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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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Need help to edit the Haskell script in Winhugs and documentation.
by RAJESH DALSANIYA 28 Mar '09

28 Mar '09

* * *Both sections relate to the case study: Index for a document of text.* * * *SECTION A: * * * *Given the attached Haskell code which produces an index of words, make the following alterations by modifying existing functions and including new functions where necessary – parts 1) to 5):* * * *1) **Where a word occurs N times on the same line, ensure that the line number occurs n times in the index entry for that word. * * * *2) **Allow words to be hyphenated and treat a hyphenated word as a single word. However, for those words which are split over two lines, treat a split word as a single word without the hyphen. * * * *3) **Treat a capitalised word (one or more capitals) as being different from the word in all lower case (but they should still be sorted alphabetically) – unless it is at the start of a sentence with only the initial letter capitalised. A sentence is terminated by a ‘.’, ‘?’ or ‘!’. * * * *4) **Make the output more readable in the form of an index table in columns with appropriate spacing and without brackets. * * * *5) **Include a user-friendly menu, so that the user can choose input/output file names or default files, and choose to rerun or exit. * * * *Parts 1) to 5) may be developed in any order. * * * * * *SECTION B:* * * *6) **For your version of function, makeIndex (only), show how an alternative ordering of the composed functions would provide a more efficient execution of makeIndex. Justify your answer. * * * *7) **For the parts 1) to 5) above that you have attempted, discuss the use you have made of a) higher-order functions, b) list comprehension, c) monadic input/output, d) functional composition, and/or e) partial parameterisation (or Curried functions). Include an evaluation of how useful your use of these concepts has been. * * * *------------------------------------------------------------------- * * * *import Prelude* * * *type Doc = String* *type Line = String* *type Word = String * * * *makeIndex :: Doc -> [ ([Int], Word) ]* * * *makeIndex* * = shorten . -- [([Int], Word)] -> [([Int], Word)]* * amalgamate .-- [([Int], Word)] -> [([Int], Word)]* * makeLists . -- [(Int, Word)] -> [([Int], Word)]* * sortLs . -- [(Int, Word)] -> [(Int, Word)]* * allNumWords .-- [(Int, Line)] -> [(Int, Word)]* * numLines . -- [Line] -> [(Int, Line)]* * splitUp -- Doc -> [Line]* * * *splitUp :: Doc -> [Line]* * * *splitUp [] = []* *splitUp text* * = takeWhile (/='\n') text : -- first line* * (splitUp . -- splitup other lines* * dropWhile (==’\n’) . -- delete 1st newline(s)* * dropWhile (/='\n')) text -- other lines* * * *numLines :: [Line] -> [(Int, Line)]* * * *numLines lines -- list of pairs of * * = zip [1 .. length lines] lines -- line no. & line* * * *-- for each line* *-- a) split into words* *-- b) attach line no. to each word* * * *splitWords :: Line -> [Word] -- a)* * * *splitWords [] = []* *splitWords line * * = takeWhile isLetter line : -- first word in line* * (splitWords . -- split other words* * dropWhile (not.isLetter) . -- delete separators* * dropWhile isLetter) line -- other words* * where* * isLetter ch* * = (‘a’<=ch) && (ch<=’z’)* * || (‘A’<=ch) && (ch<=’Z’)* * * *numWords :: (Int, Line) -> [(Int, Word)] -- b)* * * *numWords (number, line)* * = map addLineNum ( splitWords line) -- all line pairs* * where* * addLineNum word = (number, word) -- a pair* * * *allNumWords :: [(Int, Line)] -> [(Int, Word)]* * * *allNumWords = concat . map numWords -- doc pairs * * * *sortLs :: [(Int, Word)] -> [(Int, Word)]* * * *sortLs [ ] = [ ]* *sortLs (a:x)* * = sortLs [b | b <- x, compare b a] -- sort 1st half* * ++ [a] ++ -- 1st in middle* * sortLs [b | b <- x, compare a b] -- sort 2nd half* * where* * compare (n1, w1) (n2, w2)* * = (w1 < w2) -- 1st word less* * || (w1 == w2 && n1 < n2) -- check no. * * * *makeLists :: [(Int, Word)] -> [([Int], Word)]* * * *makeLists * * = map mk -- all pairs* * where mk (num, word) = ([num], word)* * -- list of single no.* * * *amalgamate :: [([Int], Word)] -> [([Int], Word)]* * * *amalgamate [ ] = [ ]* *amalgamate [a] = [a]* *amalgamate ((n1, w1) : (n2, w2) : rest)-- pairs of pairs* * | w1 /= w2 = (n1, w1) : amalgamate ((n2, w2) : rest)* * | otherwise = amalgamate ((n1 ++ n2, w1) : rest)* * -- if words are same grow list of numbers* * * *shorten :: [([Int], Word)] -> [([Int], Word)]* * * *shorten* * = filter long -- keep pairs >4* * where* * long (num, word) = length word > 4 -- check word >4* * *

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ByteStrings, FFI, iovec
by Antoine Latter 28 Mar '09

28 Mar '09

Folks, I'm putting together an FFI layer to a library which uses writev for its IO, and so expects an array of iovec structs as an argument to one of its calls. for more information, check out "man writev"[1]. Since I'm already using Data.Binary, the writev call almost sounds like a perfect match for lazy bytestrings, so I thought I'd put up what I have for comments. I'm using c2hs[2]. The recursive nesting of 'withForeignPtr' put me off at first, but I thought I'd at least give it a shot. I haven't put this into practice yet, so it could be buggy. I'm more looking for comments on the approach. Antoine [1] http://linux.die.net/man/3/writev [2] http://www.cse.unsw.edu.au/~chak/haskell/c2hs/ Foreign/IOVec.chs >>>>> -- -*-haskell-*- module Foreign.IOVec (IOVec() ,withLazyByteString ) where import C2HS import qualified Data.ByteString as S import qualified Data.ByteString.Internal as S import qualified Data.ByteString.Lazy as L #include <sys/uio.h> #c typedef struct iovec hs_iovec; #endc {#pointer *hs_iovec as IOVec newtype#} -- | This is intended for calling into something like writev. -- But I suppose that nothing stops you from calling into -- readv and blowing away the passed-in ByteString. withLazyByteString :: L.ByteString -> (IOVec -> Int -> IO b) -> IO b withLazyByteString b f = let bs = L.toChunks b num = length bs in allocaBytes (num * {#sizeof hs_iovec#}) $ \vecAry -> go vecAry 0 bs where go vecAry off [] = f (IOVec vecAry) off go vecAry off (b:bs) = let vec = vecAry `plusPtr` (off * {#sizeof hs_iovec#}) (fptr, bsOff, bsLen) = S.toForeignPtr b in withForeignPtr fptr $ \bsPtr -> do {#set hs_iovec->iov_base#} vec $ castPtr $ bsPtr `plusPtr` bsOff {#set hs_iovec->iov_len#} vec $ cIntConv bsLen go vecAry (off+1) bs <<<<<

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Re: Exception handling in numeric computations
by Gregory Petrosyan 28 Mar '09

28 Mar '09

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? Gregory

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putting SOE 'on the library path'
by Michael Mossey 28 Mar '09

28 Mar '09

This is a beginners question, which I have posted to beginners(a)haskell.org several times, but because I have gotten no answer, I thought I would try the cafe. I'm working through the "School of Expression" book, and I would like to install the code that comes with the book somewhere. Right now I have to write my exercise programs in the same directory where the SOE code is sitting so that my exercise programs can import SOE modules. My platform is Windows. I tried using the -i options to ghc and ghci, but to no avail. It wasn't clear on Windows in what form -i takes its arguments: backslashes as usual for Windows, forward slashes perhaps? How about paths with spaces in them? Does it need quotes around the filename? I tried all combinations. ghc never gave an error indicating there was something wrong with what I typed. It happily accepted all forms of -i. But it never found the libraries. Thanks, Mike

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Conditional compilation
by Robin Green 28 Mar '09

28 Mar '09

I am writing some code for citation support in gitit, and all the #ifdefs I'm using to do conditional compilation are a bit tiresome. Suppose you have the requirement that a certain feature of your software be disable-able at compile time, to avoid having to pull in certain dependencies (which may not be available on all platforms). Disabling a feature may entail removing certain fields from certain constructors (again, to avoid pulling in certain dependencies), and/or removing certain functions from certain modules. What is the best way to do this in Haskell? This problem description suggests that perhaps insights from Aspect-Oriented Programming and/or Software Product Lines may be relevant. However, I haven't heard of much work that relates these concepts to Haskell. Maybe this would be a good topic for an enterprising student? -- Robin

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