November 2015 - Beginners

Re: [Haskell-beginners] Beginners Digest, Vol 89, Issue 38 How to define a simple function
by Francisco Gutierrez 24 Nov '15

24 Nov '15

Dear sirs:I am defining a function halve, that breaks a list in two halfs. But I always get an error. Below is my code. What am I doing wrong?Thanks!halve xs = [take (floor (longitud/2)), drop (floor (longitud/2))] where longitud = length xs From: "beginners-request(a)haskell.org" <beginners-request(a)haskell.org> To: beginners(a)haskell.org Sent: Saturday, November 21, 2015 8:21 PM Subject: Beginners Digest, Vol 89, Issue 38 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. Re: Warning: Tab Character (Dan Stromberg) 2. Re: Warning: Tab Character (Jeon-Young Kang) 3. Re: Warning: Tab Character (Thomas Jakway) ---------------------------------------------------------------------- Message: 1 Date: Sat, 21 Nov 2015 16:44:21 -0800 From: Dan Stromberg <strombrg(a)gmail.com> To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> Subject: Re: [Haskell-beginners] Warning: Tab Character Message-ID: <CAOvKW55oxCcEhtqppC0bXQZjOEEv5vcYnBTWHaj9C5vhhm7g+w(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" I believe the first version of make required tabs, but some subsequent versions have relaxed that requirement to make things easier. Here's an example on a Linux Mint system (the 20's are of course spaces) : $ od -cx < Makefile cmd output started 2015 Sat Nov 21 04:39:47 PM PST 0000000 \n g o : \n . / t \n \n 670a 3a6f 200a 2020 2e20 742f 0a0a 0000016 above cmd output done 2015 Sat Nov 21 04:39:47 PM PST dstromberg@dell-inspiron:/var/tmp/make-tabs i686-pc-linux-gnu 25234 $ make cmd output started 2015 Sat Nov 21 04:39:48 PM PST Makefile:3: *** missing separator. Stop. above cmd output done 2015 Sat Nov 21 04:39:48 PM PST dstromberg@dell-inspiron:/var/tmp/make-tabs i686-pc-linux-gnu 25234 $ make --version cmd output started 2015 Sat Nov 21 04:39:53 PM PST GNU Make 3.81 Copyright (C) 2006 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. This program built for i686-pc-linux-gnu above cmd output done 2015 Sat Nov 21 04:39:53 PM PST dstromberg@dell-inspiron:/var/tmp/make-tabs i686-pc-linux-gnu 25234 I'll add that vim's Makefile syntax highlighting puts the space-indented line in red, suggesting an error. Changing the spaces to a single tab makes the red go away. HTH On Sat, Nov 21, 2015 at 3:34 PM, Thomas Jakway <tjakway(a)nyu.edu> wrote: > Why would make require tabs...? > Admittedly it's been a long time since I've written any makefiles by hand > but I always use spaces instead of tabs and have never had any problems. > > On Nov 21, 2015 6:01 PM, David Moberg <kaddkaka(a)gmail.com> wrote: > > > > Have you found "soft tabs"? That option will make TextMate use spaces > instead of tab characters. Read 4.11 "Using Spaces Instead of Tabs" in Dan > Strombergs link for instrucitons. > > > > 2015-11-21 21:36 GMT+01:00 Jeon-Young Kang <jykang22(a)gmail.com>: > >> > >> I've tried to choose "instead of spaces". But there is no option of > mine (Textmate 2 beta). > >> > >> On Sat, Nov 21, 2015 at 3:14 PM, Dan Stromberg <strombrg(a)gmail.com> > wrote: > >>> > >>> > >>> Perhaps enable soft tabs? > >>> https://manual.macromates.com/en/working_with_text > >>> > >>> This is an issue in Python too (at least from PEP8's perspective) - > but note that many "make" programs require tabs instead of spaces. > >>> > >>> On Sat, Nov 21, 2015 at 11:13 AM, Jeon-Young Kang <jykang22(a)gmail.com> > wrote: > >>>> > >>>> Hello everyone. > >>>> > >>>> > >>>> I am a newbie of Haskell, using Textmate on mac. > >>>> > >>>> > >>>> I got the following warning. How can I fix it? > >>>> > >>>> > >>>> Warning: > >>>> > >>>> Tab character > >>>> > >>>> Ok, modules loaded: Main. > >>>> > >>>> > >>>> > >>>> Sincerely, > >>>> Young > >>>> > >>>> _______________________________________________ > >>>> Beginners mailing list > >>>> Beginners(a)haskell.org > >>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > >>>> > >>> > >>> > >>> > >>> -- > >>> Dan Stromberg > >>> > >>> _______________________________________________ > >>> Beginners mailing list > >>> Beginners(a)haskell.org > >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > >>> > >> > >> > >> > >> -- > >> Department of Geography > >> State University of New York at Buffalo > >> > >> jykang22(a)gmail.com > >> > >> Jeon-Young Kang > >> > >> _______________________________________________ > >> Beginners mailing list > >> Beginners(a)haskell.org > >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > >> > > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > -- Dan Stromberg

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help with type error in a wxHaskell program
by Jason J. Corso 22 Nov '15

22 Nov '15

Hi: I'm new to Haskell and to wxHaskell (but found playing with wxHaskell to be a nice mid-ground between my years of imperative experience and the pure functional). Thanks for the help. I wrote the following code to create a set of staticText widgets by mapping the layout over a list. Easy enough... module Main where import Graphics.UI.WX main :: IO () main = start gui gui :: IO () gui = do f <- frame let s = map (\t -> staticText f [text := show t]) [1..10] set f [layout := margin 10 ( row 2 $ map (\x -> (widget x)) s)] But, I get a type error that I do not understand Couldn't match expected type ‘IO (Window a0)’ with actual type ‘[Prop (Frame ())] -> IO (Frame ())’ Probable cause: ‘frame’ is applied to too few arguments Thank you in advance, Jason

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Warning: Tab Character
by Jeon-Young Kang 21 Nov '15

21 Nov '15

Hello everyone. I am a newbie of Haskell, using Textmate on mac. I got the following warning. How can I fix it? Warning: Tab character Ok, modules loaded: Main. Sincerely, Young

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Ambiguous type of WriterT result I am not using
by Martin Vlk 21 Nov '15

21 Nov '15

Hi I have two functions, foldrEntries and foldrEntriesW, where the latter is a WriterT version of the former. Here are the type signatures: foldrEntries :: (Entry -> a -> a) -> a -> (String -> a) -> Entries -> a foldrEntriesW :: (Monoid w, Monad m) => (Entry -> a -> WriterT w m a) -> WriterT w m a -> (String -> WriterT w m a) -> Entries -> WriterT w m a I want to implement foldrEntries in terms of foldrEntriesW using the Identity monad and ignore/not use the writer result. I am doing this in order to reuse the foldrEntriesW implementation and avoid code duplication. This is what I have so far: http://lpaste.net/145641 But the compiler complains about ambiguous type for the writer reult I am ignoring (message in the above lpaste). Normally I think the way around this is to provide explicit type annotation for "foldIt", but in this case the result type depends on the type of "a" in foldrEntries type and I don't know how to express this and make the compiler happy. (I was able to make it work by calling "tell ()", basically writing a dummy value, which lets compiler know what the type is, but this is not so good - I don't want to make artificial function calls like this.) Can anybody help me with that? Many Thanks Martin

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Re: [Haskell-beginners] could not install "process-leksah" due to dependencies errors....
by André Verwijs 21 Nov '15

21 Nov '15

OS: OpenSUSE tumbleweed 2015-11-18 Plasma 5.4.2 QT version: 5.5.1 Kernel: 4.3.0-2 (needed for building from source) gtksourceview-devel 3.18.1-1.1 libwebkitgtk-devel 2.4.9-3.2

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could not install "process-leksah" due to dependencies errors....
by André Verwijs 21 Nov '15

21 Nov '15

remote-repos: hackage.haskell.org http://hackage.haskell.org/packages/archive >> cabal install leksah process-leksah Resolving dependencies... cabal: Could not resolve dependencies: trying: leksah-0.15.1.4 (user goal) trying: hlint-1.9.23 (dependency of leksah-0.15.1.4) next goal: haskell-src-exts (dependency of leksah-0.15.1.4) rejecting: haskell-src-exts-1.16.0.1/installed-911... (conflict: hlint => haskell-src-exts>=1.17 && <1.18) rejecting: haskell-src-exts-1.17.0 (conflict: leksah => haskell-src-exts>=1.13.5 && <1.17) rejecting: haskell-src-exts-1.16.0.1, 1.16.0, 1.15.0.1, 1.15.0, 1.14.0.1, 1.14.0, 1.13.5 (conflict: hlint => haskell-src-exts>=1.17 && <1.18) rejecting: haskell-src-exts-1.13.4, 1.13.3, 1.13.2, 1.13.1, 1.13.0, 1.12.0, 1.11.1, 1.10.2, 1.10.1, 1.10.0, 1.9.6, 1.9.5, 1.9.4, 1.9.3, 1.9.0, 1.8.2, 1.8.1, 1.8.0, 1.7.2, 1.7.1, 1.7.0, 1.6.1, 1.6.0, 1.5.3, 1.5.2, 1.5.1, 1.5.0, 1.4.0, 1.3.5, 1.3.4, 1.3.3, 1.3.2, 1.3.1, 1.3.0, 1.2.0, 1.1.4, 1.1.3.1, 1.1.3, 1.1.1, 1.1.0, 1.0.1, 1.0.0, 0.5.7, 0.5.6, 0.5.5, 0.5.4, 0.5.3, 0.5.2, 0.4.8, 0.4.6, 0.4.5, 0.4.4.1, 0.4.4, 0.4.3.1, 0.4.3, 0.4.2, 0.4.1, 0.3.12, 0.3.11, 0.3.10, 0.3.9, 0.3.7, 0.3.5, 0.3.4, 0.3.3, 0.3.2, 0.2.1 (conflict: leksah => haskell-src-exts>=1.13.5 && <1.17) Backjump limit reached (change with --max-backjumps).

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NNOUNCE: Haskell Communities and Activities Report (29th ed., November 2015)
by Mihai Maruseac 21 Nov '15

21 Nov '15

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (29th edition, November 2015) is now available, in PDF and HTML formats: http://haskell.org/communities/11-2015/report.pdf http://haskell.org/communities/11-2015/html/report.html Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of March 2015: target deadline for contributions to the May 2015 edition of the HCAR Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in March (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! Mihai Maruseac <hcar at haskell.org> -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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RDF with Haskell
by Jeon-Young Kang 20 Nov '15

20 Nov '15

Hello. I am a newbie learning Haskell. I'd like to import RDF in Haskell as well as to do SPARQL. I went through RDF4H, Swish, and so on. But it is still difficult to understand them for me. Anyone can suggest me to learn that? or recommend any kinds of books or tutorials? Sincerely, Jeon-Young --

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Monad for Pair
by Mike Houghton 19 Nov '15

19 Nov '15

Hi, I have newtype Pair a = P (a, a) deriving (Show) and have made Monoid, Applicative and Functor instances for it. I’m a bit stumped with Monad! instance Monad Pair where return x = P (x, x) and I got brain ache with >>= And really return x = P (x, x) doesn’t seem correct anyway. Would someone please write the Monad with an explanation? Many Thanks Mike

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Bug in solution for problem 19
by Nicolás Hertzulis 19 Nov '15

19 Nov '15

The penultimate solution for problem 19 of Ninety-Nine Haskell Problems <https://wiki.haskell.org/H-99:_Ninety-Nine_Haskell_Problems> is giving the result for the opposite of the given N. e.g. rotate "abcdefgh" (-3) gives the result for rotate "abcdefgh" 3 Here's the problem https://wiki.haskell.org/99_questions/11_to_20 and the solution https://wiki.haskell.org/99_questions/Solutions/19 I think it could be solved like this: rotate xs n | n <= 0 = (reverse . take (negate n) . reverse $ xs) ++ (reverse . drop (negate n) . reverse $ xs) | n > 0 = (drop n xs) ++ (take n xs) I know it's not great, just keeping the spirit of the original solution Is this the right list for reporting this kind of bugs? Nicolás

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