- Haskell-Cafe - Haskell.org

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 02 Dec '11

02 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 01 Dec '11

01 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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Re: [Haskell-cafe] How to get a file path to the program invoked?
by dokondr 01 Dec '11

01 Dec '11

Balazs, thanks! It's great that these packages exist! On Thu, Dec 1, 2011 at 11:17 PM, Balazs Komuves <bkomuves(a)gmail.com> wrote: > > Hello, > > I'm not subscribed to haskell cafe, but I browse the archives sometimes. > > As Simon Hengel wrote there, there are two packages on Hackage > trying to solve this exact problem (since GHC and the standard library > does not provide the necessary information): > > http://hackage.haskell.org/package/executable-path > http://hackage.haskell.org/package/FindBin > > (Hackage is down at the moment, so I'm not completely sure about the > second link). > I'm the author of the first one. > > Balazs > >

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Re: [Haskell-cafe] How to get a file path to the program invoked?
by dokondr 01 Dec '11

01 Dec '11

To be precise, $0 always contains the path to the program called. You are right, this path will change depending on location from which the program was called. So $0 is OK for my case, while current directory is unrelated. Try this: #!/bin/sh echo "Arg 0: $0" echo "All Parameters: [$@]" Again, any way to get the same functionality in GHC? On Thu, Dec 1, 2011 at 10:32 PM, Giovanni Tirloni <gtirloni(a)sysdroid.com>wrote: > On Thu, Dec 1, 2011 at 5:26 PM, dokondr <dokondr(a)gmail.com> wrote: > >> On the contrary, standard shell variable $0 - contains a full path to the >> program location in the directory structure, no matter from what directory >> the program was called. >> >> > Are you sure? > > $ zero.sh > ./zero.sh > > $ ./zero.sh > ./zero.sh > > $ /home/gtirloni/zero.sh > /home/gtirloni/zero.sh > > > > -- > Giovanni > > >

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Hackage down!
by Edgar Gomes Araujo 01 Dec '11

01 Dec '11

Hi everybody, Is hackage.haskell.org down? I'm trying to access it but no answer, no ping response (timeout) , nothing. Is somebody else facing the same problem? Cheers, Edgar Gomes

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