- Beginners - Haskell.org

Configuration of Environment
by juncheng_hsk＠outlook.com 22 Nov '19

22 Nov '19

Hi All, I’m a newcomer to Haskell and mailing list, so I am not sure if my question is proper. I use notepad++ to learn Haskell now. Are there any suggestions or conventions which can help me learn Haskell? Best regards, Juncheng

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How to use the Data.Map inbuild Monoid
by Leonhard Applis 18 Nov '19

18 Nov '19

Hello Francesco, > `Map k v` is already an instance of `Monoid` (when `v` is an instance of `Ord`), are you sure you need to write another one? Thanks for your answer! My "Values" is a Monoid, however it seems that the Graph cannot properly be seen correctly as a Monoid using mconcat does not (properly) work, meaning that <> is not correctly applied to the someMonoidsso instead of type G = Map Text someMonoid mconcat [gs] I have to write: foldr (Map.unionWith (<>)) Map.empty [gs] This passes the tests. Same Problem with g1 <> g2 --(Does not work properly) and Map.unionWith (<>) g1 g2 --(Does work) I have declared someMonoid myself, do I need to declare something special about it? I feel like my G is messing up with <> being about him, or about someMonoid A broken down piece of code is: importData.MapasMap dataSum=SumIntderiving (Eq,Show) instanceSemigroupSumwhere (<>) (Sum a) (Sum b)=Sum(a + b) instanceMonoidSumwhere mempty =Sum0 typeG=Map.MapStringSum And to verify my problem: GHCI > v = Map.singleton "A" (Sum 1) GHCI > u = Map.singleton "A" (Sum 1) GHCI> c= v <> u GHCI> Map.lookup "A" c Just (Sum 1) but I want Just (Sum 2) thanks Leonhard

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Export two modules from one.hs file?
by Leonhard Applis 18 Nov '19

18 Nov '19

Hey All, I currently want to make a module, which "truly" exposes 3 functions. However, there are more functions which I want to test. I would like to export two modules from the same file, "ModuleToUse" and "ModuleOpen". Then I can use ModuleToUse everywhere else, and ModuleOpen in the UnitTests. I know I can declare ModuleOpen, and export only certain functions with ModuleToUse. But I am not sure, if that is a smart / normal thing to do. In my case this would double my modules, and ... hence files? What is the "real world" approach for this? Thanks Leonhard

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importing Data.Digits
by Alexander Chen 18 Nov '19

18 Nov '19

Hi All, This is my first 'post' not quite sure if I am doing it right. I have a question about the importing of modules that are available on the Hackage.Haskell site: http://hackage.haskell.org/package/digits if I write this in my wordpad and then load it to GCI i get the following error: chapter4.hs:1:1: error: Could not find module ‘Data.Digits’ Perhaps you meant Data.Bits (from base-4.12.0.0) Use -v to see a list of the files searched for. | 1 | import Data.Digits | ^^^^^^^^^^^^^^^^^^ However if I load Data.Char than it loads fine. What am I doing wrong? best,

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Re: [Haskell-beginners] Expose every Function of a wrapped type
by 鲍凯文 16 Nov '19

16 Nov '19

Hi, Have you considered using `Control.Lens.Wrapped` or `Control.Newtype`? You'd have to move the wrapping/unwrapping bits to the sites in your code at which you use underlying `Map` functions (quite repetitive), but it could be easier than redefining all the functions you want (and definitely easier than having to redefine a new one when you think of needing it?). There's probably a canonical way to handle this sort of thing, but I can't find it ATM. Good luck! On Fri, Nov 15, 2019 at 5:00 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. Expose every Function of a wrapped type (Leonhard Applis) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 15 Nov 2019 11:36:03 +0000 > From: Leonhard Applis <Leonhard.Applis(a)protonmail.com> > To: "beginners(a)haskell.org" <beginners(a)haskell.org> > Subject: [Haskell-beginners] Expose every Function of a wrapped type > Message-ID: > > <8VyrlRTZwYrqqkI95kJ3UHQ41YSXFwhcqA4orOfVyau1ULCyCnVVkPI__Ya6rLaYFCUrVB3yM0IqT_l5NcCiMHhyfjsRg0S2xmxG3Cmvkq0=@ > protonmail.com> > > Content-Type: text/plain; charset="utf-8" > > Hi, > > I currently have a newtype definition of a typed Data.Map. > > newtype G = G Data.Map String Values > > I have done this because for my specific use-case, I can define G as a > monoid and get some nice benefits. > > However, I find myself often just re-declaring functions of Data.Map and > wrap/unwrap them in my Constructor G, such as > > singleton :: String -> Value -> G > singleton s v = G $ (Data.Map.singleton s v) > > I think I do not need to limit the scope of G's functionality, I basically > want everything from Data.Map + Bonus , like my Monoid. > Also I want G to be a specific Data.Map, there is never any other case > where it's not a Map String Values. > > However, if I have > > type G2 = Data.Map String Values > > I cannot instance Monoid. > > So, I am very sure that I'm not the first person ever doing this. Is there > a language extension providing what I want? Or is what i want "bad"? > Am I just missing a higher-order base function which does what I want? > > best regards & thanks > Leonhard >

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Expose every Function of a wrapped type
by Leonhard Applis 15 Nov '19

15 Nov '19

Hi, I currently have a newtype definition of a typed Data.Map. newtype G = G Data.Map String Values I have done this because for my specific use-case, I can define G as a monoid and get some nice benefits. However, I find myself often just re-declaring functions of Data.Map and wrap/unwrap them in my Constructor G, such as singleton :: String -> Value -> G singleton s v = G $ (Data.Map.singleton s v) I think I do not need to limit the scope of G's functionality, I basically want everything from Data.Map + Bonus , like my Monoid. Also I want G to be a specific Data.Map, there is never any other case where it's not a Map String Values. However, if I have type G2 = Data.Map String Values I cannot instance Monoid. So, I am very sure that I'm not the first person ever doing this. Is there a language extension providing what I want? Or is what i want "bad"? Am I just missing a higher-order base function which does what I want? best regards & thanks Leonhard

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What is this?
by Dimitri DeFigueiredo 13 Nov '19

13 Nov '19

Hello, Can anyone shed a light on the proper way to write the function below. It looks pretty standard, but it's not clear to me what it is? data Abortable res = Aborted | Executed res deriving Functor -- i.e. Maybe abortableThroughEither :: Abortable (Either a b) -> Either a (Abortable b) abortableThroughEither Aborted = Right Aborted abortableThroughEither (Executed (Left a)) = Left a abortableThroughEither (Executed (Right b)) = Right (Executed b) Thanks! Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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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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