May 2010 - Beginners - Haskell.org

Type ambiguity - Int vs. Integer
by Thomas 17 May '10

17 May '10

Hello! I was very surprised to see that Int and Integer seem to differ with respect to ambiguity. And I have no idea what is going on. This works as intended: data AW = AI Integer deriving (Show, Eq, Ord) class AWC a where toAW :: a -> AW fromAW :: AW -> a instance AWC Integer where toAW = AI fromAW (AI v) = v > toAW 5 5 However, the following gives an error: data AW = AI Int deriving (Show, Eq, Ord) class AWC a where toAW :: a -> AW fromAW :: AW -> a instance AWC Int where toAW = AI fromAW (AI v) = v > toAW 5 Ambiguous type variable 't' in the constraints: 'Num t' arising from the literal '5' 'AWC t' arising from the use of 'toAW' Ok, I can fix this easily with > toAW (5::Int) But I still don't understand what's going on? Why does one type work and the other does not? Is there a way around this without type annotations? Any hint would be greatly appreciated! Thanks in advance, Thomas

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diagrams installation trouble
by Scott N. Walck 17 May '10

17 May '10

Hi folks, I'm trying to install Brent Yorgey's diagrams library. I'm having trouble getting the Haskell bindings to the Cairo library installed correctly and/or visible. As you can see below, I have the Haskell Cairo stuff installed at /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1 I did this with something like $ sudo aptitude install libghc6-cairo-dev In other words, I used the ubuntu package system to install this. You can also see that ghc-pkg does not see this installation. I thought I might be able to fix the problem by registering these packages with 'ghc-pkg register', but I can't get that to work. Finally (and this is not shown below) I tried to compile gtk2hs from source. I seem to have had a successful './configure' step, but then I fail at the 'make' step, for a similar reason. The system can't find the cairo and similar files at /usr/lib/haskell-packages One more point about my system might be important. I installed GHC 6.10 using the ubuntu installer, then compiled 6.12 from source, then installed the most recent haskell-platform. I would greatly appreciate any help you all could provide. Thanks, Scott walck@walck-nsf:~/computer/haskell/Yorgey$ cd diagrams-new/ walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ runhaskell Setup.lhs configure Configuring diagrams-0.2.9... Setup.lhs: At least the following dependencies are missing: cairo >=0.9 && <0.11, colour >=2.2.1 && <2.4, containers ==0.2.* walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ runhaskell Setup.lhs configure --user Configuring diagrams-0.2.9... Setup.lhs: At least the following dependencies are missing: cairo >=0.9 && <0.11, containers ==0.2.* walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ ghc-pkg list /usr/local/lib/ghc-6.12.1/package.conf.d: Cabal-1.8.0.2 GLUT-2.1.2.1 HTTP-4000.0.9 HUnit-1.2.2.1 OpenGL-2.2.3.0 QuickCheck-2.1.0.3 array-0.3.0.0 base-3.0.3.2 base-4.2.0.0 bin-package-db-0.0.0.0 bytestring-0.9.1.5 cgi-3001.1.7.2 containers-0.3.0.0 deepseq-1.1.0.0 directory-1.0.1.0 (dph-base-0.4.0) (dph-par-0.4.0) (dph-prim-interface-0.4.0) (dph-prim-par-0.4.0) (dph-prim-seq-0.4.0) (dph-seq-0.4.0) extensible-exceptions-0.1.1.1 ffi-1.0 fgl-5.4.2.2 filepath-1.1.0.3 (ghc-6.12.1) (ghc-binary-0.5.0.2) ghc-prim-0.2.0.0 haskell-platform-2010.1.0.0 haskell-src-1.0.1.3 haskell98-1.0.1.1 hpc-0.5.0.4 html-1.0.1.2 integer-gmp-0.2.0.0 mtl-1.1.0.2 network-2.2.1.7 old-locale-1.0.0.2 old-time-1.0.0.3 parallel-2.2.0.1 parsec-2.1.0.1 pretty-1.0.1.1 process-1.0.1.2 random-1.0.0.2 regex-base-0.93.1 regex-compat-0.92 regex-posix-0.94.1 rts-1.0 stm-2.1.1.2 syb-0.1.0.2 template-haskell-2.4.0.0 time-1.1.4 unix-2.4.0.0 utf8-string-0.3.4 xhtml-3000.2.0.1 zlib-0.5.2.0 /home/walck/.ghc/x86_64-linux-6.12.1/package.conf.d: GLFW-0.4.2 binary-0.5.0.2 colour-2.3.1 darcs-2.4.1 dataenc-0.13.0.2 dimensional-0.8 gnuplot-0.3.4 hashed-storage-0.4.11 haskeline-0.6.2.2 mmap-0.4.1 monoid-transformer-0.0.2 non-negative-0.0.5.1 numeric-prelude-0.1.3.4 numtype-1.0 storable-record-0.0.2.2 terminfo-0.3.1.2 transformers-0.2.1.0 utf8-string-0.3.6 utility-ht-0.0.5.1 walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ dpkg -L libghc6-cairo-dev /. /usr /usr/share /usr/share/doc /usr/share/doc/libghc6-cairo-dev /usr/share/doc/libghc6-cairo-dev/changelog.gz /usr/share/doc/libghc6-cairo-dev/copyright /usr/share/doc/libghc6-cairo-dev/changelog.Debian.gz /usr/lib /usr/lib/haskell-packages /usr/lib/haskell-packages/ghc6 /usr/lib/haskell-packages/ghc6/lib /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1 /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/cairo.package.conf /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/libHScairo.a /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/HScairo.o /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Cairo.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Paths.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Patterns.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Text.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Drawing/Transformations.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Fonts /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Fonts/FontOptions.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/Image.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/PDF.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/PNG.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/PS.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/SVG.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Surfaces/Surface.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal/Utilities.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Matrix.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Types.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo/Internal.hi /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/imports/Graphics/Rendering/Cairo.hi walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ ls -l /usr/lib/haskell-packages/ghc6/lib/ total 20 drwxr-xr-x 3 root root 4096 2010-05-04 08:25 cairo-0.10.1 drwxr-xr-x 3 root root 4096 2010-05-04 16:48 curl-1.3.5 drwxr-xr-x 4 root root 4096 2010-05-04 08:25 glib-0.10.1 drwxr-xr-x 3 root root 4096 2010-05-04 16:00 gtk-0.10.1 drwxr-xr-x 3 root root 4096 2010-05-04 08:25 mtl-1.1.0.2 walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ ghc-pkg register /usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/cairo.package.conf Reading package info from "/usr/lib/haskell-packages/ghc6/lib/cairo-0.10.1/cairo.package.conf" ... done. cairo-0.10.1: missing id field cairo-0.10.1: dependency "base-4.1.0.0" doesn't exist (use --force to override) cairo-0.10.1: dependency "mtl-1.1.0.2" doesn't exist (use --force to override) cairo-0.10.1: dependency "glib-0.10.1" doesn't exist (use --force to override) cairo-0.10.1: dependency "bytestring-0.9.1.4" doesn't exist (use --force to override) walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ ghc-pkg register /usr/lib/haskell-packages/ghc6/lib/glib-0.10.1/glib.package.conf Reading package info from "/usr/lib/haskell-packages/ghc6/lib/glib-0.10.1/glib.package.conf" ... done. glib-0.10.1: missing id field glib-0.10.1: dependency "base-4.1.0.0" doesn't exist (use --force to override) walck@walck-nsf:~/computer/haskell/Yorgey/diagrams-new$ -- Scott N. Walck Associate Professor of Physics Lebanon Valley College Annville, PA 17003 phone: 717-867-6153 fax: 717-867-6075 email: walck(a)lvc.edu

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foldl by foldr
by Matt Andrew 16 May '10

16 May '10

Hi all, I am working my way through 'Real World Haskell' as my first introduction to Haskell and am thoroughly enjoying both it and Haskell. I have, however, hit something of a brick wall in figuring out one of the examples used in chapter 4. The example code is flagged as 'non-trivial' to understand, but I am trying to understand each thing presented reasonably well before I move on. I was hoping someone could shed some light on the issue for me; I have done a fair amount of searching and have not found an answer, so apologies if this has come up before and I have not found it. The offending code is: myFoldl :: (a -> b -> a) -> a -> [b] -> a myFoldl f zero xs = foldr step id xs zero where step x g a = g (f a x) The thing I am having trouble understanding is what the 'id' function is doing in a function that expects 3 arguments and is given 4 (foldr). Here is the breakdown of where I get stuck: My understanding of what id does as a function is that it returns the identity of the object it is passed. Playing with ghci gives me the type signature (a -> a) and as expected id returns the argument it is given unaltered. Now I believe Haskell does not allow the passing of more arguments to a function than it expects. Therefore I am driven to the conclusion that Haskell is treating the call to 'foldr' above as either: foldr step (id xs) zero (i.e. the second argument is the thunk (id xs)) or: foldr (step id) xs zero (i.e. given that 'step' is defined above as taking 3 arguments, whereas the type signature for 'foldr' expects a function taking 2 arguments it is accepting (step id) as a partially applied function taking two arguments) The problem is that having simulated the above scope in ghci for an identical call to 'foldr,' explicitly parenthesising the call to 'foldr' in either of the above ways throws an error. In addition to this either way of understanding the function doesn't make much sense to me: the first seems entirely redundant, the second would result in 'id' being called by 'step' in a position where it has no arguments and thus would surely result in an error. I'm obviously missing an important part of the puzzle and would appreciate any help in understanding this code. Appreciate you taking the time to read this, Matt Andrew

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Help with slow algorithm
by Diego Echeverri 16 May '10

16 May '10

Hi guys. I've been playing with https://www.spoj.pl/ to get better with Haskell. I'm trying to do: https://www.spoj.pl/problems/PALIN/ Basically given a number k, you have to find a number bigger than k that is palindrome. The thing is that k can be 1000000 digits long. I have learnt a bunch of things doing this exercise but I'm still getting time-limit exception. Here's the code: http://gist.github.com/401880 I believe that the bottleneck is in the addOne function (Specially in the case where you have a bunch of nines). I have rewritten it to use STArray but isn't fast enough. doing: addOne $ B.replicate 500000 'a' takes too much time! Can anybody point me out of better (faster) ways to improve the code? I don't need my code fixed, just suggestions and clues about things to try. Thanks! Diego Echeverri

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Re: [Haskell-beginners] Help with slow algorithm
by Diego Echeverri 15 May '10

15 May '10

Thanks! I checked Brandon's link and it was indeed wasting too much time doing GC (80%-70%). I did profiling by type and most of it were lists. I'm only using lists in the addOne function when I get the elements of the array. Maybe it would help to create the ByteArray directly without using that intermediate list (No idea how to do it). Or it would be even better if I could just use something like "mutable bytestrings" That would avoid some of the packing and unpacking. Even when I managed to make it pass. I feel like cheating. Instead of changing it algorithmically I changed some of the GC options to avoid GC time. In order to do this I followed this: http://www.haskell.org/haskellwiki/SPOJ In case anybody is interested, here's the code: http://gist.github.com/402282 On Sat, May 15, 2010 at 2:57 AM, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > Congrats. > But you should try to squeeze a little more out of it. > > Cheers, > Daniel > -- Att: Diego Echeverri Saldarriaga

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gnuplot problem in ghci
by Scott N. Walck 14 May '10

14 May '10

Hello, I've used the gnuplot package http://hackage.haskell.org/package/gnuplot before with great satisfaction. Now I have a new computer (a Lenovo T400 with ubuntu 9.10) and I have a problem. After I make a plot in ghci, using a command such as plotFunc [] [0.0,0.1..10.0] cos shown below, the keyboard no longer echos my input to ghci. ghci will still do what I ask, as in computing 3+4 below, but I could not see the "3+4" until after I hit return. This is gnuplot 0.3.4. Could anyone suggest an idea? Thanks, Scott walck@walck-nsf:~/courses/phy261/code$ gnuplot --version gnuplot 4.2 patchlevel 5 walck@walck-nsf:~/courses/phy261/code$ ghci GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :m +Graphics.Gnuplot.Simple Prelude Graphics.Gnuplot.Simple> plotFunc [] [0.0,0.1..10.0] cos Loading package syb-0.1.0.2 ... linking ... done. Loading package base-3.0.3.2 ... linking ... done. Loading package array-0.3.0.0 ... linking ... done. Loading package containers-0.3.0.0 ... linking ... done. Loading package monoid-transformer-0.0.2 ... linking ... done. Loading package old-locale-1.0.0.2 ... linking ... done. Loading package filepath-1.1.0.3 ... linking ... done. Loading package old-time-1.0.0.3 ... linking ... done. Loading package unix-2.4.0.0 ... linking ... done. Loading package directory-1.0.1.0 ... linking ... done. Loading package process-1.0.1.2 ... linking ... done. Loading package time-1.1.4 ... linking ... done. Loading package utility-ht-0.0.5.1 ... linking ... done. Loading package gnuplot-0.3.4 ... linking ... done. Prelude Graphics.Gnuplot.Simple> 3+4 7 Prelude Graphics.Gnuplot.Simple> -- Scott N. Walck Associate Professor of Physics Lebanon Valley College Annville, PA 17003 phone: 717-867-6153 fax: 717-867-6075 email: walck(a)lvc.edu

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Call for papers : MULTICONF-10, USA, July 2010
by John Edward 14 May '10

14 May '10

It would be highly appreciated if you could share this announcement with your colleagues, students and individuals whose research is in computer science, computer engineering, information science and related areas. Call for papers : MULTICONF-10, USA, July 2010 The 2010 multi-conference (MULTICONF-10) (website: http://www.PromoteResearch.org ) will be held during July 12-14, 2010 in Orlando, Florida, USA. The primary goal of MULTICONF is to promote research and developmental activities in computer science, information technology, control engineering, and related fields. Another goal is to promote the dissemination of research to a multidisciplinary audience and to facilitate communication among researchers, developers, practitioners in different fields. The following conferences are planned to be organized as part of MULTICONF-10. • International Conference on Artificial Intelligence and Pattern Recognition (AIPR-10) • International Conference on Automation, Robotics and Control Systems (ARCS-10) • International Conference on Bioinformatics, Computational Biology, Genomics and Chemoinformatics (BCBGC-10) • International Conference on Computer Communications and Networks (CCN-10) • International Conference on Enterprise Information Systems and Web Technologies (EISWT-10) • International Conference on High Performance Computing Systems (HPCS-10) • International Conference on Information Security and Privacy (ISP-10) • International Conference on Image and Video Processing and Computer Vision (IVPCV-10) • International Conference on Software Engineering Theory and Practice (SETP-10) • International Conference on Theoretical and Mathematical Foundations of Computer Science (TMFCS-10) MULTICONF-10 will be held at Imperial Swan Hotel and Suites. It is a full-service resort that puts you in the middle of the fun! Located 1/2 block south of the famed International Drive, the hotel is just minutes from great entertainment like Walt Disney World® Resort, Universal Studios and Sea World Orlando. Guests can enjoy free scheduled transportation to these theme parks, as well as spacious accommodations, outdoor pools and on-site dining — all situated on 10 tropically landscaped acres. Here, guests can experience a full-service resort with discount hotel pricing in Orlando. Please see the website http://www.PromoteResearch.org for more details. Sincerely John Edward

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More Data.Vector fun
by Philip Scott 13 May '10

13 May '10

Hey ho, This should be a quick one, I hope! I am mucking about with the moadic initialisers for vectors, and am experiencing all kinds of fun. For example, I think I should be able to make a new vector of, say, 3 thingies* long: Prelude Data.Vector.Generic.Mutable> new 3 Now of course, poor old haskell does not know what sort of thing I am going to put in it yet, so it gets a bit cross: <interactive>:1:0: Ambiguous type variable `m' in the constraint: `Control.Monad.Primitive.PrimMonad m' arising from a use of `new' at <interactive>:1:0-4 Probable fix: add a type signature that fixes these type variable(s) <interactive>:1:0: Ambiguous type variables `v', `a' in the constraint: `MVector v a' arising from a use of `new' at <interactive>:1:0-4 Probable fix: add a type signature that fixes these type variable(s) Now I have paid very close attention to this 'Probable Fix', but have not been able to find out precisely how to specify what sort of vector I would like (I guess this is v..) and am even more boggled by 'm'.. I guess this is asking what sort of Monad I am going to use to fill it up? An example would be greatly appreciated. I find the type of 'new' quite interesting: new :: forall (m :: * -> *) (v :: * -> * -> *) a. (PrimMonad m, Data.Vector.Generic.Mutable.MVector v a) => Int -> m (v (PrimState m) a) So I did a bit of reading up on exstential types, and I (just about) get what 'forall' does, however I don't know what the (m :: * -> *) and (v :: * -> * -> *) bits mean. Just telling me what they are called would probably be enough to get me googling! I get the feeling I am a bit out of my depth here, but hey ho, I guess that's how you learn to swim! All the best, Phil * thingies == things of some type, which doesn't really matter at the momant.

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cabal haddock problem
by Ashish Agarwal 13 May '10

13 May '10

I've setup a cabal file for a project. Building works fine but cabal haddock gives the following error: src/AFRP/AFRP.hs:1:0: File name does not match module name: Saw: `Main' Expected: `AFRP.AFRP' However, the module name specified in AFRP.hs is in fact AFRP.AFRP so why is haddock seeing Main, especially since cabal build completes successfully?

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Haskell Serialization
by Tom Hobbs 13 May '10

13 May '10

Hi all, Can anyone recommend any articles relating to de/serialization in Haskell? I've been reading these: - http://hackage.haskell.org/packages/archive/binary/0.4.1/doc/html/Data-Bina… - http://en.wikipedia.org/wiki/Serialization#Haskell - http://www.haskell.org/tutorial/stdclasses.html (The Read/Show bits) ...and I'd like some more examples etc. My situation is basically this, I have a non-Haskell black-box tuple space ( http://en.wikipedia.org/wiki/Tuple_space) which I want to use Haskell to read/write data from/into. I had trouble understanding how Read/Show could help me here. Data.Binary made more sense, but like I say, I'd really like to find some more articles and examples. Even apart from the tuple space stuff. I'm trying to piece together in my mind how I'd read some serialized data from anywhere and turn that into a Haskell data type in a nicely reusable way. I've been flicking through Real World Haskell and couldn't see much that I found helpful for this. Many thanks, Tom

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