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Today's Topics:
1. How to use GHC.Event module, main thread not exit directly
(baltern...@foxmail.com)
2. Re: Haskell Question (Seph Shewell Brockway)
3. Re: Haskell Question (Ut Primum)
4. Re: Haskell Question (Seph Shewell Brockway)
----------------------------------------------------------------------
Message: 1
Date: Fri, 18 Oct 2019 23:53:42 +0800
From: "baltern...@foxmail.com" <baltern...@foxmail.com>
To: beginners <beginners@haskell.org>
Subject: [Haskell-beginners] How to use GHC.Event module, main thread
not exit directly
Message-ID: <201910182353417471...@foxmail.com>+8C573A28B77CA83E
Content-Type: text/plain; charset="utf-8"
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.
baltern...@foxmail.com
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Message: 2
Date: Fri, 18 Oct 2019 17:16:41 +0100
From: Seph Shewell Brockway <s...@codex.scot>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Haskell Question
Message-ID: <20191018161641.pl5zbgzd2z45bxzm@leviathan>
Content-Type: text/plain; charset=utf-8
On Wed, Oct 16, 2019 at 09:09:40PM +0000, instanc...@aol.com wrote:
> 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
It would probably help you learn if you made an attempt to solve it
yourself first. If you post what you have so far, I’d be happy to take
a look.
Regards,
Seph
--
Seph Shewell Brockway, BSc MSc (Glas.)
Pronouns: she/her
------------------------------
Message: 3
Date: Fri, 18 Oct 2019 18:59:05 +0200
From: Ut Primum <utpri...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Haskell Question
Message-ID:
<CANjDmKL2=hXqiovgTmRjMeqGtJYBYZmaLvH=jiopb9whkbg...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi,
this is my solution:
productLastPart :: Int -> [Int] -> Int
productLastPart n xs = product (take n (reverse xs))
It only uses functions product, take and reverse
Regards,
Ut
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Il giorno ven 18 ott 2019 alle ore 18:16 Seph Shewell Brockway
<s...@codex.scot> ha scritto:
> On Wed, Oct 16, 2019 at 09:09:40PM +0000, instanc...@aol.com wrote:
> > 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
>
> It would probably help you learn if you made an attempt to solve it
> yourself first. If you post what you have so far, I’d be happy to take
> a look.
>
> Regards,
>
> Seph
>
> --
> Seph Shewell Brockway, BSc MSc (Glas.)
> Pronouns: she/her
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 4
Date: Fri, 18 Oct 2019 20:44:22 +0100
From: Seph Shewell Brockway <s...@codex.scot>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Haskell Question
Message-ID: <20191018194422.z2kuwm7tcvyxor6i@leviathan>
Content-Type: text/plain; charset=utf-8
On Fri, Oct 18, 2019 at 06:59:05PM +0200, Ut Primum wrote:
> Hi,
> this is my solution:
>
> productLastPart :: Int -> [Int] -> Int
> productLastPart n xs = product (take n (reverse xs))
>
> It only uses functions product, take and reverse
> Regards,
> Ut
That should work. I’d probably use point-free style, though, and avoid
all of those brackets.
productLastPart n = product . take n . reverse
The version you’ve written runs in linear time, as would a version that
worked like this:
productLastPart n xs = product (drop (length xs - n) xs)
because reverse, length and product (actually a partial application of
foldr) are all linear in the length of the list—they access each element
once. You can see the GHC implementation of reverse at
https://hackage.haskell.org/package/base-4.12.0.0/docs/src/GHC.List.html#reverse.
This turns out to be the best we can do with a regular Haskell list,
because accessing the last element of a list is a linear-time operation:
last :: [a] -> a
last [] = error "Oh no!"
last [x] = x
last (x : xs) = last xs
There do, however, exist list types like Sequence and Vector that allow
constant-time access to both ends of the list.
Hope this is helpful.
Seph
--
Seph Shewell Brockway, BSc MSc (Glas.)
Pronouns: she/her
------------------------------
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