October 2019 - Beginners

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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