September 2013 - Haskell-Cafe

Mistakes in documentation for weak pointers?
by Heinrich Apfelmus 10 Sep '13

10 Sep '13

Dear café, I'm currently studying weak pointers in order to implement garbage collection for a small JavaScript FFI used by the threepenny-gui library [1]. While the paper [2] is fairly clear, it seems that the documentation in System.Mem.Weak [3] differs in certain aspects. Could someone help me and clarify this? I have two questions: 1. The sentence in the documentation "References from the finalizer to the key are treated in the same way as references from the value to the key: they do not keep the key alive." seems incorrect in the subordinate clause. Namely, the paper only states that the weak pointer object does not keep the key alive. However, I understood that it is perfectly acceptable for the value to keep the key alive, and that this relation is not changed by mkWeak . Is this still correct? (The memo table example in the paper never stores the value itself, only a weak pointer to it, so it doesn't matter whether the value is reachable from the key or not.) 2. The sentence in the documentation "A heap object is reachable if: ... It is a weak pointer object whose key is reachable." seems to differ from the paper, which states that while weak pointers are always retained if their keys are alive, they can very much be unreachable. But if I think about it, the formulation in the documentation seems to be equivalent to the paper (as far as the semantics are concerned, it does not matter whether unreachable weak pointers are tombstoned or not.). Is that correct? [1]: http://www.haskell.org/haskellwiki/Threepenny-gui [2]: http://community.haskell.org/~simonmar/papers/weak.pdf [3]: http://hackage.haskell.org/packages/archive/base/latest/doc/html/System-Mem… Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

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Transforming AST parametrized with type
by Sergey Mironov 10 Sep '13

10 Sep '13

Hi. Pat asked a question [1] about AST parametrized with types. People suggest to use Functor machinery if possible. Have anything changed since them? Do we have a way to safely transform the tree like data Expr a = Id { id :: Id a } | Op { op :: Char, expra :: (Expr a) , exprb :: (Expr a) } data Id a = Id { name :: a } from Expr String to, say, Expr Int where Int names may represent some keys in the hash map? Regards, Sergey [1] - http://stackoverflow.com/questions/5434889/is-it-possible-to-use-syb-to-tra… PS sclv suggested to use synthesize from SYB to solve the problem. Could anyone provide me with example of this method?

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Looking for GUI examples
by Sergey Mironov 10 Sep '13

10 Sep '13

Hi, Cafe. I'd like to write simple GUI utility containing tray icon and the menu. Could you please suggest Haskell example to make my start easier? Regards, Sergey

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Lenses: Should I declare Getters?
by Charlie Paul 10 Sep '13

10 Sep '13

I've been looking through Edward Kmett's lens library, and I'm a bit befuddled about Getters. In my own code, why would I want to have something be a Getter instead of a plain function? As far as I can see, a plain function is simpler to use, and can be converted to a Getter with "to" if we want to use it as a Fold. Is there a situation where writing a Getter is superior than writing a function, then lifting it as needed? Thank you, Charles Paul

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Re: [Haskell-cafe] [Haskell] Hackage 2 now available for beta testing
by Ross Paterson 10 Sep '13

10 Sep '13

On Mon, Sep 09, 2013 at 07:23:59PM +0100, Duncan Coutts wrote: > Well-Typed and the Industrial Haskell Group (IHG) are very pleased to > announce that Hackage 2 is now available for public beta testing. The > plan is to do the final switchover in late September, to coincide > with ICFP. > > http://beta.hackage.haskell.org/ What's the story with haddock documentation? I see that some packages have docs imported from the old server, some have newly generated docs, and some have none, but no indication whether a bot has tried to build it or not. There's mention of maintainer uploads of docs as a fallback, but I couldn't find where one would do that. (It would also need to document the flags needed to get the links right.)

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resources on static analysis
by Maarten Faddegon 10 Sep '13

10 Sep '13

Dear list, I am interested in learning more about static analysis of Haskell code. Specifically of the relation between arguments of recursive and non-recursive calls. For example if we look at the ++ function from Prelude: (++) [] ys = ys (++) (x:xs) ys = x : xs ++ ys amongst others, we could infer the relations: ys_i+1 = ys_i (x:xs)_i+1 = xs_i Searching the web I found several tools (HLint, Haskabelle, Sourcegraph), but I am interested in the theory behind this. If you could recommend a paper or a book on this topic I would be grateful. Thanks, Maarten Faddegon

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Please excuse brief service disruption
by John Wiegley 10 Sep '13

10 Sep '13

The e-mail services at haskell.org were switched from exim4 to postfix tonight, please excuse the service disruption. For about one hour e-mails were not being accepted to the mailing lists. All should be resolved now. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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This is a mail system test, please ignore
by John Wiegley 10 Sep '13

10 Sep '13

Testing the new e-mail services at haskell.org. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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Cabal --enable-tests
by satvik chauhan 09 Sep '13

09 Sep '13

Hi cafe, I wanted to ask this as I couldn't find this in cabal documentation. Is there any CCP macro set when a package is configured with --enable-testing? If not is there a way to do that? -Satvik

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An Extra Empty Line
by Hong Yang 09 Sep '13

09 Sep '13

Hi Dear Haskellers, I have this small program to grep recursively in parallel. It works fine, but generates the last line empty. Is this empty line coming from mapConcurrently()? Thanks, Hong -- mygrepr.hs -- lrf.pl is an old Perl script to get all non-duplicate files recursively under the current directory, since some of directories have tons of links -- the program runs concurrently on either specified number of threads or 3/4 of the number of available cores module Main (main) where import Control.Concurrent.Async import Control.Monad import Data.List import Data.List.Split import GHC.Conc import System.Environment ( getArgs ) import System.Exit import System.Process import Text.Regex.Posix main :: IO () main = do hs_argv <- getArgs if null hs_argv || any (=~ "-h") hs_argv || any (=~ "--h") hs_argv then putStrLn "mygrepr [+RTS -N[x] -RTS] [OPTION]... PATTERN" else do numCores <- getNumProcessors numCapas <- getNumCapabilities let numT | numCapas > 1 = numCapas | otherwise = max numCapas (numCores `div` 4 * 3) _ <- setNumCapabilities numT let numThreads = fromIntegral numT :: Double findResult <- readProcess "lrf.pl" [] [] let files = lines findResult let num_of_files = fromIntegral $ length files :: Double let chunks = chunksOf (ceiling (num_of_files/numThreads)) files results <- mapConcurrently (grep hs_argv) chunks let (_, grepResult, _) = unzip3 results putStr $ unlines $ nub $ filter (\line -> not (line =~ "Binary file .* matches")) $ lines (concat grepResult) grep :: [String] -> [String] -> IO (ExitCode, String, String) grep hs_argv files = readProcessWithExitCode "/tool/pandora64/.package/grep-2.5.4/bin/grep" (hs_argv ++ files) []

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