- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] ANN: HalfInteger-1.1.1
by Andrew Coppin 04 Jul '09

04 Jul '09

Alexander Dunlap wrote: > Couple of suggestions: > > - You should put an (Integer i) => constraint on the halve function so > that it becomes impossible to create invalid HalfIntegers. > Right. Currently you can *make* such a HalfInteger. You just won't be able to *do* anything with it afterwards. It would probably be more sensible to just add a constraint there. > - The documentation for toHalfInteger is truncated. Also, why can't > you make the rounding more predictable? > ...wow. OK, I'm looking at the source, and it seems I must have got distracted at that moment or something, because I just 100% didn't actually finish writing the documentation! o_O That's pretty special. ;-) As I *should* have written, the rounding for something like toHalfInteger 0.25 is kind of unpredictable; but if you do toHalfInteger 0.5, the result is *guaranteed* to be exact. All the functions are carefully tuned to work correctly on things which really are half integers, and to give plausible results otherwise. Just don't let your life depend on 0.25 being mapped to exactly a half or to exactly zero... > Nice work! > Heh, thanks.

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Flow Graphs for Language-independent Code Representation
by Cetin Sert 04 Jul '09

04 Jul '09

Hi *^o^*, I'm working on a source code transformation project for numerical automatic differentiation for Fortran and C. I would love to know the best Haskell way/package available today to represent procedural (non-OO) code in a language-independent manner. Any tips or resource, paper references are most welcome! Regards, Cetin Sert

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Small Haddock question
by Andrew Coppin 04 Jul '09

04 Jul '09

This is irritating me now... Suppose I have something like the following: zero = 0 :: Int one = 1 :: Int two = 2 :: Int three = 3 :: Int How do I add Haddock comments to the end of each line? For some reason, Haddock doesn't like either of zero = 0 :: Int -- | Zero zero = 0 :: Int -- ^ Zero Either way it whinges about parse errors. How do I make it shut up and stop being so dense at me? :-} It's damned obvious what I want it to do...

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Haskell Weekly News: Issue 124 - July 4, 2009
by Brent Yorgey 04 Jul '09

04 Jul '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090704 Issue 124 - July 04, 2009 --------------------------------------------------------------------------- Welcome to issue 124 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements HLint 1.6. Neil Mitchell [2]announced the release of [3]HLint 1.6, a tool for automatically suggesting improvements to Haskell code. Haskell Implementers Workshop: accepted talks. Simon Marlow [4]announced that the list of talks at the [5]Haskell Implementers Workshop 2009 has now been posted. bloxorz clone. Patai Gergely [6]announced a [7]Haskell clone of the game "bloxorz", written by Viktor Devecseri. Fun with type functions. Simon Peyton-Jones [8]announced that he, Ken Shan, and Oleg have finished Version 2 of their [9]paper "Fun with Type Functions", which gives a programmer's tour of what type functions are and how they are useful. If you have a moment to look at, and wanted to help them improve it, leave comments on the linked wiki page. package Boolean: Generalized booleans. Conal Elliott [10]announced [11]Boolean, a new package for generalized booleans, which provides type classes with generalizations of Boolean values and operations, if-then-else, Eq and Ord. TernaryTrees-0.1.1.1 - An efficient ternary tree implementation of Sets and Maps. Alex Mason [12]announced the release of [13]TernaryTrees, a package that extends Data.Set ad Data.Map with some ternary tree structures, one of the more efficient ways of storing strings in a set. 6.12.1 planning. Simon Marlow [14]announced plans for a release of GHC 6.12.1, sometime around September. If you have the time and inclination to help with any of the listed features, please get involved! regular-0.1. José Pedro Magalhães [15]announced the release of the [16]regular library. Many generic programs require information about the recursive positions of a data type, such as generic fold, generic rewriting, and the Zipper data structure. Regular provides a fixed point view on data which allows these definitions for regular data types. It also serves as the basis for a [17]generic rewriting library. Google Summer of Code Progress updates from participants in the 2008 [18]Google Summer of Code. Haddock improvements. Isaac Dupree has made it easier to generate Haddock documentation [19]for non-exported functions, posted an [20]overview of the issues involved in getting proper cross-package documentation working, and his current [21]plan. EclipseFP. Thomas Ten Cate has done a lot of work on EclipseFP, including some [22]cosmetic updates and getting [23]error reporting to work better. space profiling. Gergely Patai is [24]working on a network protocol for his profiling grapher tool, so that other tools can monitor the profiling information. haskell-src-exts. Niklas Broberg has [25]released [26]haskell-src-exts version 1.0.0! fast darcs. Petr Rockai has completed [27]quite a bit of work on darcs, including a [28]beta release of darcs 2.3. Discussion Monoid wants a (++) equivalent. Bryan O'Sullivan [29]suggested adding a more concise operator to the Monoid class for 'mappend', leading to a long, bike-shed-ish (but hopefully still useful) discussion. Reflections on the ICFP 2009 programming contest. Justin Bailey began a [30]discussion on results and experiences from the ICFP 2009 programming contest. Blog noise [31]Haskell news from the [32]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Gergely Patai: [33]Playing and learning. * Ketil Malde: [34]A set of tools for working with 454 sequences. * Sebastian Fischer: [35]FP Overview. * Magnus Therning: [36]Making a choice from a list in Haskell, Vty (part 1). * David Amos: [37]Conjugacy classes, part 1. * Well-Typed.Com: [38]GHC and Windows DLLs. * Manuel M T Chakravarty: [39]Converting typed term representations: from HOAS to de Bruijn.. * >>> Ivan Uemlianin: [40]Haskell: sort and sortBy. * Gregory Collins: [41]Building a website with Haskell, part 3. * Michael Snoyman: [42]Hack sample- chat server. * Luke Palmer: [43]On the By functions. * Magnus Therning: [44]Dataenc finally making it into Debian. * Thomas ten Cate: [45]New build instructions. * Erik de Castro Lopo: [46]Three More for the Debian New Queue. * >>> Yuval Kogman: [47]What Haskell did to my brain. * Greg Bacon: [48]FFI: calling into kernel32.dll. * Greg Bacon: [49]Setting up a simple test with Cabal. * Ketil Malde: [50]Dephd updates. * Bryan O'Sullivan: [51]What's in a text API?. * Brent Yorgey: [52]2009 ICFP programming contest reflections. * Galois, Inc: [53]Galois, Inc. Wins Two Small Business Research Awards from Federal Agencies. * Greg Bacon: [54]Cleaning up your Haskell imports. * Douglas M. Auclair (geophf): [55]Realized Constants are Comonadic. Quotes of the Week * KF8NH: all monads are functors, but for Hysterical Raisins not all Monads are Functors. * lilac: lambda actually is just the greek letter l. it stands for lilac. * lilac: before mauke we all implemented map with a fold every time we needed it. * luqui: I'll just stick to my religion: I have a personal relationship with our lord and savior, the untyped lambda calculus. * copumpkin: I think I was implemented in haskell. I mean, my parents never used seq, ever. * Benjamin Russell: Haskell. "Avoid success at all costs." Made with dinosaur technology. About the Haskell Weekly News New editions are posted to [56]the Haskell mailing list as well as to [57]the Haskell Sequence and [58]Planet Haskell. [59]RSS is also available, and headlines appear on [60]haskell.org. To help create new editions of this newsletter, please see the information on [61]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [62]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/17332 3. http://community.haskell.org/~ndm/hlint/ 4. http://article.gmane.org/gmane.comp.lang.haskell.general/17331 5. http://haskell.org/haskellwiki/HaskellImplementorsWorkshop 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/60907 7. http://just-bottom.blogspot.com/2009/07/playing-and-learning.html 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/60846 9. http://haskell.org/haskellwiki/Simonpj/Talk:FunWithTypeFuns 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/60716 11. http://hackage.haskell.org/package/Boolean 12. http://www.haskell.org//pipermail/haskell-cafe/2009-June/063625.html 13. http://hackage.haskell.org/package/TernaryTrees 14. http://www.haskell.org//pipermail/haskell-cafe/2009-June/063681.html 15. http://www.haskell.org//pipermail/haskell/2009-July/021468.html 16. http://www.cs.uu.nl/wiki/bin/view/GenericProgramming/Regular 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rewriting 18. http://hackage.haskell.org/trac/summer-of-code/wiki/SoC2008 19. http://haddock2009.wordpress.com/2009/06/23/how-to-navigate-your-code/ 20. http://haddock2009.wordpress.com/2009/06/24/cross-package-documentation-par… 21. http://haddock2009.wordpress.com/2009/06/25/cross-package-plan-a/ 22. http://eclipsefp.wordpress.com/2009/06/20/cosmetics/ 23. http://eclipsefp.wordpress.com/2009/06/30/compiling-and-error-reporting/ 24. http://just-bottom.blogspot.com/2009/06/short-term-hp2any-plans.html 25. http://nibrofun.blogspot.com/2009/06/gsoc-status-report-week-4.html 26. http://hackage.haskell.org/package/haskell%2Dsrc%2Dexts 27. http://web.mornfall.net/blog/soc_progress_5.html 28. http://web.mornfall.net/blog/darcs_2.3_beta_1.html 29. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/60754 30. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/60690 31. http://planet.haskell.org/ 32. http://haskell.org/haskellwiki/Blog_articles 33. http://just-bottom.blogspot.com/2009/07/playing-and-learning.html 34. http://blog.malde.org/index.php/2009/07/03/a-set-of-tools-for-working-with-… 35. http://www-ps.informatik.uni-kiel.de/~sebf/haskell/fp-overview.html 36. http://therning.org/magnus/archives/685 37. http://haskellformaths.blogspot.com/2009/06/conjugacy-classes-part-1.html 38. http://blog.well-typed.com/2009/07/ghcdlls/ 39. http://justtesting.org/post/134566537 40. http://llaisdy.wordpress.com/2009/07/03/haskell-sort-and-sortby/ 41. http://gregorycollins.net/posts/2009/07/02/building-a-website-part-3 42. http://blog.snoyman.com/2009/07/01/hack-sample-chat-server/ 43. http://lukepalmer.wordpress.com/2009/07/01/on-the-by-functions/ 44. http://therning.org/magnus/archives/672 45. http://eclipsefp.wordpress.com/2009/07/01/new-build-instructions/ 46. http://www.mega-nerd.com/erikd/Blog/CodeHacking/Debian/polyparse_dataenc_js… 47. http://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_na_2009/haske… 48. http://feedproxy.google.com/~r/gbacon/~3/o-lAJeIoUbk/ffi-calling-into-kerne… 49. http://feedproxy.google.com/~r/gbacon/~3/7rlf4Dd1JKU/setting-up-simple-test… 50. http://blog.malde.org/index.php/2009/06/16/dephd-updates/ 51. http://www.serpentine.com/blog/2009/06/30/python-and-haskell-text-apis-comp… 52. http://byorgey.wordpress.com/2009/06/29/2009-icfp-programming-contest-refle… 53. http://www.galois.com/blog/2009/06/29/galois-awarded-two-federal-sbirs/ 54. http://feedproxy.google.com/~r/gbacon/~3/AGNKaCB2-GI/cleaning-up-your-haske… 55. http://logicaltypes.blogspot.com/2009/06/realized-constants-are-comonadic.h… 56. http://www.haskell.org/mailman/listinfo/haskell 57. http://sequence.complete.org/ 58. http://planet.haskell.org/ 59. http://sequence.complete.org/node/feed 60. http://haskell.org/ 61. http://haskell.org/haskellwiki/HWN 62. http://code.haskell.org/~byorgey/code/hwn/

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Cont, ContT and IO()
by Günther Schmidt 04 Jul '09

04 Jul '09

Hi, I've got an IO action, some file system IO, traversing one level only and iterating over files found. I wish to build in an "early" exit, ie. if an IO action in the loop encounters a particular value I want it to abort the loop. Now so far, pls don't shoot, I have done this by throwing IO Exceptions and catching them. I'm trying to rewrite this using Continuatios / callCC but can't figure out where to place what. I certainly don't have the intuition yet and funny enough not even in RWH I could find some Cont/ContT examples. Would someone please draw me an example? Günther

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Half-integer
by Andrew Coppin 04 Jul '09

04 Jul '09

I just wrote a small module for dealing with half-integers. (That is, any number I/2 where I is an integer. Note that the set of integers is a subset of this; Wikipedia seems to reserve "half-integer" for such numbers that are *not* integers.) module HalfInteger where data HalfInteger i instance (Eq i) => Eq (HalfInteger i) instance (Ord i) => Ord (HalfInteger i) instance (Integral i) => Show (HalfInteger i) instance (Integral i) => Num (HalfInteger i) half :: (Num i) => HalfInteger i fromNum :: (Integral i, RealFrac x) => x -> HalfInteger i toNum :: (Integral i, Fractional x) => HalfInteger i -> x isInteger :: (Integral i) => HalfInteger i -> Bool Note carefully that the set of half-integers is *not* closed under multiplication! This means that for certain arguments, there are two reasonable products that could be returned. (E.g., 1/2 * 1/2 = 1/4, so 0 or 1/2 would be a reasonable rounding.) I haven't put a lot of effort into the rounding details of (*) or fromNum; which answer you get is kind of arbitrary. (However, addition and subtraction are exact, and for multiplications where an exact result is possible, you will get that result.) The Show instance outputs strings such as fromInteger 5 fromInteger 5 + half fromInteger (-5) - half depending on the isInteger predicate. Now, the question is... Is this useful enough to be worth putting on Hackage?

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ORM for haskell?
by Marc Weber 04 Jul '09

04 Jul '09

Some time ago I stumbled upon SQLAlchemy which is a great ORM wrapper library for python. It has a nice syntax I'd like to see in a haskell library as well. SQLAlchemy already provides some lazy features such as loading subitems on access etc. All haskell SQL libraries I know only let you run SQL statements but not much more. To start real business you no longer want to write many SQL commands. Example why it matters: schools - 1:n - teachers - 1:n - pupils If you want to list all schools which have a pupil with age > 3 you'd write an sql query like this: SELECT dictinct * FROM schools as s JOIN teachers t ON (t.school_id = s.id) JOIN pupils as p ON (p.teacher_id = t.id) WHERE p.age > 3 in SQLAlchemy it looks like this: session.query(School).join(School.teachers).join(Teacher.pupils).filter(Pupil.age > 3).all() difference? Because SQLAlchemy knows about the relations you don't have to remember alias names. So there is no chance to get that wrong. Another example: Updating the age of a pupil: row = SELECT * FROM pupils where age = 13; UPDATE pupils SET age = 14 WHERE id = <the id you got above> p = session.query(Pupil).filter(Pupil.age==13).one().age=14 session.commit() difference? You don't have to care about ids. you just assign a new value and tell the engine that it should commit. So again less chances to get something wrong. What about trees (eg web site navigation) id | title | parent_id 1 | top | null 2 | submenu | 1 3 | submenu2 | 1 should result in top - submenu - submenu2 using SQLAlchemy you can just do parent = session.query('nodes').filter(Node.id = 1) def print(node): print node.title print node.subnodes # this will run a subquery automatically for you returning submenu{,2} Again no sql. No chance to get something wrong? You can skim the manual to get a better idea how SQLAlchemy works http://www.sqlalchemy.org/docs/05/sqlalchemy_0_5_5.pdf I have to admit that I haven't used SQLAlchemy in a real project yet. However I can imagine doing so. Comparing this to what we have on hackage I'd say some work has to be done to get close to SQLAlchemy. The backend doesn't have to be a relational database. However I'd like to use this kind of abstraction in haskell. Is there anyone interested in helping building a library which a) let's you define kind of model of you data b) let's you store you model in any backend (maybe a relational database) c) does static checking of your queries at compilation time? Right now I'd say the best way to go is define the model in the application and not get the scheme from an existing database because there is not way to store all scheme details within a relational model. I think SQLAlchemy does it right by providing a way to define the model in python. Of course haskell doesn't have "objects" to store. But GADTs could be stored (data Foo = ...) So are there any volunteers who are interested in helping writing this kind of storage solution for haskell which could be used in real world business apps? Maybe this does already exist and I've missed it? Marc Weber

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space leak hints?
by Uwe Hollerbach 04 Jul '09

04 Jul '09

Good evening, all, I wonder if I could tap your collective wisdom regarding space leaks? I've been messing about with haskeem, my little scheme interpreter, and I decided to see if I could make it run reasonably space-efficiently. So far... no. Here's what I tried: I wrote a tiny scheme program to compute Collatz sequences for successive numbers, starting from 1 and incrementing forever (well, in principle). Because real scheme implementations are fully tail-call-optimized, this'll run in constant memory; I checked that with mzscheme, and it does indeed work. With my little interpreter, that's not the case: memory usage grows continually, although apparently less-than-linearly. I've built the interpreter with the profiling stuff described on the wiki and in RWH Ch 25 turned on and have made a few runs with that; I stuck the postscript plot that's the result of one of those runs onto my web site at http://www.korgwal.com/haskeem/run2.ps. The full source to the interpreter is a little large to paste into this message; it's available on my web site, and also on hackage. But according to the plot, there appear to be three main memory allocation issues, and they seem to all be related, if I'm reading stuff correctly. The core of the interpreter is a function, evalLisp, which evaluates scheme forms. There are of course a fair number of different forms, but the largest generic usage is "evaluate each of a list of forms, returning the value of the last of them as the overall result". In order to express that in a couple of different places, and to accomodate the possibility of an empty list, I have a really tiny function lastOrNil which just calls "last" on a (haskell) list, checking for the possibility of an empty list, and returning a haskeem LispVal object: > lastOrNil = liftM lON > where lON [] = List [] > lON l = last l (sorry, proportional fonting may be throwing this off). It's this function which is directly implicated in two of the top three memory pigs, and nearly directly in the third one as well. If I could eliminate the memory growth in these three cost centers, I would already capture over 90% of the growth in this benchmark, which would make me very happy indeed. But I don't really understand what is going on here. It seems entirely plausible, indeed likely, that the list which I'm traversing there is not fully evaluated. So I've tried adding 'seq' to this function. Uhhh... from memory, I dumped it after it didn't work, I had > lastOrNil = liftM lON > where lON [] = List [] > lON (l:[]) = l > lON (l:ls) = seq l (lON ls) (again, proportional fonting might mess me up here.) As I said, I dumped this after it made no difference whatsoever. I also tried bang-patterns in a couple of places, strictness annotation of my basic LispVal types... nothing. It all made exactly no difference, as far as I could tell. I've tried a couple of google searches, but haven't come up with anything better than what I've already described. So I'm a stumped chump! I'd be grateful for any suggestions... thanks in advance! Uwe

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How to present the commonness of some objects?
by Magicloud Magiclouds 03 Jul '09

03 Jul '09

Hi, I thought "class" was for this purpose. But it turns out not. Code as following could not compiled. 1 main = do 2 mapM_ (\(x, y, widget) -> do 3 a <- widgetRun widget 4 putStrLn $ show a 5 ) widgetList 6 7 widgetList :: (Widget w) => [(Integer, Integer, w)] 8 widgetList = [] 9 10 class Widget w where 11 widgetRun :: w -> IO () --- % ghc --make tmp/test.hs [1 of 1] Compiling Main ( tmp/test.hs, /tmp/Main.o ) tmp/test.hs:3:16: Ambiguous type variable `t' in the constraint: `Widget t' arising from a use of `widgetRun' at tmp/test.hs:3:16-31 Probable fix: add a type signature that fixes these type variable(s) -- 竹密岂妨流水过山高哪阻野云飞

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Help needed to pick a better name then "hack"
by Jinjing Wang 03 Jul '09

03 Jul '09

Hack is such an inconvenient name for a package, may I get some inspiration from renaming it? Also, is there an idiom to use when upgrading package name on hackage? Best, -- jinjing

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