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Re: [Haskell-cafe] Rational and % operator remix
by michael rice 30 Mar '09

30 Mar '09

Works like a charm. Here's a final copy for those interested. Michael ===== import Data.Ratio cf2 :: Rational -> [Int] cf2 a = let ai = floor a air = toRational ai in ai : if a == air then [] else cf2 (1 / (a - air)) --- On Sun, 3/29/09, Miguel Mitrofanov <miguelimo38(a)yandex.ru> wrote: From: Miguel Mitrofanov <miguelimo38(a)yandex.ru> Subject: Re: [Haskell-cafe] Rational and % operator remix To: "michael rice" <nowgate(a)yahoo.com> Cc: "Lennart Augustsson" <lennart(a)augustsson.net>, haskell-cafe(a)haskell.org Date: Sunday, March 29, 2009, 4:25 PM cf2 a = let ai = floor a air = toRational ai in ai : if a == air then [] else cf2 (1 / (a - air)) On 30 Mar 2009, at 00:19, michael rice wrote: > I'm a Lisper, kind of feeling my way around here in Haskell, so please bear with me. > > I did the things you suggested, but I think the last one may have gotten garbled. Anyway, this is what I ended up with > > cf2 :: Rational -> [Int] > cf2 a = let ai = floor a > in > if a == (toRational ai) > then [ai] > else ai : cf2 (1 / (a - ai)) > > but I'm still at least one error short of a clean run > > Main> :load cf.hs > ERROR "cf.hs":7 - Type error in application > *** Expression : ai : cf2 (1 / (a - ai)) > *** Term : ai > *** Type : Ratio Integer > *** Does not match : Int > > > Where did I go wrong or what did I leave out? > > Michael > > > --- On Sun, 3/29/09, Lennart Augustsson <lennart(a)augustsson.net> wrote: > > From: Lennart Augustsson <lennart(a)augustsson.net> > Subject: Re: [Haskell-cafe] Rational and % operator remix > To: "michael rice" <nowgate(a)yahoo.com> > Cc: haskell-cafe(a)haskell.org > Date: Sunday, March 29, 2009, 2:29 PM > > You can use floor in a Rational directly, no need to take it apart and divide. > > There is no need to write (toRational 1), just write 1. > > Don't write (subtract ai a), write (ai - i). > > You also have a type error; the ai should no be a Rational, so you > need to move to toRational call to the comparison. > > -- Lennart > > 2009/3/29 michael rice <nowgate(a)yahoo.com>: > > 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)) > > > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Rational and % operator remix
by michael rice 30 Mar '09

30 Mar '09

I'm a Lisper, kind of feeling my way around here in Haskell, so please bear with me. I did the things you suggested, but I think the last one may have gotten garbled. Anyway, this is what I ended up with cf2 :: Rational -> [Int] cf2 a = let ai = floor a in if a == (toRational ai) then [ai] else ai : cf2 (1 / (a - ai)) but I'm still at least one error short of a clean run Main> :load cf.hs ERROR "cf.hs":7 - Type error in application *** Expression : ai : cf2 (1 / (a - ai)) *** Term : ai *** Type : Ratio Integer *** Does not match : Int Where did I go wrong or what did I leave out? Michael --- On Sun, 3/29/09, Lennart Augustsson <lennart(a)augustsson.net> wrote: From: Lennart Augustsson <lennart(a)augustsson.net> Subject: Re: [Haskell-cafe] Rational and % operator remix To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Sunday, March 29, 2009, 2:29 PM You can use floor in a Rational directly, no need to take it apart and divide. There is no need to write (toRational 1), just write 1. Don't write (subtract ai a), write (ai - i). You also have a type error; the ai should no be a Rational, so you need to move to toRational call to the comparison. -- Lennart 2009/3/29 michael rice <nowgate(a)yahoo.com>: > 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)) > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Re: [Haskell-cafe] Rational and % operator remix
by michael rice 30 Mar '09

30 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 30 Mar '09

30 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 30 Mar '09

30 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 29 Mar '09

29 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/

4 3

Need help to edit the Haskell script in Winhugs and documentation.
by RAJESH DALSANIYA 29 Mar '09

29 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* * *

2 1

ByteStrings, FFI, iovec
by Antoine Latter 29 Mar '09

29 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 <<<<<

3 2

Re: Exception handling in numeric computations
by Gregory Petrosyan 29 Mar '09

29 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

2 1

putting SOE 'on the library path'
by Michael Mossey 29 Mar '09

29 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

3 3

Conditional compilation
by Robin Green 29 Mar '09

29 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

4 3

Building GHC under cygwin
by Zachary Turner 28 Mar '09

28 Mar '09

Has anyone had any luck building a windows version of GHC under cygwin? I followed the instruction on the building guide to build it under MinGW/msys, which works, but when I try to build it under cygwin instead of msys I get this: cabal-bin.exe: Cannot find the program 'ghc' at '/cygdrive/c/dev/haskell/ghc/ghc-6.10.1/bin/ghc' or on the path make[1]: *** [bootstrapping.conf] Error 1 make[1]: Leaving directory `/cygdrive/c/dev/haskell/ghc/ghc-6.10.1-src/libraries' make: *** [stage1] Error 2 Fairly early in the build process. Looking in that folder, there is a ghc.exe there, but not a ghc. I'm not too familiar with cygwin so I feel like this is something fairly obvious, but I didn't see it covered on the wiki anywhere. A google search indicated that I might have to override the host mode in the configure state but didn't elaborate, and also I'm not sure how to do that. I also want to make sure that I get a *windows* version of GHC out of the build, as the post I found suggested that might not be the case.

1 0

Haxr doesn't compile from cabal (HTTP > 4000 breaks it)
by Jeff Heard 28 Mar '09

28 Mar '09

The haxr cabal library dependencies seem to be off. I wonder, since haxr would benefit highly from the HTTP 4k series of performance improvements, is it trivial to make it compatible with the latest library? -- Jeff

2 1

Re: [Haskell-cafe] Definition of "tail recursive" wrt Folds
by Brent Yorgey 28 Mar '09

28 Mar '09

On Wed, Mar 25, 2009 at 05:54:17PM +1030, Mark Spezzano wrote: > > What, strictly speaking, is the definition of ”tail recursive” as opposed to > just “recursive”? A recursive function is tail recursive if the final result of the recursive call is the final result of the function itself. If the result of the recursive call must be further processed (say, by adding 1 to it, or consing another element onto the beginning of it), it is not tail recursive. With that said, tail recursion is not that useful of a concept in a lazy language like Haskell. The important concept to know in Haskell is 'guarded recursion', where any recursive calls occur within a data constructor (such as foldr, where the recursive call to foldr occurs as an argument to (:)). This allows the result of the function to be consumed lazily, since it can be evaluated up to the data constructor and the recursive call delayed until needed. -Brent

4 3

Haxr doesn't compile from cabal (HTTP > 4000 breaks it)
by Henning Thielemann 28 Mar '09

28 Mar '09

Most breakage caused by new HTTP package is due to the renaming from Response to Response_String and Request to Request_String. I think it was not a good idea to do that. There could have well be two types named Response from different modules, one with a type parameter and the other one without. Or the parametrized variant could have be named ResponseGeneric or so. That would have allowed packages to work both with version 3001 and 4000.

2 1

Lazy counting and printing
by Anakim Border 28 Mar '09

28 Mar '09

Hi, I have written a code like this: import Control.Monad.State countedRecords :: [Int] -> StateT Int IO [Int] countedRecords [] = return [] countedRecords (r:rs) = do modify (+1) rest <- countedRecords rs return $ r:rs printEven :: [Int] -> StateT Int IO () printEven records = do let even = filter (\n -> n `mod` 2 == 0) records lift $ mapM_ (putStrLn . show) even combine :: [Int] -> StateT Int IO () combine records = countedRecords records >>= printEven main = do let numbers = [1..1000000] count <- execStateT (combine numbers) 0 putStrLn $ "Processed " ++ show count ++ " numbers." My intent is that of processing a large collection of records (in this example, integers) doing two things at once: count their total number and print those that satisfy a certain condition. I would like to keep the definitions of the two functions performing such tasks independent. As a result I've used the State monad to hide intermediate counts. Unfortunately the code above breaks laziness, in that records are first completely counted (and stored in memory) and only then filtered and printed. If I redefine main as: main = do let numbers = [1..] .. i.e. I produce an unbounded number of records, I get a stack overflow. Moreover, I don't like the way countedRecords and printEven are defined, because their type signatures are too "broad". I would prefer them to be: countedRecords :: [Int] -> State Int [Int] printEven :: [Int] -> IO () and let "combine" do some sort of lifting into the StateT monad. Is there any way to perform both operations lazily? Thanks

1 0

ANN: smartword 0.0.0.5 Web based flash card for Word Smart I and II vocabularies
by Ahn, Ki Yung 28 Mar '09

28 Mar '09

Name: smartword Synopsis: Web based flash card for Word Smart I and II vocabularies Version: 0.0.0.5 Homepage: http://kyagrd.dyndns.org/~kyagrd/project/smartword/ Category: Web,Education License: BSD3 License-file: LICENSE Author: Ahn, Ki Yung Maintainer: Ahn, Ki Yung <kya(a)pdx.edu> =================================================================== Web based online study tool for all vocabularies in Word Smart I and II, a poular book series for studying GRE vocabularies. I typed the vocabulary data and wrote the program in 2004, because I got too boring just going over the strange English words. If you don't read Korean, you can just ignore the Korean translation. Source code is outdated, so never even think of using it as a web programming reference. However, it will still be helpful as a neat web application when one tries to squeeze GRE vocabularies into the volatile memory of human brain. Installation: You can either compile CGI binaries with GHC or use Hugs to run lhs as a CGI script. Copy all .cgi files and data directories (book1, book1.ans, book2, book2.ans) into your webserver CGI directory (usually cgi-bin). Usage: If you get it wright the flash card goes away, but if you didn't get it the flash card goes to the bottom of the deck again. So, it won't end until you get all of them right.

1 0

Use unsafePerformIO to catch Exception?
by Xiao-Yong Jin 28 Mar '09

28 Mar '09

Hi, I just feel it is not comfortable to deal with exceptions only within IO monad, so I defined > tryArith :: a -> Either ArithException a > tryArith = unsafePerformIO . try . evaluate and it works quite good as > map (tryArith . (div 5)) [2,1,0,5] evaluates to > [Right 2,Right 5,Left divide by zero,Right 1] However, I guess unsafePerformIO definitely has a reason for its name. As I read through the document in System.IO.Unsafe, I can't convince myself whether the use of 'tryArith' is indeed safe or unsafe. I know there have been a lot of discussion around unsafePerformIO, but I still can't figure it out by myself. Can someone share some thoughts on this particular use of unsafePerformIO? Is it safe or not? And why? Thanks, Xiao-Yong -- c/* __o/* <\ * (__ */\ <

17 49

is there any reason why Language.C.Syntax.AST.CTranslUnit doesn't derive show?
by Anatoly Yakovenko 28 Mar '09

28 Mar '09

is there any reason why Language.C.Syntax.AST.CTranslUnit doesn't derive show? I would like to look at the data structure it generates. It's a lot easier to experiment it when i can write a template C file, print out the AST and then modify that data structure directly, instead of trying to grok the library. Thanks for your great work btw, the parser is pretty sweet. Anatoly

2 2

Parallel term reduction---solution
by John D. Ramsdell 28 Mar '09

28 Mar '09

On February 1, I sent a message to this list asking for advice on how to write a parallel term reduction system. I now have a semi-explicit parallelised version of a term reduction system that makes effective use of multiple CPUs. The enclosed code describes the solution, even though its performance on the example reduction rule does not show a speed up. In the actual reduction system. profiling shows that 99% of the runtime and 98% of the allocations occur in the equivalent of the branch function below, for a representative test case. It's no surprise that just one parallel function, parMap, is all that's needed. John > module Main (main) where > import System.Time (ClockTime(..), getClockTime) > import Data.Tree (Tree(..), flatten) > import Data.Maybe (isNothing) > import Control.Parallel The reduction system takes a rule, a step count, and an initial value, and computes a tree of reductions. > reduce :: (Eq a, Monad m) => (a -> [a]) -> Int -> a -> m (Tree a) > reduce rule limit root = > step rule limit [top] [top] > where > top = Item { item = root, parent = Nothing } The Item data structure stores the information about a reduction step in a form that can be used to construct the final answer as a tree. > data Eq a => Item a > = Item { item :: a, > parent :: Maybe (Item a) } > instance Eq a => Eq (Item a) where > x == y = item x == item y The step function is where nearly all of the time is used. It is called as: step rule limit seen todo where seen is the items already seen, and todo is the items on the queue. The order of the items in the seen list is irrelevant, because the tree is assembled as a post processing step. > step :: (Eq a, Monad m) => (a -> [a]) -> Int -> > [Item a] -> [Item a] -> m (Tree a) > step _ _ seen [] = tree seen > step rule limit seen todo = > combine rule seen limit [] [] (parMap (branch rule seen) todo) Each branch of the rule derivation tree can be evaluated in parallel. Furthermore, most of the checking to see if an item has been seen before can be done in parallel. > branch :: Eq a => (a -> [a]) -> [Item a] -> Item a -> Reduct a > branch rule seen parent = > Reduct parent (seqList kids) > where > kids = [ kid | kid <- rule (item parent), not (dejaVu kid seen) ] This data structure ensures as much computation as possible is done in parallel. > data Eq a => Reduct a = Reduct !(Item a) ![a] Has x been seen before? > dejaVu :: Eq a => a -> [Item a] -> Bool > dejaVu x seen = any (\i-> x == item i) seen Combine the results of one level of the rule derivation tree. The combination function is not associative, so there are no opportunities for parallelisation. > combine :: (Eq a, Monad m) => (a -> [a]) -> [Item a] -> Int -> > [Item a] -> [Item a] -> [Reduct a] -> m (Tree a) > combine rule seen' limit seen todo [] = > step rule limit (seen ++ seen') (reverse todo) > combine _ _ limit _ _ _ > | limit <= 0 = fail "Step limit exceeded" > combine rule oseen limit seen todo (Reduct p kids : reducts) = > combine rule oseen (limit - 1) seen' todo' reducts > where > (seen', todo') = foldr f (seen, todo) kids > f kid (seen, todo) = > if dejaVu kid seen then > (seen, todo) > else > (x:seen, x:todo) > where > x = Item { item = kid, parent = Just p } > seqList :: [a] -> [a] > seqList xs = > loop xs > where > loop [] = xs > loop (y : ys) = seq y (loop ys) > parMap :: (a -> b) -> [a] -> [b] > parMap _ [] = [] > parMap f (x:xs) = > par y (pseq ys (y:ys)) > where > y = f x > ys = parMap f xs The next two functions assemble the answer into a tree. Sequential search is just fine for tree building. > tree :: (Eq a, Monad m) => [Item a] -> m (Tree a) > tree items = > case filter (isNothing . parent) items of > [root] -> return (build items (item root)) > _ -> fail "bad tree" > build :: Eq a => [Item a] -> a -> Tree a > build items root = > Node { rootLabel = root, > subForest = map (build items . item) children } > where > children = filter child items > child i = maybe False ((== root) . item) (parent i) A silly rule > rule :: Int -> [Int] > rule n = filter (>= 0) [n - 1, n `quot` 2, n `quot` 3] > secDiff :: ClockTime -> ClockTime -> Float > secDiff (TOD secs1 psecs1) (TOD secs2 psecs2) > = fromInteger (psecs2 - psecs1) / 1e12 + fromInteger (secs2 - secs1) > main :: IO () > main = > do > t0 <- getClockTime > t <- reduce rule 20000 5000 > let ns = length (flatten t) > t1 <- getClockTime > putStrLn $ "length: " ++ show ns > putStrLn $ "time: " ++ show (secDiff t0 t1) ++ " seconds" The makefile ------------ PROG = parreduce GHCFLAGS = -Wall -package containers -package parallel -threaded \ -fno-warn-name-shadowing -O %: %.lhs ghc $(GHCFLAGS) -o $@ $< all: $(PROG) clean: -rm *.o *.hi $(PROG)

1 0

Haddock GSoC project
by Isaac Dupree 28 Mar '09

28 Mar '09

I'm interested in being a GSoC student, and the Haddock-related tickets looked like a good place to start http://hackage.haskell.org/trac/summer-of-code/ticket/1567 http://hackage.haskell.org/trac/summer-of-code/ticket/1568 http://hackage.haskell.org/trac/summer-of-code/ticket/1569 ... haddock could use some love! And I love documentation. I think I'd start by hacking on some relatively easy Haddock tickets to find my way around the code and the hacking process. Then it might be time to move into one of the bigger projects (I'm tempted to say #1567, "Haddock: allow documentation to propagate between packages"), depending on priorities; probably still working on some smaller but important Haddock tickets. Then it depends how much time we have left in the summer, there's a lot that can be done! - I've hacked on the GHC lexing/parsing code a bit, and I know darcs, haskell, etc., and I've been around watching on mailing-lists since before Haddock 2.x, so I feel like I have a fair amount of context with which to approach this project. What do you think, is this a good project to look towards? What's the next step... should I elaborate my proposal by looking at Haddock tickets and their priorities? But I should have your feedback first; what do the mentors, or the Haskell community, want most to be improved about Haddock? -Isaac

3 4

Re: Exception handling in numeric computations
by John Lato 28 Mar '09

28 Mar '09

> > Message: 8 > Date: Fri, 27 Mar 2009 10:41:05 -0700 > From: Jonathan Cast <jonathanccast(a)fastmail.fm> > Subject: Re: [Haskell-cafe] Re: Exception handling in numeric > computations > To: Gregory Petrosyan <gregory.petrosyan%2Bhaskell(a)gmail.com> > Cc: haskell-cafe(a)haskell.org > Message-ID: <1238175665.20367.12.camel@jcchost> > Content-Type: text/plain; charset=utf-8 > > On Fri, 2009-03-27 at 20:38 +0300, Gregory Petrosyan wrote: >> On Fri, Mar 27, 2009 at 7:31 PM, Donn Cave <donn(a)avvanta.com> wrote: >> > Quoth John Lato <jwlato(a)gmail.com>, >> > >> >> An exception is caused by some sort of interaction with the run-time >> >> system (frequently a hardware issue). The programmer typically can't >> >> check for these in advance, but can only attempt to recover after >> >> they've happened. >> >> >> >> An error is some sort of bug that should be fixed by the programmer. >> > >> > I have never felt that I really understood that one. >> >> Me too :-) >> >> BTW, John, how often do you encounter _hardware_ issues compared to "errors"? > > Can't speak for anyone else, but I usually encounter hardware issues > just before I replace the hardware... This is getting confusing with two "John"'s replying. For myself, I do not encounter hardware issues often at all (unless you count buggy drivers). They are the exceptional condition. > >> Is an "out of memory" thing an error or exception? >> You will say "exception, for sure", wouldn't you? :-) > > No. GHC possesses an out-of-memory exception that (IIRC) it never > throws, because it's simply not worth trying to recover from heap > exhaustion. Maybe 20 years ago it was, but these days a program that > manages to exhaust space is almost certainly either buggy or poorly > optimized. I would have said exception, although I agree with the assessment that it's not worth trying to recover from. > > An `error' is any condition where the correct response is for the > programmer to change the source code :) > >> And if it is a >> result of applying >> known-to-be-very-memory-hungry algorithms to non-trivial input? Looks like >> programmer's error, doesn't it? > > See above. Due to their brevity, I sometimes find it difficult to understand Jonathan's replies. So at the risk of putting words in his mouth, I will take an attempt to explain this further. Keep in mind these are my words, not his, and it's possible I have completely misunderstood his point since I've only been using Haskell intensively for about a year (and I have no formal CS training, and other biographical points apply as well). Running out of memory is an exceptional condition. Doing something that is likely to cause memory exhaustion is a programming error, because it was the wrong solution to the problem. Memory exhaustion, the exceptional condition, is an end result of making the programming error. In this case (but not all), it is possible to treat the symptoms by handling the exception, but the general solution is to avoid the problem in the first place. > >> And I think I can provide lots of similar examples. >> >> If there exists separation between errors and exceptions, it should be >> very strong >> and evident — otherwise "casual programmers" like myself will need to >> stare at the >> ceiling every time they write something to decide what suits best. > I am not sure when I would every throw an exception in Haskell code, short of re-raising from a handler. Things that I would have used exceptions for in C-sharp can be handled explicitly with Maybe or Either, a style that I greatly prefer. John > Protip: try pacing instead of staring at the ceiling. > > jcc >

1 0

Re: Use unsafePerformIO to catch Exception?
by John Lato 28 Mar '09

28 Mar '09

> From: Jules Bean <jules(a)jellybean.co.uk> > wren ng thornton wrote: >> The type of head should not be [a] -> a + Error, it should be (a:[a]) -> >> a. With the latter type the compiler can ensure the precondition will be >> proved before calling head, thus eliminating erroneous calls. > > Yes, but you know and I know that's not haskell. > > I'm talking about haskell. > > In haskell - a language which does not fully support dependent types - > head is both necessary and useful. > I could follow the rest of this, but I don't understand why 'head' is necessary. Couldn't you always replace it with a case statement, with undefined on [] if necessary? I won't deny that it's extremely useful, though! John Lato

3 3

Internationalization of Haskell code
by Vasyl Pasternak 28 Mar '09

28 Mar '09

In my blog http://progandprog.blogspot.com/2009/03/i18n-and-haskell.html, I've described process of internationalization Haskell programms using GNU GetText. I wrote hgettext tool and Text.I18N.GetText library to help with internationalization (latest sources on http://hgettext.googlecode.com/files/hgettext-0.1.2.tar.gz) A bit later I plan to put it to hackage and wiki (waiting for account). -- Best regards, Vasyl Pasternak

4 3

The infamous evil monomorphism restriction (was: A bit of a shock - Memoizing functions)
by Peter Verswyvelen 28 Mar '09

28 Mar '09

>From a previous email in the beginners list I more or less understood that the monomorphism restriction will not exist anymore in Haskell Prime. Is this correct? On Fri, Mar 27, 2009 at 10:32 PM, Jonathan Cast <jonathanccast(a)fastmail.fm>wrote: > On Fri, 2009-03-27 at 14:26 -0700, Kirk Martinez wrote: > > Your powersOfTwo function, since it gets memoized automatically (is > > this the case for all functions of zero arguments?), > > It is the case for all functions which have zero arguments *at the time > they are presented to the code generator*. The infamous evil > monomorphism restriction arises from the fact that overloaded > expressions, such as > > negative_one = exp(pi * sqrt(-1)) > > look like functions of zero arguments, but are not, and hence do not get > memoized. This behavior was considered sufficiently surprising, when it > was discovered in early Haskell compilers, that the construct was > outlawed from the language entirely. > > jcc > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 1

A bit of a shock - Memoizing functions
by Gü?nther Schmidt 28 Mar '09

28 Mar '09

Hi, um, well, I'm not even sure if I have correctly understood this. Some of the memoizing functions, they actually "remember" stuff *between* calls? Günther

9 9

toSelem takes no format parameter
by Kemps-Benedix Torsten 28 Mar '09

28 Mar '09

Sterling, the toSElem function of ToSElem does not take a parameter that allows specification of e.g. the locale that shall be used for rendering a specific attribute. If one wants to change this, one also has to change setAttribute and the like. Do you have any plans to extend HStringTemplate in this way? Schöne Grüße Torsten Kemps-Benedix (Geschäftsführer) SKS Unternehmensberatung GmbH & Co KG Geheimrat-Hummel-Platz 4 D-65239 Hochheim am Main Germany M +49.163.36017.01 T +49.700.36017.000 F +49.700.36017.011 torsten.kemps-benedix(a)sks-ub.de <blocked::mailto:torsten.kemps-benedix@sks-ub.de> http://www.sks-ub.de <blocked::http://www.sks-ub.de/>

2 1

wxHaskell
by Tsunkiet Man 28 Mar '09

28 Mar '09

Hello, I've just installed wxHaskell as follow: - Download wxHaskell from the following page: http://haskell.org/haskellwiki/WxHaskell/Download and downloaded the binairy version for Windows XP. - Registered using the batch file provided. Now I have this small program from my education page that I want to run just to make sure my installation is correct. ( http://www.cs.uu.nl/wiki/pub/FP/Practicum/Balls.hs) I've also got Helium (2003) and winHugs (sept 2006) (I'm so sorry for using the old versions, but I just can't find new versions) My GHC compiler is version: GHC 6.10.1 and my wxHaskell is named like: wxhaskell-bin-msw2.8.9-ghc6.10.1-0.11.0-0.zip. (If you need more information just contact me) Now my error message in winHugs is: And if ERROR file:.\Balls.hs - Can't find imported module "Graphics.UI.WX" And if I run that in Helium I get some error message that the . in "Graphics.UI.WX" is unexpected. (Which means by my views that it didn't import wxHaskell.) And if I run it in ghci it compiles perfect. So I thought let's type in "main" to start it in a interpreter. So I get the following: http://img10.imageshack.us/img10/5118/testzrs.jpg. I don't think that program is running correctly. Can someone point out what went wrong or what I am doing wrong? And some small question, do I always have to compile those programs in order for it to run correctly? Thanks for your time and effort! Tsunkiet Man

2 2

want to upgrade to 6.10.1 ghc
by Vasili I. Galchin 28 Mar '09

28 Mar '09

Hello, I am running Ubuntu Linux. I have forgotten what is the preferred means of upgrading. Download the source and compile it using my current compiler .. 6.8.2? Regards, Vasili

2 1

ANN: HackMail 0.0 -- Procmail + Monads = Awesome!
by Joe Fredette 28 Mar '09

28 Mar '09

I'm happy to announce my very second Hackage upload!* HackMail is a Procmail-alike, though it doesn't (yet) support promail syntax. It dynamically loads a haskell source file (literate or vanilla, eithers okay) 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. As well as some configuration data. Not everything is working just yet -- the program is operational, but still lacking some of the features I want, namely a pipe mode, and some other things. It also uses a homebrew email parser, and lacks Haddock and Tests, but I wanted to release it mostly for comments/questions/helpful criticism from interested parties. Also, I wanted to upload something to hackage... It's a good feeling. Questions/Comments/Donations of Money, Vestal Virgins, or Cookies welcome! Package is on Hackage @ http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HackMail Source is on Patch-tag @ http://patch-tag.com/repo/Hackmail /Joe Fredette * (I uploaded a little tool called addLicenseInfo to test out the procedure, so... yah.)

1 0

Arch Linux now provides more than 1000 Haskell libraries and apps
by Don Stewart 28 Mar '09

28 Mar '09

A small milestone in the packaging business: http://archhaskell.wordpress.com/2009/03/27/arch-haskell-news-mar-14-2009-1… More than 1000 Haskell packages packaged up for Arch Linux. Hackage now has 1163 (+41) Haskell packages, of which 1007 (+33) have been natively packaged for Arch in AUR. That’s 33 new packages in two weeks, and lots of updates as well. Read this week's updates on the blog. -- Don

1 0

typeOf for polymorphic value
by oleg＠okmij.org 27 Mar '09

27 Mar '09

> Does anyone know of a trick to accomplish `typeOf id'? > Using something else than TypeRep as the representation, of course. Yes. The analysis of polymorphic types has been used in the inverse type-checker http://okmij.org/ftp/Haskell/types.html#de-typechecker The enclosed code computes TypeRep of polymorphic values. The code returns real TypeRep. Here are a few examples: tt2 = mytypof not -- Bool -> Bool tt3 = mytypof [True] -- [Bool] tt4 = mytypof id -- any1 -> any1 tt5 = mytypof [] -- [any1] tt6 = mytypof const -- any1 -> any2 -> any1 tt7 = mytypof Just -- any1 -> Maybe any1 tt8 = mytypof map -- (any1 -> any2) -> [any1] -> [any2] tt9 = mytypof maybe -- any1 -> (any2 -> any1) -> Maybe any2 -> any1 The code was tested on GHC 6.8.2. I don't have access to any computer with GHC 6.10, so I can't say how it works with that version of GHC. {-# LANGUAGE ScopedTypeVariables, EmptyDataDecls #-} {-# LANGUAGE FlexibleInstances, FlexibleContexts #-} {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE IncoherentInstances #-} module Typeof where import Data.Typeable -- tests tt1 = mytypof True -- Bool tt2 = mytypof not -- Bool -> Bool tt3 = mytypof [True] -- [Bool] tt4 = mytypof id -- any1 -> any1 tt5 = mytypof [] -- [any1] tt6 = mytypof const -- any1 -> any2 -> any1 tt7 = mytypof Just -- any1 -> Maybe any1 tt8 = mytypof map -- (any1 -> any2) -> [any1] -> [any2] tt9 = mytypof maybe -- any1 -> (any2 -> any1) -> Maybe any2 -> any1 class MyTypeable a where mytypof :: a -> TypeRep -- Gamma is the sequence of type varaibles. It is used to assign -- different indices to different type variables instance (Analyze a result, MyTypeable' HNil gout result) => MyTypeable a where mytypof a = fst $ mytypof' HNil (analyze a) class MyTypeable' gin gout classification | gin classification -> gout where mytypof' :: gin -> classification -> (TypeRep,gout) instance Typeable a => MyTypeable' gin gin (TAtomic a) where mytypof' gin _ = (typeOf (undefined::a), gin) instance Typeable a => MyTypeable' gin gin (TAtomic1 a) where mytypof' gin _ = (typeOf (undefined::a), gin) instance (MyTypeable' gin g a, MyTypeable' g gout b) => MyTypeable' gin gout (TFunction a b) where mytypof' gin _ = let (tr1,g) = mytypof' gin (undefined::a) (tr2,gout) = mytypof' g (undefined::b) in (mkFunTy tr1 tr2,gout) instance (MyTypeable' gin g c, MyTypeable' g gout a) => MyTypeable' gin gout (TConstructed c a) where mytypof' gin _ = let (tr1,g) = mytypof' gin (undefined::c) (tr2,gout) = mytypof' g (undefined::a) in (mkTyConApp (typeRepTyCon tr1) [tr2],gout) instance (HIndex a gin n, MyTypeable'' n gin gout (TVariable a)) => MyTypeable' gin gout (TVariable a) where mytypof' = mytypof'' (undefined::n) class MyTypeable'' n gin gout classification | n gin classification ->gout where mytypof'' :: n -> gin -> classification -> (TypeRep,gout) instance HLen gin n1 => MyTypeable'' Z gin (HCons a gin) (TVariable a) where mytypof'' _ gin _ = (mkany (undefined::S n1), HCons (undefined::a) gin) instance Nat n => MyTypeable'' (S n) gin gin (TVariable a) where mytypof'' _ gin _ = (mkany (undefined::S n), gin) mkany :: Nat n => n -> TypeRep mkany n = mkTyConApp (mkTyCon ts) [] where ts = "any" ++ show (nat n) -- Lookup the index of an item x in the list l -- The index is 1-based. If not found, return 0 class Nat n => HIndex x l n | x l -> n instance HIndex x HNil Z instance (Nat n, TypeEq x a f, HIndex' f x (HCons a l) n) => HIndex x (HCons a l) n class HIndex' f x l n | f x l -> n instance HLen l n => HIndex' HTrue x l n instance HIndex x l n => HIndex' HFalse x (HCons a l) n -- Length of a list class Nat n => HLen l n | l -> n instance HLen HNil Z instance HLen l n => HLen (HCons a l) (S n) -- Analyze a type -- Result of the type analysis data TAtomic a data TVariable a data TFunction a b data TConstructed c a -- only one level: kind * -> * data TAtomic1 a data TVariable1 a data Level1 a data W a = W a -- We can have more levels, cf Typeable2, etc. class Analyze a b | a -> b analyze:: Analyze a b => a -> b analyze = undefined instance TypeCast f (TAtomic Int) => Analyze Int f instance TypeCast f (TAtomic Bool) => Analyze Bool f instance TypeCast f (TAtomic Char) => Analyze Char f instance TypeCast f (TAtomic Int) => Analyze (W Int) f instance TypeCast f (TAtomic Bool) => Analyze (W Bool) f instance TypeCast f (TAtomic Char) => Analyze (W Char) f instance (Analyze (c x) rx, Analyze (d y) ry, TypeCast f (TFunction rx ry)) => Analyze (c x->d y) f instance (Analyze (W x) rx, Analyze (d y) ry, TypeCast f (TFunction rx ry)) => Analyze (x->d y) f instance (Analyze (c x) rx, Analyze (W y) ry, TypeCast f (TFunction rx ry)) => Analyze (c x->y) f instance (Analyze (W x) rx, Analyze (W y) ry, TypeCast f (TFunction rx ry)) => Analyze (x->y) f instance (Analyze (W x) rx, Analyze (W y) ry, TypeCast f (TFunction rx ry)) => Analyze (W (x->y)) f instance (Analyze (Level1 (c ())) rc, Analyze (W x) rx, TypeCast f (TConstructed rc rx)) => Analyze (c x) f instance TypeCast f (TAtomic1 ([] ())) => Analyze (Level1 ([] ())) f instance TypeCast f (TAtomic1 (Maybe ())) => Analyze (Level1 (Maybe ())) f instance TypeCast f (TVariable1 (c ())) => Analyze (Level1 (c ())) f -- the most general instance instance TypeCast f (TVariable a) => Analyze (W a) f -- instance TypeCast f (TVariable a) => Analyze a f -- the most general instance {- ta1 = analyze True -- ta1 :: TAtomic Bool ta2 = analyze not -- ta2 :: TFunction (TAtomic Bool) (TAtomic Bool) ta3 = analyze [True] -- ta3 :: TConstructed (TAtomic1 [()]) (TAtomic Bool) ta4 = analyze id -- ta4 :: TFunction (TVariable GHC.Prim.Any) (TVariable GHC.Prim.Any) ta5 = analyze [] -- ta5 :: TConstructed (TAtomic1 [()]) (TVariable GHC.Prim.Any) ta6 = analyze const -- ta6 :: TFunction -- (TVariable GHC.Prim.Any) -- (TFunction (TVariable GHC.Prim.Any) (TVariable GHC.Prim.Any)) -} -- Standard HList stuff data Z data S a class Nat a where nat :: a -> Int instance Nat Z where nat _ = 0 instance Nat a => Nat (S a) where nat _ = succ (nat (undefined::a)) data HTrue data HFalse instance Show HTrue where show _ = "HTrue" instance Show HFalse where show _ = "HFalse" data HNil = HNil deriving Show data HCons a b = HCons a b deriving Show -- literally lifted from the HList library class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x class TypeEq' () x y b => TypeEq x y b | x y -> b where type'eq :: x -> y -> b type'eq _ _ = undefined::b class TypeEq' q x y b | q x y -> b class TypeEq'' q x y b | q x y -> b instance TypeEq' () x y b => TypeEq x y b instance TypeCast b HTrue => TypeEq' () x x b instance TypeEq'' q x y b => TypeEq' q x y b instance TypeEq'' () x y HFalse

2 1

typeOf for polymorphic value
by Lennart Augustsson 27 Mar '09

27 Mar '09

Using Data.Typeable.typeOf we can get a representation of the the type of a monomorphic value, for instance Prelude Data.Typeable> typeOf not Bool -> Bool But if we try using it on a polymorphic value it fails Prelude Data.Typeable> typeOf id <interactive>:1:0: Ambiguous type variable `a' in the constraint: `Typeable a' arising from a use of `typeOf' at <interactive>:1:0-8 Probable fix: add a type signature that fixes these type variable(s) And understandably so. Does anyone know of a trick to accomplish `typeOf id'? Using something else than TypeRep as the representation, of course. Any tricks and existing language extensions are welcome. (As ghc moves towards first class polymorphic values this question gets more interesting.) -- Lennart

2 1

Re: [Haskell-cafe] Grouping - Map / Reduce
by Peter Verswyvelen 27 Mar '09

27 Mar '09

Well, this approach has the problem that the running sum of key k blocks until a new value for that k arrives in the input stream. If you wanted to know the sum of the values of each key after you got nelements in the input stream, we could change the valuesWithKey inner function into: runningSumsOfValuesPerKey :: (Eq k, Num v) => [k] -> [(k, v)] -> [[v]] runningSumsOfValuesPerKey allPossibleKeys = runningSums . allValuesPerKey where runningSums = map (scanl (+) 0) allValuesPerKey pairs = [ valuesWithKey key pairs | key <- allPossibleKeys ] valuesWithKey key = map (\(k,v) -> if k==key then v else 0) then map (!!n) on the result of runningSumsOfValuesPerKey gives you the sum after n elements arrived. I think if you now generalize this so you don't use 0 but mempty, mconcat and other Monoid methods, that you might get something like Luke's trie solution, not sure, Luke is a fair bit smarter than I am :-) But this code is very inefficient I'm afraid, I guess the blueprint stuff that was posted really builds a map incrementally, but I don't understand that yet. Ik spreek Nederlands ja ('t is te zeggen, "Antwerps"). Yes I'm still learning Haskell, but I think with Haskell this is a never ending process, since there's soo much stuff to discover and the language evolves (which makes it both exciting and frustrating, but that's the dilemma of knowledge anyway, the more you know the better you realize the vast amount of knowledge that you don't know yet :-) On Fri, Mar 27, 2009 at 12:53 AM, Guenther Schmidt <gue.schmidt(a)web.de>wrote: > Dear Peter, > > wow, thanks, this is a very ... interesting ... approach, I hadn't thought > about that yet ;) > > Ben je nederlands? > > In case you'd be interested to share the "road to Haskell" experience with > another newbie just ask. > > Günther > > Peter Verswyvelen schrieb: > > I'm also learning Haskell so the solution below might be (1) inefficient >> and (2) incorrect, but hey, let's give it a try :-) >> >> For simplicity, in the testing code, I assume an infinite list of >> key/value pairs where they keys are of type Char between 'a' and 'z' and the >> values are Integers (the code also seems to work for keys with just a lower >> bound but no upper bound) >> I think the code speaks for itself >> >> import System.Random >> runningSumsOfValuesPerKey :: (Eq k, Num v) => [k] -> [(k, v)] -> [[v]] >> runningSumsOfValuesPerKey allPossibleKeys = runningSums . allValuesPerKey >> where >> runningSums = map (scanl (+) 0) >> allValuesPerKey pairs = [ valuesWithKey key pairs | key <- >> allPossibleKeys ] >> valuesWithKey key = map snd . filter ((==key) . fst) >> -- Testing >> randomPairs :: [(Char, Integer)] >> randomPairs = zip keys values >> where >> keys = randomRs ('a','z') rnd1 >> values = randomRs (0,9) rnd2 >> (rnd1,rnd2) = split (mkStdGen 0) >> test = map (take 10) [rs `atKey` 'c', rs `atKey` 'z'] >> where >> rs = runningSumsOfValuesPerKey ['a'..] randomPairs >> xs `atKey` k = xs !! (fromEnum k - fromEnum 'a') >> >> >> >> > >

2 1

ANN: FallingBlocks 0.1
by Ben Sanders 27 Mar '09

27 Mar '09

Hello, I just uploaded fallingblocks to Hackage. It is another Tetris clone, but it uses SDL, and I thought there could be more SDL examples. Any and all comments and suggestions will be extremely appreciated! There is a darcs repo at http://patch-tag.com/publicrepos/fallingblocks Cheers! -Ben

2 1

Re: about Haskell code written to be "too smart"
by John Lato 27 Mar '09

27 Mar '09

> From: wren ng thornton <wren(a)freegeek.org> > Dan Weston wrote: >> So to be clear with the terminology: >> >> inductive = good consumer? >> coinductive = good producer? >> >> So fusion should be possible (automatically? or do I need a GHC rule?) with >> inductive . coinductive >> >> Or have I bungled it? > > Not quite. Induction means starting from base cases and building things > upwards from those. Coinduction is the dual and can be thought of as > starting from the ceiling and building your way downwards (until you hit > the base cases, or possibly forever). > ( quite a lot of text trimmed for brevity) Wren, thank you for contributing this post. Coming from the point of view of someone who doesn't grok it all yet, this is the best commentary I've read on deforestation/fusion, and in fact is helpful for category theory newbies as well (at least I found it so). It really should go on a wiki somewhere. John Lato

2 1

Grouping - Map / Reduce
by Gü?nther Schmidt 27 Mar '09

27 Mar '09

Hi, let say I got an unordered lazy list of key/value pairs like [('a', 99), ('x', 42), ('a', 33) ... ] and I need to sum up all the values with the same keys. So far I wrote a naive implementation, using Data.Map, foldl and insertWith. The result of this grouping operation, which is effectively another list of key/value pairs, just sums this time, needs to be further processed. The building of this map is of course a bottleneck, the successive processing needs to wait until the entire list is eventually consumed the Map is built and flattened again. Is there another way of doing this, something more "streaming architecture" like? Is Googles "Map - Reduce" related to this? Günther

8 10

an OS-independent executeFile??
by Vasili I. Galchin 27 Mar '09

27 Mar '09

Hello, I have been looking through Hackage and using Hoogle to "fork and execute" a program in an OS-independent way, i.e. neutral from POSIX and Win32 APIs. Does such a library function exist? Regards, Vasili

2 4

Language.Haskell.Parser question
by Vasyl Pasternak 27 Mar '09

27 Mar '09

Hi, I want to parse haskell file to find all calls to function 'foo' and gathers a create a list of all argumets, which passed to it. E.g. from the following code: f1 = foo 5 f2 = foo 8 f3 = foo 9 I want to extract a list [5, 8, 9] (suppouse function takes only one argument) The most obvious way is to use Language.Haskell for this task. The parser works pretty good, but its output data type is terrible. As I understand, I need to extract all objects that looks like HsApp (HsVar (UnQual (HsIdent "foo"))) .... The question is, is there a method to do it quickly or I have to process each object of different type separately ? Maybe it is a work for a template Haskell ? Thanks. -- Best regards, Vasyl Pasternak

4 7

Compile OpenGL for jhc
by Csaba Hruska 27 Mar '09

27 Mar '09

Hi! As OpenGL haskell binding depends only on base, it would be great to compile it with jhc. I've tried to do that, but I had some problems with preprocessor directives. It would be also great to add jhc support for OpenGL bindings build scripts. Or, it would be better to add jhc support for cabal-install. :) Is there anyone who has any idea any of this things? (for soulution)

2 3

ANN: io-capture-0.2 capturing std(out|err) in IO action
by Yusaku Hashimoto 27 Mar '09

27 Mar '09

Hi, I announce the release of io-capture package of version 0.2 io-capture is a Library to capturing stdout and stderr in IO action. It exports a function named `capture', It takes an IO and an String, the action to run and the given whole stdin, and returns whole stdout and stderr in the action as String. For example: ghci> :m +System.IO.Capture ghci> :m +System.IO ghci> capture (getLine >>= putStr >> getLine >>= hPutStr stderr) "foo \nbar\n" ("foo","bar") io-capture is available on hackage, repository is on patch-tag. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/io- capture-0.2 http://patch-tag.com/repo/io-capture/browse It behaves almost fine, but It's still experimental, I know some trouble about when given lazy reading action such as getContents. And there may be unknown bugs. If you find it, feel free to report it and any feedbacks are welcome. Thanks for reading, Yusaku Hashimoto

1 0

Re: Exception handling in numeric computations
by John Lato 27 Mar '09

27 Mar '09

> Message: 15 > From: Xiao-Yong Jin <xj2106(a)columbia.edu> > John Lato <jwlato(a)gmail.com> writes: > > So I have another question. Is the following function safe > and legitimate? > >> safeDiv :: (Exception e, Integral a) => >> a -> a -> Either e a >> safeDiv x y = unsafePerformIO . try . evaluate $ div x y > > I believe it should be okay to use this 'safeDiv'. What do > you think? Your question is unclear. Are you asking if this function can be safely used with current Haskell standards and implementations or are you asking should the Haskell specification make a guarantee that this function will do what you want? I would answer "no" either way, but for different reasons. Common to both is: Don't use unsafe* functions unless you can prove that your usage is safe. That is proof as in mathematical usage; it must be rigorous and complete. John Lato

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Copying data files when installing, using Cabal
by Henk-Jan van Tuyl 27 Mar '09

27 Mar '09

L.S., I am trying to install an application with data files (I am developing that application). These data files are mentioned in the .cabal file, after the "Data-files:" keyword. When I give command "runhaskell setup install", the data files are not copied; how can I correct this? -- Regards, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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[ANN] I/O library for Windows
by Felix Martini 26 Mar '09

26 Mar '09

Hi all, winio is an I/O library for Windows using Windows API functions and has 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. Because the library uses IOCP instead of select it is not limited to 1024 open sockets. Try for example the thread-ring program where each Haskell thread passes a UDP message around (Change the MaxUserPort field in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters to 65534 to enable all ports). The winio package is available on Hackage. Notice that it uses a development version of the network package which is available at http://darcs.haskell.org/packages/network/. The library has not been tested much and should be considered experimental so please try it if you use Windows and notify me of any issue or corner case. Kind Regards, Felix

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