February 2014 - Beginners

Data.Stream interleave implementation question
by Bryan Brady 19 Aug '14

19 Aug '14

I'm in the process of learning Haskell and am implementing my own Stream type. ( I'm working my way through homework problems I found at http://www.seas.upenn.edu/~cis194/hw/06-laziness.pdf ) One of the questions is: "Deﬁne the stream ruler :: Stream Integer which corresponds to the ruler function 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, . . . where the nth element in the stream (assuming the ﬁrst element corresponds to n = 1) is the largest power of 2 which evenly divides n. " I got pretty close, but ended up looking at Data.Stream to nail down my mistake. My original solution was: ----- data Stream a = Cons a (Stream a) deriving (Eq, Ord) streamRepeat :: a -> Stream a streamRepeat a = Cons a (streamRepeat a) interleaveStreams :: Stream a -> Stream a -> Stream a interleaveStreams (Cons a as) (Cons b bs) = Cons a (Cons b (interleaveStreams as bs)) ruler :: Stream Integer ruler = foldr1 interleaveStreams (map streamRepeat [0..]) ----- Attempting to evaluate 'ruler' in ghci does not finish. I knew something was wrong with my implementation of interleaveStreams because it worked when I expanded it out manually for a few steps. The way Data.Stream implements interleave is: interleaveStreams (Cons a as) bs = Cons a (interleaveStreams bs as) I don't understand why this works and why my original implementation doesn't. I figure if the latter works, the former should as well. Here is my reasoning: In the latter definition, Cons a (interleaveStreams bs as), (interleaveStreams bs as) is a thunk. The thunk should only be evaluated when it is needed. In my original definition, (interleaveStreams as bs) is a thunk. The difference is an extra Cons (e.g., Cons b). It seems like the additional Cons is causing problems, I just don't understand why. Can anyone point out what I'm missing? thanks, bb

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web client/server app - what to choose
by Miroslav Karpis 01 Mar '14

01 Mar '14

Hi, please can you help me with following: I would like to make a server/client web app, but I'm not sure what packages/modules would do the best. I did several client/server applications not in Haskell, where I have used REST (JSON) API as a communication between client and server. In Haskell I'm not sure what would be best, but for future use most probably also REST/JSON would be best. Server will do some realtime mathematical calculations . After its done it will send to client results. Client will visualize the results via WebGL/Threejs. I was thinking about Snap for server but for client I'm not really not sure. Any ideas/comments more than appreciated ;-) Cheers, M.

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Cabal update possible?
by Obscaenvs 28 Feb '14

28 Feb '14

Have: * The Glorious Glasgow Haskell Compilation System, version 7.6.3 * cabal-install version 1.16.0.2 using version 1.16.0 of the Cabal library Can I update to Cabal 1.18 so I can use sandboxes without breaking my dependencies? I think that that is what broke my Haskell toolchain 2h ago. Now I redid it so everything's fine, but I'd like to use sandboxes. If impossible, hsenv has worked for me in the past, but... I did not want to use Haskell Platform, since it pulls in several OpenGL libs completely unnecessary *for* and potentially detrimental *to* my server. "Haskell Platform, Server Edition", anyone? fredrik

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Failure to install cabal-install.
by Derek Mahar 27 Feb '14

27 Feb '14

How can I tell the Haskell compiler where to find module Control.Monad.Error? After building GHC 7.6.3 and Haskell Platform 2013.2 on Red Hat Linux 6, I found that 'cabal install cabal-install' fails when it attempts to build library HTTP-4000.2.11 because the Haskell compiler cannot find module Control.Monad.Error. ******************************************************* * Installation completed successfully. * * Programs installed into: * /usr/local/bin * * Now do "cabal update" to initialize the package list * * Additional packages may be found at http://hackage.haskell.org * or via "cabal list <pattern>" * * Use "cabal install <foo>" to install additional packages * ******************************************************* [dmahar@dmahar haskell-platform-2013.2.0.0]$ cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install [dmahar@dmahar haskell-platform-2013.2.0.0]$ cabal install cabal-install Resolving dependencies... Configuring HTTP-4000.2.11... Building HTTP-4000.2.11... Preprocessing library HTTP-4000.2.11... Network/Stream.hs:35:8: Could not find module `Control.Monad.Error' There are files missing in the `mtl-2.1.2' package, try running 'ghc-pkg check'. Use -v to see a list of the files searched for. Failed to install HTTP-4000.2.11 cabal: Error: some packages failed to install: HTTP-4000.2.11 failed during the building phase. The exception was: ExitFailure 1 cabal-install-1.18.0.2 depends on HTTP-4000.2.11 which failed to install. Thank you, Derek

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copy file and create directory
by Renah Scarowsky 26 Feb '14

26 Feb '14

Is there a version of copyFile that will create the directory structure if it does not exist yet? Thanks, Renah Scarowsky Suite Solutions Create>Manage>Deploy http://www.suite-sol.com<http://www.suite-sol.com/>

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Re: [Haskell-beginners] Beginners Digest, Vol 68, Issue 23
by Patrick Lynch 26 Feb '14

26 Feb '14

Good morning, Re: Today's Topic 1 - see below. My apology for not including the error message. The following is what I did: I downloaded the source code and did the following [from the O'Reilly book]: 1. download was done on a Mac. 2. I saved the file to helloWorld.hs 3. I went to my terminal screen and executed: runhaskell helloworld.hs 4. on the terminal the following appeared: hell yeah 5. so far, so good 6. I went to my browser and entered the url: http://localhost:3000 7. I was expecting to see the following on my browser: <!DOCTYPE html> <html><head><title></head><body>hell yeah</body></html> 8. Unfortunately, I get the following message: "Safari can't connect to Server Safari can't open page "http://localhost:3000" because Safari can't connect to server "localhost" Good day, Patrick ----- Original Message ----- From: <beginners-request(a)haskell.org> To: <beginners(a)haskell.org> Sent: Tuesday, February 25, 2014 7:00 AM Subject: Beginners Digest, Vol 68, Issue 23 > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/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. Re: Beginners Digest, Vol 68, Issue 22 (KMandPJLynch) > 2. Re: Beginners Digest, Vol 68, Issue 22 (Mateusz Kowalczyk) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 24 Feb 2014 20:32:40 -0500 > From: KMandPJLynch <kmandpjlynch(a)verizon.net> > To: beginners(a)haskell.org > Subject: Re: [Haskell-beginners] Beginners Digest, Vol 68, Issue 22 > Message-ID: <609E40BD-3267-4F21-88A6-1BDC4DEDC609(a)verizon.net> > Content-Type: text/plain; charset=us-ascii > > Good evening, > > I'm trying to learn how to implement Haskell on a web site. > I've got the O'Reilly book: "Developing Web Applications with Haskell and > Yesod". > I'd appreciate some guidance in regard to this. > I'm having a bit of a problem getting the helloWorld.hs app up and running > on Yesod. > > Thank you > > On Feb 24, 2014, at 2:30 PM, 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://www.haskell.org/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. Failure to install cabal-install. (Derek Mahar) >> 2. Re: Failure to install cabal-install. (Derek Mahar) >> >> >> ---------------------------------------------------------------------- >> >> Message: 1 >> Date: Mon, 24 Feb 2014 13:20:45 -0500 >> From: Derek Mahar <derek.mahar(a)gmail.com> >> To: beginners(a)haskell.org >> Subject: [Haskell-beginners] Failure to install cabal-install. >> Message-ID: >> <CABtfiZb7+p=DtGoz57GVXwgHCdK1vPL175UxmirRY1EXM4aEBA(a)mail.gmail.com> >> Content-Type: text/plain; charset="utf-8" >> >> How can I tell the Haskell compiler where to find module >> Control.Monad.Error? After building GHC 7.6.3 and Haskell Platform >> 2013.2 >> on Red Hat Linux 6, I found that 'cabal install cabal-install' fails when >> it attempts to build library HTTP-4000.2.11 because the Haskell compiler >> cannot find module Control.Monad.Error. >> >> ******************************************************* >> * Installation completed successfully. >> * >> * Programs installed into: >> * /usr/local/bin >> * >> * Now do "cabal update" to initialize the package list >> * >> * Additional packages may be found at http://hackage.haskell.org >> * or via "cabal list <pattern>" >> * >> * Use "cabal install <foo>" to install additional packages >> * >> ******************************************************* >> [dmahar@dmahar haskell-platform-2013.2.0.0]$ cabal update >> Downloading the latest package list from hackage.haskell.org >> Note: there is a new version of cabal-install available. >> To upgrade, run: cabal install cabal-install >> [dmahar@dmahar haskell-platform-2013.2.0.0]$ cabal install cabal-install >> Resolving dependencies... >> Configuring HTTP-4000.2.11... >> Building HTTP-4000.2.11... >> Preprocessing library HTTP-4000.2.11... >> >> Network/Stream.hs:35:8: >> Could not find module `Control.Monad.Error' >> There are files missing in the `mtl-2.1.2' package, >> try running 'ghc-pkg check'. >> Use -v to see a list of the files searched for. >> Failed to install HTTP-4000.2.11 >> cabal: Error: some packages failed to install: >> HTTP-4000.2.11 failed during the building phase. The exception was: >> ExitFailure 1 >> cabal-install-1.18.0.2 depends on HTTP-4000.2.11 which failed to install. >> >> Thank you, >> >> Derek >>

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Re: [Haskell-beginners] Beginners Digest, Vol 68, Issue 22
by KMandPJLynch 25 Feb '14

25 Feb '14

Good evening, I'm trying to learn how to implement Haskell on a web site. I've got the O'Reilly book: "Developing Web Applications with Haskell and Yesod". I'd appreciate some guidance in regard to this. I'm having a bit of a problem getting the helloWorld.hs app up and running on Yesod. Thank you On Feb 24, 2014, at 2:30 PM, 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://www.haskell.org/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. Failure to install cabal-install. (Derek Mahar) > 2. Re: Failure to install cabal-install. (Derek Mahar) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 24 Feb 2014 13:20:45 -0500 > From: Derek Mahar <derek.mahar(a)gmail.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] Failure to install cabal-install. > Message-ID: > <CABtfiZb7+p=DtGoz57GVXwgHCdK1vPL175UxmirRY1EXM4aEBA(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > How can I tell the Haskell compiler where to find module > Control.Monad.Error? After building GHC 7.6.3 and Haskell Platform 2013.2 > on Red Hat Linux 6, I found that 'cabal install cabal-install' fails when > it attempts to build library HTTP-4000.2.11 because the Haskell compiler > cannot find module Control.Monad.Error. > > ******************************************************* > * Installation completed successfully. > * > * Programs installed into: > * /usr/local/bin > * > * Now do "cabal update" to initialize the package list > * > * Additional packages may be found at http://hackage.haskell.org > * or via "cabal list <pattern>" > * > * Use "cabal install <foo>" to install additional packages > * > ******************************************************* > [dmahar@dmahar haskell-platform-2013.2.0.0]$ cabal update > Downloading the latest package list from hackage.haskell.org > Note: there is a new version of cabal-install available. > To upgrade, run: cabal install cabal-install > [dmahar@dmahar haskell-platform-2013.2.0.0]$ cabal install cabal-install > Resolving dependencies... > Configuring HTTP-4000.2.11... > Building HTTP-4000.2.11... > Preprocessing library HTTP-4000.2.11... > > Network/Stream.hs:35:8: > Could not find module `Control.Monad.Error' > There are files missing in the `mtl-2.1.2' package, > try running 'ghc-pkg check'. > Use -v to see a list of the files searched for. > Failed to install HTTP-4000.2.11 > cabal: Error: some packages failed to install: > HTTP-4000.2.11 failed during the building phase. The exception was: > ExitFailure 1 > cabal-install-1.18.0.2 depends on HTTP-4000.2.11 which failed to install. > > Thank you, > > Derek >

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make https request with client certificate
by Stanisław Findeisen 24 Feb '14

24 Feb '14

Hi How can you issue a HTTPS request using client X.509 certificate for authentication? Is it supported? -- http://people.eisenbits.com/~stf/ http://www.eisenbits.com/ OpenPGP: 80FC 1824 2EA4 9223 A986 DB4E 934E FEA0 F492 A63B

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Dynamic Loading of Haskell Code!
by Gilberto Melfe 22 Feb '14

22 Feb '14

Hi there All! I would like to ask: Is it possible for a Haskell program to load code while it's running, in order for it to facilitate the implementation of Plugins? (like it's possible in Perl or Python) Can someone enlighten me, please? Thank You Gilberto

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Need help with HXT
by Philippe Sismondi 21 Feb '14

21 Feb '14

Greetings. I am trying to move some very old code from using HaXml to HXT. This is experimental; I have reasons which I think are not relevant to this post. The website that purports to document HXT has broken links. This is Uwe Schmidt's website. I have emailed Dr. Schmidt. Meanwhile, can anyone elucidate the use of readDocument with its withCurl and withHTTP options? What may be passed in the list to e.g. withHTTP? - Phil -

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