November 2016 - Haskell-Cafe

Atomically modify two MVars
by Lian Hung Hon 10 Nov '16

10 Nov '16

Dear cafe, Is it possible to atomically modify two MVars? (I know it can be done with TVars) Regards, Hon

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ghc installation on redhat7
by CRUMEYROLLE Pierre 10 Nov '16

10 Nov '16

Hello y want to install from source ghc 7.10 on red hat 7 y have installed by rpm method ghc 7.6.3 on my host because for generate ghc it's necessary to have pre install a ghc compiler. y have also perl version 5.16.3 installed. y have downloaded on https://www.haskell.org/ghc/download_ghc_7_10_1 the source distribution ghc-7.10.1-src.tar.bz2 after tar xvf ghc-7.10.1-src.tar.bz2 y launch ./configure --prefix=$HOME y have this error "Could not find module `System.Directory'" my trace below, thanks for explain $ ./configure --prefix=$HOME checking for gfind... no checking for find... /usr/bin/find checking for sort... /usr/bin/sort checking for GHC Git commit id... given ca00def1d7093d6b5b2a937ddfc8a01c152038eb checking for ghc... /usr/bin/ghc checking version of ghc... 7.6.3 checking build system type... x86_64-unknown-linux-gnu checking host system type... x86_64-unknown-linux-gnu checking target system type... x86_64-unknown-linux-gnu Build platform inferred as: x86_64-unknown-linux Host platform inferred as: x86_64-unknown-linux Target platform inferred as: x86_64-unknown-linux GHC build : x86_64-unknown-linux GHC host : x86_64-unknown-linux GHC target : x86_64-unknown-linux configure: Building in-tree ghc-pwd utils/ghc-pwd/Main.hs:4:8: Could not find module `System.Directory' Use -v to see a list of the files searched for. configure: error: Building ghc-pwd failed best regards

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What does intero add to GHCi?
by Yuji Yamamoto 10 Nov '16

10 Nov '16

Hello, Haskellers! I've recently seen many tools for Haskell IDE backend such as ghc-mod, hsdev, haskell-ide-engine, ide-backend, and intero. In addition, I found GHCi added commands to support IDEs: :complete, :type-at, etc. Then, I heard intero is a fork of GHCi. Actually intero's commands look same with GHCi's. So, what does intero add to GHCi? Thanks in advance. -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep Facebook: http://www.facebook.com/igrep Google+: https://plus.google.com/u/0/+YujiYamamoto_igrep

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Asynchronous computations and error handling in the free monad
by Damian Nadales 09 Nov '16

09 Nov '16

Yesterday I posted the following question to SO: http://stackoverflow.com/questions/40495304/abstract-result-types-in-free-m… As discussion with Benjamin ensued, in which he stated that it is better to be explicit about the fact that we want parts of our code to run asynchronously. I am not so sure. I also have a similar question about how would you deal with fatal errors that could happen at the interpreter. I guess this could be modeled in the free monad by having a command returning a `Maybe` or `Either` type. However we're adding details about the interpreters in our DSL, since we are stating that certain command could cause fatal errors. So my question is what approach do you or would you use?

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Avoid sharing
by Michael Roth 08 Nov '16

08 Nov '16

Hello! A short question, given: data Seed = ... data Value = ... someGenerator :: Seed -> [Value] createTwo :: Seed -> ([Value], [Value]) createTwo s = (as, bs) where as = someGenerator s bs = drop 1000000000 (someGenerator s) Is it guaranteed that 'someGenerator s' is created twice and not shared between 'as' and 'bs'? Is this by language design? Are there any GHC options that change the behaviour? Thank you, Michael

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TIOBE Index: Is Haskell finally going to hit the top 20?
by Henk-Jan van Tuyl 08 Nov '16

08 Nov '16

L.S., I know the TIOBE index[0] is not very scientific, but it is nice to see Haskell moving up in the TIOBE Index. From tiobe.com: " TIOBE Index for November 2016 November Headline: Is Haskell finally going to hit the top 20? Some people say that Haskell is the most mature purely functional programming language available nowadays. It has quite a long history, dating back from 1990 and its community is growing, although slowly. This month Haskell is only 0.255% away from the top 20 at position 23. Let's see what happens the next few months. " Haskell ranked 34th last month. Regards, Henk-Jan van Tuyl [0] http://www.tiobe.com/tiobe-index/ -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Strange space leak
by Tom Ellis 07 Nov '16

07 Nov '16

Can anyone explain why this leaks space: main :: IO () main = let loop () = return () *> loop () in loop () whilst this doesn't: main :: IO () main = let loop = return () *> loop in loop I'm stumped.

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FFT, via homogeneous functions?
by David Banas 07 Nov '16

07 Nov '16

Hi all, I’m trying to implement the fast Fourier transform (FFT), using the homogeneous functions described in Sec. 4.2 of *Arrows and Computation* by Ross Paterson: -- FFT implementation -- Note my reversal of Ross' even/odd convention, below. -- I did this, to remain consistent with the explanation, above, which came from a pre-existing work. fft :: RealFloat b => Hom (Complex b) (Complex b) fft = id :&: proc (e, o) -> do e' <- fft -< e o' <- fft >>> twiddle -< o unriffle -< f (e', o') where f (x, y) = (x + y, x - y) twiddle :: RealFloat b => Hom (Complex b) (Complex b) twiddle = id :&: twiddle' 1 twiddle' :: RealFloat b => Int -> Hom (Pair (Complex b)) (Pair (Complex b)) twiddle' n = (id *** (* phi)) :&: (((twiddle' n') *** ((twiddle' n') >>> (arr ((* phi) `prod` (* phi))))) >>> unriffle) where phi = cis (-pi / (fromIntegral (2 ^ n))) n' = n + 1 I’m getting this output, which means that, while my *twiddle* function is working for all tree depths tested, my *fft* function fails for tree depths greater than 3: Tree depth: 0 Testing twiddles...True Testing FFT...True Tree depth: 1 Testing twiddles...True Testing FFT...True Tree depth: 2 Testing twiddles...True Testing FFT...True Tree depth: 3 Testing twiddles...True Testing FFT...True Tree depth: 4 Testing twiddles...True Testing FFT...False Tree depth: 5 Testing twiddles...True Testing FFT...False And I’m wondering if anyone sees the flaw in my code. More here: https://htmlpreview.github.io/?https://github.com/capn-freako/Haskell_Misc/… <https://htmlpreview.github.io/?https://github.com/capn-freako/Haskell_Misc/…> Thanks! -db

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Guidance on symbol error
by Christopher Howard 07 Nov '16

07 Nov '16

Hi list. I'm trying to build gruff on Debian Jessie with GHC 7.6.3. I'm building gruff-0.3.3 because gruff-0.3.4 dependencies seem to be too new for Jessie. I was able to build all the dependencies of gruff-0.3.3 (after installing a ton of -dev packages). But when cabal tries to build the gruff package itself, I get this error: /usr/bin/ld: dist/build/gruff/gruff-tmp/src/compute.o: undefined reference to symbol '_ZdaPv@@GLIBCXX_3.4' //usr/lib/x86_64-linux-gnu/libstdc++.so.6: error adding symbols: DSO missing from command line It's been quite a while since I did any C++ programming... could somebody clarify for me what this means? Thank you. -- https://qlfiles.net My PGP public key ID is 0x340EA95A (pgp.mit.edu)

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ResourceT "unmasked" during allocation?
by Viktor Dukhovni 07 Nov '16

07 Nov '16

I am looking at using ResourceT to manage the acquisiition and deallocation of network sockets. In comparing resourceT's `allocate` with `bracket` I was somewhat perplexed to find that `allocate` in Control.Monad.Trans.Resource seems to not mask exceptions during allocation (see "No mask" comment I inserted): allocateRIO :: IO a -> (a -> IO ()) -> ResourceT IO (ReleaseKey, a) allocateRIO acquire rel = ResourceT $ \istate -> liftIO $ E.mask $ \restore -> do a <- restore acquire -- ^^^ No mask ^^^ key <- register' istate $ rel a return (key, a) allocate :: MonadResource m => IO a -- ^ allocate -> (a -> IO ()) -- ^ free resource -> m (ReleaseKey, a) allocate a = liftResourceT . allocateRIO a By contrast in Exception.Base we see that `bracket` masks the initialization action: bracket before after thing = mask $ \restore -> do a <- before -- ^^^ masked! ^^^ r <- restore (thing a) `onException` after a _ <- after a return r This seems to suggest that ResourceT is more exposes to resource leaks via asynchronous exceptions. Say if a file descriptor is a allocated, but an exception interrupts "allocate" before it can register the resource. Perhaps I am missing something, or there are good reasons for the difference in approach. Can anyone shed some light on the reason why ResourceT differs (seemingly) from bracket in this regard. -- Viktor.

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