May 2014 - Beginners - Haskell.org

Using Either and Maybe
by Alan Buxton 06 May '14

06 May '14

So I had a function that would leave me with a Nothing or a Just, and I wanted to use the result of this function to lookup in a Map. So something like this was very easy to do. h> let list = Data.Map.fromList [(1,2),(3,4),(5,6)] h> Just 4 >>= flip Data.Map.lookup list Nothing h> Just 3 >>= flip Data.Map.lookup list Just 4 Now, however, my source function gives me an Either. I ended up writing this function to let me chain Either with Data.Map.lookups: (>>?=) :: (Either a b, a) -> (b -> Maybe c) -> (Either a c,a) (>>?=) (Left x,e) _ = (Left x,e) (>>?=) (Right y,e) f = case f y of Nothing -> (Left e,e) Just z -> (Right z,e) But I can't help thinking I'm reinventing the wheel here somehow. There must be a better way, right? Thanks in advance for any pointers. Alan

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Re: [Haskell-beginners] cabal problems
by Patrick Wheeler 05 May '14

05 May '14

> But downloading and installing the cabal.exe worked. Everything seems okay now. Thanks. Great glad it worked. On Mon, May 5, 2014 at 7:48 AM, Kees Bleijenberg <k.bleijenberg(a)lijbrandt.nl > wrote: > Thanks Patrick. > First I tried cabal install cabal cabal-install => errors. > But downloading and installing the cabal.exe worked. Everything seems okay > now. Thanks. > > Kees > > =========== > > Van: Beginners [mailto:beginners-bounces@haskell.org] Namens Patrick > Wheeler > Verzonden: maandag 5 mei 2014 14:21 > Aan: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell > Onderwerp: Re: [Haskell-beginners] cabal problems > > You could try to download cabal binary from: > http://www.haskell.org/cabal/download.html > > This might be a this issue, the error reported looks similar: > https://github.com/haskell/cabal/issues/1561 > > If it is the same then using a newer version of cabal should fix the > problem. > > Patrick > --- snip --- > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > -- Patrick Wheeler Patrick.John.Wheeler(a)gmail.com Patrick.J.Wheeler(a)rice.edu Patrick.Wheeler(a)colorado.edu

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Re: [Haskell-beginners] cabal problems
by Patrick Wheeler 05 May '14

05 May '14

You could try to download cabal binary from: http://www.haskell.org/cabal/download.html This might be a this issue, the error reported looks similar: https://github.com/haskell/cabal/issues/1561 If it is the same then using a newer version of cabal should fix the problem. Patrick On Mon, May 5, 2014 at 4:32 AM, Kees Bleijenberg <k.bleijenberg(a)lijbrandt.nl > wrote: > Thanks Patrick. I think cabal on Windows is different. There is no > index.tar.gz on my computer. Maybe I'll wait for a new release of the > Haskell Platform to install GHC again :-( > > Additonal: a few days ago I tried to install eclipseFP and the > installation failed with a cabal error (don't know what error). Maybe that > is the source of my problems….? > > Kees > ========================= > > Van: Beginners [mailto:beginners-bounces@haskell.org] Namens Patrick > Wheeler > Verzonden: zondag 4 mei 2014 23:35 > Aan: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell > Onderwerp: Re: [Haskell-beginners] cabal problems > > I have not run into that problem before. > > I would download: > http://hackage.haskell.org/packages/index.tar.gz > > There is a link for it near the bottom of the main page: > http://hackage.haskell.org > > On my mac i would copy it to: > > ~/.cabal/packages/hackage.haskell.org/. > > I am not sure where the cabal directory is on windows though. > > If that does not work another option wold but to download the source for > cabal-install and compile it. > > cabal unpack cabal-install, or grab the tarball at the bottom of: > http://hackage.haskell.org/package/cabal-install > > cd into the directory and use `cabal install` > > That should work if you have all of the dependancies already on your > computer. If you don't and the missing dependency list is large(cabal > configure can tell you what you are missing) then it might just be quicker > to reinstall the haskell platform. > > Hope that helps, > > Patrick > > On Sat, May 3, 2014 at 1:09 PM, Kees Bleijenberg < > k.bleijenberg(a)lijbrandt.nl> wrote: > I don’t know exactly how, but suddenly I get cabal errors. > If I do cabal update there are no errors, everything is fine. > Cabal install caball-install => cabal: internal error when reading > packages index: could not read tar file entry. The package index or > package cache is probably corrupt. Running cabal update might fix it. > Every install or update shows the same error message. > > I’am using win7 64 bits, Haskell platform 2013.2.0.0. If i do cabal - - > version => cabal install version 1.18.0.2 using version 1.18.1 of the cabal > library. > On Google the advice is to clear the directory > c:\users\<user>\appData\roaming\cabal empty. But that removes all my > packages. Isn’t there an easier way? > I have never used cabal sandboxes (at least not on purpose) > > Kees > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > > > > -- > Patrick Wheeler > Patrick.John.Wheeler(a)gmail.com > Patrick.J.Wheeler(a)rice.edu > Patrick.Wheeler(a)colorado.edu > ________________________________________ > No virus found in this message. > Checked by AVG - www.avg.com > Version: 2014.0.4570 / Virus Database: 3931/7439 - Release Date: 05/04/14 > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > -- Patrick Wheeler Patrick.John.Wheeler(a)gmail.com Patrick.J.Wheeler(a)rice.edu Patrick.Wheeler(a)colorado.edu

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Re: [Haskell-beginners] cabal problems
by Patrick Wheeler 05 May '14

05 May '14

I have not run into that problem before. I would download: http://hackage.haskell.org/packages/index.tar.gz There is a link for it near the bottom of the main page: http://hackage.haskell.org On my mac i would copy it to: ~/.cabal/packages/hackage.haskell.org/. I am not sure where the cabal directory is on windows though. If that does not work another option wold but to download the source for cabal-install and compile it. cabal unpack cabal-install, or grab the tarball at the bottom of: http://hackage.haskell.org/package/cabal-install cd into the directory and use `cabal install` That should work if you have all of the dependancies already on your computer. If you don't and the missing dependency list is large(cabal configure can tell you what you are missing) then it might just be quicker to reinstall the haskell platform. Hope that helps, Patrick On Sat, May 3, 2014 at 1:09 PM, Kees Bleijenberg <k.bleijenberg(a)lijbrandt.nl > wrote: > I don’t know exactly how, but suddenly I get cabal errors. > > If I do cabal update there are no errors, everything is fine. > > Cabal install caball-install => cabal: internal error when reading > packages index: could not read tar file entry. The package index or > package cache is probably corrupt. Running cabal update might fix it. > > Every install or update shows the same error message. > > > > I’am using win7 64 bits, Haskell platform 2013.2.0.0. If i do cabal - - > version => cabal install version 1.18.0.2 using version 1.18.1 of the cabal > library. > > On Google the advice is to clear the directory > c:\users\<user>\appData\roaming\cabal empty. But that removes all my > packages. Isn’t there an easier way? > > I have never used cabal sandboxes (at least not on purpose) > > > > Kees > > > > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > > -- Patrick Wheeler Patrick.John.Wheeler(a)gmail.com Patrick.J.Wheeler(a)rice.edu Patrick.Wheeler(a)colorado.edu

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cabal problems
by Kees Bleijenberg 03 May '14

03 May '14

I don't know exactly how, but suddenly I get cabal errors. If I do cabal update there are no errors, everything is fine. Cabal install caball-install => cabal: internal error when reading packages index: could not read tar file entry. The package index or package cache is probably corrupt. Running cabal update might fix it. Every install or update shows the same error message. I'am using win7 64 bits, Haskell platform 2013.2.0.0. If i do cabal - - version => cabal install version 1.18.0.2 using version 1.18.1 of the cabal library. On Google the advice is to clear the directory c:\users\<user>\appData\roaming\cabal empty. But that removes all my packages. Isn't there an easier way? I have never used cabal sandboxes (at least not on purpose) Kees

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Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse
by Robert Weisser 02 May '14

02 May '14

Hi Vlatko, Thanks again for your help. I'm actually using a Mac, which should be almost exactly like linux. Robert ----- Original Message ----- From: Vlatko Basic Sent: 05/01/14 01:23 PM To: Robert Weisser, The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse Hi Robert, The two solutions below are exclusive. Either first or second. When you install ghc, it creates two directories in your $HOME. Those are .cabal and .ghc. In those directories cabal installs packages. cabal-dev and SourceGraph are executable programs only and you do not need their source files, just binary file. So, the proposed solution creates those executable programs, but without disturbing packages you already have installed. Let me know if you need further help. I'm assuming you're on Linux. But the solution for Windows should be similar, but not sure. vlatko -------- Original Message -------- Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse From: Robert Weisser Robert.Weisser(a)gmx.com To: vlatko.basic(a)gmail.com , The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell beginners(a)haskell.org Date: 01.05.2014 16:44 Thanks, Vlatko. I'm not sure I understand everything you said, but I'll give it a try. Robert ----- Original Message ----- From: Vlatko Basic Sent: 05/01/14 03:59 AM To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse Hi Robert, Current version supports only cabal-dev for sandboxing. New version that is about to come out will support cabal for sandboxing as well. The problem is that installed ghc depends on cabal version being shipped with. And it looks like it'll stay like that for a while, and until it is solved, you'll depend on cabal being shipped with ghc. The problematic packages are bins only, so you can - build them sandboxed and move the bins to .cabal/bin or - move/rename your .cabal and .ghc, cabal update, build cabal-dev and sourcegraph as usual, copy the bins to moved/renamed .cabal/bin, delete newly created .cabal and .ghc, move/rename back your .cabal and .ghc You can go with 3. also, but for a bit larger project you'll run into cabal-hell, so better to use it. It is quite comfortable to work in eclipseFP, so I think it is worth some initial troubles. :-) Best regards, vlatko -------- Original Message -------- Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse From: Robert Weisser Robert.Weisser(a)gmx.com To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell beginners(a)haskell.org Date: 01.05.2014 05:17 Thanks. That's great info. ----- Original Message ----- From: Norbert Melzer Sent: 04/30/14 07:19 PM To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse Omit cabal-dev it is not maintained anymore and most of its functionality is part of cabal already (cabal sandbox) Am 01.05.2014 00:18 schrieb "Robert Weisser" < Robert.Weisser(a)gmx.com >: I decided to try out Eclipse with the EclipseFP plug-in for Haskell. I downloaded Eclipse, and then started to install EclipseFP. At some point, I got a screen which said that I needed to install buildwrapper and scion-browser. It offered to install both of them, and optionally install hoogle, hlint, stylish-haskell, SourceGraph, and cabal-dev. The option to install the additional items was pre-checked. I left it checked. The installation of buildwrapper, etc. took a long time. It seemed to be going well until I saw the messages below, which say that installing SourceGraph will likely break several other packages, most of which were part of the EclipseFP installation. I am not a complete novice with Haskell but I am a complete novice with Cabal. I'm not sure what to do. Here are some options I have considered: 1. Use --force-reinstalls and hope for the best. 2. Try to resolve the problem with Cabal, although I don't know to do this. 3. Use EclipseFP without SourceGraph and cabal-dev. 4. Use EclipseFP without SourceGraph, but install cabal-dev using cabal on the command line. 5. Forget about Eclipse altogether, and go back to using vim. Any advice would be appreciated. By the way, I am using Haskell Platform 7.6.3. Here are the messages I saw in the Eclipse console window: <terminated> Installing executable SourceGraph Resolving dependencies... In order, the following would be installed: asn1-types-0.2.3 (new package) asn1-encoding-0.8.1.3 (new package) asn1-parse-0.8.1 (new package) bktrees-0.3.1 (new package) byteable-0.1.1 (new package) cereal-0.4.0.1 (new package) colour-2.3.3 (new package) cpphs-1.18.2 (new version) crypto-pubkey-types-0.4.2.2 (new package) cryptohash-0.11.4 (new package) digest-0.0.1.2 (new package) dlist-0.5 (new version) aeson-0.7.0.3 (reinstall) changes: dlist-0.7.0.1 -> 0.5 data-default-instances-dlist-0.0.1 (reinstall) changes: dlist-0.7.0.1 -> 0.5 data-default-0.5.3 (reinstall) cookie-0.4.1.1 (new package) haskell-src-exts-1.13.5 (new version) hslua-0.3.12 (new package) mime-types-0.1.0.4 (new package) multiset-0.2.2 (new package) pandoc-types-1.12.3.2 (new package) pem-0.2.2 (new package) polyparse-1.8 (new version) publicsuffixlist-0.1 (new package) http-client-0.3.2 (new package) regex-pcre-builtin-0.94.4.8.8.34 tel:0.94.4.8.8.34 (new package) highlighting-kate-0.5.6.1 (new package) resourcet-0.4.10.2 (new version) securemem-0.1.3 (new package) crypto-cipher-types-0.0.9 (new package) cipher-aes-0.2.7 (new package) cipher-rc4-0.1.4 (new package) crypto-random-0.0.7 (new package) cprng-aes-0.5.2 (new package) crypto-numbers-0.2.3 (new package) crypto-pubkey-0.2.4 (new package) socks-0.5.4 (new package) temporary-1.1.2.5 (new package) text-stream-decode-0.1.0.5 (new package) conduit-1.0.17.1 (new version) http-client-conduit-0.2.0.1 (new package) wl-pprint-text-1.1.0.2 (new package) graphviz-2999.16.0.0 (new package) x509-1.4.11 (new package) x509-store-1.4.4 (new package) x509-system-1.4.5 (new package) x509-validation-1.5.0 (new package) tls-1.2.6 (new package) connection-0.2.1 (new package) http-client-tls-0.2.1.1 (new package) http-conduit-2.0.0.10 (new package) xml-1.3.13 (new package) texmath-0.6.6.1 (new package) yaml-0.8.8.2 (reinstall) changes: conduit-1.1.1 -> 1.0.17.1, resourcet-1.1.2 -> 0.4.10.2 zip-archive-0.2.2.1 (new package) pandoc-1.12.3.3 (new package) Graphalyze-0.14.0.2 (new package) SourceGraph-0.7.0.5 (new package) cabal: The following packages are likely to be broken by the reinstalls: stylish-haskell-0.5.10.0 scion-browser-0.3.1 persistent-template-1.3.1.3 persistent-sqlite-1.3.0.5 persistent-1.3.0.6 buildwrapper-0.8.0 dynamic-cabal-0.3.1 Use --force-reinstalls if you want to install anyway. _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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replacing fold with scan!
by raffa f 02 May '14

02 May '14

hi everyone! here's my new problem. i wrote my version of filter: filter' :: (a -> Bool) -> [a] -> [a] filter' f = foldr (\x acc -> if f x then x:acc else acc) [] and it works! however, i wanted to use scan too. so i just replaced foldr with scanr, to see what would happen: filter'' :: (a -> Bool) -> [a] -> [a] filter'' f = scanr (\x acc -> if f x then x:acc else acc) [] but that doesn't work! ghci gives me this: folds.hs:15:59: Couldn't match expected type `a' with actual type `[a0]' `a' is a rigid type variable bound by the type signature for filter'' :: (a -> Bool) -> [a] -> [a] at folds.hs:14:13 In the second argument of `scanr', namely `[]' In the expression: scanr (\ x acc -> if f x then x : acc else acc) [] In an equation for filter'': filter'' f = scanr (\ x acc -> if f x then x : acc else acc) [] Failed, modules loaded: none. the problem seems to be with the start value of [], it seems? i don't understand, i thought scan and fold worked pretty much the same. i learned about these functions today, so i'm still trying to wrap my head around them... thank you!

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Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse
by Robert Weisser 01 May '14

01 May '14

Thanks, Vlatko. I'm not sure I understand everything you said, but I'll give it a try. Robert ----- Original Message ----- From: Vlatko Basic Sent: 05/01/14 03:59 AM To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse Hi Robert, Current version supports only cabal-dev for sandboxing. New version that is about to come out will support cabal for sandboxing as well. The problem is that installed ghc depends on cabal version being shipped with. And it looks like it'll stay like that for a while, and until it is solved, you'll depend on cabal being shipped with ghc. The problematic packages are bins only, so you can - build them sandboxed and move the bins to .cabal/bin or - move/rename your .cabal and .ghc, cabal update, build cabal-dev and sourcegraph as usual, copy the bins to moved/renamed .cabal/bin, delete newly created .cabal and .ghc, move/rename back your .cabal and .ghc You can go with 3. also, but for a bit larger project you'll run into cabal-hell, so better to use it. It is quite comfortable to work in eclipseFP, so I think it is worth some initial troubles. :-) Best regards, vlatko -------- Original Message -------- Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse From: Robert Weisser Robert.Weisser(a)gmx.com To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell beginners(a)haskell.org Date: 01.05.2014 05:17 Thanks. That's great info. ----- Original Message ----- From: Norbert Melzer Sent: 04/30/14 07:19 PM To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse Omit cabal-dev it is not maintained anymore and most of its functionality is part of cabal already (cabal sandbox) Am 01.05.2014 00:18 schrieb "Robert Weisser" < Robert.Weisser(a)gmx.com >: I decided to try out Eclipse with the EclipseFP plug-in for Haskell. I downloaded Eclipse, and then started to install EclipseFP. At some point, I got a screen which said that I needed to install buildwrapper and scion-browser. It offered to install both of them, and optionally install hoogle, hlint, stylish-haskell, SourceGraph, and cabal-dev. The option to install the additional items was pre-checked. I left it checked. The installation of buildwrapper, etc. took a long time. It seemed to be going well until I saw the messages below, which say that installing SourceGraph will likely break several other packages, most of which were part of the EclipseFP installation. I am not a complete novice with Haskell but I am a complete novice with Cabal. I'm not sure what to do. Here are some options I have considered: 1. Use --force-reinstalls and hope for the best. 2. Try to resolve the problem with Cabal, although I don't know to do this. 3. Use EclipseFP without SourceGraph and cabal-dev. 4. Use EclipseFP without SourceGraph, but install cabal-dev using cabal on the command line. 5. Forget about Eclipse altogether, and go back to using vim. Any advice would be appreciated. By the way, I am using Haskell Platform 7.6.3. Here are the messages I saw in the Eclipse console window: <terminated> Installing executable SourceGraph Resolving dependencies... In order, the following would be installed: asn1-types-0.2.3 (new package) asn1-encoding-0.8.1.3 (new package) asn1-parse-0.8.1 (new package) bktrees-0.3.1 (new package) byteable-0.1.1 (new package) cereal-0.4.0.1 (new package) colour-2.3.3 (new package) cpphs-1.18.2 (new version) crypto-pubkey-types-0.4.2.2 (new package) cryptohash-0.11.4 (new package) digest-0.0.1.2 (new package) dlist-0.5 (new version) aeson-0.7.0.3 (reinstall) changes: dlist-0.7.0.1 -> 0.5 data-default-instances-dlist-0.0.1 (reinstall) changes: dlist-0.7.0.1 -> 0.5 data-default-0.5.3 (reinstall) cookie-0.4.1.1 (new package) haskell-src-exts-1.13.5 (new version) hslua-0.3.12 (new package) mime-types-0.1.0.4 (new package) multiset-0.2.2 (new package) pandoc-types-1.12.3.2 (new package) pem-0.2.2 (new package) polyparse-1.8 (new version) publicsuffixlist-0.1 (new package) http-client-0.3.2 (new package) regex-pcre-builtin-0.94.4.8.8.34 tel:0.94.4.8.8.34 (new package) highlighting-kate-0.5.6.1 (new package) resourcet-0.4.10.2 (new version) securemem-0.1.3 (new package) crypto-cipher-types-0.0.9 (new package) cipher-aes-0.2.7 (new package) cipher-rc4-0.1.4 (new package) crypto-random-0.0.7 (new package) cprng-aes-0.5.2 (new package) crypto-numbers-0.2.3 (new package) crypto-pubkey-0.2.4 (new package) socks-0.5.4 (new package) temporary-1.1.2.5 (new package) text-stream-decode-0.1.0.5 (new package) conduit-1.0.17.1 (new version) http-client-conduit-0.2.0.1 (new package) wl-pprint-text-1.1.0.2 (new package) graphviz-2999.16.0.0 (new package) x509-1.4.11 (new package) x509-store-1.4.4 (new package) x509-system-1.4.5 (new package) x509-validation-1.5.0 (new package) tls-1.2.6 (new package) connection-0.2.1 (new package) http-client-tls-0.2.1.1 (new package) http-conduit-2.0.0.10 (new package) xml-1.3.13 (new package) texmath-0.6.6.1 (new package) yaml-0.8.8.2 (reinstall) changes: conduit-1.1.1 -> 1.0.17.1, resourcet-1.1.2 -> 0.4.10.2 zip-archive-0.2.2.1 (new package) pandoc-1.12.3.3 (new package) Graphalyze-0.14.0.2 (new package) SourceGraph-0.7.0.5 (new package) cabal: The following packages are likely to be broken by the reinstalls: stylish-haskell-0.5.10.0 scion-browser-0.3.1 persistent-template-1.3.1.3 persistent-sqlite-1.3.0.5 persistent-1.3.0.6 buildwrapper-0.8.0 dynamic-cabal-0.3.1 Use --force-reinstalls if you want to install anyway. _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Re: [Haskell-beginners] replacing fold with scan!
by Zhi An Ng 01 May '14

01 May '14

Hi, The types of `foldr` and `scanr` are different: > :t scanr scanr :: (a -> b -> b) -> b -> [a] -> [b] > :t foldr foldr :: (a -> b -> b) -> b -> [a] -> b I believe the reason is that `scanr` does something like a prefix sum, https://en.wikipedia.org/wiki/Prefix_sum So to get this to work, you need to change the type signature of filter'' filter'' :: (a -> Bool) -> [a] -> [[a]] That's all. Anyway you can try entering the definition of filter'' into ghci, and use :t to let ghc figure our the type for you as well :) Best, Zhi An On Thu, May 1, 2014 at 10:42 AM, raffa f <freitasraffa(a)gmail.com> wrote: > hi everyone! here's my new problem. i wrote my version of filter: > > filter' :: (a -> Bool) -> [a] -> [a] > filter' f = foldr (\x acc -> if f x then x:acc else acc) [] > > and it works! however, i wanted to use scan too. so i just replaced foldr > with scanr, to see what would happen: > > filter'' :: (a -> Bool) -> [a] -> [a] > filter'' f = scanr (\x acc -> if f x then x:acc else acc) [] > > but that doesn't work! ghci gives me this: > > folds.hs:15:59: > Couldn't match expected type `a' with actual type `[a0]' > `a' is a rigid type variable bound by > the type signature for filter'' :: (a -> Bool) -> [a] -> [a] > at folds.hs:14:13 > In the second argument of `scanr', namely `[]' > In the expression: > scanr (\ x acc -> if f x then x : acc else acc) [] > In an equation for filter'': > filter'' f = scanr (\ x acc -> if f x then x : acc else acc) [] > Failed, modules loaded: none. > > the problem seems to be with the start value of [], it seems? i don't > understand, i thought scan and fold worked pretty much the same. i learned > about these functions today, so i'm still trying to wrap my head around > them... > > thank you! > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > >

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Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse
by Robert Weisser 01 May '14

01 May '14

Thanks. That's great info. ----- Original Message ----- From: Norbert Melzer Sent: 04/30/14 07:19 PM To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell Subject: Re: [Haskell-beginners] Problem installing EclipseFp in Eclipse Omit cabal-dev it is not maintained anymore and most of its functionality is part of cabal already (cabal sandbox) Am 01.05.2014 00:18 schrieb "Robert Weisser" < Robert.Weisser(a)gmx.com >: I decided to try out Eclipse with the EclipseFP plug-in for Haskell. I downloaded Eclipse, and then started to install EclipseFP. At some point, I got a screen which said that I needed to install buildwrapper and scion-browser. It offered to install both of them, and optionally install hoogle, hlint, stylish-haskell, SourceGraph, and cabal-dev. The option to install the additional items was pre-checked. I left it checked. The installation of buildwrapper, etc. took a long time. It seemed to be going well until I saw the messages below, which say that installing SourceGraph will likely break several other packages, most of which were part of the EclipseFP installation. I am not a complete novice with Haskell but I am a complete novice with Cabal. I'm not sure what to do. Here are some options I have considered: 1. Use --force-reinstalls and hope for the best. 2. Try to resolve the problem with Cabal, although I don't know to do this. 3. Use EclipseFP without SourceGraph and cabal-dev. 4. Use EclipseFP without SourceGraph, but install cabal-dev using cabal on the command line. 5. Forget about Eclipse altogether, and go back to using vim. Any advice would be appreciated. By the way, I am using Haskell Platform 7.6.3. Here are the messages I saw in the Eclipse console window: <terminated> Installing executable SourceGraph Resolving dependencies... In order, the following would be installed: asn1-types-0.2.3 (new package) asn1-encoding-0.8.1.3 (new package) asn1-parse-0.8.1 (new package) bktrees-0.3.1 (new package) byteable-0.1.1 (new package) cereal-0.4.0.1 (new package) colour-2.3.3 (new package) cpphs-1.18.2 (new version) crypto-pubkey-types-0.4.2.2 (new package) cryptohash-0.11.4 (new package) digest-0.0.1.2 (new package) dlist-0.5 (new version) aeson-0.7.0.3 (reinstall) changes: dlist-0.7.0.1 -> 0.5 data-default-instances-dlist-0.0.1 (reinstall) changes: dlist-0.7.0.1 -> 0.5 data-default-0.5.3 (reinstall) cookie-0.4.1.1 (new package) haskell-src-exts-1.13.5 (new version) hslua-0.3.12 (new package) mime-types-0.1.0.4 (new package) multiset-0.2.2 (new package) pandoc-types-1.12.3.2 (new package) pem-0.2.2 (new package) polyparse-1.8 (new version) publicsuffixlist-0.1 (new package) http-client-0.3.2 (new package) regex-pcre-builtin-0.94.4.8.8.34 tel:0.94.4.8.8.34 (new package) highlighting-kate-0.5.6.1 (new package) resourcet-0.4.10.2 (new version) securemem-0.1.3 (new package) crypto-cipher-types-0.0.9 (new package) cipher-aes-0.2.7 (new package) cipher-rc4-0.1.4 (new package) crypto-random-0.0.7 (new package) cprng-aes-0.5.2 (new package) crypto-numbers-0.2.3 (new package) crypto-pubkey-0.2.4 (new package) socks-0.5.4 (new package) temporary-1.1.2.5 (new package) text-stream-decode-0.1.0.5 (new package) conduit-1.0.17.1 (new version) http-client-conduit-0.2.0.1 (new package) wl-pprint-text-1.1.0.2 (new package) graphviz-2999.16.0.0 (new package) x509-1.4.11 (new package) x509-store-1.4.4 (new package) x509-system-1.4.5 (new package) x509-validation-1.5.0 (new package) tls-1.2.6 (new package) connection-0.2.1 (new package) http-client-tls-0.2.1.1 (new package) http-conduit-2.0.0.10 (new package) xml-1.3.13 (new package) texmath-0.6.6.1 (new package) yaml-0.8.8.2 (reinstall) changes: conduit-1.1.1 -> 1.0.17.1, resourcet-1.1.2 -> 0.4.10.2 zip-archive-0.2.2.1 (new package) pandoc-1.12.3.3 (new package) Graphalyze-0.14.0.2 (new package) SourceGraph-0.7.0.5 (new package) cabal: The following packages are likely to be broken by the reinstalls: stylish-haskell-0.5.10.0 scion-browser-0.3.1 persistent-template-1.3.1.3 persistent-sqlite-1.3.0.5 persistent-1.3.0.6 buildwrapper-0.8.0 dynamic-cabal-0.3.1 Use --force-reinstalls if you want to install anyway. _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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