August 2012 - Haskell-Cafe

ANNOUNCE basic-prelude-0.3
by Dan Burton 23 Aug '12

23 Aug '12

We are pleased to announce a new release of basic-prelude. http://hackage.haskell.org/package/basic-prelude-0.3.0.0 We apparently weren't entirely clear about the original purpose of basic-prelude, and we got repeated feedback of the form "where are the list functions?" Somewhat in response to this, we decided to rename the BasicPrelude module to CorePrelude. This module is an *incomplete* prelude, and the goal of this module is to provide enhanced core Prelude functionality *without* making any controversial decisions. CorePrelude serves as the core of Michael's ClassyPrelude, my ModularPrelude, and now the new BasicPrelude module. (If you ever feel the need to create your own Prelude replacement, we encourage you to build it off of CorePrelude. If there is any reason that CorePrelude would not be well-suited to this purpose, then please let us know!) I believe that by using the same Core, it will be easier for these various Prelude replacements to play nice with each other. We created a new module, BasicPrelude (not to be confused with the old BasicPrelude), which *is* a fully-featured Prelude, emphasizing the use of Text and FilePath, while also providing most of the List functions that you know and love, with a few exceptions, as documented. BasicPrelude is among the simplest examples of how we intend CorePrelude to be used. The next time you are working on a hobby project, please try using BasicPrelude instead of regular Prelude, and let us know how it goes! {-# LANGUAGE NoImplicitPrelude #-} {-# LANGUAGE OverloadedStrings #-} import BasicPrelude -- Dan Burton

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Rigid skolem type variable escaping scope
by oleg＠okmij.org 23 Aug '12

23 Aug '12

Matthew Steele asked why foo type-checks and bar doesn't: > class FooClass a where ... > > foo :: (forall a. (FooClass a) => a -> Int) -> Bool > foo fn = ... > > newtype IntFn a = IntFn (a -> Int) > > bar :: (forall a. (FooClass a) => IntFn a) -> Bool > bar (IntFn fn) = foo fn This and further questions become much simpler if we get a bit more explicit. Imagine we cannot write type signatures like those of foo and bar (no higher-ranked type signatures). But we can define higher-rank newtypes. Since we can't give foo the higher-rank signature, we must re-write it, introducing the auxiliary newtype FaI. > {-# LANGUAGE Rank2Types #-} > class FooClass a where m :: a > > instance FooClass Int where m = 10 > > newtype FaI = FaI{unFaI :: forall a. (FooClass a) => a -> Int} > foo :: FaI -> Bool > foo fn = ((unFaI fn)::(Char->Int)) m > 0 We cannot apply fn to a value: we must first remove the wrapper FaI (and instantiate the type using the explicit annotation -- otherwise the type-checker has no information how to select the FooClass instance). Let's try writing bar in this style. The first attempt > newtype IntFn a = IntFn (a -> Int) > newtype FaIntFn = FaIntFn{unFaIntFn:: forall a. FooClass a => IntFn a} > > bar :: FaIntFn -> Bool > bar (IntFn fn) = foo fn does not work: Couldn't match expected type `FaIntFn' with actual type `IntFn t0' In the pattern: IntFn fn Indeed, the argument of bar has the type FaIntFn rather than IntFn, so we cannot pattern-match on IntFn. We must first remove the IntFn wrapper. For example: > bar :: FaIntFn -> Bool > bar x = case unFaIntFn x of > IntFn fn -> foo fn That doesn't work for another reason: Couldn't match expected type `FaI' with actual type `a0 -> Int' In the first argument of `foo', namely `fn' In the expression: foo fn foo requires the argument of the type FaI, but fn is of the type a0->Int. To get the desired FaI, we have to apply the wrapper FaI: > bar :: FaIntFn -> Bool > bar x = case unFaIntFn x of > IntFn fn -> foo (FaI fn) And now we get the desired error message, which should become clear: Couldn't match type `a0' with `a' because type variable `a' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: FooClass a => a -> Int The following variables have types that mention a0 fn :: a0 -> Int (bound at /tmp/h.hs:16:16) In the first argument of `FaI', namely `fn' In the first argument of `foo', namely `(FaI fn)' When we apply FaI to fn, the type-checker must ensure that fn is really polymorphic. That is, free type variable in fn's type do not occur elsewhere type environment: see the generalization rule in the HM type system. When we removed the wrapper unFaIntFn, we instantiated the quantified type variable a with some specific (but not yet determined) type a0. The variable fn receives the type fn:: a0 -> Int. To type-check FaI fn, the type checker should apply this rule G |- fn :: FooClass a0 => a0 -> Int ----------------------------------- G |- FaI fn :: FaI provided a0 does not occur in G. But it does occur: G has the binding for fn, with the type a0 -> Int, with the undesirable occurrence of a0. To solve the problem then, we somehow need to move this problematic binding fn out of G. That binding is introduced by the pattern-match. So, we should move the pattern-match under the application of FaI: > bar x = foo (FaI (case unFaIntFn x of IntFn fn -> fn)) giving us the solution already pointed out. > So my next question is: why does unpacking the newtype via pattern > matching suddenly limit it to a single monomorphic type? Because that's what removing wrappers like FaI does. There is no way around it. That monomorphic type can be later generalized again, if the side-condition for the generalization rule permits it. You might have already noticed that `FaI' is like big Lambda of System F, and unFaI is like System F type application. That's exactly what they are: http://okmij.org/ftp/Haskell/types.html#some-impredicativity My explanation is a restatement of the Simon Peyton-Jones explanation, in more concrete, Haskell terms.

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Haskell/F#/Ocaml Developer Opportunity in Singapore
by mani maran 23 Aug '12

23 Aug '12

Hello Everyone, I am currently recruiting a Functional Programming Developer for one of the bank in Singapore. Anyone interested please contact me at +65-83991283 or please drop a note to me at manimaran_elumalai(a)kellyit.com.sgwith your contact details. Below are some skills my client is looking for... * Strong in a functional programming language (Haskell, OCaml, F#) * Additional useful languages: C++ / javascript * Maths skills useful. * Good communication skills. * Willingness to interact with traders & quants * 1st degree in Computer Science (preferably with some maths courses) * Post grad degree in Computer Science (Msc/Phd) * Publicly available code that can be reviewed. * No finance background required. Regards, Manimaran

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Cloud Haskell real usage example
by Thiago Negri 23 Aug '12

23 Aug '12

Hello everyone. I'm taking my first steps in Cloud Haskell and got some unexpected behaviors. I used the code from Raspberry Pi in a Haskell Cloud [1] as a first example. Did try to switch the code to use Template Haskell with no luck, stick with the verbose style. I changed some of the code, from ProcessId-based messaging to typed channel to receive the Pong; using "startSlave" to start the worker nodes; and changed the master node to loop forever sending pings to the worker nodes. The unexpected behaviors: - Dropping a worker node while the master is running makes the master node to crash. - Master node do not see worker nodes started after the master process. In order to fix this, I tried to "findSlaves" at the start of the master process and send ping to only these ones, ignoring the list of NodeId enforced by the type signature of "startMaster". Now the master finds new slaves. The bad thing is that when I close one of the workers, the master process freezes. It simply stop doing anything. No more messages and no more Pings to other slaves. :( My view of Cloud Haskell usage would be something similar to this: a master node sending work to slaves; slave instances getting up or down based on demand. So, the master node should be slave-failure-proof and also find new slaves somehow. Am I misunderstanding the big picture of Cloud Haskell or doing anything wrong in the following code? Code (skipped imports and wiring stuff): -- newtype Ping = Ping (SendPort Pong) deriving (Typeable, Binary, Show) newtype Pong = Pong ProcessId deriving (Typeable, Binary, Show) worker :: Ping -> Process () worker (Ping sPong) = do wId <- getSelfPid say "Got a Ping!" sendChan sPong (Pong wId) master :: Backend -> [NodeId] -> Process () master backend _ = forever $ do workers <- findSlaves backend say $ "Slaves: " ++ show workers (sPong, rPong) <- newChan forM_ workers $ \w -> do say $ "Sending a Ping to " ++ (show w) ++ "..." spawn w (workerClosure (Ping sPong)) say $ "Waiting for reply from " ++ (show (length workers)) ++ " worker(s)" replicateM_ (length workers) $ do (Pong wId) <- receiveChan rPong say $ "Got back a Pong from " ++ (show $ processNodeId wId) ++ "!" (liftIO . threadDelay) 2000000 -- Wait a bit before return main = do prog <- getProgName args <- getArgs case args of ["master", host, port] -> do backend <- initializeBackend host port remoteTable startMaster backend (master backend) ["worker", host, port] -> do backend <- initializeBackend host port remoteTable startSlave backend _ -> putStrLn $ "usage: " ++ prog ++ " (master | worker) host port" -- [1] http://alenribic.com/writings/post/raspberry-pi-in-a-haskell-cloud

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Haskell Weekly News: Issue 241
by Daniel Santa Cruz 23 Aug '12

23 Aug '12

Welcome to issue 241 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of August 12 to 18, 2012. Quotes of the Week * monochrom: seq is a suggestion. pseq is an order. * johnw: Haskell is like using a finely crafted blade to whittle yourself a canoe, as your friends pass by on their cruise liner and wonder what you're up to Top Reddit Stories * Yesod gets financial backing. Domain: yesodweb.com, Score: 76, Comments: 23 On Reddit: [1] http://goo.gl/isVxZ Original: [2] http://goo.gl/aTtqC * The category design pattern Domain: haskellforall.com, Score: 65, Comments: 32 On Reddit: [3] http://goo.gl/Owrgh Original: [4] http://goo.gl/iDEen * I remember the time before monads | free variable Domain: free-variable.org, Score: 64, Comments: 5 On Reddit: [5] http://goo.gl/41Utn Original: [6] http://goo.gl/febU9 * GHC 7.6.1 Release Candidate 1 Domain: haskell.org, Score: 55, Comments: 33 On Reddit: [7] http://goo.gl/weAei Original: [8] http://goo.gl/QHtd7 * PVP: upper bounds are not our friends Domain: haskell.org, Score: 49, Comments: 94 On Reddit: [9] http://goo.gl/8ihwE Original: [10] http://goo.gl/0YbGl * MongoDB on the hunt for Haskell Evangelist Domain: haskellers.com, Score: 36, Comments: 17 On Reddit: [11] http://goo.gl/X1WLL Original: [12] http://goo.gl/TYDdE * Raspberry Pi in a Haskell Cloud Domain: alenribic.com, Score: 33, Comments: 6 On Reddit: [13] http://goo.gl/esAHP Original: [14] http://goo.gl/me7ob * Practical Foundations for Programming Languages (first impressions) Domain: blog.ezyang.com, Score: 30, Comments: On Reddit: [15] http://goo.gl/FjrY1 Original: [16] http://goo.gl/8AzpC * Scoutess GSoC project finished! Domain: self.haskell, Score: 28, Comments: 4 On Reddit: [17] http://goo.gl/hRzvC Original: [18] http://goo.gl/hRzvC Top StackOverflow Questions * Subtype polymorphism in Haskell votes: 17, answers: 2 Read on SO: [19] http://goo.gl/xERPq * Is it possible to use a bracketing syntactic sugar for an applicative functor? votes: 14, answers: 2 Read on SO: [20] http://goo.gl/3EH63 * Useful operations on free arrows votes: 11, answers: 0 Read on SO: [21] http://goo.gl/BQ6ry * Against cold boot attacks: how to restrain sensitive information in Haskell votes: 9, answers: 1 Read on SO: [22] http://goo.gl/kUOn2 * GHCi and compiled code seem to behave differently votes: 7, answers: 1 Read on SO: [23] http://goo.gl/q8yJi * Implementing Iota in Haskell votes: 7, answers: 1 Read on SO: [24] http://goo.gl/KonKf * Understanding STG votes: 6, answers: 1 Read on SO: [25] http://goo.gl/uQmKn * Quickcheck for non-boolean tests votes: 6, answers: 0 Read on SO: [26] http://goo.gl/on6jt * Screen capture in Haskell? votes: 6, answers: 3 Read on SO: [27] http://goo.gl/7xIX5 * typeclass challenge: having both variadic arguments and results votes: 6, answers: 2 Read on SO: [28] http://goo.gl/8aA6Z Until next time, Daniel Santa Cruz References 1. http://www.yesodweb.com/blog/2012/08/joining-forces-advance-haskell 2. http://www.reddit.com/r/haskell/comments/y62j3/yesod_gets_financial_backing/ 3. http://www.haskellforall.com/2012/08/the-category-design-pattern.html 4. http://www.reddit.com/r/haskell/comments/yfhem/the_category_design_pattern/ 5. http://www.free-variable.org/2012/08/i-remember-the-time-before-monads/ 6. http://www.reddit.com/r/haskell/comments/y87rh/i_remember_the_time_before_m… 7. http://www.haskell.org/pipermail/glasgow-haskell-users/2012-August/022671.h… 8. http://www.reddit.com/r/haskell/comments/y3vzd/ghc_761_release_candidate_1/ 9. http://www.haskell.org/pipermail/haskell-cafe/2012-August/102885.html 10. http://www.reddit.com/r/haskell/comments/ydkcq/pvp_upper_bounds_are_not_our… 11. http://www.haskellers.com/jobs/18 12. http://www.reddit.com/r/haskell/comments/yc1no/mongodb_on_the_hunt_for_hask… 13. http://alenribic.com/writings/post/raspberry-pi-in-a-haskell-cloud 14. http://www.reddit.com/r/haskell/comments/ydvn2/raspberry_pi_in_a_haskell_cl… 15. http://blog.ezyang.com/2012/08/practical-foundations-for-programming-langua… 16. http://www.reddit.com/r/haskell/comments/yda6c/practical_foundations_for_pr… 17. http://www.reddit.com/r/haskell/comments/y9v83/scoutess_gsoc_project_finish… 18. http://www.reddit.com/r/haskell/comments/y9v83/scoutess_gsoc_project_finish… 19. http://stackoverflow.com/questions/12002979/subtype-polymorphism-in-haskell 20. http://stackoverflow.com/questions/12014524/is-it-possible-to-use-a-bracket… 21. http://stackoverflow.com/questions/12001350/useful-operations-on-free-arrows 22. http://stackoverflow.com/questions/11932246/against-cold-boot-attacks-how-t… 23. http://stackoverflow.com/questions/11926398/ghci-and-compiled-code-seem-to-… 24. http://stackoverflow.com/questions/11960809/implementing-iota-in-haskell 25. http://stackoverflow.com/questions/11921683/understanding-stg 26. http://stackoverflow.com/questions/11942554/quickcheck-for-non-boolean-tests 27. http://stackoverflow.com/questions/11963561/screen-capture-in-haskell 28. http://stackoverflow.com/questions/12014420/typeclass-challenge-having-both…

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regex-pcre is not working with UTF-8
by José Romildo Malaquias 22 Aug '12

22 Aug '12

Hello. It seems that the regex-pcre has a bug dealing with utf-8: Prelude> :m + Text.Regex.PCRE Prelude Text.Regex.PCRE> "país:Brasil" =~ "país:(.*)" :: (String,String,String,[String]) ("","pa\237s:Brasil","",["rasil"]) Notice the missing 'B' in the result of the regex matching. With regex-posix this does not happen: Prelude> :m + Text.Regex.Posix Prelude Text.Regex.Posix> "país:Brasil" =~ "país:(.*)" ::(String,String,String,[String]) ("","pa\237s:Brasil","",["Brasil"]) I hope this bug can be fixed soon. Is there a bug tracker to report the bug? If so, what is it? Romildo

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salvia
by Sergey Mironov 22 Aug '12

22 Aug '12

Hi. Does anybody know anything about Sebastiaan Visser, the maintainer of Salvia-* packages (web server) ? Looks like his email is dead. Sergey

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Galois internships available
by Lee Pike 22 Aug '12

22 Aug '12

**************************************** * ANNOUNCING: * INTERNSHIP AT GALOIS, Inc. **************************************** Galois, Inc. <www.galois.com> has an internship available in Portland, Oregon, USA. PROJECT OVERVIEW: The project is a 4+ year research project on build high-assurance autonomous vehicles. Galois will be working on three aspects of this problem: * Synthesizing software components from Haskell-based embedded DSLs. * Building/porting a hardware platform for testing our prototypes. * Performing static/dynamic analysis on C/C++ code to detect vulnerabilities. We intend to release open-source most (if not all) source code developed on the project and we will be publishing on our research results. Being an intern for repeated terms is a possibility. LOGISTICS: The length and start date of the internship are all negotiable: anytime from this fall through next fall is acceptable. Ideally, you can be at Galois for at least 3 continuous months. The internship is paid competitively, and the intern will be responsible for her own living arrangements (although we can certainly help you find arrangements). Galois is located in the heart of downtown, with multiple public transportation options available and world-class bicycle infrastructure, so living here without an automobile is a viable option. QUALIFICATIONS: * MUST-HAVES: * The ability to be geographically located in Portland during the internship. * Good knowledge of C/C++. * Some experience with low-level/embedded design. * Excellent software engineering ability and aptitude. * NICE-TO-HAVES (not necessary, but let me know if you have these qualifications!): * Experience with Haskell and particularly embedded DSLs. * Experience with microcontrollers (particularly ARM Cortex M). * Experience with control systems. * Interest in/experience with real-time systems and RTOSes. * Interest in/experience with software security. * Interest in/experience with static analysis. * Experience with ArduPilot <http://code.google.com/p/ardupilot-mega/> or other autopilot systems. * Good writing skills/experience in writing technical papers. * DO NOT NEED: * A specific degree (we're interested in hearing from anyone from post-docs to undergrads). ABOUT GALOIS: Galois, Inc. is located in Portland, Oregon with a mission to create trustworthiness in critical systems. We're in the business of taking blue-sky ideas and turning them into real-world technology solutions. We've been developing real-world systems for over 10 years using Haskell. To get a sense of life at Galois, one of our previous interns documented his internship here: <http://blog.ezyang.com/2010/08/day-in-the-life-of-a-galois-intern/>. TO APPLY: In one email, * Please attach a C.V. (PDF, plain text, or markdown only). * In the body of the email, include a brief non-HTML note stating your interest and experience and any other relevant details. Send this to Lee Pike (leepike at galois.com) with the subject line "Internship 2012". If you follow these directions, you'll get a confirmation from me.

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fclabels > 0.5
by Sergey Mironov 21 Aug '12

21 Aug '12

Hi. I'm porting old code, which uses fclabels <0.5. Old fclabels define Iso typeclass as follows: class Iso f where iso :: a :<->: b -> f a -> f b iso (Lens a b) = osi (b <-> a) osi :: a :<->: b -> f b -> f a osi (Lens a b) = iso (b <-> a) Newer one defines iso: class Iso (~>) f where iso :: Bijection (~>) a b -> f a ~> f b instance Arrow (~>) => Iso (~>) (Lens (~>) f) where iso bi = arr ((\a -> lens (fw bi . _get a) (_set a . first (bw bi))) . unLens) instance Arrow (~>) => Iso (~>) (Bijection (~>) a) where iso = arr . (.) but no osi. I'm not a guru in categories, can you help me define osi? Thanks Sergey.

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Pure and monadic functions using the Repa arrays
by felipe zapata 21 Aug '12

21 Aug '12

Hi Haskellers, I have been playing with the Repa functions and trying the Repa-examples. In order to gain experience with the Repa functions I have written some small linear algebra utilities and import this module to a bigger project. In the beginning of my project I used the mmultP function from the repa-examples to calculate a big matrix, therefore I have and array of type: arr :: Monad m => m (Array U DIM2 Double) Then I carried this array in a lot of functions which become Monadic function and then it is necessary to introduce the monadic machinery for manipulating this functions . The Question is then if there is the possibility to work with a pure function in place of the monadic version? There is something like a runRepa function? runRepa :: Monad m => m (Array U DIM2 Double) - > Array U DIM2 Double or could I used the unsafePerformIO function ? or the evaluation of the parallel arrays must be postponed until the Repa.Array is called in the main function? Thanks in Advance, Felipe.

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