- Haskell-Cafe - Haskell.org

Generics & pattern matching
by Joerg Fritsch 06 Dec '12

06 Dec '12

I have the code below that I pluged from SO. My problem is that although I sort of understand how with rewrite True and rewrite False it is suppressed that always the first element of the tuple stays as-is; I cannot find any way to apply this to the last element also. In essence there are times when I would need to change all elements of the tuple but the first and at other times I would need to change all elements except the first and the last. I don't quite understand (yet) how I could do that. Is there any way to match for a specific element eg. the third from the right or so? --Joerg {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE TypeOperators #-} import Data.Typeable import GHC.Generics rewrite_ :: (Generic a, Generic b, Rewriter (Rep a), Rewrite (Rep a) ~ Rep b) => a -> b rewrite_ = to . rewrite False . from class Rewriter f where type Rewrite f :: * -> * rewrite :: Bool -> f a -> (Rewrite f) a instance Rewriter f => Rewriter (M1 i c f) where type Rewrite (M1 i c f) = M1 i c (Rewrite f) rewrite x = M1 . rewrite x . unM1 instance Typeable c => Rewriter (K1 i c) where type Rewrite (K1 i c) = K1 i String rewrite False (K1 x) | Just val <- cast x = K1 val rewrite _ _ = K1 "NIL" instance (Rewriter a, Rewriter b) => Rewriter (a :*: b) where type Rewrite (a :*: b) = Rewrite a :*: Rewrite b rewrite x (a :*: b) = rewrite x a :*: rewrite True b y0 :: (String, Int, Double) y0 = ("something", 3, 4.788) y1 :: (String, String, String, (Int, Int)) y1 = ("something else", "Hello", "NIL", (4,6)) main :: IO () main = do print (rewrite_ y0 :: (String, String, String)) print (rewrite_ y1 :: (String, String, String, String))

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Are there REPA linear algebra routines? e.g. Eigenvalues?
by KC 06 Dec '12

06 Dec '12

:) -- -- Regards, KC

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The end of an era, and the dawn of a new one
by Simon Peyton-Jones 06 Dec '12

06 Dec '12

Friends You'll have seen Simon Marlow's recent announcement (included below): | Today I'm announcing that I'm leaving Microsoft Research. Simon Marlow and I have worked together on GHC for nearly two decades. During much of that time we have shared a cup of coffee every morning (one cup each, since you ask), to discuss what's going on in GHC-land. We had frequent detailed and illuminating debates about design alternatives. Simon is the absolute master of GHC's code generation, runtime system, garbage collector, support for parallel execution, and much else besides. His sheer programming skill in rooting out obscure and hard-to-reproduce runtime system bugs has always amazed me. He is more than just a great hacker, of course. He is a leader in our community, has a strong publication record, chaired the Haskell Prime committee for a year, and is sought after for tutorials about parallel programming. I owe Simon a great debt, as I think we all do, for the care and skill he has lavished on GHC. He's not going to disappear entirely, but he will be spending much less time on GHC than in the past. That is a change, but it's a change that was always going to happen sometime, and while change is sometimes uncomfortable, it can also open up new possibilities. GHC is over 20, and like my own children (who are actually all a bit younger than GHC), that's not a bad time to start to head out into the world. Over time, more and more people have started to help with the task of developing and maintaining GHC. We'll have to do more of that. These days GHC has a very large "surface area" (running on different platforms, dynamic linking, debugger, profiler, GHCi, libraries and Cabal, etc etc), and has grown far beyond what any single person or research group can manage. Lest you should wonder, I myself will certainly continue to work on GHC. But my job is to be a *researcher*, and I am not well equipped -- in temperament or expertise -- to deal with the myriad complexities of deployment, upgrade policy, portability, and the like. Well Typed (mostly in the guise of Ian Lynagh) will continue to handle some of these issues, but again there is too much for one person to do. So we need your help. Seriously. Rather than just thinking "GHC HQ will sort it out", I would welcome you saying "Shall I take responsibility for X?". Johan, Bryan, and others offering to be GHC Performance Tsars is a great recent example. But don't wait to be asked: just make an offer if you see a gap or an open ticket --- and I think there will be plenty of those. Particularly valuable are offers to take responsibility for a particular area (eg the LLVM code generator, or the FFI). I'm hoping that this sea change will prove to be quite empowering, with GHC becoming more and more a community project, more resilient with fewer single points of failure. I am going to miss that cup of coffee though. Thank you Simon! Facebook is lucky to have you. Onward and upward. Simon (PJ) | -----Original Message----- | From: glasgow-haskell-users-bounces(a)haskell.org [mailto:glasgow-haskell- | users-bounces(a)haskell.org] On Behalf Of Simon Marlow | Sent: 22 November 2012 11:27 | To: haskell; glasgow-haskell-users(a)haskell.org | Subject: Leaving Microsoft | | Today I'm announcing that I'm leaving Microsoft Research. | | My plan is to take a break to finish the book on Parallel and Concurrent | Haskell for O'Reilly, before taking up a position at Facebook in the UK | in March 2013. | | This is undoubtedly a big change, both for me and for the Haskell | community. I'll be stepping back from full-time GHC development and | research and heading into industry, hopefully to use Haskell. It's an | incredibly exciting opportunity for me, and one that I hope will | ultimately be a good thing for Haskell too. | | What does this mean for GHC? Obviously I'll have much less time to work | on GHC, but I do hope to find time to fix a few bugs and keep things | working smoothly. Simon Peyton Jones will still be leading the project, | and we'll still have support from Ian Lynagh, and of course the | community of regular contributors. Things are in a reasonably stable | state - there haven't been any major architectural changes in the RTS | lately, and while we have just completed the switchover to the new code | generator, I've been working over the past few weeks to squeeze out all | the bugs I can find, and I'll continue to do that over the coming months | up to the 7.8.1 release. | | In due course I hope that GHC can attract more of you talented hackers | to climb the learning curve and start working on the internals, in | particular the runtime and code generators, and I'll do my best to help | that happen. | | Cheers, | Simon

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Maintainer of hgettext
by Ivan Perez 06 Dec '12

06 Dec '12

Hello haskellers, A few months ago I sent an email to Vasyl Pasternak regarding a couple of bugs in hgettext [1], together with a small patch that fixes them. I never received an answer and I can see that the error persists. Does anyone know where he is? What would you recommend at this point? Cheers, Ivan. [1] http://hackage.haskell.org/package/hgettext

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Vector mysteries
by Fixie Fixie 06 Dec '12

06 Dec '12

Hi I am trying to implement Show for a storable-mutable-vector. Does anyone have a clue why the type-system is killing me :-) Code is below... Cheers Felix ---- {-# LANGUAGE ExistentialQuantification #-} import Control.Monad (liftM2) import qualified Data.Vector.Unboxed as V import qualified Data.Vector.Unboxed.Mutable as MV import Data.Vector.Storable.Mutable import GHC.Prim (RealWorld) import Control.Monad.Primitive import Control.Monad import qualified Data.Vector as VEC import qualified Data.Vector.Generic.Mutable as GM import Data.Int import Data.Typeable.Internal import Data.Primitive data Row = forall m s. (Storable s, MV.Unbox s, Prim s, PrimMonad m) => Row (MV.MVector (PrimState m) s) instance Show Row where show (Row row) = do let xx = MV.read row 0 "Done" main :: IO () main = do print "Done" I get this error: Could not deduce (PrimState m ~ PrimState m0) from the context (Storable s, MV.Unbox s, Prim s, PrimMonad m) bound by a pattern with constructor Row :: forall (m :: * -> *) s. (Storable s, MV.Unbox s, Prim s, PrimMonad m) => MV.MVector (PrimState m) s -> Row, in an equation for `show' NB: `PrimState' is a type function, and may not be injective Expected type: MV.MVector (PrimState m0) s Actual type: MV.MVector (PrimState m) s In the first argument of `MV.read', namely `row' In the expression: MV.read row 0

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sequential logic
by Christopher Howard 06 Dec '12

06 Dec '12

Hi. I was wondering what the various (and especially most simple) approaches one could take for working with (simulating or calculating) sequential logic in Haskell. By sequential logic, I mean like wikipedia describes, where a system is made up of logic gates, whose output is dependent not only on the logic operation of the gate, but on the previous state of the gate. (Like in electronics where the system can be driven by a clock signal or have memory.) Does one have to get into FRP for that sort of thing? Or use some kind of FSM framework? Or is there some kind of fancy state monad suitable for that? Or...? I'm no electronic or digital engineer, but for learning purposes I've been trying to see if I could build an (extremely simple) virtual processor or calculator in Haskell, using only the Bool type and a few of the boolean operators (and functions composed of the aforementioned), reproducing things like half adders and full adders as functions in the program. Of course, I quickly ran into the stateful aspect of the problem, in subjects like clock signal and flip flops. -- frigidcode.com

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Haskell Weekly News: Issue 253
by Daniel Santa Cruz 06 Dec '12

06 Dec '12

Welcome to issue 253 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of November 25 to December 01, 2012. Quotes of the Week * edwardk: this breaks my previous record of 6 unsafeCoerce's in a line * bos:Harpers gona harp. Top Reddit Stories * Pretty Output in GHCi (HOWTO in comments) Domain: i.imgur.com, Score: 82, Comments: 14 On Reddit: [1] http://goo.gl/2c7Ns Original: [2] http://goo.gl/uYZAs * Stable, Vetted Hackage: Call for Participation Domain: yesodweb.com, Score: 59, Comments: 24 On Reddit: [3] http://goo.gl/QrJEz Original: [4] http://goo.gl/1PlBK * Strict vs non-strict semantics: "Why can’t I just be a little lazy?" Domain: blog.ezyang.com, Score: 42, Comments: 20 On Reddit: [5] http://goo.gl/P2nSm Original: [6] http://goo.gl/WDhk6 * io-stream: simple and easy to use primitives for I/O using streams Domain: github.com, Score: 37, Comments: 10 On Reddit: [7] http://goo.gl/uDbGG Original: [8] http://goo.gl/sGqtY * Toward a Secret Sky: Building a Haskell web app with Snap: Snap quickstart guide Domain: unpythonic.blogspot.ca, Score: 36, Comments: 15 On Reddit: [9] http://goo.gl/6UdY8 Original: [10] http://goo.gl/Y6wli * Hackage: unittyped-0.1, an extendable library for type-safe computations including units Domain: hackage.haskell.org, Score: 34, Comments: 23 On Reddit: [11] http://goo.gl/f7b1M Original: [12] http://goo.gl/jmZCi * Easy JSON parsing in Haskell with Aeson Domain: blog.raynes.me, Score: 34, Comments: 18 On Reddit: [13] http://goo.gl/JERS0 Original: [14] http://goo.gl/Ccfbm * 1st São Paulo Haskell Meeting Domain: plus.google.com, Score: 29, Comments: 4 On Reddit: [15] http://goo.gl/MKVPb Original: [16] http://goo.gl/yaLUi * Conal Elliott: "Parallel speculative addition via memoization" Domain: conal.net, Score: 26, Comments: 11 On Reddit: [17] http://goo.gl/EQZxj Original: [18] http://goo.gl/wg0KD * Elm: Making the Web Functional [video/slides] Domain: infoq.com, Score: 26, Comments: 1 On Reddit: [19] http://goo.gl/0qAJM Original: [20] http://goo.gl/bldR0 * From traced monoidal to compact closed: geometry of interaction explained with (OCaml) code Domain: semantic-domain.blogspot.fr, Score: 18, Comments: 22 On Reddit: [21] http://goo.gl/auq1p Original: [22] http://goo.gl/4XvRw * Static Analysis with Applicatives Domain: gergo.erdi.hu, Score: 17, Comments: 13 On Reddit: [23] http://goo.gl/sNLm9 Original: [24] http://goo.gl/Lvxpn * 24 Pull Requests - Does Anyone Have Project Suggestions? Domain: self.haskell, Score: 16, Comments: 3 On Reddit: [25] http://goo.gl/U2bsY Original: [26] http://goo.gl/U2bsY * what if haskell-platform was a hackage metapackage? Domain: self.haskell, Score: 16, Comments: 7 On Reddit: [27] http://goo.gl/SPIlO Original: [28] http://goo.gl/SPIlO * Does anyone know if more videos are going to be posted to Haskell Live TV? Domain: youtube.com, Score: 15, Comments: 2 On Reddit: [29] http://goo.gl/jJxWb Original: [30] http://goo.gl/NVk3D * Data-driven hacks for DB querying — is this evil? Domain: hpaste.org, Score: 13, Comments: 22 On Reddit: [31] http://goo.gl/0J5eJ Original: [32] http://goo.gl/gfRLS * Why does this result not appear in this search in Hoogle? Domain: self.haskell, Score: 12, Comments: 10 On Reddit: [33] http://goo.gl/DGTu2 Original: [34] http://goo.gl/DGTu2 Top StackOverflow Questions * Is putStrLn thread-safe? votes: 12, answers: 1 Read on SO: [35] http://goo.gl/rjRyN * How do you prove that a function is unique for its type? votes: 11, answers: 1 Read on SO: [36] http://goo.gl/odOSy * Laziness/strictness between data and newtype votes: 10, answers: 2 Read on SO: [37] http://goo.gl/Dkqv8 * Why do we use folds to encode datatypes as functions? votes: 10, answers: 1 Read on SO: [38] http://goo.gl/RRVuE * Haskell repa - how to reduce array and return index? votes: 7, answers: 2 Read on SO: [39] http://goo.gl/HpXhR * How much is applicative really about applying, rather than “combining”? votes: 7, answers: 1 Read on SO: [40] http://goo.gl/zq6j3 * Why is that not lazy votes: 7, answers: 2 Read on SO: [41] http://goo.gl/F8H0m * Biapplicative and Bimonad? votes: 6, answers: 1 Read on SO: [42] http://goo.gl/7C01J * reactive-banana-wx `sink` does not generate an event enabling post-sink processing votes: 6, answers: 1 Read on SO: [43] http://goo.gl/pXdTu * Expand type synonyms, type families with GHCi votes: 6, answers: 1 Read on SO: [44] http://goo.gl/G6Sks * Converting expression to pointfree style (Haskell) votes: 6, answers: 1 Read on SO: [45] http://goo.gl/36gLO * ADTs vs. Typeclasses — Canonical use votes: 6, answers: 1 Read on SO: [46] http://goo.gl/ofpbX * Haskell: Monitor a file without polling (à la inotify in linux) votes: 6, answers: 3 Read on SO: [47] http://goo.gl/JGzI0 Until next time, Daniel Santa Cruz References 1. http://i.imgur.com/t6hOx.png 2. http://www.reddit.com/r/haskell/comments/144biy/pretty_output_in_ghci_howto… 3. http://www.yesodweb.com/blog/2012/11/stable-vetted-hackage 4. http://www.reddit.com/r/haskell/comments/141hww/stable_vetted_hackage_call_… 5. http://blog.ezyang.com/2012/11/why-cant-i-just-be-a-little-lazy/ 6. http://www.reddit.com/r/haskell/comments/13tt44/strict_vs_nonstrict_semanti… 7. https://github.com/snapframework/io-streams 8. http://www.reddit.com/r/haskell/comments/13z9i4/iostream_simple_and_easy_to… 9. http://unpythonic.blogspot.ca/2012/11/building-haskell-web-app-with-snap-sn… 10. http://www.reddit.com/r/haskell/comments/13sb85/toward_a_secret_sky_buildin… 11. http://hackage.haskell.org/package/unittyped-0.1 12. http://www.reddit.com/r/haskell/comments/13t795/hackage_unittyped01_an_exte… 13. http://blog.raynes.me/blog/2012/11/27/easy-json-parsing-in-haskell-with-aes… 14. http://www.reddit.com/r/haskell/comments/141ch4/easy_json_parsing_in_haskel… 15. https://plus.google.com/events/cng3rcv1tjl84g2juddk1i36icg 16. http://www.reddit.com/r/haskell/comments/13vctv/1st_s%C3%A3o_paulo_haskell_… 17. http://conal.net/blog/posts/parallel-speculative-addition-via-memoization 18. http://www.reddit.com/r/haskell/comments/13yenz/conal_elliott_parallel_spec… 19. http://www.infoq.com/presentations/Elm 20. http://www.reddit.com/r/haskell/comments/1423uc/elm_making_the_web_function… 21. http://semantic-domain.blogspot.fr/2012/11/in-this-post-ill-show-how-to-tur… 22. http://www.reddit.com/r/haskell/comments/13zzxl/from_traced_monoidal_to_com… 23. http://gergo.erdi.hu/blog/2012-12-01-static_analysis_with_applicatives 24. http://www.reddit.com/r/haskell/comments/143wpd/static_analysis_with_applic… 25. http://www.reddit.com/r/haskell/comments/143c0e/24_pull_requests_does_anyon… 26. http://www.reddit.com/r/haskell/comments/143c0e/24_pull_requests_does_anyon… 27. http://www.reddit.com/r/haskell/comments/1445jh/what_if_haskellplatform_was… 28. http://www.reddit.com/r/haskell/comments/1445jh/what_if_haskellplatform_was… 29. http://www.youtube.com/user/HaskellLiveTV 30. http://www.reddit.com/r/haskell/comments/13x0z9/does_anyone_know_if_more_vi… 31. http://hpaste.org/78401 32. http://www.reddit.com/r/haskell/comments/13xi63/datadriven_hacks_for_db_que… 33. http://www.reddit.com/r/haskell/comments/13r9ii/why_does_this_result_not_ap… 34. http://www.reddit.com/r/haskell/comments/13r9ii/why_does_this_result_not_ap… 35. http://stackoverflow.com/questions/13549706/is-putstrln-thread-safe 36. http://stackoverflow.com/questions/13575583/how-do-you-prove-that-a-functio… 37. http://stackoverflow.com/questions/13566673/laziness-strictness-between-dat… 38. http://stackoverflow.com/questions/13575894/why-do-we-use-folds-to-encode-d… 39. http://stackoverflow.com/questions/13570354/haskell-repa-how-to-reduce-arra… 40. http://stackoverflow.com/questions/13625905/how-much-is-applicative-really-… 41. http://stackoverflow.com/questions/13663430/why-is-that-not-lazy 42. http://stackoverflow.com/questions/13556314/biapplicative-and-bimonad 43. http://stackoverflow.com/questions/13580121/reactive-banana-wx-sink-does-no… 44. http://stackoverflow.com/questions/13595773/expand-type-synonyms-type-famil… 45. http://stackoverflow.com/questions/13596335/converting-expression-to-pointf… 46. http://stackoverflow.com/questions/13610164/adts-vs-typeclasses-canonical-u… 47. http://stackoverflow.com/questions/13642619/haskell-monitor-a-file-without-…

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QuickCheck shrink
by warrensomebody＠gmail.com 05 Dec '12

05 Dec '12

Can someone point me at some documentation or examples of how to write and use a QuickCheck shrink function. I can't seem to find anything online. Thanks, Warren

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Re: [Haskell-cafe] The end of an era, and the dawn of a new one
by Johan Tibell 05 Dec '12

05 Dec '12

On Wed, Dec 5, 2012 at 12:37 PM, David Terei <davidterei(a)gmail.com> wrote: > I have always considered the LLVM code generator my responsibility and > will continue to do so. I don't seem to find the time to make > improvements to it but make sure to keep it bug free and working with > the latest LLVM releases. So if others want are interested in working > on it then there is plenty of room for that but I'll continue to > maintain it and be responsible for it at the least. I will maintain the I/O manager as per usual, probably together with Bryan and Andreas (but I cannot speak for them of course).

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Design of a DSL in Haskell
by Joerg Fritsch 05 Dec '12

05 Dec '12

This is probably a very basic question. I am working on a DSL that eventuyally would allow me to say: import language.cwmwl main = runCWMWL $ do eval ("isFib::", 1000, ?BOOL) I have just started to work on the interpreter-function runCWMWL and I wonder whether it is possible to escape to real Haskell somehow (and how?) either inside ot outside the do-block. I thought of providing a defautl-wrapper for some required prelude functions (such as print) inside my interpreter but I wonder if there are more elegant ways to co-loacate a DSL and Haskell without falling back to being a normal library only. --Joerg

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