- Haskell-Cafe - Haskell.org

Network.CGI and SQL Queries
by william murphy 21 Jul '11

21 Jul '11

Hello there, There's an odd error every time I try to run this code: import Network.CGI import Text.XHtml import qualified Text.Html as H import Database.HDBC import Database.HDBC.Sqlite3 import System.Random ... sqlReaderIO = do handle <- connectSqlite3 "table2.db" scoresSql <- quickQuery' handle "SELECT * FROM pachinkoScores ORDER BY score, name" [] let scores = map (map (\x -> fromSql x :: String)) scoresSql return (show scores) ... cgiTMain = do ... scoresPassed <- sqlReaderIO output . renderHtml $ (inputForm n ss) maybe (inputForm n ss) (\mT -> page "Loading..." (visual t v a mT ss) n v a (t + 1) mT) maxT main = runCGI $ handleErrors cgiTMain When I remove the "scoresPassed <- sqlReaderIO" line, the code run smoothly. Nothing else references "scoresPassed". The ghc, however, gives me this error when the line is included: tankwSql.hs:144:15: No instance for (MonadCGI IO) arising from a use of `output' Possible fix: add an instance declaration for (MonadCGI IO) In the first argument of `(.)', namely `output' In the expression: output . renderHtml In the expression: output . renderHtml $ inputForm n tankwSql.hs:147:30: Couldn't match expected type `CGIT m0 CGIResult' with actual type `IO CGIResult' In the first argument of `handleErrors', namely `cgiTMain' In the second argument of `($)', namely `handleErrors cgiTMain' In the expression: runCGI $ handleErrors cgiTMain Thanks, Will

3 2

Re: [Haskell-cafe] pointer equality
by wren ng thornton 21 Jul '11

21 Jul '11

On 7/20/11 1:22 PM, Chris Smith wrote: > If the latter, then it seems this would be a > pretty serious garbage collector bug, and that it would be impossible > that such a bug wouldn't also break other code that doesn't use pointer > equality at all. After all, we've got a running user thread, which if > it were to force those thunks now they would necessarily be observably > equal, but if it doesn't and waits until later they may be different? Not at all. Some amount of time passes between obtaining the pointer of x and obtaining the pointer of y in order to test whether they're equal. Given that time passes between these two steps, it's possible for GC to happen after getting one of the pointers and then when returning to user code we get the other pointer. It so happens that GC will never happen at that point on current versions of GHC, but that's not to say it couldn't (as it mustn't if the API specified no false positives). I can't think of any plausible optimization that would lead to a design which allows false positives, but I'm not sure pointer equality should really be offered in the first place. That's what StableNames are for. Pointer equality may be a reasonable implementation of StableName equality on some systems, but I really think that the two ideas should be kept separate. -- Live well, ~wren

1 0

Re: [Haskell-cafe] pointer equality
by wren ng thornton 21 Jul '11

21 Jul '11

On 7/20/11 5:08 PM, Paul Johnson wrote: > I would have thought that the compiler, as a matter of optimisation, > could insert a check to see if (==) is comparing an object with itself. > The only way I can see this breaking is with perverse instances of Eq > that would return False for "f == f". Like the instance for Double (where f = NaN)? -- Live well, ~wren

1 0

NetSNMP; FreeBSD 8.1
by Комар Максим 21 Jul '11

21 Jul '11

I want to install package NetSNMP: > cabal install NetSNMP Resolving dependencies... Configuring NetSNMP-0.1.6... cabal: Missing dependency on a foreign library: * Missing C library: netsnmp This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: NetSNMP-0.1.6 failed during the configure step. The exception was: ExitFailure 1 Package net-mgmt/net-snmp already installed: > pkg_info | grep snmp net-snmp-5.5_4 An extendable SNMP implementation With options --extra-include-dirs= and --extra-lib-dirs= result is same: > cabal install NetSNMP --extra-include-dirs=/usr/local/include > --extra-lib-dirs=/usr/local/lib Resolving dependencies... Configuring NetSNMP-0.1.6... cabal: Missing dependency on a foreign library: * Missing C library: netsnmp This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: NetSNMP-0.1.6 failed during the configure step. The exception was: ExitFailure 1 How to install NetSNMP package?

3 3

Haskell fgl example
by Rohit Agrawalla 20 Jul '11

20 Jul '11

I am new to haskell and the haskell fgl (functional graph library). l am looking for some examples of the haskell fgl. Would appreciate any pointers regarding the same. -- Rohit

3 2

Haskell Implementors Workshop talk proposals due this Friday!
by Ben Lippmeier 20 Jul '11

20 Jul '11

Call for Talks ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2011 Tokyo, Japan, September 23rd, 2011 The workshop will be held in conjunction with ICFP 2011 http://www.icfpconference.org/icfp2011/ Important dates Proposal Deadline: 22nd July 2011 Notification: 8th August 2011 Workshop: 23rd September 2011 The Haskell Implementors' Workshop is to be held alongside ICFP 2011 this year in Tokyo, Japan. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2011. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop we hope to study the underlying technology. We want to bring together anyone interested in the nitty gritty details necessary to turn a text file into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and Tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 min talks with 10 mins for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words to benl(a)cse.unsw.edu.au We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Rebekah Leslie (Portland State University) * Ben Lippmeier - co-chair (University of New South Wales) * Andres Loeh (Well-Typed LLP) * Oleg Lobachev (University of Marburg) * Neil Mitchell - co-chair (Standard Chartered) * Dimitrios Vytiniotis (Microsoft Research)

1 0

Re: [Haskell-cafe] pointer equality
by David Barbour 20 Jul '11

20 Jul '11

On Tue, Jul 19, 2011 at 11:14 PM, yi huang <yi.codeplayer(a)gmail.com> wrote: > 2011/7/20 Eugene Kirpichov <ekirpichov(a)gmail.com> > >> reallyUnsafePointerEq#, and it really is as unsafe as it sounds :) >> >> Why is it so unsafe? i can't find any documentation on it. > I think always compare pointer first is a good optimization. > Any number of conditions could make it fail, but most especially GC. It is almost always better to explicitly model a property when you want it. Uniqueness based on construction, for example, can be modeled with a monad and protected by an ADT. > module Unique ( HasUnique(..), Unique, newUnique, openUnique) where > class HasUnique m uid where > newUniqueID :: m uid > > data Unique uid a = U !uid a > > newUnique :: (HasUnique m uid, Monad m) => a -> m (Unique uid a) > newUnique a = newUniqueID >>= \ uid -> return (U uid a) > > openUnique :: (Unique m a) -> a > openUnique (U _ x) = x > > instance (Eq uid) => Eq (Unique uid a) > instance (Ord uid) => Ord (Unique uid a)

1 0

derive + quickCheck
by bob zhang 20 Jul '11

20 Jul '11

Hi, all, I found derive + quickCheck very useful but I came across some problems. I used derive to derive instance of Arbitrary immeditaely, but sometimes the sample is non-terminating, which I mean the result is very very big. I used samples <- take 10 <$> sample' in ghci to test the result, it's non-terminating.. another problem is that $(derive makeArbitrary ''JValue) uses reify, so I can not see the generated code, any better way to have a look at the generated code in ghci? my sample code {-# LANGUAGE FlexibleInstances ,MultiParamTypeClasses ,GeneralizedNewtypeDeriving ,FunctionalDependencies ,TypeSynonymInstances ,TemplateHaskell #-} module JsonParse where import Text.ParserCombinators.Parsec import Text.Parsec.String() import Control.Applicative hiding ( (<|>) , many, optional ) import Control.Monad import Test.QuickCheck -- unGen, sample -- import Language import Data.DeriveTH import Data.Binary import Data.Derive.Arbitrary import Language.Haskell.TH import Language.Haskell.TH.Syntax data JValue = JString String | JNumber Double | JBool Bool | JNull | JObject [(String, JValue)] -- | JArray [JValue] -- deriving (Eq,Ord,Show) $(derive makeArbitrary ''JValue)

2 3

partial inheritance
by Patrick Browne 19 Jul '11

19 Jul '11

Hi, Is it possible to model partial inheritance using Haskell type classes? For example, if the class Bird has a flying method can we represent Penguins as a sub-class of Bird without a flying method? Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

11 15

Parsing cabal files to calculate average number of dependencies
by Gwern Branwen 19 Jul '11

19 Jul '11

Athas on #haskell wondered how many dependencies the average Haskell package had. I commented that it seemed like some fairly simple scripting to find out, and as these things tend to go, I wound up doing a complete solution myself. First, we get most/all of Hackage locally to examine, as tarballs: for package in `cabal list | grep '\*' | tr -d '\*'`; do cabal fetch $package; done Then we cd .cabal/packages/hackage.haskell.org Now we can run a command which extracts the .cabal file from each tarball to standard output: find . -name "*.tar.gz" -exec tar --wildcards "*.cabal" -Oxf {} \; We could grep for 'build-depends' or something, but that gives unreliable dirty results. (>80k items, resulting in a hard to believe 87k total deps and an average of 27 deps.) So instead, we use the Cabal library and write a program to parse Cabal files & spit out the dependencies, and we feed each .cabal into that: find . -name "*.tar.gz" -exec sh -c 'tar --wildcards "*.cabal" -Oxf {} | runhaskell ~/deps.hs' \; And what is deps.hs? Turns out to be surprisingly easy to parse a String, extract the Library and Executable AST, and grab the [Dependency] field, and then print it out (code is not particularly clean): import Distribution.Package import Distribution.PackageDescription import Distribution.PackageDescription.Parse main :: IO () main = do cbl <- getContents let desc = parsePackageDescription cbl case desc of ParseFailed _ -> return () ParseOk _ d -> putStr $ unlines $ map show $ map (\(Dependency x _) -> x) $ extractDeps d extractDeps :: GenericPackageDescription -> [Dependency] extractDeps d = ldeps ++ edeps where ldeps = case (condLibrary d) of Nothing -> [] Just c -> condTreeConstraints c edeps = concat $ map (condTreeConstraints . snd) $ condExecutables d So what are the results? (The output of one run is attached.) I get 18,134 dependencies, having run on 3,137 files, or 5.8 dependencies per package. -- gwern http://www.gwern.net

3 5