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Re: [Haskell-cafe] Rational and % operator remix
by michael rice 29 Mar '09

29 Mar '09

Correct, if it can be stated as a Rational then it terminates. I was messing around last night with the first function on a sequence that approximates sqrt(2) cf (take 25 (1: [2,2..])) forgetting that while the sequence is infinite, I only grabbed the first 25 elements. Your replacement code works fine except for a missing import Data.Ratio It's quite removed from what I was trying so I'm going to have to read ahead a bit to understand it. Thanks, Michael --- On Sun, 3/29/09, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: From: Daniel Fischer <daniel.is.fischer(a)web.de> Subject: Re: [Haskell-cafe] Rational and % operator remix To: haskell-cafe(a)haskell.org Cc: "michael rice" <nowgate(a)yahoo.com> Date: Sunday, March 29, 2009, 2:35 PM -----Inline Attachment Follows----- Am Sonntag 29 März 2009 19:40:19 schrieb michael rice: > Hi, > > Thanks again for the help last night. > > The second function cf2 is an attempt to reverse the process of the first > function, i.e., given a rational number it returns a list of integers, > possibly infinite, Not for rational numbers. > but you shouldn't get into trouble if you use 98%67 as > input (output should be [1,2,6,5]). The interpreter is complaining about > the '=' following the 'in' keyword. That should be '=='. > Is there a better way to state this? > > Michael > > import Data.Ratio > cf :: [Int] -> Rational > cf (x:[]) = toRational x > cf (x:xs) = toRational x + 1 / cf xs > > cf2 :: Rational -> [Int] > cf2 a = let ai = toRational (floor ((numerator a) / (denominator a))) > in > if a = ai > then [a] > else ai : cf2 ((toRational 1) / (subtract ai a)) import Data.List (unfoldr) cf3 :: Rational -> [Integer] -- Int may overflow cf3 0 = [0] cf3 x = a0:unfoldr f r where a0 = floor x r = x - fromInteger a0 f 0 = Nothing f y = Just (properFraction $ recip y)

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Template Haskell messes up scoping?
by Stephan Friedrichs 29 Mar '09

29 Mar '09

Hi, when I tried to reorganize some code to use the data-accessor and data-accessor-template packages, i stumbled across a strange effect: When using template haskell some things are out of scope that really shouldn't be. Let me give an example: ============== T.hs ============== {-# LANGUAGE TemplateHaskell #-} data Foo = Foo { bar :: Bar } $( return [] ) data Bar = Bar ================================== > ghci T.hs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( T.hs, interpreted ) T.hs:4:13: Not in scope: type constructor or class `Bar' Failed, modules loaded: none. No matter what you do in $( ... ), the code doesn't compile. Even in the simple case above, it doesn't work. What's happening here? Bug or feature? And do you know a workaround? //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr

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Rational and % operator remix
by michael rice 29 Mar '09

29 Mar '09

Hi, Thanks again for the help last night. The second function cf2 is an attempt to reverse the process of the first function, i.e., given a rational number it returns a list of integers, possibly infinite, but you shouldn't get into trouble if you use 98%67 as input (output should be [1,2,6,5]). The interpreter is complaining about the '=' following the 'in' keyword. Is there a better way to state this? Michael import Data.Ratio cf :: [Int] -> Rational cf (x:[]) = toRational x cf (x:xs) = toRational x + 1 / cf xs cf2 :: Rational -> [Int] cf2 a = let ai = toRational (floor ((numerator a) / (denominator a))) in if a = ai then [a] else ai : cf2 ((toRational 1) / (subtract ai a))

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Re: [Haskell-cafe] Re: Exception handling in numeric computations
by Jonathan Cast 29 Mar '09

29 Mar '09

On Sat, 2009-03-28 at 12:51 +0300, Gregory Petrosyan wrote: > On Sat, Mar 28, 2009 at 10:53 AM, Ketil Malde <ketil(a)malde.org> wrote: > > So the difference between an exception or an error type is mainly what > > you intend to do about it. There's no point in wrapping divisions in > > Maybe unless you actually are able to do something useful to recover > > from a zero denominator. > > That is exactly the point I was trying to make. > > When I write a code, I can't say in advance, in what way it will be used. > So, for dealing with errors, I have to choose one way or another, mostly > without that knowledge. When I'm using e.g. C++, it's easy: > something like mantra "when in doubt, throw an exception" :-) > combined with RAII, works good (but not ideal, of course). > > So, I'll ask again: when I program in Haskell, what mechanism should I use? If you don't know, use a (true) exception. That is, Left or Exception or throwIO. Only use error or throw when you *know* the condition is un-recoverable. jcc

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Re: [Haskell-cafe] type Rational and the % operator
by michael rice 29 Mar '09

29 Mar '09

This seems to work: import Data.Ratio cf :: [Integer] -> Rational cf (x:[]) = toRational x cf (x:xs) = toRational x + (1 / cf xs) Hugs> :load cf.hs Main> cf [1,2,6,5] 98 % 67 Main> cf [2,3] 7 % 3 Main> Thanks, guys! Michael --- On Sun, 3/29/09, Luke Palmer <lrpalmer(a)gmail.com> wrote: From: Luke Palmer <lrpalmer(a)gmail.com> Subject: Re: [Haskell-cafe] type Rational and the % operator To: "michael rice" <nowgate(a)yahoo.com> Cc: "Ryan Ingram" <ryani.spam(a)gmail.com>, haskell-cafe(a)haskell.org Date: Sunday, March 29, 2009, 12:23 AM 2009/3/28 michael rice <nowgate(a)yahoo.com> Still the same problem. The book says the answer is supposed to be an exact 98/67. Michael import Data.Ratio cf :: [Integer] -> Rational cf (x:xs) = (x % 1) + (1 % (cf xs)) cf (x:[]) = x % 1 cf [] = 0 % 1 Use (/) for division: cf :: [Integer] -> Rationalcf (x:xs) = toRational x + 1 / cf xscf [x] = toRational x cf [] = 0 Luke

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Re: [Haskell-cafe] type Rational and the % operator
by michael rice 29 Mar '09

29 Mar '09

Still the same problem. The book says the answer is supposed to be an exact 98/67. Michael import Data.Ratio cf :: [Integer] -> Rational cf (x:xs) = (x % 1) + (1 % (cf xs)) cf (x:[]) = x % 1 cf [] = 0 % 1 Hugs> :load cf.hs ERROR "cf.hs":3 - Type error in application *** Expression : x % 1 + 1 % cf xs *** Term : x % 1 *** Type : Ratio Integer *** Does not match : Ratio (Ratio Integer) Data.Ratio> --- On Sat, 3/28/09, Ryan Ingram <ryani.spam(a)gmail.com> wrote: From: Ryan Ingram <ryani.spam(a)gmail.com> Subject: Re: [Haskell-cafe] type Rational and the % operator To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, haskell-cafe(a)haskell.org Date: Saturday, March 28, 2009, 11:17 PM Just use "/" for division. % is for construction of rationals from the "underlying" numeric type. For example, instead of "toRational x" you can write "x % 1". -- ryan 2009/3/28 michael rice <nowgate(a)yahoo.com>: > I may be missing something here, but this is what I intended. > > An expression of the form > > 1 > a1 + ------ > 1 > a2 + ------ > 1 > a3 + -- > a4 + ... > > Where the ai's are positive integers is called > a continued fraction. > > Function cf should take [1,2,6,5] to produce > > 1 > 1 + ----- > 1 > 2 + ----- > 1 > 6 + -- > 5 > > Michael > > > --- On Sat, 3/28/09, Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> wrote: > > From: Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> > Subject: Re: [Haskell-cafe] type Rational and the % operator > To: "michael rice" <nowgate(a)yahoo.com> > Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "Duane Johnson" > <duane.johnson(a)gmail.com>, haskell-cafe(a)haskell.org > Date: Saturday, March 28, 2009, 10:39 PM > > On 2009 Mar 28, at 22:36, michael rice wrote: > > import Data.Ratio > cf :: [Integer] -> Rational > cf (x:xs) = (toRational x) + (1 % (cf xs)) > cf (x:[]) = toRational x > cf [] = toRational 0 > > Data.Ratio> :load cf.hs > ERROR "cf.hs":3 - Type error in application > *** Expression : toRational x + 1 % cf xs > *** Term : toRational x > *** Type : Ratio Integer > *** Does not match : Ratio (Ratio Integer) > > Your function cf produces a Rational (Ratio Int); you're using it in the > denominator of another Ratio, which makes that Ratio's type Ratio (Ratio > Int). This is almost certainly not what you intended, but I couldn't say > what you actually want. > -- > brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.com > system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu > electrical and computer engineering, carnegie mellon university KF8NH > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] type Rational and the % operator
by michael rice 29 Mar '09

29 Mar '09

I may be missing something here, but this is what I intended. An expression of the form 1 a1 + ------ 1 a2 + ------ 1 a3 + -- a4 + ... Where the ai's are positive integers is called a continued fraction. Function cf should take [1,2,6,5] to produce 1 1 + ----- 1 2 + ----- 1 6 + -- 5 Michael --- On Sat, 3/28/09, Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> wrote: From: Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> Subject: Re: [Haskell-cafe] type Rational and the % operator To: "michael rice" <nowgate(a)yahoo.com> Cc: "Brandon S. Allbery KF8NH" <allbery(a)ece.cmu.edu>, "Duane Johnson" <duane.johnson(a)gmail.com>, haskell-cafe(a)haskell.org Date: Saturday, March 28, 2009, 10:39 PM On 2009 Mar 28, at 22:36, michael rice wrote:import Data.Ratio cf :: [Integer] -> Rational cf (x:xs) = (toRational x) + (1 % (cf xs)) cf (x:[]) = toRational x cf [] = toRational 0 Data.Ratio> :load cf.hs ERROR "cf.hs":3 - Type error in application *** Expression : toRational x + 1 % cf xs *** Term : toRational x *** Type : Ratio Integer *** Does not match : Ratio (Ratio Integer) Your function cf produces a Rational (Ratio Int); you're using it in the denominator of another Ratio, which makes that Ratio's type Ratio (Ratio Int). This is almost certainly not what you intended, but I couldn't say what you actually want. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.comsystem administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.eduelectrical and computer engineering, carnegie mellon university KF8NH

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Re: [Haskell-cafe] type Rational and the % operator
by michael rice 29 Mar '09

29 Mar '09

That fixed the problem with recognizing the % operator, but what's it trying to tell me now? Michael ============ import Data.Ratio cf :: [Integer] -> Rational cf (x:xs) = (toRational x) + (1 % (cf xs)) cf (x:[]) = toRational x cf [] = toRational 0 Data.Ratio> :load cf.hs ERROR "cf.hs":3 - Type error in application *** Expression : toRational x + 1 % cf xs *** Term : toRational x *** Type : Ratio Integer *** Does not match : Ratio (Ratio Integer) --- On Sat, 3/28/09, Duane Johnson <duane.johnson(a)gmail.com> wrote: From: Duane Johnson <duane.johnson(a)gmail.com> Subject: Re: [Haskell-cafe] type Rational and the % operator To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Saturday, March 28, 2009, 9:44 PM I believe it's import Data.Ratio I found it on hayoo... http://holumbus.fh-wedel.de/hayoo/hayoo.html?query=%25#0:%25 -- Duane Johnson On Mar 28, 2009, at 7:39 PM, michael rice wrote: Hi, I've been away from Haskell Land for a while, but I think the function cf below, given a list of Ints should calculate a continuous fraction. I'm using Hugs 98 and get errors when loading and also when trying to use the % operator at the command prompt (see below). What must I do to get this to work? Michael =============== cf :: [Int] -> Rational cf [] = 0 cf (x:[]) = 1 % x cf (x:xs) = x + (1 % cf xs) Hugs> :load cf.hs ERROR "cf.hs":2 - Undefined variable "%" Hugs> Hugs> 1 % 5 ERROR - Undefined variable "%" Hugs> _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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type Rational and the % operator
by michael rice 29 Mar '09

29 Mar '09

Hi, I've been away from Haskell Land for a while, but I think the function cf below, given a list of Ints should calculate a continuous fraction. I'm using Hugs 98 and get errors when loading and also when trying to use the % operator at the command prompt (see below). What must I do to get this to work? Michael =============== cf :: [Int] -> Rational cf [] = 0 cf (x:[]) = 1 % x cf (x:xs) = x + (1 % cf xs) Hugs> :load cf.hs ERROR "cf.hs":2 - Undefined variable "%" Hugs> Hugs> 1 % 5 ERROR - Undefined variable "%" Hugs>

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Haskell Weekly News: Issue 111 - March 28, 2009
by Brent Yorgey 28 Mar '09

28 Mar '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090328 Issue 111 - March 28, 2009 --------------------------------------------------------------------------- Welcome to issue 111 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements CHP package. Neil Brown [2]announced the release of version 1.2.0 of the [3]CHP library (which supports explicit message-passing concurrency in Haskell), with various bug-fixes and a new "clock" synchronisation primitive. smartword 0.0.0.5 Web based flash card for Word Smart I and II vocabularies. Ki Yung Ahn [4]announced the release of [5]smartword 0.0.0.5, a web based flash card system for Word Smart I and II, a popular book series for studying GRE vocabularies. HackMail 0.0 -- Procmail + Monads = Awesome!. Joe Fredette [6]announced his very second Hackage upload, [7]HackMail. Hackmail is a Procmail-alike, though it doesn't (yet) support procmail syntax. It dynamically loads a haskell source file and then sits as a daemon watching a directory for new emails. The source file contains a function which sorts email and delivers it to some directory. FallingBlocks 0.1. Ben Sanders [8]announced [9]fallingblocks, a Tetris clone using SDL. io-capture-0.2 capturing std(out|err) in IO action. Yusaku Hashimoto [10]announced the release of [11]io-capture 0.2, a library to capture stdout and stderr in an IO action. It exports a function capture, which takes an IO action and a String representing the entire input, and returns Strings representing the data written to stdout and stderr. wxAsteroids 1.0. Henk-Jan van Tuyl [12]announced [13]wxAsteroids, a game demonstrating the wxHaskell GUI. The votes are in!. Eelco Lempsink [14]announced that the [15]results of the Haskell logo competition are in! Congratulations to Jeff Wheeler on his winning design. Making videos of your project. Don Stewart [16]described how to create short screencasts showing off your latest awesome Haskell project. WinGhci, a GUI for GHCI on Windows. Pepe Gallardo [17]announced the first release of [18]WinGhci, a simple GUI for GHCI on Windows. It is closely based on WinHugs, and provides similar functionality. hranker: Basic utility for ranking a list of items (e.g. for the logo poll). Robin Green [19]announced [20]hranker, a command-line utility that helps the user rank a list of items (of any type implementing Show, Eq and Ord). The hope is that the code is sufficiently clear that it could also serve as an educational piece of code, especially for people wanting to learn how to use the HCL library. salvia-0.1, salvia-extras-0.1. Sebastiaan Visser [21]announced a new version of [22]Salvia, a lightweight Haskell Web Server Framework. Changes in this release include easier dependencies, some new default handler environments that simplify setting up a server application, support for keep-alive, a great deal of additional documentation, support for Windows, and various cleanup and bug fixes. Haddock 2.4.2. David Waern [23]announced a new release of [24]Haddock, the Haskell documentation tool. This is a bug fix release only, and it's the same version that will ship with GHC 6.10.2, unless any important problems are discovered before the GHC release. Because the .haddock file format has changed, links to previously installed documentation will not work when generating documentation using this version. ansi-terminal, ansi-wl-pprint - ANSI terminal support for Haskell. Max Bolingbroke [25]announced the [26]ansi-terminal and [27]ansi-wl-pprint packages, which allow Haskell programs to produce much richer console output by allowing colorisation, emboldening and so on. Both Unix-like (OS X, Linux) and Windows operating systems are supported (via a pure Haskell ANSI emulation layer for Windows). I/O library for Windows. Felix Martini [28]announced the [29]package, an I/O library for Windows using Windows API functions with I/O completion port support. The main goal of this library is to support Simon Marlow's new Handle API once he has added that to GHC. The library also has a compatibility module for socket functions from the network-bytestring package. Discussion Grouping - Map / Reduce. Günther Schmidt [30]asked about a way to lazily group an unordered list of key/value pairs, leading to some interesting solutions and discussion of preserving laziness. about Haskell code written to be "too smart". Manlio Perillo began an [31]epic discussion about Haskell coding style, idioms, pedagogy, and much, much more. Blog noise [32]Haskell news from the [33]blogosphere. * >>> : [34]I18n and Haskell. Internationalization of Haskell programs. * Holumbus: [35]Holumbus-Distribution Update. * Roman Cheplyaka: [36]Recent activity. Ongoing Hpysics development. * Jason Dagit: [37]Type-Correct Changes---A Safe Approach to Version Control Implementation. Jason is now a Master of Computer Science. * Sebastian Fischer: [38]Fair conjunction and disjunction. * Luke Palmer: [39]Certificate Design Pattern. * >>> Daniel Burrows: [40]Sometimes, tools are useful.. * GHC / OpenSPARC Project: [41]Pinning threads to cores. * Galois, Inc: [42]One Million Haskell Downloads.... * Thomas M. DuBuisson: [43]Yet another proposal for Haskell - the ever growing language. * Christopher Lane Hinson: [44]Twilight Hero. * Xmonad: [45]xmonad download statistics. * LHC Team: [46]Ease of implementation.. * Darcs: [47]darcs weekly news #22. * Well-Typed.Com: [48]Regression testing with Hackage. * Neil Mitchell: [49]Concise Generic Queries. * >>> Vo Minh Thu: [50]First stab at Template Haskell. * >>> Nax: [51]HStringTemplate. Quotes of the Week * ddarius: fmap: Because getting functions to the values is half the battle. * monochrom: Monad is about postmodernism. There are laws but no one owns them. You can interpret them any way you want. You can write about your own understanding and the meaning of your writing is not fixed. * conal: Recursion is the goto of functional programming * monochrom: "Monad is about computation." "Our company is about synergy." "iPod is about coolness." Godawful postmodernism nothingness. * olsner: nah, SkyNet is just a zygohistomorphic prepromorphism, nothing fancy * vixey: put some restriction like every token has a neighbourhood locally homeomophic to algol * gwern: Unfortunately Neo, no one can be *told* what the Monad is. About the Haskell Weekly News New editions are posted to [52]the Haskell mailing list as well as to [53]the Haskell Sequence and [54]Planet Haskell. [55]RSS is also available, and headlines appear on [56]haskell.org. To help create new editions of this newsletter, please see the information on [57]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [58]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://blogs.kent.ac.uk/crg-group/2009/03/chp-120-now-with-clocks/ 3. http://www.cs.kent.ac.uk/projects/ofa/chp/ 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55899 5. http://kyagrd.dyndns.org/~kyagrd/project/smartword/ 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55851 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HackMail 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55812 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fallingblocks 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55786 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/io%2Dcapture 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55783 13. http://www.haskell.org/haskellwiki/wxAsteroids 14. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55500 15. http://www.cs.cornell.edu/w8/~andru/cgi-perl/civs/results.pl?num_winners=1&… 16. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55495 17. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55446 18. http://code.google.com/p/winghci/ 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55434 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hranker 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55424 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/salvia 23. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55345 24. http://www.haskell.org/haddock/ 25. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55324 26. http://batterseapower.github.com/ansi-terminal/ 27. http://batterseapower.github.com/ansi-wl-pprint/ 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/55322 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/winio 30. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/55579 31. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/55539 32. http://planet.haskell.org/ 33. http://haskell.org/haskellwiki/Blog_articles 34. http://progandprog.blogspot.com/2009/03/i18n-and-haskell.html 35. http://holumbus.fh-wedel.de/blog/?p=21 36. http://physics-dph.blogspot.com/2009/03/recent-activity.html 37. http://blog.codersbase.com/2009/03/25/type-correct-changes-a-safe-approach-… 38. http://www-ps.informatik.uni-kiel.de/~sebf/projects/fair-predicates.html 39. http://lukepalmer.wordpress.com/2009/03/24/certificate-design-pattern/ 40. http://algebraicthunk.net/~dburrows/blog/entry/sometimes-tools-are-useful/ 41. http://ghcsparc.blogspot.com/2009/03/pinning-threads-to-cores.html 42. http://www.galois.com/blog/2009/03/23/one-million-haskell-downloads/ 43. http://tommd.wordpress.com/2009/03/22/yet-another-proposal-for-haskell-the-… 44. http://blog.downstairspeople.org/2009/03/22/twilight-hero/ 45. http://xmonad.wordpress.com/2009/03/22/xmonad-download-statistics/ 46. http://lhc-compiler.blogspot.com/2009/03/ease-of-implementation.html 47. http://blog.darcs.net/2009/03/darcs-weekly-news-22.html 48. http://blog.well-typed.com/2009/03/regression-testing-with-hackage/ 49. http://neilmitchell.blogspot.com/2009/03/concise-generic-queries.html 50. http://thu.selfip.org/entry/first-stab-th.html 51. http://nax.is-a-geek.net/blog/entries/10/hstringtemplate 52. http://www.haskell.org/mailman/listinfo/haskell 53. http://sequence.complete.org/ 54. http://planet.haskell.org/ 55. http://sequence.complete.org/node/feed 56. http://haskell.org/ 57. http://haskell.org/haskellwiki/HWN 58. http://code.haskell.org/~byorgey/code/hwn/

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