- Beginners - Haskell.org

incoherent instance question
by Graham Gill 08 Jun '18

08 Jun '18

Please see the paste: https://pastebin.com/zBim7Zkx I'm experimenting with defining UpperBounded and LowerBounded typeclasses. An example type belonging to the latter that is not also Bounded would be type Natural from Numeric.Natural. I want to say that if a type is Bounded, then it is also UpperBounded and LowerBounded. If a type is both UpperBounded and LowerBounded, then it is also Bounded. To express the constraints, I need FlexibleInstances and UndecidableInstances extensions. These allow the module to load into ghci (8.4.2) with only a warning, but, without the INCOHERENT pragmas, I get an overlapping instance error if I try to evaluate minBound, maxBound, upperBound or lowerBound instantiated to either of the types Foo or Bar. A solution is to apply the INCOHERENT pragma to the instances at lines 11, 14 and 17. Reading over section 10.8.3.6. Overlapping instances in the GHC User Guide, I believe I understand. (Is there a better solution?) In the paste, I have INCOHERENT pragmas only at lines 11 and 17. This gives me the following behaviour in ghci: 1. minBound, maxBound, upperBound and lowerBound instantiated to type Foo all function as expected, evaluating to the appropriate lower or upper bound. 2. upperBound and maxBound instantiated at Bar give overlapping instance errors for UpperBounded, as expected. 3. lowerBound :: Bar evaluates to C, as expected. 4. minBound :: Bar gives an overlapping instance error for UpperBounded: *UpperLowerBounded> minBound :: Bar <interactive>:141:1: error: • Overlapping instances for UpperBounded Bar arising from a use of ‘minBound’ Matching instances: instance [safe] Bounded a => UpperBounded a -- Defined at UpperLowerBounded.hs:14:10 instance [safe] UpperBounded Bar -- Defined at UpperLowerBounded.hs:31:10 • In the expression: minBound :: Bar In an equation for ‘it’: it = minBound :: Bar It's #4 that I don't understand. An explanation would be very much appreciated. (Also, what's a [safe] instance?) Regards, Graham

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How to get IO String from Network.Socket.ByteString.recv method
by Dinesh Amerasekara 05 Jun '18

05 Jun '18

Hi, I am unable to compile the below code. import Network.Socket hiding(recv) import Network.Socket.ByteString as S (recv) import qualified Data.ByteString.Lazy.Char8 as Char8 getMessage :: Socket -> IO String getMessage sock = Char8.unpack <$> S.recv sock 8888 It gives the below error. Couldn't match type ‘Data.ByteString.Internal.ByteString’ with ‘ByteString’NB: ‘ByteString’ is defined in ‘Data.ByteString.Lazy.Internal’ ‘Data.ByteString.Internal.ByteString’ is defined in ‘Data.ByteString.Internal’ Expected type: IO ByteString Actual type: IO Data.ByteString.Internal.ByteString In the second argument of ‘(<$>)’, namely ‘recv sock 8888’In the expression: unpack <$> recv sock 8888In an equation for ‘getMsg’: getMsg sock = unpack <$> recv sock 8888 Can somebody tell me how I can return the IO String using Network.Socket.ByteString.recv? Best Regards,Dinesh.

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ghc-8.4.3 at Ubuntu
by Andreas Röhler 04 Jun '18

04 Jun '18

While installing ghc-8.4.3-i386-deb8-linux.tar.xz at Debian8 went smoothly, can't se how to install ghc-8.4.3 at Ubuntu resp. Xubuntu: in dir ghc-8.4.3/ ./configure checking for gfind... no checking for find... /usr/bin/find checking for sort... /usr/bin/sort checking for GHC Git commit id... given 51abb1c88b53e2989a2a8c2939ac4abc04bef194 checking for ghc... /usr/bin/ghc checking version of ghc... 7.10.3 configure: error: GHC version 8.0 or later is required to compile GHC. Did cabal update and tried haskell-platform for ubuntu https://www.haskell.org/platform/linux.html#linux-ubuntu which seems broken Tried Haskell-platform from source: ./platform.sh *** *** Building hptool *** cabal: unrecognised command: new-build (try --help) Any help? Cheers, Andreas

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Conflict with global lib from sandbox
by Andreas Röhler 28 May '18

28 May '18

Hi, called "cabal sandbox init" from inside an empty directory. When running cabal install... see errors like that: /home/my-name/.cabal/lib/i386-linux-ghc-7.6.3/unix-2.7.2.2/libHSunix-2.7.2.2.a(HsUnix.o):(.text+0x20): first defined here /usr/lib/ghc/unix-2.6.0.1/libHSunix-2.6.0.1.a(HsUnix.o): In function `__hsunix_grantpt': Shouldn't /usr/lib/... be hidden from inside a sandbox? Thanks, Andreas

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Re: [Haskell-beginners] ghci timings
by Doug McIlroy 25 May '18

25 May '18

> Just use last to print the last in the list. Because of lazy evaluation, this doesn't guarantee that the earlier elements have been computed. You might use "sum" to assure that they are. Doug

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ghci timings
by mike h 25 May '18

25 May '18

Hi, I have a function that takes a while to execute and its output is a list of 1000 numbers. I’ve setup ghci to show timings. So in ghci I run the function, it churns away and then starts display the output list and finally shows the timings. I’m assuming that the execution time includes the time taken to output the result. Is it possible to get the execution time that does not include the time to output the result? Thanks Mike

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Error stack install ghc-mod
by Dinesh Amerasekara 24 May '18

24 May '18

Hi, When I executed the command stack install ghc-mod, it gave me the following error. In the dependencies for ghc-mod-5.8.0.0: Cabal-2.0.1.1 from stack configuration does not match >=1.18 && <1.25 (latest matching version is 1.24.2.0) base-4.10.1.0 from stack configuration does not match >=4.6.0.1 && <4.10 (latest matching version is 4.9.1.0) cabal-helper must match <0.8 && >=0.7.3.0, but the stack configuration has no specified version (latest matching version is 0.7.3.0) extra-1.6.2 from stack configuration does not match <1.6 && >=1.4 (latest matching version is 1.5.3) ghc-8.2.2 from stack configuration does not match >=7.6 && <8.2 monad-journal-0.8.1 from stack configuration does not match <0.8 && >=0.4 (latest matching version is 0.7.2) optparse-applicative-0.14.0.0 from stack configuration does not match >=0.13.0.0 && <0.14 (latest matching version is 0.13.2.0) needed since ghc-mod is a build target. Can I get an advice to solve this error? Thanks And Best Regards,Dinesh

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Understanding type variables in Haskell class declarations
by PATRICK BROWNE 21 May '18

21 May '18

I am trying to understand how to interpret type variables in Haskell class declarations from a paper <http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=1EA31D76684217BC72…> . With respect to the code below I have the following questions: 1. What is the difference between "a b" in the header of the Container class and "a b" in the signature of the class methods? Does the whitespace in header mean 2 distinct types and in the methods mean function application? 2. In the Boathouse class what is the difference between the first ocurance of "b p" and the second bracketed "(b p)"? 3. When I try to make instance of these classes I seem to need FlexibleInstances. Why is this? {-# LANGUAGE MultiParamTypeClasses #-} -- Containers a b stands for all container types a holding things of type b. -- from :info command a has kind *->*, b has kind * class Containers a b where insert :: b -> a b -> a b remove :: b -> a b -> a b whatsIn :: a b -> [b] class Surfaces a b where put :: b -> a b -> a b takeOff :: b -> a b -> a b whatsOn :: a b -> [b] -- from :info command "p" has kind *, "h" and "b" have kind *->* class People p class Containers h p => Houses h p where class (People p, Surfaces h p) => Boats h p where class (Boats b p,Houses h (b p)) => BoatHouses h b p where class (People p, Houses h (b p),Boats b p) => HouseBoats h b p -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie <http://www.dit.ie/> Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie <http://www.dit.ie/> Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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merge error
by trent shipley 18 May '18

18 May '18

The below produces an error. And I am very proud that I could use the GHCi debugging tools to get this far. merge [] [] works. merge [1] [] works. I don't know why the failing example fails. It should return: [4,5] Help to unstuck is appreciated. :step merge [4,5] [] *** Exception: ex6_8.hs:(12,1)-(16,66): Non-exhaustive patterns in function merge Given: merge :: Ord a => [a] -> [a] -> [a] merge [] [] = [] merge [x] [] = [x] merge [] [y] = [y] merge first@(x:xs) second@(y:ys) | x <= y = x : merge xs second | otherwise = y : merge first ys

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ANNOUNCE: Haskell Communities and Activities Report (34th ed., May 2018)
by Mihai Maruseac 15 May '18

15 May '18

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (34th edition, May 2018) is now available, in PDF and HTML formats: http://haskell.org/communities/05-2018/report.pdf http://haskell.org/communities/05-2018/html/report.html All previous editions of HCAR can be accessed on the wiki at https://wiki.haskell.org/Haskell_Communities_and_Activities_Report Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of September 2018: target deadline for contributions to the October 2018 edition of the HCAR Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in November (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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