October 2013 - Beginners

Re: [Haskell-beginners] monads do not fit together?
by Brandon Allbery 15 Oct '13

15 Oct '13

On Tue, Oct 15, 2013 at 5:00 AM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > I wonder, do you always have to create a monadtransformer if your program > uses 2 kinds of IO? > In one sense the answer is "yes"; but the real answer is "that's not how it works". There is only one IO. Monad transformers allow you to add functionality on top of it, but at some point you need to do actual I/O in the IO monad. One reason it's done this way is that CGIT could in theory be written such that you could swap out the raw IO monad for something that lets you do testing. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

1 0

Re: [Haskell-beginners] monads do not fit together?
by Brandon Allbery 15 Oct '13

15 Oct '13

On Mon, Oct 14, 2013 at 10:58 AM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > I get the same message (Couldn't match expected type `CGIT IO a0' with > actual type `IO [String]'). I think I do understand where this message is > coming from. But what can I do to fix it? > For now, you just correct the type of runRefreshFirewall; it should be runRefreshFirewall :: String -> CGIT IO [String] If you are intending to run actual IO actions in the real thing, you'll need to use liftIO to "reach" the IO embedded in the CGIT IO a. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

2 1

Re: [Haskell-beginners] Exercise of "Programming with Arrows"
by Kim-Ee Yeoh 14 Oct '13

14 Oct '13

On Tue, Oct 8, 2013 at 4:08 AM, Thiago Negri <evohunz(a)gmail.com> wrote: > I agree with "no-op assembly line", but when I'm using `first` on a processor, I want to process the first stream *only*. The second stream should remain as it was not touched, so future processors will receive the same sequence from the second stream. We are in violent agreement! > I mean, I think I need to guarantee that this definition holds: > > `g *** f` is the same as `first g >>> swap >>> first f >>> swap` Excellent! Let's turn to laws and definitions to be _precise_ in what we're saying. > If my implementation of `first` uses a real no-op assembly line for `c` (i.e., `arr id`), then I would lose the stream. To that end, consider this easy problem: using (1) the arrow laws that you know off the top of your head, and (2) the above definition of (***) can you show that g *** (arr id) = first g? Because arr id, as you noted, is a no-op. Do you see what's going on? -- Kim-Ee

1 0

Re: [Haskell-beginners] monads do not fit together?
by Kim-Ee Yeoh 14 Oct '13

14 Oct '13

On Mon, Oct 14, 2013 at 6:51 PM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > The error at the problem line is: parse error on input `<-' **** > > If I move the line above the ’if’ I get: Couldn't match expected type > `CGIT IO t0' with actual type `IO [String]'. **** > Looks like you're missing a "do". You're writing code like this: if blah then x <- foo bar x else quux when you need to write if blah then do x <- foo bar x else quux Recommended exercises: * can you see in your mind's eye the desugaring of do-syntax? * how would you write it in a single line, i.e. without using indentation a.k.a. "layout" -- hint: look up curly braces -- Kim-Ee

2 1

monads do not fit together?
by Kees Bleijenberg 14 Oct '13

14 Oct '13

The program belows reads the get fields 'un' and 'ip'. If these get fields both exist, the program modifies a firewall script. The problem is in cgiMain. The error at the problem line is: parse error on input `<-' If I move the line above the 'if' I get: Couldn't match expected type `CGIT IO t0' with actual type `IO [String]'. How can I extract values from the IO [String] monad in cgiMain? I don't want to alter the type of runRefreshFirewall. Kees module Main( main ) where import Network.CGI import Text.XHtml import Text.XHtml.Transitional import Data.Maybe runRefreshFirewall :: String -> String -> IO [String] runRefreshFirewall un ip = do return ["test" ] inputCgiOkPage :: String -> String -> [String] -> Html inputCgiOkPage un ip msgs = body << h1 << ("IP: " ++ (show ip)) undefinedFieldsPage :: Html undefinedFieldsPage = body << h1 << "Undefined Fields" cgiMain :: CGI CGIResult cgiMain = do maybeUn <- getInput "un" maybeIp <- getInput "ip" if ((isJust maybeUn) && (isJust maybeIp)) then msgs <-runRefreshFirewall (fromJust maybeUn) (fromJust maybeIp) -- problem output $ renderHtml $ inputCgiOkPage (fromJust maybeUn) (fromJust maybeIp) msgs else output $ renderHtml undefinedFieldsPage main :: IO () main = runCGI $ handleErrors cgiMain

1 0

Control.Exception.bracket-like for MonadIO ?
by Olivier Iffrig 14 Oct '13

14 Oct '13

Hello, I'm trying to build an application with a monad encapsulating its state, environment, ... For the sake of simplicity, let's say data MyState type Main = StateT MyState IO But I would like to use something like bracket with my Main monad, and I did not find how to to it. I'm looking for something like bracket' :: MonadIO m => m a -> (a -> m b) -> (a -> m c) -> m c or in my case myBracket :: Main a -> (a -> Main b) -> (a -> Main c) -> Main c Searching for that on hoogle did not give me anything interesting... Did someone encounter this problem, and have a solution ? -- Olivier

2 2

Turing machine that decides if a string is an element of an-bn-cn
by Costello, Roger L. 13 Oct '13

13 Oct '13

Hi Folks, I implemented, in Haskell, a Turing Machine that decides if a string is a member of the language consisting of an equal number of a's, b's, and c's, i.e., anbncn. http://www.xfront.com/Turing-Machines/an-bn-cn/TM.hs.txt /Roger

2 1

daemons and signals
by Jeff Lasslett 13 Oct '13

13 Oct '13

Hello, I'm trying to write a daemon that handles some signals. My problem is currently with the handling of posix signals. I don't think my signal handlers are being run. All they do at present is write to syslog and a file (for diagnostic purposes). I'm not seeing anything in /var/log/system.log (I'm on a mac). My code is attached. Any advice would be greatly appreciated. Thanks, Jeff

2 1

cabal issues
by emacstheviking 12 Oct '13

12 Oct '13

Fresh install via homebrew on osx, brew doctor reports no problems. After installing ghc and cabal-install I then tried a cabal update and it failed because of permissions issues in the ~/.cabal folder I fixed that as stated on the brew site and tried again, no joy... Last 10 lines of the build log ( /Users/seancharles/.cabal/logs/OpenGLRaw-1.4.0.0.log ): cabal: /Users/seancharles/.cabal/logs/OpenGLRaw-1.4.0.0.log: permission denied I also did a Verify and Repair from Disk Utilities just in case, that fixed some things but no this. Have I done something wrong or is there an issue with cabal because every time I do a cabal update it seems to me the problem keeps coming back?!?! This is very frustrating because I initially installed the latest Haskell Platform for OSX .pkg file and had exactly the same problems. Sometimes I despair at how hard it seems to get a working Haskell installation...I never have these problems on Linux, only OSX and I don't know why! Thanks in advance. Sean. Some information (I did notice that seancharles:staff and seancharles:admin are in existence) that may help. I have no idea about :staff and :admin, not seen that before! all of the log file: Installing library in /Users/seancharles/.cabal/lib/x86_64-osx-ghc-7.6.3/OpenGLRaw-1.4.0.0 setup-Cabal-1.16.0-x86_64-osx-ghc-7.6.3: dist/build/HSOpenGLRaw-1.4.0.0.o: permission denied ghc --version The Glorious Glasgow Haskell Compilation System, version 7.6.3 Seans-iMac:~ seancharles$ cabal --version cabal-install version 1.18.0.2 using version 1.18.1 of the Cabal library Folder permissions: drwxr-xr-x 9 seancharles staff 306 12 Oct 20:37 .cabal brew folder: drwxr-xr-x 6 seancharles admin 204 12 Oct 20:34 Cellar

1 0

Re: [Haskell-beginners] Error when starting GHCI (Arie van Wingerden)
by Arie van Wingerden 12 Oct '13

12 Oct '13

Hi Howard, thanks for your help. I guess this is a nasty problem which may take quit some time to solve. Since I only wish to learn a new functional language, I'll try my hands on F# for now. Perhaps later on this problem is solved and I will retry starting with Haskell. For now i wish you luck and say goodbye. 2013/10/11 Arie van Wingerden <xapwing(a)gmail.com> > C:\Windows\system32>cabal update > Downloading the latest package list from hackage.haskell.org > cabal: Failed to download > http://hackage.haskell.org/packages/archive/00-index.tar.gz : ErrorMisc > "Unsucessful HTTP code: 404" > > Which is another error; sigh... > > I already uninstalled 2 applications that might have interfered; but the > problems remain. > > > 2013/10/11 Howard B. Golden <howard_b_golden(a)yahoo.com> > > Arie, >> >> Try this from the command line: >> >> cabal update >> cabal install hs-dotnet >> >> That should install hs-dotnet. (I'm not sure that will solve your ghci >> problem, but it's worth a try.) >> >> Howard >> >> ------------------------------ >> *From:* Arie van Wingerden <xapwing(a)gmail.com> >> *To:* Howard B. Golden <howard_b_golden(a)yahoo.com> >> *Cc:* "beginners(a)haskell.org" <beginners(a)haskell.org> >> *Sent:* Friday, October 11, 2013 1:03 AM >> >> *Subject:* Re: [Haskell-beginners] Error when starting GHCI (Arie van >> Wingerden) >> >> Hi Howard, >> >> maybe you are spot on! >> I only have this: "C:\Users\Arie\AppData\Roaming\ghc" only containing >> ghci_history !! >> But how to repair this? >> >> The install gave me no errors. >> >> Kind regards, >> Arie >> >> >> 2013/10/10 Howard B. Golden <howard_b_golden(a)yahoo.com> >> >> Arie, >> >> The only difference I have with your output on my computer is a line >> after the "Using binary package database..." line: >> >> Using binary package database: >> C:\Users\hgolden\AppData\Roaming\ghc\i386-mingw32-7.6.3\package.conf.d\package.cache >> >> That package cache contains hs-dotnet-0.4.0. >> >> I have no idea if this is why you are getting the error message at the >> end ("Loading package base ... "). >> >> Did you get any error messages when you installed the Haskell Platform? >> >> Howard >> >> ------------------------------ >> *From:* Arie van Wingerden <xapwing(a)gmail.com> >> *To:* Howard B. Golden <howard_b_golden(a)yahoo.com>; The >> Haskell-Beginners Mailing List - Discussion of primarily beginner-level >> topics related to Haskell <beginners(a)haskell.org> >> *Sent:* Wednesday, October 9, 2013 11:41 AM >> *Subject:* Re: [Haskell-beginners] Error when starting GHCI (Arie van >> Wingerden) >> >> Hi, >> following Howards guidance I tried to start GHCI with the -v option and >> get: >> >> GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help >> Glasgow Haskell Compiler, Version 7.6.3, stage 2 booted by GHC version >> 7.4.1 >> Using binary package database: C:\Program Files\Haskell >> Platform\2013.2.0.0\lib\ >> package.conf.d\package.cache >> wired-in package ghc-prim mapped to >> ghc-prim-0.3.0.0-9d603ae4e062e1401099a3daa89 >> 31d0f >> wired-in package integer-gmp mapped to >> integer-gmp-0.5.0.0-4093e6ff1e8ebdba04ec8 >> 92df9ac108f >> wired-in package base mapped to >> base-4.6.0.1-f0c2cc6dcf0e12bf75312a2e7f354095 >> wired-in package rts mapped to builtin_rts >> wired-in package template-haskell mapped to >> template-haskell-2.8.0.0-42f1e6e5551 >> 1ef4dff7e4249580faf8f >> wired-in package dph-seq not found. >> wired-in package dph-par not found. >> Hsc static flags: -static >> Loading package ghc-prim ... linking ... done. >> Loading package integer-gmp ... linking ... done. >> *** gcc: >> "C:\Program Files\Haskell Platform\2013.2.0.0\lib/../mingw/bin/gcc.exe" >> "-fno-st >> ack-protector" "-Wl,--hash-size=31" "-Wl,--reduce-memory-overheads" >> "-LC:\Progra >> m Files\Haskell Platform\2013.2.0.0\lib\base-4.6.0.1" "--print-file-name" >> "wsock >> 32.dll" >> *** gcc: >> "C:\Program Files\Haskell Platform\2013.2.0.0\lib/../mingw/bin/gcc.exe" >> "-fno-st >> ack-protector" "-Wl,--hash-size=31" "-Wl,--reduce-memory-overheads" >> "-LC:\Progra >> m Files\Haskell Platform\2013.2.0.0\lib\base-4.6.0.1" "--print-file-name" >> "user3 >> 2.dll" >> *** gcc: >> "C:\Program Files\Haskell Platform\2013.2.0.0\lib/../mingw/bin/gcc.exe" >> "-fno-st >> ack-protector" "-Wl,--hash-size=31" "-Wl,--reduce-memory-overheads" >> "-LC:\Progra >> m Files\Haskell Platform\2013.2.0.0\lib\base-4.6.0.1" "--print-file-name" >> "shell >> 32.dll" >> Loading package base ... ghc.exe: : Kan opgegeven module niet vinden. >> *** Deleting temp files: >> Deleting: >> *** Deleting temp dirs: >> Deleting: >> <command line>: can't load .so/.DLL for: (addDLL: could not load DLL) >> >> Any ideas why this happens? >> TIA, >> Arie >> >> >> 2013/10/8 Howard B. Golden <howard_b_golden(a)yahoo.com> >> >> Hi Arie, >> >> My guess is that you don't have a necessary library in your path. Here >> are the Haskell libraries I have in my path (change as necessary for your >> setup): >> >> C:\Program Files\Haskell Platform\2013.2.0.0\lib\extralibs\bin >> C:\Program Files\Haskell Platform\2013.2.0.0\bin >> C:\Program Files\Haskell Platform\2013.2.0.0\mingw\bin >> >> HTH, >> >> Howard >> >> ------------------------- >> Message: 2 >> >> Date: Tue, 8 Oct 2013 18:12:41 +0200 >> From: Arie van Wingerden <xapwing(a)gmail.com> >> To: beginners(a)haskell.org >> Subject: [Haskell-beginners] Error when starting GHCI >> Message-ID: >> <CADkALSHpG7s89tT7rXJw56q7hkDyEcwv1=xu5Tg3_qLqoqtDxg(a)mail.gmail.com> >> Content-Type: text/plain; charset="iso-8859-1" >> >> Hi, >> >> today I downloaded the Haskell platform for 32 bits Windows >> (HaskellPlatform-2013.2.0.0-setup.exe) and installed it. >> >> The first time I started GHCI all worked perfectly. >> >> When I closed GHCI and later tried to start it again I constantly get >> however: >> C:\Users\Arie>ghci >> GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help >> Loading package ghc-prim ... linking ... done. >> Loading package integer-gmp ... linking ... done. >> Loading package base ... ghc.exe: : Kan opgegeven module niet vinden. >> <command line>: can't load .so/.DLL for: (addDLL: could not load DLL) >> WHERE Kan opgegeven module niet vinden MEANS Can't find the specified >> module >> >> Any ideas? >> >> Kind regards, >> Arie >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> >> >> >> >> >> >> >> >> >

1 0