- Haskell-Cafe - Haskell.org

ContT and MonadSafe
by PICCA Frederic-Emmanuel 16 Nov '21

16 Nov '21

Hello, I have some code like this data Hdf5Path sh e = H5RootPath (Hdf5Path sh e) | H5GroupPath ByteString (Hdf5Path sh e) | H5GroupAtPath Int (Hdf5Path sh e) | H5DatasetPath ByteString | H5DatasetPathAttr (ByteString, ByteString) | H5Or (Hdf5Path sh e) (Hdf5Path sh e) deriving (Show) Then I implement this kind of method withHdf5PathP :: (MonadSafe m, Location l) => l -> Hdf5Path sh e -> (Dataset -> m r) -> m r withHdf5PathP loc (H5RootPath subpath) f = withHdf5PathP loc subpath f withHdf5PathP loc (H5GroupPath n subpath) f = withGroupP (openGroup loc n Nothing) $ \g -> withHdf5PathP g subpath f withHdf5PathP loc (H5GroupAtPath i subpath) f = withGroupAtP loc i $ \g -> withHdf5PathP g subpath f withHdf5PathP loc (H5DatasetPath n) f = withDatasetP (openDataset' loc n Nothing) f withHdf5PathP loc (H5DatasetPathAttr (a, c)) f = withDatasetP (openDatasetWithAttr loc a c) f withHdf5PathP loc (H5Or l r) f = withHdf5PathP loc l f `catchAll` const (withHdf5PathP loc r f) I decided to switch to the ContT transfomer and try to implement this like this withHdf5PathP :: (MonadSafe m, Location l) => l -> Hdf5Path sh e -> ContT r m Dataset withHdf5PathP loc (H5RootPath subpath) = withHdf5PathP loc subpath withHdf5PathP loc (H5GroupPath n subpath) = do g <- withGroupP (openGroup loc n Nothing) withHdf5PathP g subpath withHdf5PathP loc (H5GroupAtPath i subpath) = do g <- withGroupAtP loc i withHdf5PathP g subpath withHdf5PathP loc (H5DatasetPath n) = withDatasetP (openDataset' loc n Nothing) withHdf5PathP loc (H5DatasetPathAttr (a, c)) = withDatasetP (openDatasetWithAttr loc a c) withHdf5PathP loc (H5Or l r) = ??? during the process, I also changed all the withXXX method to use Continuation. -bracket' :: MonadSafe m => (a -> IO ()) -> IO a -> (a -> m r) -> m r -bracket' r a = bracket (liftIO a) (liftIO . r) +bracket' :: MonadSafe m => (a -> IO ()) -> IO a -> ContT r m a +bracket' r a = ContT (bracket (liftIO a) (liftIO . r)) -withBytes :: MonadSafe m => Int -> (ForeignPtr a -> m r) -> m r +withBytes :: MonadSafe m => Int -> ContT r m (ForeignPtr a) withBytes n = bracket' touchForeignPtr (mallocForeignPtrBytes n) -withFileP :: MonadSafe m => IO File -> (File -> m r) -> m r +withFileP :: MonadSafe m => IO File -> ContT r m File withFileP = bracket' closeFile -withGroupP :: MonadSafe m => IO Group -> (Group -> m r) -> m r +withGroupP :: MonadSafe m => IO Group -> ContT r m Group withGroupP = bracket' closeGroup -withGroupAtP :: (Location l, MonadSafe m) => l -> Int -> (Group -> m r) -> m r -withGroupAtP l i f = do +withGroupAtP :: (Location l, MonadSafe m) => l -> Int -> ContT r m Group +withGroupAtP l i = do es <- liftIO $ nxEntries' l - withGroupP (openGroup l (es !! i) Nothing) f + withGroupP (openGroup l (es !! i) Nothing) -withDatasetP :: MonadSafe m => IO Dataset -> (Dataset -> m r) -> m r +withDatasetP :: MonadSafe m => IO Dataset -> ContT r m Dataset withDatasetP = bracket' closeDataset -withDataspaceP :: MonadSafe m => IO Dataspace -> (Dataspace -> m r) -> m r +withDataspaceP :: MonadSafe m => IO Dataspace -> ContT r m Dataspace withDataspaceP = bracket' closeDataspace The H5Or is a sort of Alternative which try to extract the info from l and if any exception is triggered switch to the right part r. Would you be so kind to help me write the H5Or part with the continuation. I am not sure that I understand all the ContT arcanes. thanks for your help Frederic

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haskell-ci problems
by Sven Panne 16 Nov '21

16 Nov '21

I am (finally) about to switch my Haskell projects on GitHub from Travis CI to GitHub Actions, so I've tried haskell-ci (from HEAD) for creating the workflow files from the .cabal files. But something went wrong, see https://github.com/haskell-opengl/StateVar/actions/runs/1456427818/workflow Initially, I thought that this failure was related to using the latest and greatest GHC version, so I removed that, but the failure stays: https://github.com/haskell-opengl/StateVar/actions/runs/1456437461/workflow Any hints on what I'm doing wrong? Or is it a haskell-ci bug? Cheers, S.

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About adding msync and anonymous mapping (virtual allocation) API to mmap package
by YueCompl 15 Nov '21

15 Nov '21

Dear Gracjan Polak, and Cafe for wider attention, I'm using the great [mmap](https://hackage.haskell.org/package/mmap <https://hackage.haskell.org/package/mmap>) package, and now I need 2 more features closely related to its existing features: - API to perform `msync` so as for: - A long running process to checkpoint the persistence of modifications - Parallel readers to see other writer's modification with cache coherence, i.e. invalidate cache as notified - API to do anonymous mmap-ping for volatile (virtual) memory allocation I expect the coding work to be trivial and intend to do it myself, however it's better to ask your opinion meanwhile I experiment with the ideas. Would you like to accept such features into [mmap] package? How do you think about details of such features? For the background of my needs: I'm working on a shared immutable heap implementation, persistency will be provided by disk backed storage servers, parallel work horse servers will build heap contents in rather coarse grained fashion, first as volatile heaps in one's private memory, then publish into shared storage with bulk writing, so all servers will be able to mmap such public heaps into their own memory address space, with kernel pages as cache shared by all processes on a same physical node. I hope it's okay to ask in the Cafe list at the same time, as I also anticipate more feedbacks/opinions from the list with broader expertise, will much appreciate! Thanks with best regards, Compl

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Breaking change to Eq accepted by Core Libraries Committee
by Rune K. Svendsen 14 Nov '21

14 Nov '21

Dear list, I would like to bring attention to the fact that the Haskell “Core Libraries Committee” has just accepted a breaking change to the Eq type class in exchange for very little apparent benefit. Please use this GitHub issue to voice your opinion on the issue: https://github.com/haskell/core-libraries-committee/issues/3 I don’t think it’s been sufficiently discussed yet. /Rune

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clz / bsr functions on Integer
by james faure 13 Nov '21

13 Nov '21

Why is there no `countLeadingZeros` function for Integer? This seems particularly strange because gmp provides mp_bitcnt_t mpz_scan0 (const mpz_t op, mp_bitcnt_t starting_bit) More importantly, what is the recommended way to access it?

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Should circular deriving "via" be rejected?
by Viktor Dukhovni 12 Nov '21

12 Nov '21

GHC accepts: {-# LANGUAGE DerivingStrategies, DerivingVia #-} newtype Foo = Foo Int deriving (Show, Eq) via (Bar) newtype Bar = Bar Int deriving (Show, Eq) via (Foo) should there be any effort to avoid circular definitions? Of course one could always implement an explicit `show` method that diverges, so perhaps this is no worse, but on the other hand, statically deciding divergence looks potentially easier in this case... On the other hand actual definitions like this seem unlikely in practice, so perhaps not worth burning precious compiler cycles to detect them? -- Viktor.

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Want to join a Team and Looking for good first issues to contribute
by Hima Bindu 10 Nov '21

10 Nov '21

Hi There! I am Hima Bindu, a CS Undergrad in GVPCE(A), AP, India. I like to be a polyglot and started learning a few languages, and came across Haskell. I was fascinated by its laziness and learned it for a couple of months. I skimmed through the repos, GSoC projects and a few Twitter threads. I want to contribute to anything that is beginner-friendly. It's overwhelming but I want to give it a try. Looking for any kind of suggestions or a walkthrough. This is very new to me, so please feel free to point out if I'm doing something wrong or moving in the wrong direction. *My area of interests:* Compilers(Currently working on one by following Bob Nystrom's book), Operating Systems/Kernals(have no experience but interested), anything related specifically to Haskell language(Data Structures, error handling etc) *Also, I eagerly want to join monthly meetings or something of that sort if there are any. How can I do that?* Really looking forward to your replies. Thank you Have a good day :D -- *Hima Bindu Tenneti* *Reach me here! <https://portfolios.talentsprint.com/~hima_bindu/>* *Programs must be written for people to read, and only incidentally for machines to execute. --Hal Abelson, SICP*

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[meetup announcement] Haskell Utrecht Meetup Group
by Ida Bzowska 10 Nov '21

10 Nov '21

Hi Everyone, Just letting you know that the freshly formed Haskell meetup from the Netherlands will have the first meetup on the 22nd of Nov; if you are in the area - don't hesitate: come! https://www.meetup.com/haskell-utrecht/events/281978445/ λCheers Ida Bzowska

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[ANN] linear-generics-0.2
by David Feuer 09 Nov '21

09 Nov '21

This new major version improves the GHCGenerically1 DerivingVia target. Previously, it worked on strictly fewer types than GHC could derive Generic1 for. Now it works on strictly more types than that. It does this by using a derived Generic instance instead of a derived Generic1 instance, thanks to the magic of quantified constraints. For anyone already using the package, the upgrade path is trivial: just derive any additional Generic instances GHC asks for.

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Re: [Haskell-cafe] Locate errors in TH for Haskell code
by Michal J Gajda 07 Nov '21

07 Nov '21

Dear Anton, > I have a task to convert one Haskell expression to another. > And I'd like to report errors properly. > > [interpret| -- error reported here > valid haskell code > invalid code <- error > |] > > I can parse it with haskell-src-exts or haskell-src-meta. > but if I have error in the code (type-check) error is positioned on the > first line of the QQ-expression. > > Do you know is it possible to report error at the line where it has > happened or > to keep original source code location in the Exp. > > In the function I do transformation Exp -> Exp. > > Or maybe there is a better way to do it instead of QQ? QQ is a good way. The simplest way would be to use `Language.Haskell.TH.location :: Q Loc` [1] and update it with the `Language.Haskell.Exts.SrcLoc.Loc` you get from `haskell-src-exts`[2]. [1] https://hackage.haskell.org/package/template-haskell-2.18.0.0/docs/Language… [2] https://hackage.haskell.org/package/haskell-src-exts-1.23.1/docs/Language-H… Note that [1] is available since template-haskell version 2.8.0.0. -- Best regards Michał

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