk, Belarus?
by Yuras Shumovich 06 Oct '13

06 Oct '13

Hi, I just read an article (sorry, it is in russian: http://habrahabr.ru/post/196454/ ). The idea I found interesting: even in big citied developers complain that nothing happens at their location, but when you try to make an event -- only few of them want to participate. I never participate in events. What is wrong with me/us? So, any haskellers in Minsk, Belarus? What about beer, coffee or co-hacking? I don't have solid idea how to organize an event, but I'm sure haskellers are interesting people, both personally and professionally. Thanks, Yuras

2 1

Re: [Haskell-cafe] Poll & plea: State of GUI & graphics libraries in Haskell
by Paul Liu 06 Oct '13

06 Oct '13

Thanks. I've just built GHC HEAD on Mac OS X Lion, and tested by installing libraries with --enable-shared and loading a GLFW program into GHCi. Using ghci -fno-ghci-sandbox, everything works great including closing and restarting GL window multiple times. Can't wait for the official release of GHC 7.8! On Tue, Oct 1, 2013 at 12:09 PM, Carter Schonwald <carter.schonwald(a)gmail.com> wrote: > thats the linker bug. > > the glfw stuff has been tested on ghc HEAD / 7.7 by folks on #haskell-game > in recent memory. GHCI + foreign libs should work fine now (modulo thread > local storage related thing). > > the historical element doesn't matter any more. > > To the best of my knowledge, all such issues should be gone. Anyone who > cares about making sure GHCI+ gui libs play nice, PLEASE test with HEAD. > > the better this issue is properly tested (which i believe it has been), the > more we can actually prevent it from happening. This requires people to test > with HEAD GHCi now, rather than doing archaeology. > > anyone who cares, please play with GHCI in HEAD. If your lib doesn't work > with ghci, please report a bug. It would be a new bug because it wont' be > the previous reasons it hasnt' worked. > > > tl;dr to the best of my knowledge this issue is resolved in HEAD. Test HEAD. > Help us make sure it stays resolved by testing HEAD. > > thanks > -Carter > > > > > On Tue, Oct 1, 2013 at 1:20 PM, Paul Liu <ninegua(a)gmail.com> wrote: >> >> I reported a problem with statically linked GLFW library on Mac OS X >> Lion in this thread: >> >> http://www.haskell.org/pipermail/haskell-cafe/2012-January/097355.html >> >> I do not know why this is broken on Mac OS X Lion, but not on Linux or >> Windows. There was an EnableGUI hack for GHC 7.2 (and previous >> versions) and OS X version before Lion, but it no longer works. So I'm >> not sure if it is OS X Lion, or GLFW, or GHC, or a combination of them >> that caused this problem. >> >> Regards, >> Paul Liu >> >> On Tue, Oct 1, 2013 at 7:04 AM, Carter Schonwald >> <carter.schonwald(a)gmail.com> wrote: >> > Hey simon, the two issues that have recurrently bit ghci interaction >> > with >> > foreign GUI libs are >> > 1) the ghci linker. This is fixed in head by now having ghci use the >> > system >> > linker >> > 2) some GUI libs require thread local state, and ghci has a flag for >> > that >> > 3) I'm not aware of anyone reporting newly broken libs wrt GUI bindings >> > when 7.6 rolled out. The only fix that's relevant to 7.8 is the >> > dylinker >> > bit, but that would have been a problem historically too. >> > >> > I believe a number of folks in #haskell-game have recently tested point >> > one. >> > (Though I should ask to double check) >> > >> > At the very least, I'm not aware of hearing of such a 7.6 specific ghci >> > breakage before. >> > >> > >> > On Tuesday, October 1, 2013, Simon Peyton-Jones wrote: >> >> >> >> Dear GHC devs >> >> >> >> >> >> >> >> See below (in red). I do not know the details of this, but it sounds >> >> like >> >> a pretty serious problem, and it used to work. Is whatever-it-is >> >> confirmed >> >> fixed in 7.8? Do we have a test that’ll trip if it breaks again? (I’m >> >> guessing that the latter might be hard.) >> >> >> >> >> >> >> >> Thanks >> >> >> >> >> >> >> >> Simon >> >> >> >> >> >> >> >> -----Original Message----- >> >> From: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] On Behalf >> >> Of >> >> Paul Liu >> >> Sent: 30 September 2013 07:18 >> >> To: Conal Elliott >> >> Cc: Haskell Cafe >> >> Subject: Re: [Haskell-cafe] Poll & plea: State of GUI & graphics >> >> libraries >> >> in Haskell >> >> >> >> >> >> >> >> Hi Conal, >> >> >> >> >> >> >> >> I wasn't able to make it to last Saturday's FARM track, but I think >> >> >> >> there was a good chance that Paul would have demonstrated his Euterpea >> >> >> >> music library, which includes a GUI interface (called MUI) written on >> >> >> >> top of GLFW. I wrote its initial implementation (around 2009?) with a >> >> >> >> monadic interface that let you wire together UI components with >> >> >> >> signals (I believe Dan later wrote an arrow interface, but I could be >> >> >> >> wrong). It was actually inspired by the ideas behind your Phooey UI >> >> >> >> library. It should be very easy to extract this part out as a >> >> >> >> standalone package if there is enough interest. >> >> >> >> >> >> >> >> The only issue with it (and all other UI libraries) is that it doesn't >> >> >> >> play nicely in GHCi. It used to work pretty well with GHC 7.2 and 7.4 >> >> >> >> on almost all platforms (Mac needs an extra hack), but GHC 7.6 broke >> >> >> >> Mac (and perhaps Windows too). GHC 7.8 supposedly should fix this >> >> >> >> problem. >> >> >> >> >> >> >> >> BTW, as also the author of the GLFW library on HackageDB, I've done >> >> >> >> barely minimal to keep this Haskell binding afloat. I'm actually >> >> >> >> leaning towards GLFW-b library, which is better maintained, and >> >> >> >> provides similar binding for GLFW C library but with a saner interface >> >> >> >> (no dependency on the OpenGL library, for example). If you don't need >> >> >> >> the two extra things that GLFW does (choice of either dynamic or >> >> >> >> static linking to GLFW C, and an embedded bitmap font), I suggest you >> >> >> >> try out GLFW-b if you are only looking for a think graphics layer with >> >> >> >> input+window+OpenGL. >> >> >> >> >> >> >> >> The only thing keeping GLFW-b from becoming a good foundation for a >> >> >> >> pure Haskell UI lib is IMHO the lack of a light-weight, >> >> >> >> cross-platform, and full-featured font rendering solution. I believe >> >> >> >> many other libraries (including Diagram) are having the same problem. >> >> >> >> >> >> >> >> >> >> >> >> On Thu, Sep 26, 2013 at 8:32 PM, Conal Elliott <conal(a)conal.net> wrote: >> >> >> >> > I'm polling to see whether there are will and expertise to reboot >> >> > graphics >> >> >> >> > and GUIs work in Haskell. I miss working on functional graphics and >> >> > GUIs >> >> > in >> >> >> >> > Haskell, as I've been blocked for several years (eight?) due to the >> >> > absence >> >> >> >> > of low-level foundation libraries having the following properties: >> >> >> >> > >> >> >> >> > * cross-platform, >> >> >> >> > * easily buildable, >> >> >> >> > * GHCi-friendly, and >> >> >> >> > * OpenGL-compatible. >> >> >> >> > >> >> >> >> > The last several times I tried Gtk2hs, I was unable to compile it on >> >> > my >> >> > Mac. >> >> >> >> > Years ago when I was able to compile, the GUIs looked and interacted >> >> > like a >> >> >> >> > Linux app, which made them awkward and upleasant to use. wxHaskell >> >> > (whose >> >> >> >> > API and visual appearance I prefered) has for years been incompatible >> >> > with >> >> >> >> > GHCi, in that the second time I open a top-level window, the host >> >> > process >> >> >> >> > (GHCi) dies abruptly. Since my GUI & graphics programs are often >> >> > one-liners, >> >> >> >> > and I tend to experiment a lot, using a full compilation greatly >> >> > thwarts >> >> > my >> >> >> >> > flow. For many years, I've thought that the situation would >> >> > eventually >> >> >> >> > improve, since I'm far from the only person who wants GUIs or >> >> > graphics >> >> > from >> >> >> >> > Haskell. >> >> >> >> > >> >> >> >> > About three years ago, I built a modern replacement of my old Pan and >> >> >> >> > V >> > >> > >> > _______________________________________________ >> > ghc-devs mailing list >> > ghc-devs(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/ghc-devs >> > >> >> >> >> -- >> Regards, >> Paul Liu > > -- Regards, Paul Liu

8 12

Haskell Madrid meetup: Web development with Haskell. October 9th
by Alberto G. Corona 06 Oct '13

06 Oct '13

Hi Spanish haskellers. Maybe it is too late for the announcement, but we will have a meetup the 9th (next Tuesday). Since the meetup group is devoted to functional programming in general, not specifically Haskell, I will give an introduction to web programming in Haskell: major platforms, libraries technologies coding examples etc. Overall, it will be an opportunity to know people interested in FP, Haskell and Web programming near you. This group meet up every second tuesday of each month. So if you can not attend, perhaps the next month you could. Nos vemos! http://www.meetup.com/FP-Madrid/events/138571062/?_af_eid=138571062&a=uc1_v… -- Alberto.

1 0

Announce: HDBI-1.2 and friends
by Aleksey Uymanov 05 Oct '13

05 Oct '13

Hello, haskellers! Here is HDBI-1.2 and some friends There is class `FromRow` and `ToRow` from this version as well as hdbi-conduit package. So, you can write your code like this: {-# LANGUAGE OverloadedStrings , TemplateHaskell #-} import Control.Monad.IO.Class import Data.Conduit import Data.Conduit.HDBI import Database.HDBI import Database.HDBI.SQlite import Language.Haskell.TH.HDBI import qualified Data.Conduit.List as L import qualified Data.Text as T data Animal = Animal { aName :: T.Text , aAge :: Double , aWeight :: Double } $(deriveFromRow ''Animal) $(deriveToRow ''Animal) animalsList :: [Animal] animalsList = [Animal "Puffy" 15 0.3 ,Animal "Puppy" 1 0.5 ,Animal "Rex" 3 5 ,Animal "Cat" 2 2.2] foldAnimals :: (Animal, Animal, Double) -> Animal -> (Animal, Animal, Double) foldAnimals (a, b, sumw) c@(Animal name age weight) = (newa, newb, sumw + weight) where newa | age > (aAge a) = c | otherwise = a newb | weight > (aWeight b) = c | otherwise = b main = do (aged, weighted, wsum) <- runResourceT $ do (_, c) <- allocConnection $ connectSqlite3 ":memory:" liftIO $ do runRaw c "create table animals (name, age, weight)" runManyRows c "insert into animals(name, age, weight) values (?,?,?)" animalsList selectRawAllRows c "select name, age, weight from animals" $$ L.fold foldAnimals (none, none, 0) putStrLn $ "The most aged is " ++ (T.unpack $ aName aged) ++ " with age " ++ (show $ aAge aged) putStrLn $ "The most weighted is " ++ (T.unpack $ aName weighted) ++ " with weigh " ++ (show $ aWeight weighted) putStrLn $ "Total biomass is " ++ show wsum where none = Animal "" 0 0 the result will be: The most aged is Puffy with age 15.0 The most weighted is Rex with weigh 5.0 Total biomass is 8.0 This is much more type safe way to work with raw SQL queries. Links: http://hackage.haskell.org/package/hdbi http://hackage.haskell.org/package/hdbi-postgresql http://hackage.haskell.org/package/hdbi-sqlite http://hackage.haskell.org/package/hdbi-conduit And you are welcome on GitHub: https://github.com/s9gf4ult/hdbi https://github.com/s9gf4ult/hdbi-postgresql https://github.com/s9gf4ult/hdbi-sqlite https://github.com/s9gf4ult/hdbi-conduit -- Aleksey Uymanov <s9gf4ult(a)gmail.com>

1 0

building a FFI library with cabal
by Tad Doxsee 05 Oct '13

05 Oct '13

Hi, I'm trying to create an FFI library to CHOLMOD ( http://www.cise.ufl.edu/research/sparse/cholmod/) but am having problems. My project is here: github.com/tdox/hcholmod. I can link an executable with the build script in the examples directory (line 4). But line 7 does not work (I get undefineds for functions that are in cholmod.a. Also, I can't build example1 with cabal. (Uncommenting the -- lines in hcholmod.cabal results is similar undefineds.) I would appreciate any help or hints. Thanks, Tad

1 1

Seeking Control.Lens Combinator
by Charlie Paul 05 Oct '13

05 Oct '13

Hello, I'm looking for a combinator along the lines of (&&&) :: Lens' a b -> Lens' a b' -> Lens' a (b,b') I can see how it could lead to lenses that don't follow the laws, but for Lenses which are somehow independent (like _1 and _2), it works perfectly well. Is there a way in lens to specify this "independence"? Thank you, Charles Paul

3 2

New command line parsing package
by Simon bergot 05 Oct '13

05 Oct '13

Hello all, I have recently uploaded argparser<http://hackage.haskell.org/package/argparser-0.3.2>, a command line parser library. I made it because of 3 reasons: - I am writing more and more scripts in haskell instead of python - I did not want to use the ones presented here<http://www.haskell.org/haskellwiki/Command_line_option_parsers>. I found that they were either lacking features I wanted, or using overkill extensions such as TemplateHaskell. - I wanted to learn how to create a package. So this is a beginner's work. I have uploaded it anyway because I feel it has very few dependencies and a satisfying set of features. Any feedback is welcome (especially about the documentation). -- Simon

1 0

Newclasses
by Wvv 05 Oct '13

05 Oct '13

"Newclasses" are not a new vision of classes! Not at all! Newclasses could elegant solve several instance problems! 1) we want to have "partly applied instances", like Parent2Child: Parent a => Child a like instance Applicative m => Monad m where return = pure -- we won't define here (>>=) 2) we want to have superclass' instances, like Child2Parent: Child a => Parent a like instance Monad m => Applicative m where pure = return (<*>) = ap 3) we want to have default instances outside of class and as many as possible, not the only one. like class Foo a where foo :: ... default foo :: ... foo = ... 4) we want to have multi-class instances to separate (or unite) classes like type Stringy a = (Show a, Read a) instance Stringy SomeData where read = ... show = ... (4)th problem we could solve separately, but maybe it isn't easy enough to do such de-sugaring, and it could much easier to add them in newclasses. (3)rd problem is solved partly, but not in universal way, non-flexible and a bit ugly. (2)nd problem is solved, but it is mostly impossible to use them and it is not recommend to use it for overlapping and incoherent issues. (1)st problem is unsolved at all (partly, it is possible to make depended classes, but checker don't check if we implement parent classes). This is a compose proposal. Newclasses solve these problems at once! As newtype is a data looking like a type, same newclass is looking like a class, but is mostly an instance! Mostly, but not full. To best understanding what "newclasses" is, let's look at (1)st problem: -- we wish to write -- instance Applicative m => Monad m where -- return = pure -- we write "newclass" instead of "instance" -- add "=>" and giving a name to newclass like a class -- this is not instance, so newclass can't overlap with any instance newclass (Applicative m) => Monad m => ApMonad m where return = pure data D a .... instance Applicative D where pure = ... (<*>) = ... -- creating instances from newclass is intuitive -- we implement here Monad class, not "ApMonad"; ApMonad is a just newclass instance ApMonad D where -- we already have 'return = pure', so we define only (>>=) (>>=) = ... What do we see here? Newclass looks like class, but it's mostly an instance! Newclass: Grammar: newclass constraint => Parent a => NewClassName a where ... As class, newclass has a name, which is unique and can't conflict with any other newclass or class names. As class, methods of newclass could be empty or implemented. As class, methods of newclass are not use in function inference. As class we can make an instance of newclasses and overwrite any of his functions! But, instance of newclass IS an instance of the parent (!)class! So, newclasses is like a de-sugaring. As instance, newclass contains only parents methods! On contrary to instancess, we don't use newclasse directly, it only help us to create instances. If we allow for newclass to be as Parent not only classes, but newclasses, then newclass can't be recursive: neither of his (Grand)Parent could be he by himself. If we allow for newclass muliple Parents, we solve (4)th problem too. Examples: We have: class Functor f where fmap :: (a -> b) -> f a -> f b class Applicative f where -- without "Functor f =>", this is a misfeature pure :: a -> f a (<*>) :: f (a -> b) -> f a -> f b class Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b class Ord' a where -- without "Eq a =>", this is a misfeature ... compare a b -- not as in Prelude | a < b = LT | a > b = GT | otherwise = EQ (1)st, Parent2Child: Common pattern to write newclass like: newclass Parent a => Child a => ParChild a where ... Examples: newclass (Applicative m) => Monad m => ApMonad m where return = pure data C1 a ... instance Applicative C1 where pure = ... (<*>) = ... instance ApMonad C1 where -- return is already defined (>>=) = ... -- newclass (Ord' a) => Eq a => OEq a where a == b = case compare a b of EQ -> True _ -> False data C2 ... instance Ord' C2 where (>) = ... (<) = ... instance OEq C2 -- empty instance for Eq (2)nd, Child2Parrent: Common pattern to write newclass like: newclass Child a => Parent a => ChParent a where ... Examples: newclass (Eq a) => Ord' a => Ord a where compare a b | a == b = EQ | a >= b = GT | otherwise = LT data C3 ... instance Eq C3 where (==) = ... instance Ord C3 where -- Ord with much effective `compare`, than Ord' (>) = ... (<) = ... -- newclass Monad m => Applicative m => MApplicative m where pure = return (<*>) = ap data C4 a ... instance Monad C4 where return = ... (>>=) = ... instance MApplicative C4 -- empty, everything is already defined ! -- newclass Monad m => Functor m => MFunctor m where fmap = liftM newclass Applicative f => Functor f => ApFunctor f where fmap f x = pure f <*> x data C5 a ... instance Applicative C5 where pure = ... (<*>) = ... instance ApMonad C5 where (>>=) = ... -- without return instance MFunctor C5 --(!) or instance ApFunctor C5 --(!!!) but not both (3)rd, default instances: Common pattern to write newclass like: newclass GenericConstraint a => Class a => GNewclass a where ... Examples: class ToJSON a where toJSON :: a -> Value newclass (Generic a, GToJSON (Rep a)) => ToJSON a => GenToJSON a where toJSON = genericToJSON defaultOptions newclass (Data a) => ToJSON a => DataToJSON a where toJSON = dataToJSON defaultOptions data C6 ... deriving (Generic, Data) -- don't forget to create an empty instance! instance GenToJSON C6 --(!) or instance DataToJSON C6 --(!!!) but not both (4)th, multi-class instances Common pattern to write newclass: newclass (Class1 a, Class2 a, Class3 a)=> Newclass a where ... Examples: newclass (Read a, Show a) => Stringy a -- without where data C7 ... instance Stringy C7 where read = ... show = ... -- class MinBounded a where minBound :: a class MaxBounded a where maxBound :: a newclass (MinBounded a, MaxBounded a) => Bounded a -- class Additive a where (+) :: a -> a -> a class Additive a => AdditiveZero a where zero :: a class Mulipicative a where (*) :: a -> a -> a class Mulipicative a => MulipicativeOne a where one :: a newclass (Additive a, Mulipicative a, Substravive a, FromInteger a) => Num a -- without where newclass (AdditiveZero a, MulipicativeOne a, Substravive a, FromInteger a) => NumFull a -- Benefits: - "newclass" is a very powerful tool - it is universal solution without extra assumptions! - solve 4 big problems with instances - it is a Huge step - it is a step forward, not aside - it is developing function's muscles to Haskell Disadvantages: - new reserved word "newclass" - with Child2Parrent problem it is needed to write empty instances - main difficulty - it is not easy to implement this extension! Newclasses could also help to reorganize some Prelude classes, like Ord, Applicative, Functor, Bound, Num, ... What do you think of this proposal? You opinion is important and significant! Let's help to develop Haskell together! Do you like "newclasses"? Do you want to use them? Do you think newclasses are appropriate solution for written problems? Could newclasses help to resolve some other problems? Are they useless? Do you see huge difficulties of implementation? Do you have more elegant ideas, which correspondent with newclasses? If it is not clear enough, I write more clearly. Any feedback is welcome! -- View this message in context: http://haskell.1045720.n5.nabble.com/Newclasses-tp5737596.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 9

haskell platform broken in ubuntu
by Rustom Mody 05 Oct '13

05 Oct '13

I just upgraded my ubuntu laptop to 13.04 and haskell platform is gone!! http://askubuntu.com/questions/286764/how-to-install-haskell-platform-for-u… What is the current status on this? Is 13.10 going to correct this?

3 6

Calling Python from Haskell
by Manuel Gómez 05 Oct '13

05 Oct '13

Hi list, What’s the preferred way of calling into Python from Haskell? I’ve found MissingPy[0], but it seems to be somewhat bitrotten and a couple of experiments yielded segfaults. There’s also the cpython package[1], but that seems to require Python 3.3, and I’m trying to call into code written for 2.7. Are there any other alternatives, apart from direct execution of a Python interpreter with forkProcess and such? [0]: <http://hackage.haskell.org/package/MissingPy> [1]: <http://hackage.haskell.org/package/cpython>

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Machine Learning Startup Seeks Software Architect
by Charles Weitzer 04 Oct '13

04 Oct '13

My name is Charles Weitzer. I have a client with a startup quantitative hedge fund located in Northern California. The Fund currently manages a very healthy amount of capital. The founders have PhD's in Computer Science from Stanford and in Statistics from Berkeley. They have made unpublished discoveries in the field of Machine Learning and in other areas as well. This is a small, nimble company that is going to be the next big thing in terms of quantitative strategies and systems. The team is looking for an extremely talented software architect to design and build a new trading system and to rebuild their current system. The position is very challenging intellectually and involves architecting and implementing a diverse set of core infrastructures, including new production systems, scientific-computing environments, and modern data stores. From the company's point of view, this is a distributed computing problem. The position is definitely high-impact and the scope of work mostly greenfield. There is quite a bit of freedom in the choice of software tooling (e.g. it doesn't have to be C++ or Java and they are very open to Erlang, Scala, Haskell, and others). Qualifications include a proven track record of writing correct, well-designed software, solving hard problems and delivering complex projects on time. You should preferably have experience with high assurance, distributed, fault-tolerant systems. Experience with functional programming as well as real-time, low-latency, cache-friendly systems is a very big bonus. They are getting big fast, so a willingness to take initiative and a gritty determination to produce is essential. The title, scope, and level of seniority is very flexible. As well, given their returns, this is a chance to become extremely wealthy. Join a team that includes faculty at premier universities and PhD's from top-tier schools, led by the founder and CEO of a successful Internet infrastructure startup. You will have high-impact, and can expect frequent interaction with the researchers, officers, and founders. Compensation and benefits are highly competitive. If you are interested or just curious, let me know the best way to start a dialog. If you know of someone who you would highly recommend, I would appreciate it if you could forward this on to them. Talk soon, Charles Weitzer CEO\Senior Recruiter Charles Weitzer and Associates, Inc. Global Financial Recruiting Services <mailto:Charles@CharlesWeitzer.com> Charles(a)CharlesWeitzer.com Voice: USA (510) 558-9182

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Store type-class polymorphic values generically
by Christopher Done 04 Oct '13

04 Oct '13

It's very easy to state this problem without enough details and mislead people into providing a solution for a different problem, so I'll try to include all the information and use-case. I need a function that can store a value in a concrete opaque type. I know the type of the value when I store it, and I know the type of the value when I extract it, insofar as I know the type inclusive of a class constraint context. But the container must store many different things at once. Given that I know the type of the thing when I store it and when I extract it, there ought to be a “safe’ way to do this, in the same way that Data.Dynamic is “safe”. The use-case is I want to store a table of IO-like actions in an opaque like [(Name,Dynamic)] and I write into it statically, e.g. in GHCi. The reason is due to this approach: go_ref = newIORef go_ go_ = putStrLn "Hello, World!" go = do go' <- liftIO $ readIORef go_ref go' main = forkIO $ forever $ go then in GHCi I writeIORef go_ (putStrLn "What's up?") then go will now start printing "What's up?" This works now, I can do this presently. However, I want to write this as a core-to-core translation as a ghc-plugin. I want the definition go = putStrLn "Hello World!" to be translated to what I wrote above. Core cannot generate new names to be exported from a module, so go_ is now gone. So how does the `go' function read and write the IORef? With a table: table = unsafePerformIO $ newIORef [("go",Dynamic go_ref)] Hurrah! I can now put table in some module like DynamicUpdate.table and then read/write to it from Core in the generated definition of go. Done. But how do I update it from the GHCi REPL? What follows is where I'm stuck. So with Data.Dynamic, I have: toDyn :: Typeable a => a -> Dynamic With toDyn I can store concrete values and functions and then extract them: λ> fmap ($ 12) (fromDynamic (toDyn ((*2) :: Int -> Int)) :: Maybe (Int -> Int)) Just 24 But the problem with toDyn is that it can only store concrete values due to the Typeable constraint. So then I turn to existentials and unsafeCoerce: λ> data Anything = forall a. Anything a λ> let x = Anything id λ> case x of Anything (unsafeCoerce -> (id' :: Int -> Int)) -> id' 123 123 Great, I was able to store a value which contained non-concrete types. But now consider the case of a value with type-class polymorphic type in it: y = Anything (print :: Show a => a -> IO ()) Which cannot be type checked, because the Show instance is ambiguous. So you might propose to use rank-N types, like this: data Anything = Anything (Show a => a -> IO ()) Now I can store print in there happily. But now the type is not Anything, it's not generic anymore. Maybe I want to store `print', maybe I want to store `readLn'. Dead end. I don't want to have to generate existential wrappers for all functions I might possibly want to store. The support of Constraints in this page http://blog.omega-prime.co.uk/?p=127 makes me think that it's still possible. It would be cool to somehow store a tuple of the dictionary of the constraint and the value itself and then I'd later be able to extract it. But I'm finding it difficult developing anything even simple. Any ideas/help appreciated!

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Using lenses
by Simon Peyton-Jones 04 Oct '13

04 Oct '13

(I sent this to 'libraries' but Kim-Ee suggested adding Café, where so many smart people hang out.) Friends Some of you will know that I've promised to give a talk about Edward's lens library<http://hackage.haskell.org/package/lens> at the Haskell Exchange<http://skillsmatter.com/event/scala/haskell-exchange> in London next Wednesday (9th). I did this to give everyone (including me) a break from GHC hackery, and also to force me to learn about this lens voodoo that everyone is twittering about. Edward generously gave me quite a bit of one-to-one attention last week (my hair is still standing on end), but this message is to ask your help too. Specifically, I'd like to give some compelling use-cases. If you are using the lens library yourself, could you spare a few minutes to tell me how you are using it? I expect to cover Lens and Traversal but not Prism. The use-case everyone starts with is nested records, but I'd like to go beyond that. The next levels seem to be: · Lenses as views of data that isn't "really there" e.g. regarding a record with rectangular coordinates as having polar coordinates too. · Lenses and Traversals that focus on elements of finite maps (Control.Lens.At) What else? I'm sure you are using them in all sorts of cool ways that I would never think of, and I'd love to know. Please don't tell me anything secret! To give everyone the benefit I may just concatenate all the replies and send to you all, so please say if you don't want me to do that with yours. And don't burn too many cycles on this...I don't want to waste your time, and I can always get back to you if I can't understand what you say. Sooner is better than later...Weds is coming. Simon "Edward's prophet" PJ

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ANN: Nomyx 0.3 beta, the game where you can change the rules
by Corentin Dupont 04 Oct '13

04 Oct '13

Hello everybody! I released the third beta of Nomyx <http://www.nomyx.net>, the only game where You can change the rules! The second beta was a success. Great players (byorgey, nomeata, Toxaris...) proposed amazing rules, effectively building a nice universe. For example, a banking system in ecu was proposed, with cash monies and even a way to buy pies! Other rules like different democratic systems were proposed. You can check those rules in this thread: http://www.nomyx.net/forum/viewtopic.php?f=4&t=1520 Let's start a new game! I propose to set it in the medieval era. Dear rulers, I'll let your imagination speak! Please login to the game here: www. nomyx.net:8000/Nomyx. The corresponding forum thread is here: http://www.nomyx.net/forum/viewtopic.php?f=4&t=1523 Some background: this is an implementation of a Nomic [2] game in Haskell(I believe the first complete implementation of a Nomic game on a computer). In a Nomyx game you can change the rules of the game itself while playing it. The players can submit new rules or modify existing ones, thus completely changing the behaviour of the game through time. The rules are managed and interpreted by the computer. They must be written in the Nomyx language <http://hackage.haskell.org/package/Nomyx-Language>, which is a subset of Haskell. At the beginning, the initial rules are describing: - how to add new rules and change existing ones. For example a unanimity vote is necessary to have a new rule accepted. - how to win the game. For example you win the game if you have 5 rules accepted. But of course even that can be changed! Here is a video introduction and first tutorial of the game: http://vimeo.com/58265498 The example file gives a lot of examples of rules that you can submit: www. nomyx.net:8000/src/Language/Nomyx/Examples.hs changes in V0.3: - creation of admin role - inputs made by the rules now include checkboxes, buttons, textarea - outputs made by the rules are now managed (create, update, delete) - refactored the GUI - bumped to GHC 7.6 - compilation errors are displayed in context - bug fixes Cheers, Corentin [1] www.nomic.net

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