- Beginners - Haskell.org

Putrady 2018 Ch. 3 - STM
by Site Administrator 28 Oct '19

28 Oct '19

Hello, I am new not only to Haskell, but to programming in general. I hope that I am posting in the correct forum. I am working through "Practical Web Development With Haskell" by Ecky Putrady (2018), New York: Apress, and I am having some difficulty understanding the STM portion of Chapter three. More specifically, I cannot get the following code snippet to return the expected result in the REPL: -- snippet from Adapter.InMemory.Auth module as composed on pages 53-64 in the book import ClassyPrelude import qualified Domain.Auth as D -- as composed on pages 35-52 in the book import Conrol.Monad.Except import Text.StringRandom import Data.Has -- ... data State = State { stateAuths :: [(D.UserId, D.Auth)] -- ... initialState :: State initialState = State { stateAuths = [] -- ... type InMemory r m = (Has (TVar State) r, MonadReader r m, MonadIO m) addAuth :: InMemory r m => D.Auth -> m (Either D.RegistrationError D.VerificationCode) addAuth auth = do tvar <- asks getter -- ... rest of function These are the instructions for "Verification in REPL" provided on page 64 of the book: > :l Adapter.InMemory.Auth > let email = D.mkEmail "ecky(a)test.com" > let passw = D.mkPassword "1234ABCDefgh" > let auth = either undefined id $ D.Auth <$> email <*> passw > s <- newTVarIO initialState > addAuth s auth Calling the addAuth function is supposed to print the following: Right "aBNhtG653Bga9kas" -- (or whatever random vCode stringRandomIO generates) However, GHCI instead tells me: <interactive>:6.9: error: * Couldn't match expected type 'D.Auth' with actual type 'TVar State' * In the first argument of 'addAuth', namely 's' In the expression: addAuth s auth -- ... The source code for the book at https://github.com/Apress/practical-webdev-haskell was created with stack resolver lts-9.11, while I am using resolver lts-14.11. I am doing this intentionally, because debugging the compilation errors helps guide me to become more familiar with the documentation in the packages being implemented, as well as helping me to understand general Haskell concepts little by little. I have tried looking through the changelogs for the stm and classy-prelude packages on https://hackage.haskell.org, for any clues which would be intelligible to me (an extremely restrictive constraint, admittedly), that would help me understand why the addAuth function thinks the TVar State we pass to it should be a D.Auth, and why this presumably would not have been the case had I built the project with the lts-9.11 resolver, but I have not been able to understand it thus far.

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Haskell Question
by instanct95＠aol.com 18 Oct '19

18 Oct '19

I need help with this haskell functionWrite a function productLastPart which, only using library functions, returns the product of thelast n numbers in the list, where n is the first argument to the function. productLastPart :: Int -> [Int] -> Int

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How to use GHC.Event module, main thread not exit directly
by BalterNotz＠foxmail.com 18 Oct '19

18 Oct '19

Hello everyone: I write a server use GHC.Event module, Refer to this article https://wiki.haskell.org/Simple_Servers {-# LANGUAGE OverloadedStrings #-} -- A simple example of an epoll based http server in Haskell. -- -- Uses two libraries: -- * network-bytestring, bytestring-based socket IO. -- - cabal install network-bytestring: -- -- * haskell-event, epoll-based scalable IO events -- - git clone git://github.com/tibbe/event.git -- - autoreconf ; then cabal install import Network hiding (accept) import Network.Socket (fdSocket, accept) import Network.Socket.ByteString import Data.ByteString.Char8 import System.Event import System.Posix import System.Posix.IO main = withSocketsDo $ do sock <- listenOn $ PortNumber 5002 let fd = fromIntegral (fdSocket sock) mgr <- new registerFd mgr (client sock) fd evtRead loop mgr client sock _ _ = do (c,_) <- accept sock sendAll c msg sClose c msg = "HTTP/1.0 200 OK\r\nContent-Length: 5\r\n\r\nPong!\r\n" but I not find the definition of 'loop' function: loop mgr the after is my program: btnz@vmubuntuserver:~/work/code/echo-server-event-model$ cat Main.hs module Main where import Network.Socket import Network.Socket.ByteString (recv, sendAll) import GHC.Event as Event import qualified Control.Exception as E (bracket) import Data.ByteString.Internal (packChars) hints = defaultHints {addrFlags = [AI_PASSIVE], addrSocketType = Stream} msg = "HTTP/1.0 200 OK\r\nContent-Length: 5\r\n\r\nPong!\r\n" main :: IO () main = do putStrLn "Hello, Haskell!" addr <- head <$> getAddrInfo (Just hints) Nothing (Just "3000") E.bracket (sock addr) close eventServer return () where sock addr = do s <- socket (addrFamily addr) (addrSocketType addr) (addrProtocol addr) setSocketOption s ReuseAddr 1 withFdSocket s setCloseOnExecIfNeeded bind s $ addrAddress addr listen s 1024 return s eventServer s = do fd <- withFdSocket s (\ci -> return (fromIntegral ci)) mgr <- Event.new Event.registerFd mgr (service s) fd Event.evtRead Event.OneShot -- loop mgr service :: Socket -> FdKey -> Event -> IO () service s _ _ = do (c, _) <- accept s sendAll c $ packChars msg close c btnz@vmubuntuserver:~/work/code/echo-server-event-model$ cabal run Up to date Hello, Haskell! when I run cabal run the program exit directly. BalterNotz(a)foxmail.com

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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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