July 2011 - Haskell-Cafe

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)

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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)

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derive + quickCheck
by bob zhang 19 Jul '11

19 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)

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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

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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

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ANN: cabal-dev 0.8
by Rogan Creswick 19 Jul '11

19 Jul '11

We're happy to announce the release of cabal-dev 0.8! This version is available on hackage now, and contains many bug fixes and improvements, as outlined in the full release notes below. --Rogan cabal-dev release 0.8 ================================================== The 0.8 release of `cabal-dev` fixes many of the bugs that have been reported by `cabal-dev` users. It also provides more flexible, easier configuration and improves tool support for working within the `cabal-dev` sandbox (ghc-pkg suport and better ghci support). Upgrading is recommended for all users. Please report any bugs or submit enhancement requests to <https://github.com/creswick/cabal-dev/issues>. Much of the work on this release (and previous releases) of `cabal-dev` was funded by Galois, Inc. <http://www.galois.com/> About cabal-dev ================================================== `cabal-dev` is a Haskell build tool that augments Cabal and cabal-install to make it convenient to work with a sandboxed set of packages. It limits the effect of your build on unrelated parts of your system. Among other things, `cabal-dev` satisfies the following needs: * Avoid conflicts between libraries by working only with the set of packages that are needed for the current project. * Enable depending on a patched dependency without that patched dependency accidentally being used by another project's build. * Enable the simultaneous development of multiple interdependent packages without the development versions leaking to other project build environments. Significant effort has been made to ensure that `cabal-dev` works across platforms (Mac, Windows, and Linux) and with the versions of GHC that are most commonly in use (6.10-7.0). What's new ================================================== Details can be found on github at <https://github.com/creswick/cabal-dev>. Bugs fixed: * `cabal-dev` no longer passes `--verbose` to cabal-install commands that do not accept it. <https://github.com/creswick/cabal-dev/issues/18> * `cabal-dev` now recognizes and passes through short flags to cabal-install. In particular, this bug frequently came up when attempting to pass a flag (`-f`) to `cabal-dev install`. <https://github.com/creswick/cabal-dev/issues/6> * `cabal-dev` now supports all cabal-install commands, including `test`. <https://github.com/creswick/cabal-dev/issues/10> * We also made changes that should address inconsistencies with the remote repository (Hackage) cache location, but those issues remain open until we have confirmation from end users (you!) that these changes actually fix those issues. <https://github.com/creswick/cabal-dev/issues/15> and <https://github.com/creswick/cabal-dev/issues/14> Enhancements and new features: * `cabal-dev ghci` now invokes ghci with the exact same set of flags that would be used in a cabal-install build (using a trick copied from Leksah). This means that it will now have the appropriate package visibility, language settings, and source directories set when it is invoked. Note that in order for `cabal-dev ghci` to work, all dependencies must already be installed in the sandbox, and the package must already be configured. * `cabal-dev buildopts` will print the set of build options that would be passed to GHC when invoking `cabal build`. You can then use these flags for other tools that invoke GHC (e.g. the doctest package <http://hackage.haskell.org/package/doctest>). * `cabal-dev ghc-pkg` will invoke `ghc-pkg` with the appropriate flags to examine or alter the package database inside the `cabal-dev` sandbox. * `cabal-dev` now has more flexible support for cabal-install configuration: * By default `cabal-dev` will now use any settings from your cabal-install configuration file, except those that must be overridden in order to sandbox the build. This makes it more convenient to e.g. use an alternate remote repository with `cabal-dev`. * `cabal-dev` now supports merging of additional cabal-install configuration settings, to make it easy to work with multiple cabal-install configurations (`--extra-config-file=...`). * `cabal-dev` now has a flag to pass explicit options to cabal-install in case there are any conflicts between `cabal-dev` flags (`--cabal-install-arg`). * `cabal-dev` now has a flag to specify a particular cabal-install executable to use (`--with-cabal-install=...`).

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I confused myself with generics, and now I confused ghc too
by Ari Rahikkala 19 Jul '11

19 Jul '11

I want to implement generic comparison on top of syb-with-class, and based on looking at syb, it seems I could base it on something like gzipWithQ from syb's Data.Generics.Twins. However, there's no Data.Generics.SYB.WithClass.Twins. So I set out to see if I can write one myself (well, one with just enough stuff to support gzipWithQ anyway) Here's what I've got so far: http://hpaste.org/49168 The problem is that GHCi seems to be somewhat confused, or at least confusing, about the type of gzipWithQ here. When I ask with :t, it reports the type as: gzipWithQ :: (Data ctx a2, Data ctx a3, Data ctx a) => Proxy ctx -> (forall a0. Data ctx a0 => a0 -> forall a1. Data ctx a1 => a1 -> r) -> a2 -> a3 -> [r] If I actually replace gzipWithQ's annotated type with this, GHCi will reject the program. And in any case with GenericQ being defined as "type GenericQ ctx r = Data ctx a => a -> r", isn't this closer to gzipWithQ's type with the type synonyms expanded? gzipWithQ :: forall ctx a r. Data ctx a => Proxy ctx -> (Data ctx a0 => a0 -> (Data ctx a1 => a1 -> r)) -> (Data ctx a2 => a2 -> (Data ctx a3 => a3 -> [r])) (GHCi accepts this as gzipWithQ's type so I assume it's the same one. I assume those class constraints after the => come with implicit foralls? Because it seems to mean the same thing if I put foralls in there) I have no idea how to even start figuring out how to make gzipWithQ and its type work. I only got this far by copying code from Data.Generics.Twins and passing down the proxy parameter and/or context type parameter (is it even possible for this approach to work?). What I can tell is that GHCi will still change the type like that even if gzipWithQ = undefined, so I'll have to change the type and not just the body of the function. Is this the kind of thing that all those newtypes in Data.Generics.Twins are for? Should I read up on type theory?

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Failure to install Network.CGI
by william murphy 18 Jul '11

18 Jul '11

Hi all, I tried to install the Network.CGI module by inputting: cabal install CGI into the terminal but got this error: Resolving dependencies... Configuring cgi-3001.1.8.2... Preprocessing library cgi-3001.1.8.2... Building cgi-3001.1.8.2... <command line>: cannot satisfy -package-id network-2.3.0.2-24fdc6b92867c7236e81708f93cae7d0 (use -v for more information) cabal: Error: some packages failed to install: cgi-3001.1.8.2 failed during the building phase. The exception was: ExitFailure 1 I'm almost positive I have the latest version of cabal, and have tried many things, but can't seem to see what I'm leaving out. Thanks, William

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How to use Yesod OAuth
by Haisheng Wu 18 Jul '11

18 Jul '11

Hello, I'm trying to use Yesod oAuth plugin to a SNS site which is very similar to Twitter. I add oAuth into the authPlugin and I'm able to see the auth URL at login page. I click the URL and it forwards me to a page ask me input that SNS site account. I fill in my account then the browser navigate to the authorize page display a authorised code (8 digits). Then, how I suppose to continue? (I think this is the 2nd phase of the oauth process) I expect there will be a text box in my App where I can fill in the authorised code. Or even the oAuth plugin automatically get the authorised code and continue to the next oauth phase. Appreciate your help! Haisheng

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Trying to make a calculator
by Emanuil Boyanov 18 Jul '11

18 Jul '11

Hi guys, I'm new to Haskell, I learn it for couple of months now and we've learned only simple functions like map, zip etc. But now my assignment is a lot tougher than I thought so I'll need some guidelines. I've googled my assignment and I saw a person with the exact assignment so I am gonna copy the task then I'm gonna show you what I've done so far. Here it is: Make calculator function. The function argument is a list of strings and also form such list, as each string of the argument made definite action. - If the string has the form of an arithmetic expression - calculate this expression. The string result becomes part of the list- result. If the expression contains a variable which is not assigned value, the result is displayed "undefined". - If the string has the form Name = value calculated from the last expression is assigned to the variable with the corresponding name in the list, and in the result list is formed a string with type name = ... where the site stands blank corresponding value. If there is not a calculated expression to be assigned to form a string "no value". - If the string is non-blank, but there is a species different from the above two case, form the string "error". - If the string is empty, incl. when it contains only spaces, in the result there is not form a string. Expressions consist of integers without sign variables, operations + (Addition), - (subtraction), * (multiplication) and / (divide) In the beginning, end or between the elements of each row can have spaces - they are irrelevant to its correctness. Example: the list-argument ["3 +7 / 2", "2 + x", "= s", "2 * s +4", "", "2 + +4 / 5"] function should provide a result-list ["6", "undefined", "s = 6", "16", "error"]. ========= So what I've done so far is: calculator [] = [] calculator [" "] = [] calculator [c] = [c] calculator (c:" ":cs) = c:calculator cs calculator (" ":cs) = calculator cs calculator (c:" ":" ":cs) = calculator (c:" ":cs) calculator (c:cs) | (head c `elem` ['a'..'z']) && (head(tail c) `elem` ['+','-','*','/']) && (last(take (length c-2)c) `elem` ['1','2','3','4','5','6','7','8','9']) = calculator cs | otherwise = "error":calculator cs I try at first to use pattern matching for the blank strings in the first list, so that I can get rid off them in the second list. Am I even on the right way here? I found the "elem" function and I'm trying to check if the first char of the string is a letter or a number, then if the second char is one of the operations and so on. Problem is that I have no idea how can I check if the variable is defined already or how to save a variable and then give that variable the new value - is that even possible? :] And another thing `elem` [1..9] isn't working - how can I make a single char of a string to hold an integer.. Could you give me some guidelines or at least tell me if I'm on the right way here so I can move forward and improve my program. Thanks a lot!

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