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Re: [Haskell-beginners] Haskell Question
by Leonhard Applis 17 Oct '19

17 Oct '19

As I think you´re learning Haskell, I´m just giving some hints: There is a function for dropping elements: http://zvon.org/other/haskell/Outputprelude/drop_f.html Also there is similiar a "take" which may helps you with "reverse". And for multiplying everything you might want to use a fold: http://zvon.org/other/haskell/Outputprelude/foldr_f.html best regards Leonhard

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Import errors from a very simple program
by John Sturdy 26 Sep '19

26 Sep '19

Hi, I'm right at the start of learning Haskell, and I decided to jump from reading LYAH to writing a simple real-world program. It's based very closely on https://github.com/haskell-hvr/cassava/blob/master/examples/NamedBasedGener… (I removed the bit that constructs the test file, as I already have a file), and I've done "cabal install cassava", first as root, and then when I first go the error, as myself using the sandbox (which didn't change the output). I first tried this with running ghc directly, and then followed the instructions on https://wiki.haskell.org/How_to_write_a_Haskell_program, and all these get the same errors: Main.hs:4:1: error: Could not find module ‘Data.Csv’ Locations searched: Data/Csv.hs Data/Csv.lhs Data/Csv.hsig Data/Csv.lhsig | 4 | import Data.Csv | ^^^^^^^^^^^^^^^ Main.hs:6:1: error: Could not find module ‘Data.Vector’ Perhaps you meant Data.Functor (from base-4.11.1.0) Locations searched: Data/Vector.hs Data/Vector.lhs Data/Vector.hsig Data/Vector.lhsig | 6 | import qualified Data.Vector as V | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ I don't understand the connection between the name "cassava" and "Data/Csv", although looking around in the directories that have been produced, I see that Data/Csv is within the cassava directories. I'd like not only to know how to fix this, but also a pointer for what I should look at to understand what is going on. This is on a Debian system. thanks, __John (completely new to Haskell, experienced in C, Lisp, Python and others, if that helps to pitch the level of answers)

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Re: [Haskell-beginners] how to convert an array of foreignPtr a into a list of Ptr a
by 鲍凯文 03 Sep '19

03 Sep '19

Hi Frederic, I tried writing my thoughts on your question in this gist here: https://gist.github.com/darcykimball/74dd7973b2dbad22b7091135a3b9d472. There's probably much more to it than what I wrote, but I'm figuring that stuff out the more I think about it. I'll try to add more when I'm convinced I get it. Best, Kevin On Tue, Aug 27, 2019 at 5:56 AM <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. how to convert an array of foreignPtr a into a list of Ptr a > (PICCA Frederic-Emmanuel) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 27 Aug 2019 07:19:25 +0000 > From: PICCA Frederic-Emmanuel > <frederic-emmanuel.picca(a)synchrotron-soleil.fr> > To: "The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell" <beginners(a)haskell.org> > Subject: [Haskell-beginners] how to convert an array of foreignPtr a > into a list of Ptr a > Message-ID: > < > A2A20EC3B8560D408356CAC2FC148E53016CF535AB(a)SUN-DAG3.synchrotron-soleil.fr> > > Content-Type: text/plain; charset="us-ascii" > > Hello, I am writing some code whcih use FFI. > > one of my C functions has an array of pointer has imput > > extern HklBinocularsCube *hkl_binoculars_cube_new(int32_t n_spaces, > > const HklBinocularsSpace **spaces); > > > from Haskell, I have a list of ForeignPtr > so my question how can I have something like withForeignPtr for an array > of ForeignPtr > > [ForeignPtr a] -> ([Ptr a] -> IO b) -> IO b > > Thanks for your help. > > Frederic > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > ------------------------------ > > End of Beginners Digest, Vol 134, Issue 4 > ***************************************** >

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how to convert an array of foreignPtr a into a list of Ptr a
by PICCA Frederic-Emmanuel 27 Aug '19

27 Aug '19

Hello, I am writing some code whcih use FFI. one of my C functions has an array of pointer has imput extern HklBinocularsCube *hkl_binoculars_cube_new(int32_t n_spaces, const HklBinocularsSpace **spaces); from Haskell, I have a list of ForeignPtr so my question how can I have something like withForeignPtr for an array of ForeignPtr [ForeignPtr a] -> ([Ptr a] -> IO b) -> IO b Thanks for your help. Frederic

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Network PortNumber deprecation warning
by Jeff C. Britton 16 Aug '19

16 Aug '19

I have just installed Haskell using Stack on Windows. I wanted to try and write some code to do network programming with TCP/IP. I found a simple example on Stack Overflow to start me off. I'm having some problems with PortNumber and deprecated warnings. I don't know how to modify the code to be deprecation free. stack install network - installed version 2.8.0.1 ------------------------- package.yaml network >= 2.8.0.1 --------------------------- The following code gives a deprecation warning, but ultimately my sample compiles and runs. import qualified Network import qualified Network.Socket main = Network.withSocketsDo $ do handle <- Network.connectTo "192.168.1.2" (Network.PortNumber 7200) talk handle `finally` hClose handle D:\work\hsSocket\app\Main.hs:14:46: warning: [-Wdeprecations] In the use of data constructor `PortNumber' (imported from Network): Deprecated: "The high level Network interface is no longer supported. Please use Network.Socket." ----------------------------------------- So, now I try and use Network.Socket as the warning indicates. main = Network.withSocketsDo $ do handle <- Network.connectTo "192.168.1.2" (Network.Socket.PortNumber 7200) talk handle `finally` hClose handle D:\work\hsSocket\app\Main.hs:15:46: error: Not in scope: data constructor `Network.Socket.PortNumber' Module `Network.Socket' does not export `PortNumber'. | 15 | handle <- Network.connectTo "192.168.1.2" (Network.Socket.PortNumber 7200)

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Storable which is only Peekable
by PICCA Frederic-Emmanuel 04 Aug '19

04 Aug '19

Hello, I would like to know what is the best way to create a type which is only peekable and pokable. When I use Storable, I "need" to define peek and poke and ghc complain that I did not defined the poke part. So how can I write something nice. Cheers Frederic #include "hkl-binoculars.h" data Space = Space (ForeignPtr Space) Int deriving Show foreign import ccall safe "fromImage" c_fromImage :: Ptr Double -> Int -> Ptr Double -> Ptr Int -> Int -> Ptr Word16 -> Ptr Int -> Int -> IO (Ptr Space) foreign import ccall unsafe "&space_free" c_space_free :: FunPtr (Ptr Space -> IO ()) instance Storable Space where alignment _ = #{alignment struct space_t} sizeOf _ = #{size struct space_t} peek ptr = Space <$> newForeignPtr c_space_free ptr <*> #{peek struct space_t, ndim} ptr

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Stack doesn't find downloaded module
by Miguel Angel Ordoñez Silis 23 Jul '19

23 Jul '19

Hi! I downloaded the following package https://www.stackage.org/lts-13.29/package/multiset-0.3.4.1 using the following cli (Linux) command: stack install multiset. It was supposedly successfully installed but when I try to import it stack says it couldn't find it ('Could not find module ‘Data.MultiSet’). I've tried reading haskellstack.org's documentation but I've not been able to find a solution. Well, thanks in advance for your help Mike

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binding vs. parameter passing
by James Jones 22 Jul '19

22 Jul '19

One problem in *Programming Haskell from First Principles* confuses me. (So far.) It's one where you start with the declaration and binding i :: Num a => a i = 1 (which of course works because Num a => a is the type of 1) and then change the declaration to i :: a and first try to predict and then see what happens. It fails, with ghci suggesting that you put the Num a => back. That seemed reasonable at first, but then I considered this: - id has type a -> a, and if you try to evaluate id 1 it works without complaint. - In all the work I've done on compilers, parameter passing has effectively been assignment of actual parameters to the corresponding formal parameters. In Haskell, that might mean passing along the appropriate thunk, but the principle is the same, isn't it? So, if I can't bind 1 to i which is declared to have type a, why can I successfully pass 1 to id?

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Question about code I found
by Terry Phelps 17 Jul '19

17 Jul '19

I found this code on the net somewhere. It compiles and works properly: import qualified Data.ByteString as BS import Text.Printf (printf) toHex :: BS.ByteString -> String toHex bytes = do hex <- BS.unpack bytes printf "%02x" hex I cannot understand the 'do' notation is required, because it seems to be a pure function. I guess there's a monad hiding somewhere that my newbie mind can't see. So, I rewrote the code to remove the 'do stuff': -- BAD import qualified Data.ByteString as BS import Text.Printf (printf) toHex :: BS.ByteString -> String toHex bytes = printf "02x" (BS.unpack bytes) WRONG. Ghci says: xx2.hs:5:15: error: • No instance for (Text.Printf.IsChar GHC.Word.Word8) arising from a use of ‘printf’ • In the expression: printf "02x" (BS.unpack bytes) In an equation for ‘toHex’: toHex bytes = printf "02x" (BS.unpack bytes) | 5 | toHex bytes = printf "02x" (BS.unpack bytes) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Can someone please explain to me: 1. Why does this simple-looking code to convert a bytestring to a string of hex characters require 'do' or equivalent code? 2. What is the specific problem with my version of the code. The error message doesn't mean much to me yet.

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Parser using the show method
by PICCA Frederic-Emmanuel 21 Jun '19

21 Jun '19

Hello, I have a data type like this data SpaceGroup = P1 | P2 | P21 | C2 | P222 | P2221 | P21212 | P212121 | C222 | C2221 | F222 | I222 | I212121 | P4 | P41 | P42 | P43 | P422 | P4212 | P4122 | P41212 | P4222 | P42212 | P4322 | P43212 | I4 | I41 | I422 | I4122 | P3 | P31 | P32 | P312 | P321 | P3112 | P3121 | P3212 | P3221 | P6 | P61 | P62 | P63 | P64 | P65 | P622 | P6122 | P6522 | P6222 | P6422 | P6322 | R3 | R32 | P23 | P213 | P432 | P4232 | P4332 | P4132 | F23 | F432 | F4132 | I23 | I213 | I432 | I4132 deriving (Show) has you can see it is quite long. I want to write a method in order to parser a string using the show method. myparser :: Parser SpaceGroup myparser = do ... I do not want to write for each constructor something like do t <- takeText if t == "P21" then P21 else ... thanks for your help Frederic

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