- Haskell-Cafe - Haskell.org

Avoid sharing
by Michael Roth 09 Nov '16

09 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 08 Nov '16

08 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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Re: [Haskell-cafe] programming style...and type classes...
by Olaf Klinke 06 Nov '16

06 Nov '16

While we have stated in this thread what typeclasses should _not_ be used for, we probably do not have carved out yet what `proper' use is. Please help me here. There are some typeclasses, e.g. Eq, Ord, Num, Monoid, Functor, Monad and Category, that arise from category theory and algebra, so these may be regarded as "good" typeclasses even though a type can have several sensible instances. In that case, newtypes may express this. (Data.Monoid is full of such newtypes.) In defense of Ord, there are in general many ways to totally order a set (read "set" as "total elements of a type"), and none of these ways can be preferred a priori. I would not say this is an inherently bad property of Ord. Similarly, there are many ways to put a monoid or ring structure on a set. Sometimes a type class can be avoided. For example, there is Data.Foldable.toList. Hence, instead of writing a Foldable instance for your type and use a function from the Data.Foldable module, you could instead write your own toList conversion function and then fold that list. (Does anyone know a function involving Foldable that can not be reduced to 'toList' followed by some list operations? Is [] the "free instance" of Fodable?) However, this reduction may be much more inefficient than using the Foldable instance directly. As a contrived example of what I mean by "free instance", consider the following class that abstracts traversal over a sequence of unknown length. class ListLike l where patternmatch :: l a -> Maybe (a,l a) You can write ListLike instances for [], Vector, Sequence and Set, but only [a] is the solution to the type equation x = Maybe (a,x). Then there are multi-parameter type classes. This has been described as metaphorical to a type-level Prolog language [1], since ultimately multi-parameter type classes (without methods) are relations between types. Another debatable use case: You define many record types, each of which has a sort key component: data A = A {foo :: Foo, bar :: Bar, keyA :: Int} data B = B {baz :: Baz, keyB :: Int} data C = C {... , keyC :: Int} It might be convenient to define a typeclass class HasSortKey a where key :: a -> Int and endow the types A, B and C with the obvious instances. But one could also avoid the class by making a parametric type: data Keyed a = Keyed {payload :: a, key :: Int} and then let data A = A {foo :: Foo, bar :: Bar} and so forth. Which version would you say is `proper' use? -- Olaf [1] https://www.reddit.com/r/haskell/comments/ck459/typed_typelevel_programming…

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typeclass constraints?
by Gregory Guthrie 06 Nov '16

06 Nov '16

I want to define a bunch of tests in a simple demo program, where each test is locally defined, but all can be printed in one standard place. For example, each test result has a description::String, and a value, which can vary across tests. t1 = ("Sqrt(4)", sqrt(4.0)) ... t2 = ("sumList:", sum [1,2,3,4]) ... t3 = ("Description", value) ... So each test is of type: (String, value), for various value types all of which (only) have to be members of the Show class. Then for the summary of the tests, a loop: test (msg, val) = do print $ msg ++ " :: " ++ show val tests ts = mapM test ts ... Do tests [t1, t2, t3] This compiles, and assigns these types: test :: Show a => ([Char], a) -> IO () tests :: (Traversable t, Show a) => t ([Char], a) -> IO (t ()) Which works only as long as all tests have the same type for the second argument. I assume that it is somehow specializing the type to the actual encountered type of the arguments used, even though they are all show'able. So that they can vary on the actual types of the second argument, I tried something like this (pseudo code): type ATest = (Show a) => (String, a) since that wouldn't work, I tried: {-# LANGUAGE RankNTypes #-} type ATest = forall a. (Show a) => (String, a) Which compiles, but still fails on any variation in the value argument. Further, I want to abstract the type of tests from the loop which prints them, but I cannot then use it to convert from: test :: Show a => ([Char], a) -> IO () to test :: ATest -> IO () The basic idea was to define and use a polymorphic type for the tests in the definition of the testing loop. So perhaps a data structure instead; data (Show a) => ATest = Test (String, a) but that also fails, although it does give the right idea; all tests have a common structure, with a second value in the Show typeclass. I realize that generally one is recommended not to put constraints on the data type, but on the functions which use it - but here since I want a collection of tests it seems like I need a polymorphic list with consistent types. What is the right approach for this? Am I just getting the syntax wrong?

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[ANN] github-0.15
by Oleg Grenrus 05 Nov '16

05 Nov '16

Hi, today I finally released a new version of `github` package [1], providing bindings to the most of GitHub API. [2] The most notable feature, it’s now compatible with GHC-8.0.1. As GitHub doesn’t provide (or I’m not aware of) machine readable specification of their API, the feature completeness and correctness depends on the users of the package. So, please try it out, find out what’s incorrect and missing, and ideally submit a PR. The plan for the next version is to merge in definitions from github-types [3], and then split types from `github` package back into `github-types`, so others can reuse the definitions without the rest of the packages, e.g. for servant-github [4] As you can see, we are still experimenting with “how to write bindings to rest APIs”, if you have some comments, please raise an issue. Cheers, Oleg Grenrus - [1]: http://hackage.haskell.org/package/github <http://hackage.haskell.org/package/github> - [2]: https://developer.github.com/v3/ <https://developer.github.com/v3/> - [3]: http://hackage.haskell.org/package/github-types <http://hackage.haskell.org/package/github-types> - [4]: http://hackage.haskell.org/package/servant-github <http://hackage.haskell.org/package/servant-github>

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[ANN] Haskell ITA meetup in Bologna, Italy (2016-11-19)
by Francesco Ariis 04 Nov '16

04 Nov '16

Hello -cafe, the Italian Haskell Community (http://haskell-ita.it/) is happy to announce its autumn Haskell meetup! The event is going to be hosted in Bologna, on November 19th. The meeting will be in italian; we are going to form small groups/projects, to see Haskell in action and to share knowledge and opinions on functional programming. Everyone, experienced or not, is welcome, just make sure to have a laptop and a working Haskell environment! For a more detailed description of the event and to register, check: https://www.metooo.io/e/haskell-day-bologna (further information in italian follows) ----- - Dove: Bologna, Giardini Margherita, nella struttura "La Gabbia del Leone" (http://leserre.kilowatt.bo.it/gabbia-del-leone/) presso "Le Serre dei Giardini". - Quando: Sabato 19 Novembre, dalle 10:00 alle 18:00. - Chi: chiunque sia interessato alla programmazione funzionale, indipendentemente dal suo livello di esperienza. - Cosa faremo: lavoreremo in gruppi (2-6 persone circa) su diversi progetti. Ogni persona può scegliere il progetto che vuole, saranno date delle indicazioni sul livelo di conoscienza richiesto per contribuire. Lo scopo è provare a utilizzare Haskell per un progetto concreto, "toccare con mano" il modo che hanno altri di lavorare e di condividere le esperienze nelle numerose pause di gruppo. Lo scopo è puramente didattico, quindi nessuna ansia. - Cosa portare: un computer portatile e un ambiente Haskell configurato. - Come arrivare: dalla Stazione Centrale dei Treni di Bologna, prendere l'autobus 33 (frequenza 15 minuti) e scendere alla fermata "Giardini Margherita" e/o "Porta Santo Stefano" (circa 20 minuti di viaggio). Entrare nei Giardini Margherita, dall'ingresso di Via Castiglione 136 o Via Polischi 9, e recarsi presso "Le Serre dei Giardini", alla struttura "La Gabbia del Leone" (http://leserre.kilowatt.bo.it/gabbia-del-leone/) - Grazie a chi: all'associazione [Kilowatt](http://kilowatt.bo.it/about) che ha messo a disposizione gratuitamente la sala. - Per iscriversi: https://www.metooo.io/e/haskell-day-bologna

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