- Haskell-Cafe - Haskell.org

Anonymous, Unique Types, maybe
by Scott Lawrence 05 Dec '11

05 Dec '11

(Sorry if this email is rather unclear - I know my desired end result, but neither how to acheive nor explain it well. Here goes.) I'm processing lists, using them sortof as streams. (Whether that's a good idea isn't the issue here - but let me know if it isn't!) Fundamentally, there are two types of operations (at least, that are relevant here) - those that change the length of the list and those that don't. Some operators might take more than one list/stream as an argument, combining them in some way or another. Obviously, if the lists were different lengths, the operator would fail. I don't want that to happen at run time, so I want to check for it statically, presumably via the type system. I could do this manually: type AList = [Event] type BList = [Event] type CList = [Event] myMapish :: AList -> AList mySelect :: AList -> (Event -> Bool) -> BList myOtherSelect :: BList -> CList but I'd rather not have to manually define a new type for every new list length: myMapish :: List a -> List a mySelect :: List a -> List ? The '?' would be an anonymous, unique type - unless there's a better way to accomplish this. Hope that was clear, and thanks (as always) for the help (and being awesome). -- Scott Lawrence

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ismzero operator possible without equal constraint
by edgar klerks 04 Dec '11

04 Dec '11

Hi list, I am using MonadSplit (from http://www.haskell.org/haskellwiki/New_monads/MonadSplit ) for a project and now I want to make a library out of it. This seems to be straightforward, but I got stuck when I tried to move miszero out of the class: miszero :: m a -> Bool It tests if the provided monad instance is empty. My naive attempt was: miszero :: (Eq (m a), MonadPlus m) => m a -> Bool miszero = ( == mzero ) This works, but not correctly. It adds an Eq constraint that is unneeded. I would prefer to have something like: miszero :: MonadPlus m => m a -> Bool Because I am not comparing the contents of the monad. I don't even touch it. Is this possible to write? with kind regards, Edgar

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ANNOUNCE: Haskell Communities and Activities Report (21st ed., November 2011)
by Janis Voigtländer 04 Dec '11

04 Dec '11

On behalf of all the contributors, I am pleased to announce that the Haskell Communities and Activities Report (21st edition, November 2011) is now available in PDF and HTML formats: http://haskell.org/communities/11-2011/report.pdf http://haskell.org/communities/11-2011/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. I hope you will find it as interesting a read as I did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline to the call. The editor collects all the contributions into a single report and feeds that back to the community. When I try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ========================================= End of April 2012: target deadline for contributions to the May 2012 edition of the HC&A Report ========================================= Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editor for a simple e-mail reminder in April (you could point me to them as well, and I can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Janis Voigtlaender <hcar at haskell.org>

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Weird interaction between literate haskell, ghci and OverloadedStrings
by Erik de Castro Lopo 03 Dec '11

03 Dec '11

Hi, I'm working on a literate haskell document (actually TeX, but the example below is just test) and I'm using ByteStrings in the code. I know I can do: ghci -XOverloadedStrings file.lhs or, after ghci is running I can do: Main> :set -XOverloadedStrings but I'd like to embed a directive in the file so that when loaded in GHCi, I will automatically get OverloadedStrings. This is mainly so that it JustWorks(tm) when I pass the file on to someone else. Is there a way to do this? There is a short example file below. I'm using ghc-7.0.4 from Debian testing. Cheers, Erik ----------8<----------8<----------8<----------8<---------- > {-# LANGUAGE OverloadedStrings #-} This is just text that that ghc/ghci should ignore > import Data.ByteString (ByteString) > import qualified Data.ByteString.Char8 as BS Simple function: > newlineCount :: ByteString -> Int > newlineCount bs = BS.foldl foldFun 0 bs > where foldFun s ch = if ch == '\n' then s + 1 else s Once this file is loaded, I should be able to do this: newlineCount "abcd\ncdead\nasdasd\n" ----------8<----------8<----------8<----------8<---------- -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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cabal install --hyperlink-source ?
by Johannes Waldmann 03 Dec '11

03 Dec '11

Hello. I can do "cabal install --enable-documentation" which is nice because it does configure, build, haddock and copy in one go, but I don't see how to pass options from "cabal install" to "cabal haddock" (e.g., --hyperlink-source) Any hints appreciated, J.W.

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exhibit space at CeBIT Open Source 2012
by Mathias Huber 02 Dec '11

02 Dec '11

Dear Haskellers, this might interest those not too far from Hannover, Germany: http://www.linux-magazine.com/Online/News/CeBIT-Open-Source-2012-Call-for-P… "CeBIT welcomes open source projects to Hannover, Germany! The show organization and Linux Magazine are calling for open source projects to apply for free exhibit space at CeBIT Open Source 2012." There is also a call for papers: http://www.linux-magazine.com/Online/News/CeBIT-2012-Call-for-Papers-Open-S… Best, Mathias

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Program used for debugging
by Yves Parès 02 Dec '11

02 Dec '11

Hey, What do you mostly use for debugging? Simple calls to Debug.Trace.trace? Hpc? Hood?

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PLPV 2012: Call for participation
by Nikhil Swamy 02 Dec '11

02 Dec '11

Call for Participation ====================== PLPV 2012 The Sixth ACM SIGPLAN Workshop Programming Languages meets Program Verification 24th January, 2012 Philadelphia, USA (Affiliated with POPL 2012) http://research.microsoft.com/en-us/um/people/nswamy/plpv12 You are cordially invited to PLPV 2012. The goal of PLPV is to foster and stimulate research at the intersection of programming languages and program verification, by bringing together experts from diverse areas like types, contracts, interactive theorem proving, model checking and program analysis. Work in this area typically attempts to reduce the burden of program verification by taking advantage of particular semantic or structural properties of the programming language. Examples include dependently typed programming languages, which leverage a language's type system to specify and check richer than usual specifications or extended static checking systems which incorporate contracts with either static or dynamic contract checking. Registration ============ To register for PLPV 2012, please follow the instructions at: https://regmaster3.com/2012conf/POPL12/register.php The early registration deadline is December 24, 2011. Hotel Information ================= PLPV will be co-located with POPL at the Sheraton Society Hill Hotel in Philadelphia. Please visit POPL's web site to make reservations at the special conference rate. Program ======= Session 1: 9:00am-10:00am LTL types FRP: Linear-time Temporal Logic Propositions as Types, Proofs as Functional Reactive Programs Alan Jeffrey A Hoare Calculus for the Verification of Synchronous Languages Manuel Gesell and Klaus Schneider Session 2: 10:30am-12:00pm Invited talk: Could We Verify an Information-flow Computer? Benjamin C. Pierce University of Pennsylvania Lunch: 12:00pm-2:00pm (not provided) Session 3: 2:00pm-3:30pm Reflexive Toolbox for Regular Expression Matching Vladimir Komendantsky Formal Network Packet Processing with Minimal Fuss: Invertible Syntax Descriptions at Work Reynald Affeldt, David Nowak and Yutaka Oiwa The VerCors project - setting up basecamp Afshin Amighi, Stefan Blom, Marieke Huisman and Marina Zaharieva-Stojanovski Session 4: 4:00pm-5:00pm Dependent Interoperability Peter-Michael Osera, Vilhelm Sjoberg and Steve Zdancewic. Equational Reasoning about Programs with General Recursion and Call-by-value Semantics Garrin Kimmell, Aaron Stump, Harley Eades, Peng Fu, Tim Sheard, Stephanie Weirich, Chris Casinghino, Vilhelm Sjoberg, Nathan Collins and Ki Yung Ahn

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Using QuickCheck to test against a database
by Oliver Charles 02 Dec '11

02 Dec '11

Hi! I've just spent my evening trying to use QuickCheck properties to test the database accessing functions in a project I'm working on at the moment. I've got some stuff that's working, and I wanted to see what the cafe thought about it, and if anyone had any suggestions or critiscm that can help me make it even better! I got here by trying to figure out how I want to test my application. At first I saw 2 options: use a test database and assume it exists, or have each test setup the data it needs. I like the latter more, because it minimises dependence on the outside world. So I got down to writing some HUnit tests and then had a wave of inspiration... what if I didn't even define the data, but let QuickCheck do that for me? Here's what I came up with - note the code here is somewhat paraphrased for brevity: data DBState a = DBState { initDb :: IO () , entity :: a } This type stores an action that initialises the database with enough data such that it contains whatever 'entity' is. For example, 'DBState Person' knows how to insert a specific person into the database, and carries along an in-memory log. Continuing with the Person idea, I then went and made an Arbitrary instance for DBState Person: instance Arbitrary (DBState Person) where arbitrary = do l <- Person <$> arbitrary `suchThat` (not . null) return $ DBState (void $ insertPerson l) p where insertPerson p = doSql "INSERT INTO person (name) VALUES (?)" [ personName p l ] Then it's just a case of defining some properties: prop_allPersons = monadicIO $ do dbState <- pick arbitrary :: Gen [Person] fetched <- run $ initDb `mapM` dbState >> findAllPersons assert $ fetched `eqSet` (entity `map` dbState) where eqSet a b = all (`elem` a) b && all (`elem` b) a Voila! For any database, 'findAllPersons' returns all persons. Now... onto my own critiscisms with this. Firstly, way too much boiler plate. You have to remember to apply all of the states of your arbitrary instances, which is a pain, and guaranteed to be missed. At first I thought DBState looks like it's basically a writer/IO monad, but then I couldn't actually figure out the monoid to write to. In this case it's just [Person], but in more complex property we might need 2 types of entities (for example, to ensure a join worked correctly) which makes this list heterogenous. Secondly, the initDb action is sensitive to the order actions are sequenced. Presumably, I want to test against a database that's as accurate to production as possible, which means I want foreign keys enabled. If things are initialized in the wrong order, it violates the foreign key constraint, and it's a burden if people have to determine the correct order. One solution here is to make use of deferrable constraints, but support for these is flakey at best (and it doesn't work for bracketting tests with BEGIN; ROLLBACK). These problems just make the properties harder, but they don't seem to impede the quality or usefulness of the tests. I really want to make this work, because the idea of it is quite beautiful to me. Though that might be partly because this is my first time really using QuickCheck, so I'm getting the "oh wow this is sweeeet" effect at the same time ;) Would love to hear peoples thoughts! - Ollie

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List Fusion of concatMap
by Joachim Breitner 02 Dec '11

02 Dec '11

Hi, in further attempts to get a better understanding of list fusion, I am investigating under what conditions, calls to concat and concatMap succesfully fusion away. Here is my test code: import System.Exit func0 :: Int -> Bool func0 n = any (>5) [1..n] {-# NOINLINE func0 #-} func1 :: Int -> Bool func1 n = any (>5) (concatMap (\m -> [1..m]) [1..n]) {-# NOINLINE func1 #-} func2 :: Int -> Bool func2 n = any (>5) (concat (map (\m -> [1..m]) [1..n])) {-# NOINLINE func2 #-} func3 :: Int -> Bool func3 n = any (>5) [ i | m <- [1..n] , i <- [1..m]] {-# NOINLINE func3 #-} func4 :: Int -> Bool func4 n = any (>5) ( let ok m = [ i | i <- [1..m] ] in concatMap ok [1..n] ) {-# NOINLINE func4 #-} main = if func0 10 && func1 10 && func2 10 && func3 10 && func4 10 then exitSuccess else exitFailure I deliberately did not use any putStr statements in main, so that I can easily spot list types in the output of ghc -fext-core. The use of "any" and "[...]" represent any fusion-enabled producers or consumers; these are just very well-behaving and easy to spot in the core output. Here are my obeservations: * The code generated for func0 does not use any lists at all, but rather one recursive (even tail-recursive) function, as expected. This was basically a test whether I am indeed able to observe list fusion. * func1 and func2 are not fused successfully; both core outputs still contain mentions of [] (search for ZMZN; is there a tool that does this unescaping for me? ghc-core does not, it seems.) * Now the surprising fact: func3 does without any lists! The code generated contains two nested recursive functions, the inner one even tail recursive (and the outer one could have been made tail-recursive, it seems, but that would be a different issue). * func4, which is a direct one-step translation of func3 according to the semantic rules of list comprehension in the haskell report, again is not fused completely. Now I wonder: How do I need to phrase the function with concatMap such that fusion works? Does ghc treat list comprehensions differently here? Do the rewrite rules need work so that func1 and func2 do fuse correctly? Thanks for your attention, Joachim -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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