- Haskell-Cafe - Haskell.org

Cabal: updating environment variable in a dependency
by aditya siram 17 Sep '17

17 Sep '17

Hi all, I have two separate packages, a library and an app. In the library's Setup.hs I set the LIBRARY_PATH environment variable to a place where I install a custom C shared library, but when the app is built LIBRARY_PATH is empty. I realize that GHC can build and link C code but for the purposes of this project that wasn't workable. So is there a way to make this environment variable hack work or, even better, a good way a dependency can pass custom information to its dependents? Thanks! -deech

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Maintenance of streaming and streaming-bytestring
by Andrew Martin 16 Sep '17

16 Sep '17

David Feuer brought this up two weeks ago [0]. The maintainer of streaming and streaming-bytestring has been unreachable, and provided his continued absence, I would like to become a maintainer of these two libraries. I have put up PRs fixing issues [1] and I've offered on the issue tracker to become a comaintainer [2]. I use this library a lot at work, so I have an interest in keeping it in good shape. [0] https://mail.haskell.org/pipermail/libraries/2017-August/028141.html [1] https://github.com/michaelt/streaming-bytestring/pull/11 [2] https://github.com/michaelt/streaming-bytestring/issues/19 -- -Andrew Thaddeus Martin

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How to write a polymorphic serializer?
by Viktor Dukhovni 16 Sep '17

16 Sep '17

I tried to implement a polymorphic serializer that uses a enclosed MVar () to serialize an action in an arbitrary IO monad, without a need for its user to resort to explicit liftIO calls. So, as a first step I create a typeclass that has a natural default implementation, and some instances for the Monads of interest that just use the natural default: type Serially m = (MonadIO m) => forall a. IO a -> m a class (MonadIO m) => Serializable m where serialize :: MVar () -> Serially m serialize lock = liftIO . withMVar lock . const instance Serializable IO instance Serializable (StateT Int IO) ... With this, given: foo :: Serially IO -> ... -> IO () foo serially ... = do ... serially $ ... ... bar :: Serially (StateT Int IO) -> ... -> StateT Int IO () bar serially ... = do ... serially $ ... ... I can write: lock <- newMVar () foo (serialize lock) ... bar (serialize lock) and the type system figures out the correct version of serialize for foo's and bar's actual monad. Is it possible to create a single "serialize lock" closure that works for both "foo" and "bar"? Something that works along the lines of: let x = liftIO . withMVar lock . const :: ??? foo x ... bar x ... If I leave out the "liftIO", then I can of course use: x :: forall a. IO a -> IO a and the "liftIO" can be put explicitly into "foo" and "bar". foo x ... = liftIO $ x $ ... bar x ... = liftIO $ x $ ... but is it possible for "x" to both be polymorphic with respect to its user's monad and at the same time to be a *closure* around some MVar, and thus not directly a method of a type class. Of course needing to add an extra "liftI0" here and there is not onerous, I'm mostly just curious whether I'm missing something that can hide that boilerplate call in the serializer implementation. -- Viktor.

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Executing conduit streams in parallel leads to memory leaks
by Simon Hafner 13 Sep '17

13 Sep '17

When I run my conduit without any additions, it works as expected, with low constant memory usage, as advertised. It's a bit slow, so I tried to speed it up with worker pools (via parallel-io) and staged folding (via stm-conduit). However, then the memory usage indicates all the ByteString from the file readings are being fully allocated and kept in memory, even though they're not being used after a step of conduit. [1] I thought maybe because of the closing IO, the release of the file handle somehow keeps the read string in memory, so I wanted to make absolutely sure that's not the problem. [2] Switch out the `Lib.readFile` with `B.readFile` to undo that specific part. I was not using a worker pool in the beginning, so maybe the `mapConcurrently_` somehow allocated all the threads, but with the pooled solution, that should be solved as well. What else could cause all the ByteStrings to be kept in memory in the parallel version? The example is available on: https://github.com/reactormonk/non-constant-memory [1] https://github.com/reactormonk/non-constant-memory/blob/master/src/Lib.hs#L… [2] https://github.com/reactormonk/non-constant-memory/blob/master/src/Lib.hs#L…

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HPC and coverage report
by Baa 13 Sep '17

13 Sep '17

Hello Everyone. To generate coverage, Haskellers use HPC. HPC has command report [--xml-output] ... which allows to get XML output of coverage. Jenkins CI "understands" Cobertura Coverage XML format, which is shown as code source marked with red/green/etc background lines - uncovered/covered code lines. Also it's possible to mark branches (if/else/etc). It's true for Python, Java, etc. Can HPC (or other Haskell tool) does the same, i.e. generate Cobertura-compatible XML (to be understandable by Jenkins CI) ? What peoples use for coverage reporting under Jenkins CI w/ Haskell ? Is this `hpc report --xml-output` compatible w/ Jenkins CI ? === Best regards, Paul

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Is this a GHC parser error? A test case compiles from one line but not another.
by Jeffrey Brown 10 Sep '17

10 Sep '17

The test suite below[1] does what it's supposed to. The last line of it is a commented-out test case: -- assertBool "broken" $ True If I uncomment that, it fails to compile: > :reload [13 of 13] Compiling TParse ( test/TParse.hs, interpreted ) test/TParse.hs:67:3: error: parse error on input ‘assertBool’ | 67 | assertBool "broken" $ True | ^^^^^^^^^^ Failed, 12 modules loaded. > However, if I move it to somewhere earlier in the code, it works. [1] https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/e5afd39b950… -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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Re: [Haskell-cafe] Haskell Numerics / Data Science at ICFP
by Doug McIlroy 08 Sep '17

08 Sep '17

I won't be at ICFP, but would like to put in a reminder about the shabbiness of GHC floating-point output conversion. When I raised this issue once before, I got the surprising reaction that speed was more important than accuracy. Doug McIlroy

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floating point conversion
by Carter Schonwald 07 Sep '17

07 Sep '17

On Wed, Sep 6, 2017 at 5:22 PM, Doug McIlroy <doug(a)cs.dartmouth.edu> wrote: > I won't be at ICFP, but would like to put in a reminder about > the shabbiness of GHC floating-point output conversion. When I > raised this issue once before, I got the surprising reaction > that speed was more important than accuracy. > @doug: could you point me to the issue ? i'm actually allocating some of my time over the next year to improving floating point matters in ghc and the standard, and specific issues you can point me to that you've previously articulated would be super helpful :) > > Doug McIlroy > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. >

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optparse-applicative in stack LTS <= 9.3
by Viktor Dukhovni 06 Sep '17

06 Sep '17

FYI, the version of optparse-applicative (0.13.2.0) in Stack LTS-9.3 and earlier does not appear to handle the "noIntersperse" info modifier correctly. Instead of stopping parsing flags after the first non-option argument, it instead simply never parses any options. This is fixed in 0.14.0.0, which is in "Stackage Nightly". Perhaps someone else will run into the same issue. And it would of course be nice to see 0.14.0.0 appear in an upcoming LTS version (say 9.4). -- Viktor. Demo program, this fails to even process "--help" with the outdated optparse-applicative, which just reports all arguments as positional: module Main (main) where import Options.Applicative import Data.Monoid ((<>)) data Opts = Opts { bl :: Bool , fl :: Bool , st :: Maybe String , pos :: String , args :: [String] } deriving (Show) main :: IO () main = do let parse = Opts <$> switch ( long "bool" <> short 'b' ) <*> switch ( long "flag" <> short 'f' ) <*> optional ( strOption ( long "string" <> short 's' ) ) <*> strArgument ( metavar "FILE" ) <*> many ( strArgument ( metavar "ARGS..." ) ) i = info (helper <*> parse) $ fullDesc <> noIntersperse <> header "Option parser" <> progDesc "- test POSIX-style options" execParser i >>= print -- Viktor.

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[ANN] Haskell Workshop in Vancouver, BC
by Román González 06 Sep '17

06 Sep '17

`Hello, Is my pleasure to share the news about a new Study Group to learn Haskell using "The Haskell Book". If you live in Vancouver, British Columbia, or know someone that lives here and might be interested in learning Haskell, please share this link with them: https://workshops.vanfp.org/haskell Best Regards. Roman.-

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