- Haskell-Cafe - Haskell.org

Exception handling when using STUArray
by oleg＠okmij.org 11 Mar '08

11 Mar '08

Sterling Clover wrote: > there's no standard way that I know of besides "inspection" to > determine if code might throw an exception, and this is particularly > the case with the dreaded lazy IO of prelude functions. The following old message showed even two ways of doing exactly that -- in Haskell, with no external tools. Exceptions in types and exception-free programming http://www.haskell.org/pipermail/haskell/2004-June/014271.html The third way described in the message -- ensuring that exceptions will never occur -- was further developed as lightweight static capabilities. > And as per a recent discussion on libraries, it's still a royal pain > to catch IO exceptions in monads with an IO base. That is again very easy. Takusen uses a CaughtMonadIO -- which includes a wide range of monads: IO itself and various transformations of it. I am extensively using a variant written by Jesse Tov based on the Takusen's idea. His code defines two classes: EMonad and EMonadIO -- which enclose most of the interesting monads. The latter is the subclass of the former and also allows arbitrary IO (via liftIO). In either case, I use gthrow, gbracket, gcatch, ghandle, gfinally, etc. -- without even thinking which Monad I'm in and how error handling is actually implemented (via ErrorT or via IO exceptions). It works universally for most of monads of interest.

1 0

Reflective capabilities of Haskell
by Martin Hofmann 11 Mar '08

11 Mar '08

I am trying to port a programme written in Maude, which is a reflective language based on rewriting logic ( http://maude.cs.uiuc.edu/ ), to Haskell. I thought using Template Haskell might be a good idea, but I got stuck and now I am wondering if this is really possible in Haskell. Let me give an example of a Maude module defining the function last over a list of Peano numbers. fmod LAST is sorts Peano PeanoList . op 0 : -> Peano [ctor] . op s : Peano -> Peano [ctor] . op [] : -> PeanoList [ctor] . op cons : Peano PeanoList -> PeanoList [ctor] . op last : PeanoList -> Peano . vars X Y : Peano . var Xs : PeanoList . eq last(cons(X,[])) = X . eq last(cons(X,cons(Y,Xs))) = last(cons(Y,Xs)) . endfm So, last(cons(s(0),cons(s(s(0)),cons(s(s(s(0))),[])))) would reduce to s(s(s(0))). The cool thing about Maude is, that terms, modules, ... can be lifted to the meta level. For example: upModule('LAST, false) would yield

1 0

Instance selection based on a class constraint [was: Issues(Bugs?) with GHC Type Families]
by oleg＠okmij.org 11 Mar '08

11 Mar '08

> Manuel M T Chakravarty: > Hugo Pacheco: > > I would simply like the compiler not to use that instance if the > > equality constraint does not hold, like some another instance > > dependency constraint, but I assume that is not possible. > > This is independent of type families. The selection of type class > instances does *never* dependent on the instance context, only on the > instance head. I know that this is sometimes annoying, but type > checking would be a a lot more complicated/expensive without that rule. What Hugo Pacheco wants *is* possible, in GHC as it is (and even as it was back in 2004). Although the selection of type class instances indeed depends only on the instance head, on the surface, the dependence on context is quite easy to accomplish. Here's is the outline of the idea. Suppose we wish to select one of the two instances, depending on whether the constraint C x hold or not: instance C x => CMain x instance CMain x We encode this as follows: class CMain x instance (CPred x flag, CMain' flag x) => CMain x The main class has only one instance, and there is no longer any overlapping. The main selection is done in an auxiliary class class CMain' flag x instance CMain' HTrue x -- will be selected when CPred holds instance CMain' HFalse x -- otherwise Note there are no overlapping instances required. The trick is to re-write constraint C x into the form CPred x flag (where flag functionally depends on x). Whereas C x succeeds, CPred x flag should succeed and unify flag with HTrue. Should C x fail, CPred should still succeed and unify flag with HFalse. TypeEq is such a predicate in the case of type equality. CPred is easy to implement with type-classes, and probably even easier with type families. The HList and OOHaskell papers use this technique to great extent. The following message describes that even backtracking is possible: http://okmij.org/ftp/Haskell/poly2.txt

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Where does ~> come from?
by Steve Lihn 11 Mar '08

11 Mar '08

While I am reading TypeCompose module, I am having trouble finding where ~> comes from? (And its son ~~> and grandson ~~~>, etc.) This is my immediate question. Help is appreciated. A bigger (higher-order?) issue I encountered and I think other people could also have is to look up all these special "symbols" during the learning curve. As things are getting higher and higher order in Haskell, more inventive symbols are appearing. It becomes harder to remember the exact definitions are. The difficulties are: 1) I used to see a page on haskell.org that lists many symbols, but I could not find that page any more. 2) You would hope there is a quick way to search those symbols. But most search engines do not treate symbols friendly, often just ignore them. I typed ~> in Hoogle, it also returned nothing. 3) If the module defining the symbol is not in standard library, it is not possible to look up the symbol in the core library index. The only way to find the symbol seems to go to the source code, check every module being imported and if I am lucky I will find it in the same package. If I am not lucky, I will have to search that module on the Internet, and hunt it down recursively. I wonder if anybody has the same problem. If enough crowd have the same problem, maybe there could be a better way to handle this kind of documentation/learning issue! Thanks, Steve

5 4

automagically?
by Galchin Vasili 10 Mar '08

10 Mar '08

Hello, Say i am building package X that depends on A, H, D. How can I automagically check to A, H, D need to "darcs get" down and built/install before X? If this feature is not available it should be, i.e. an integration of darcs and cabal. Regards, Vasya

3 2

concurrent definitions with parsec
by Zsolt SZALAI 10 Mar '08

10 Mar '08

Hi! I'm writing a parser for a language, with a BNF like this: Type = "type" Def Def = RecordDef | RecordOfDef ... RecordDef = "record" Body RecordOfDef = "record" "of" With a perser what uses parsec module it can be mapped to haskell easily: structuredTypeDef = recordDef <|> recordOfDef But this way, the recordOfDef case will never be parsed at a line like "type record <somethings>", because recordDef will win in the first place always. Is there an option or an other operation instead of <|> to do the trick? I cant see if the problem can be solved with parsecperm or not. It is a possibility to do something like swap "recordDef <|> recordOfDef" with "recordlikeDefs " where recordlikeDefs = do { reserved "record" ; others } but that would be the dirty way... Do you have any advise on the case? Thanks, -- Zsolt Szalai

2 3

Issues(Bugs?) with GHC Type Families
by Hugo Pacheco 10 Mar '08

10 Mar '08

Hi all, I have recently tried to replicate some examples from in the articles about type families but found some possible bugs. In [2], the example class C a where type S a (k :: * -> *) :: * instance C [a] where type S [a] k = (a,k a) does not compile under the claim that the type variable k is not in scope. However, if we extract the type family from the class type family S a (k :: * -> *) :: * type instance S [a] k = (a, k a) class C a it compiles correctly. According to [3], the difference is purely syntactic sugar, does that mean that both examples should compile and behave the same or is there some subtlety that justifies the class example not to compile? Another issue is that data kinds (used in both [2] and [3]) do not seem to be supported at all by the compiler, are they already implemented in GHC? Simple examples such as datakind Nat = Zero or datakind Nat = Zero | Succ Nat fail to compile. Perhaps some of these should be submitted to the GHC Bug Tracker. I have tested both GHC 6.8.2 and 6.9.20080218. References: 1. Associated Types with Class.<http://www.cse.unsw.edu.au/~chak/papers/CKPM05.html> Manuel M. T. Chakravarty, Gabriele Keller, Simon Peyton Jones, and Simon Marlow. In *Proceedings of The 32nd Annual ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages (POPL'05)*, pages 1-13, ACM Press, 2005. 2. Associated Type Synonyms.<http://www.cse.unsw.edu.au/~chak/papers/CKP05.html> Manuel M. T. Chakravarty, Gabriele Keller, and Simon Peyton Jones. In *Proceedings of The Tenth ACM SIGPLAN International Conference on Functional Programming *, ACM Press, pages 241-253, 2005. 3. Towards Open Type Functions for Haskell.<http://www.cse.unsw.edu.au/~chak/papers/SSPC07.html> Tom Schrijvers, Martin Sulzmann, Simon Peyton-Jones, and Manuel M. T. Chakravarty. Unpublished manuscript, 2007.

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Haskell Weekly News: March 09, 2008
by Don Stewart 09 Mar '08

09 Mar '08

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20080309 Issue 71 - March 09, 2008 --------------------------------------------------------------------------- Welcome to issue 71 of HWN, a newsletter covering developments in the [1]Haskell community. Another busy week on the Haskell library front, with around 100 new and updated libraries and tools on Hackage. 1. http://haskell.org/ Announcements Google Summer of Code. Malcolm Wallace [2]announced Google is running its 'Summer of Code' project again this year, and Haskell.org is once again going to apply to be a mentoring organisation. If you're interested in earning money to hack on Haskell, and helping out the community, [3]take a look at the wiki. 2. http://article.gmane.org/gmane.comp.lang.haskell.cafe/37273 3. http://hackage.haskell.org/trac/summer-of-code Haskell in the browser. Dimitry Golubovsky [4]announced that the YHC JavaScript backend is now in alpha testing, and is open to experimentation for those wanting to write Haskell directly for the browser 4. http://article.gmane.org/gmane.comp.lang.haskell.cafe/37299 Hackage New and updated libraries in [5]the Hackage library database. 5. http://hackage.haskell.org/ * typalyze 0.1.1. Uploaded by Matthew Danish. [6]typalyze: Analyzes Haskell source files for easy reference. * lax 0.0.0.1. Uploaded by Wolfgang Jeltsch. [7]lax: Lax arrows. * truelevel 0.1.1. Uploaded by Barton Massey. [8]truelevel: Audio file compressor-limiter. * WAVE 0.1. Uploaded by Barton Massey. [9]WAVE: WAVE audio file IO library. * parseargs 0.1. Uploaded by Barton Massey. [10]parseargs: Command-line argument parsing library for Haskell programs. * conjure 0.1. Uploaded by Gwern Branwen. [11]conjure: A BitTorrent client. * Diff 0.1.1. Uploaded by Sterling Clover. [12]Diff: O(ND) diff algorithm in haskell.. * simseq 0.0. Uploaded by Gwern Branwen. [13]simseq: Simulate sequencing with different models for priming and errors. * rbr 0.8.3. Uploaded by Gwern Branwen. [14]rbr: Mask nucleotide (EST) sequences in Fasta format. * xml2x 0.2. Uploaded by Gwern Branwen. [15]xml2x: Convert BLAST output in XML format to CSV or HTML. 6. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/typalyze-0.1.1 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/lax-0.0.0.1 8. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/truelevel-0.1.1 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/WAVE-0.1 10. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parseargs-0.1 11. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/conjure-0.1 12. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Diff-0.1.1 13. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/simseq-0.0 14. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/rbr-0.8.3 15. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xml2x-0.2 * estreps 0.1. Uploaded by Gwern Branwen. [16]estreps: Repeats from ESTs. * clustertools 0.1. Uploaded by Gwern Branwen. [17]clustertools: Tools for manipulating sequence clusters. * xsact 1.6. Uploaded by Gwern Branwen. [18]xsact: Cluster EST sequences. * HsJudy 0.2. Uploaded by Gwern Branwen. [19]HsJudy: Judy bindings, and some nice APIs. * prof2dot 0.3.1. Uploaded by Gregory Wright. [20]prof2dot: Convert GHC profiles into GraphViz's dot format. * strict 0.3.2. Uploaded by Roman Leshchinskiy. [21]strict: Strict data types and String IO.. * Emping 0.4. Uploaded by Hans VanThiel. [22]Emping: derives heuristic rules from nominal data. * GuiHaskell 0.1.1. Uploaded by Neil Mitchell. [23]GuiHaskell: A graphical REPL and development environment for Haskell. * simpleargs 0.1. Uploaded by Ketil Malde. [24]simpleargs: Provides a more flexible getArgs function with better error reporting.. * parsec 3.0.0. Uploaded by Derek Elkins. [25]parsec: Monadic parser combinators. 16. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/estreps-0.1 17. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/clustertools-0.1 18. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xsact-1.6 19. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HsJudy-0.2 20. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/prof2dot-0.3.1 21. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/strict-0.3.2 22. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Emping-0.4 23. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/GuiHaskell-0.1.1 24. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/simpleargs-0.1 25. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parsec-3.0.0 * hetris 0.2. Uploaded by Gwern Branwen. [26]hetris: Text Tetris. * hscurses 1.3. Uploaded by Gwern Branwen. [27]hscurses: NCurses bindings for Haskell. * photoname 2.0. Uploaded by Dino Morelli. [28]photoname: Rename JPEG photo files based on shoot date. * mage 1.1.0. Uploaded by Gwern Branwen. [29]mage: Rogue-like. * infix 0.1.1. Uploaded by Gwern Branwen. [30]infix: Infix expression re-parsing (for HsParser library). * bio 0.3.3. Uploaded by Gwern Branwen. [31]bio: A bioinformatics library. * dephd 0.0. Uploaded by Gwern Branwen. [32]dephd: Analyze 'phred' output (.phd files). * hybrid 2.0. Uploaded by Gwern Branwen. [33]hybrid: A implementation of a type-checker for Lambda-H. * propgrid 0.1. Uploaded by Gwern Branwen. [34]propgrid: GUI propertygrid. * gravatar 0.3. Uploaded by Donald Stewart. [35]gravatar: Find the url of the gravatar associated with an email address.. 26. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hetris-0.2 27. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hscurses-1.3 28. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/photoname-2.0 29. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mage-1.1.0 30. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/infix-0.1.1 31. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bio-0.3.3 32. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dephd-0.0 33. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hybrid-2.0 34. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/propgrid-0.1 35. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/gravatar-0.3 * himerge 0.17.9. Uploaded by Luis Araujo. [36]himerge: Haskell Graphical User Interface for Emerge. * Takusen 0.8. Uploaded by Alistair Bayley. [37]Takusen: Database library with left-fold interface, for PostgreSQL, Oracle, SQLite, ODBC.. * irc 0.4.1. Uploaded by Trevor Elliott. [38]irc: A small library for parsing IRC messages.. * hexpat 0.2. Uploaded by Evan Martin. [39]hexpat: wrapper for expat, the fast XML parser. * microbench 0.1. Uploaded by Evan Martin. [40]microbench: Microbenchmark Haskell code. * hxt 7.5. Uploaded by Uwe Schmidt. [41]hxt: A collection of tools for processing XML with Haskell.. * hmatrix 0.2.1.0. Uploaded by Alberto Ruiz. [42]hmatrix: Linear algebra and numerical computations. * binary-strict 0.3.1. Uploaded by Adam Langley. [43]binary-strict: Binary deserialisation using strict ByteStrings. * category-extras 0.1. Uploaded by Dan Doel. [44]category-extras: Various modules and constructs inspired by category theory.. * pcap 0.4.3. Uploaded by Bryan OSullivan. [45]pcap: A system-independent interface for user-level packet capture. 36. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/himerge-0.17.9 37. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Takusen-0.8 38. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/irc-0.4.1 39. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hexpat-0.2 40. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/microbench-0.1 41. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hxt-7.5 42. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hmatrix-0.2.1.0 43. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/binary-strict-0.… 44. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras-… 45. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/pcap-0.4.3 * curl 1.3.1. Uploaded by Eric Mertens. [46]curl: Haskell binding to libcurl. * fastcgi 3001.0.2. Uploaded by Bjorn Bringert. [47]fastcgi: A Haskell library for writing FastCGI programs. * hslogger 1.0.5. Uploaded by John Goerzen. [48]hslogger: Versatile logging framework. * HAppS-Server 0.9.2.1. Uploaded by David Himmelstrup. [49]HAppS-Server: Web related tools and services.. * HAppS-IxSet 0.9.2.1. Uploaded by David Himmelstrup. [50]HAppS-IxSet: Added by DavidHimmelstrup, Fri Feb 29 07:27:13 PST 2008.. * HAppS-State 0.9.2.1. Uploaded by David Himmelstrup. [51]HAppS-State: Event-based distributed state.. * HAppS-Data 0.9.2.1. Uploaded by David Himmelstrup. [52]HAppS-Data: HAppS data manipulation libraries. * HAppS-Util 0.9.2.1. Uploaded by David Himmelstrup. [53]HAppS-Util: Web framework. * sessions 2008.2.28. Uploaded by Matthew Sackman. [54]sessions: Session Types for Haskell. * utf8-string 0.3. Uploaded by Eric Mertens. [55]utf8-string: Support for reading and writing UTF8 Strings. 46. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/curl-1.3.1 47. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/fastcgi-3001.0.2 48. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslogger-1.0.5 49. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Server-0.9… 50. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-IxSet-0.9.… 51. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-State-0.9.… 52. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Data-0.9.2… 53. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HAppS-Util-0.9.2… 54. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sessions-2008.2.… 55. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/utf8-string-0.3 * EdisonCore 1.2.1.2. Uploaded by Robert Dockins. [56]EdisonCore: A library of efficent, purely-functional data structures (Core Implementations). * parameterized-data 0.1. Uploaded by Alfonso Acosta. [57]parameterized-data: Parameterized data library implementing lightweight dependent types. * unix 2.3.0.0. Uploaded by Ross Paterson. [58]unix: POSIX functionality. * hoogle 3.1. Uploaded by Neil Mitchell. [59]hoogle: Haskell API Search. * ftshell 0.2. Uploaded by Janis Voigtlaender. [60]ftshell: Shell interface to the FreeTheorems library.. * free-theorems 0.2. Uploaded by Janis Voigtlaender. [61]free-theorems: Automatic generation of free theorems.. * special-functors 1.0. Uploaded by Henning Thielemann. [62]special-functors: Control.Applicative, Data.Foldable, Data.Traversable (compatibility package). * type-level 0.1. Uploaded by Alfonso Acosta. [63]type-level: Type-level programming library. * nymphaea 0.2. Uploaded by Gwern Branwen. [64]nymphaea: An interactive GUI for manipulating L-systems. * hsc3 0.2. Uploaded by Rohan Drape. [65]hsc3: Haskell SuperCollider. 56. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/EdisonCore-1.2.1… 57. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parameterized-da… 58. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/unix-2.3.0.0 59. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hoogle-3.1 60. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/ftshell-0.2 61. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/free-theorems-0.2 62. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/special-functors… 63. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/type-level-0.1 64. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/nymphaea-0.2 65. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsc3-0.2 * hosc 0.2. Uploaded by Rohan Drape. [66]hosc: Haskell Open Sound Control. * hslackbuilder 0.0.1. Uploaded by Andrea Rossato. [67]hslackbuilder: HSlackBuilder automatically generates slackBuild scripts from a cabal package. * hsparklines 0.1.0. Uploaded by Hitesh Jasani. [68]hsparklines: Sparklines for Haskell. * sat-micro-hs 0.1.1. Uploaded by Denis Bueno. [69]sat-micro-hs: A minimal SAT solver. * interlude 0.1.1. Uploaded by Gwern Branwen. [70]interlude: Replaces some Prelude functions for enhanced error reporting. * parse-dimacs 1.0.1. Uploaded by Denis Bueno. [71]parse-dimacs: DIMACS CNF parser library. * bitset 0.5. Uploaded by Denis Bueno. [72]bitset: A functional data structure for efficient membership testing.. * special-functors 1.0. Uploaded by Henning Thielemann. [73]special-functors: Control.Applicative, Data.Foldable, Data.Traversable (compatibility package). * condorcet 0.0.1. Uploaded by Gwern Branwen. [74]condorcet: Library for Condorcet voting. * heap 0.2.3. Uploaded by Stephan Friedrichs. [75]heap: Heaps in Haskell. 66. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hosc-0.2 67. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hslackbuilder-0.… 68. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsparklines-0.1.0 69. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sat-micro-hs-0.1… 70. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/interlude-0.1.1 71. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parse-dimacs-1.0… 72. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bitset-0.5 73. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/special-functors… 74. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/condorcet-0.0.1 75. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/heap-0.2.3 * hspr-sh 0.3. Uploaded by Gwern Branwen. [76]hspr-sh: Session handler for HSP. * hsp 0.2. Uploaded by Gwern Branwen. [77]hsp: Haskell Server Pages is a library for writing dynamic server-side web pages.. * trhsx 0.2.1. Uploaded by Gwern Branwen. [78]trhsx: trhsx is the preprocessor for Harp and HSP. * haskell-src-exts 0.2.1. Uploaded by Gwern Branwen. [79]haskell-src-exts: Manipulating Haskell source: abstract syntax, lexer, parser, and pretty-printer. * harp 0.2.1. Uploaded by Gwern Branwen. [80]harp: HaRP allows pattern-matching with regular expressions. * HTF 0.1. Uploaded by Gwern Branwen. [81]HTF: The Haskell Test Framework. * hsdip 0.1. Uploaded by Gwern Branwen. [82]hsdip: hsdip - a Diplomacy parser/renderer. * mpdmate 0.1. Uploaded by Gwern Branwen. [83]mpdmate: MPD/PowerMate executable. * powermate 0.1. Uploaded by Gwern Branwen. [84]powermate: PowerMate bindings. * syb-with-class 0.4. Uploaded by David Himmelstrup. [85]syb-with-class: Scrap Your Boilerplate With Class. 76. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hspr-sh-0.3 77. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsp-0.2 78. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/trhsx-0.2.1 79. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haskell-src-exts… 80. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/harp-0.2.1 81. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTF-0.1 82. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hsdip-0.1 83. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/mpdmate-0.1 84. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/powermate-0.1 85. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/syb-with-class-0… * whim 0.1. Uploaded by Gwern Branwen. [86]whim: A Haskell window manager. * memcached 0.1. Uploaded by Gwern Branwen. [87]memcached: haskell bindings for memcached. * HaLeX 1.1. Uploaded by Gwern Branwen. [88]HaLeX: HaLeX enables modelling, manipulation and animation of regular languages. 86. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/whim-0.1 87. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/memcached-0.1 88. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HaLeX-1.1 Jobs Haskell for real-time control software. Tom Hawkins [89]announced an opening for a Haskell job in real-time control software for vehicle and machinery applications 89. http://article.gmane.org/gmane.comp.lang.haskell.cafe/37093 Haskell for bioinformatics. Ketil Malde [90]announced an open position for a 3-year Ph.D. scolarship at IMR working on bioinformatics projects in Haskell 90. http://news.gmane.org/gmane.comp.lang.haskell.cafe/cutoff=37362 Blog noise [91]Haskell news from the [92]blogosphere. * [93]Barracuda P2P Chat * [94]A Lambda Calculus Reducer * [95]A Fashion Magazine in Haskell * [96]Introduction to building stateful web apps in HAppS * [97]Intro to HAppS-State * [98]Project Euler in Haskell * [99]In praise of mandatory indentation for novice programmers * [100]More Monads on the Cheap: Inlined fromMaybe * [101]A First Haskell Experience * [102]Haskell and code coverage * [103]Why I don't use Haskell for Functional Programming (monads, lifting) 91. http://planet.haskell.org/ 92. http://haskell.org/haskellwiki/Blog_articles 93. http://haskell.org/gtk2hs/archives/2008/02/24/barracuda-p2p-chat/ 94. http://www.defmacro.org/ramblings/lambda-reducer.html 95. http://www.alpheccar.org/en/posts/show/91 96. http://softwaresimply.blogspot.com/2008/02/intro-to-happs-part-1.html 97. http://softwaresimply.blogspot.com/2008/02/intro-to-happs-state.html 98. http://extempore.livejournal.com/212602.html 99. http://okasaki.blogspot.com/2008/02/in-praise-of-mandatory-indentation-for.… 100. http://osteele.com/archives/2008/02/inlined-frommaybe 101. http://the-programmers-stone.com/2008/03/04/a-first-haskell-experience/ 102. http://dukedave.blogspot.com/2008/03/back-in-action.html 103. http://jlouisramblings.blogspot.com/2008/03/why-i-dont-use-haskell-for-func… Quotes of the Week * teamonkey: the Haskell solutions that people are posting are generally so much more concise and elegant than for any other language * Dan Zwell: I am fairly new to Haskell, and I didn't realize how easy concurrent code is until I wrote this * anonymous: The Haskall (sic) language is often uses by very intelligent programmers, it often allows to use lazy computations and iterations, but it has the advantage that its iterators behave better (than in Python), and during the generation of some items you can, when you want, refer and use the items already generated. * Corun: I don't understand, what's the advantage of hugs? The uni here says to use hugs, though, but I kept finding myself going in to ghci to get a useful error message * They say that if it compiles, it will run correctly. It?s nearly true! I?m amazed. ... Such buglessness will remove a huge source of indeterminism in production environments where the work of many teams is co-ordinated by schedules. * dolio: I've made a domain specific notation for describing puddings. * cschneid: [Haskell] changed the way I look at decomposition of problems in the more corporate languages (Java and C#). I use far fewer variables, and more side-effect free methods. It's made my code clearer, and easier to test. * nicodemus: I've written some Erlang and much more Haskell. My take so far is that Erlang is good for teaching you how to fish, Haskell is good for teaching you about procuring food (including fish). * paulzork: Haskell is to functional programming like C is to imperative languages? Sort of the latin root? About the Haskell Weekly News New editions are posted to [104]the Haskell mailing list as well as to [105]the Haskell Sequence and [106]Planet Haskell. [107]RSS is also available, and headlines appear on [108]haskell.org. Headlines are available as [109]PDF. To help create new editions of this newsletter, please see the [110]contributing information. Send stories to dons at galois.com. The darcs repository is available at darcs get [111]http://code.haskell.org/~dons/code/hwn/ 104. http://www.haskell.org/mailman/listinfo/haskell 105. http://sequence.complete.org/ 106. http://planet.haskell.org/ 107. http://sequence.complete.org/node/feed 108. http://haskell.org/ 109. http://code.haskell.org/~dons/code/hwn/archives/20080309.pdf 110. http://haskell.org/haskellwiki/HWN 111. http://code.haskell.org/~dons/code/hwn/

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simple Haskell question - List comprehensions
by Philip Müller 09 Mar '08

09 Mar '08

Hi, I'm just working through Hutton's "Programming in Haskell" and I found an exercise which I can't solve, although it looks simple. Maybe someone here could give me a hint? Exercise: Show how the single comprehension [(x,y) | x <- [1,2,3], y <- [4,5,6]] with two generators can be re-expressed using two comprehensions with single generators. Hint: make use of the library function _concat_. Thank you very much, Philip

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deriving instances of Enum for tuples of bounded, enumerable types (useful for generating QuickCheck.Arbitrary instances for collection of collection types)
by Thomas Hartman 09 Mar '08

09 Mar '08

I have some code for creating instances of Enum for a tuple of bounded enumerable types. The main win with this was that it makes it easier for me to generate Test.QuickCheck.Arbitrary instances for types that are based on collections of enumerable collections -- like you might get when modeling poker for instance. In fact, I was trying to answer the question about using QuickCheck with poker, that Vincent Foley asked a few days ago at http://groups.google.com/group/fa.haskell/browse_thread/thread/8298c4d838c9… when I came up with this. (And I am posting a reply to Vincent right after I finish writing this message.) Basically I noticed that coming up with the Arbitrary instances seemed to be tricker than I would have expected. However, it was pretty straightforward to write an instance of arbitrary for instance (Enum a, Bounded a) => Arbitrary a where arbitrary = do n <- choose (fromEnum (minBound :: a), fromEnum (maxBound :: a) ) return $ toEnum n So all I needed to do was get my type into instances of Enum and Bounded, and I got my Arbitrary instance for free. Most of the work was just writing the instances for various tuples of Bounded Enumerable types. Maybe I've been spoiled by how easy many types are gotten using the magic of the deriving clause, but this seemed like a lot of work to me. I was wondering if there was an easier way? Or if not, if it would make sense for something like this to be added to the prelude? Or is there some sizeable collection of nice Arbitrary instances for QuickCheck somewheres that I can leverage off of, that hasn't made it into the standard libraries? A minor issue: I had a question if I could make the type signatures for Enum instances less verbose by doing something like type BoundedEnum = (Bounded a, Enum a) => a... I tried, and commented out my attempt as it wouldn't type check. Guidance appreciated. Thanks, Thomas. Code follows: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module EnumInstances where -- instances of Enum for tuples of types that are both Bounded and Enum -- typechecks, and does the right thing, though this signature seems messy, specially when I -- want to do this kind of thing for longer tuples instance (Enum a, Enum b,Bounded a, Bounded b) => Enum (a,b) where fromEnum (primero,segundo) = ( divFactor * (fromEnum primero) ) + (fromEnum segundo) where divFactor = fromEnum (maxBound :: b) + 1 toEnum i | (i > (fromEnum (maxBound :: (a,b)))) || (i < (fromEnum (minBound :: (a,b)))) = error "Enum (a,b) bounds error" | otherwise = (a,b) where a = toEnum baseline b = toEnum $ i - (baseline * divFactor ) baseline = i `div` divFactor divFactor = (fromEnum (maxBound :: b)) + 1 -- I tried to use a type synonym to write a cleaner type syg but... type BoundedEnumerable = (Enum a, Bounded a) => a -- does not typecheck. oh well, the verbose signatures aren't all that bad. {- instance Enum (BoundedEnumerable,BoundedEnumerable) where fromEnum (primero,segundo) = ( divFactor * (fromEnum primero) ) + (fromEnum segundo) where divFactor = fromEnum (maxBound :: b) + 1 toEnum i | (i > (fromEnum (maxBound :: (a,b)))) || (i < (fromEnum (minBound :: (a,b)))) = error "Enum (a,b) bounds error" | otherwise = (a,b) where a = toEnum baseline b = toEnum $ i - (baseline * divFactor ) baseline = i `div` divFactor divFactor = (fromEnum (maxBound :: b)) + 1 -} instance (Enum t, Enum t1, Enum t2, Bounded t, Bounded t1, Bounded t2) => Enum (t,t1,t2) where fromEnum (a,b,c) = fromEnum ((a,b),c) toEnum i = (a,b,c) where ((a,b),c) = toEnum i instance (Enum t, Enum t1, Enum t2, Enum t3, Bounded t, Bounded t1, Bounded t2, Bounded t3) => Enum (t,t1,t2,t3) where fromEnum (a,b,c,d) = fromEnum ((a,b,c),d) toEnum i = (a,b,c,d) where ((a,b,c),d) = toEnum i instance (Enum t, Enum t1, Enum t2, Enum t3, Enum t4, Bounded t, Bounded t1, Bounded t2, Bounded t3, Bounded t4) => Enum (t,t1,t2,t3,t4) where fromEnum (a,b,c,d,e) = fromEnum ((a,b,c,d),e) toEnum i = (a,b,c,d,e) where ((a,b,c,d),e) = toEnum i

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