- Haskell-Cafe - Haskell.org

Finite but not fixed length...
by Jason Dusek 13 Oct '10

13 Oct '10

Is there a way to write a Haskell data structure that is necessarily only one or two or seventeen items long; but that is nonetheless statically guaranteed to be of finite length? -- Jason Dusek Linux User #510144 | http://counter.li.org/

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a couple of cabal related questions
by Dmitry V'yal 13 Oct '10

13 Oct '10

Hello, recently I've cabalized my program and it brought some problems. First of all, I made use of 'import Paths_<package name>' functionality to get the path of installed data files. It works great, but now I can no longer run my program in ghci because this module is auto-generated by cabal. I tried to supply my own. This way ghci works, but my stub module gets compiled into the binary in place of auto-generated. And things break again. What's the best way to overcome it? And another question. I'd like to make my program output it's version string. Of course, I can hardcode it into sources, but I've already specified one in the cabal file. Is there any way to get it back in the run time? Thanks, Dmitry

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Haskell Weekly News: Issue 154 - October 13, 2010
by Daniel Santa Cruz 13 Oct '10

13 Oct '10

Welcome to issue 154 of the HWN, a newsletter covering developments in the Haskell community in the week of October 03 - 09. This community does not stand still! The last section of the newsletter contains a list of new or updated packages this passed week. All 87 of them! I had my doubts about listing them all, but there was encouragement from IRC folk to showcase them all, at least this time. If you are having a hard time filling your day with Haskell stories, make sure to jump into Reddit and help everyone by posting comments and voting on stories that are meaninful. If you manage to have time left after that, the volume of postings on the mailing lists is more than one person can keep up with. Send threads that you think the rest of the community would benefit from. For those of you who manage to have extra-extra time, you can use [1]twitterfall to try to keep up with the many tweets about Haskell! So, what was hot this week? Announcements Want to know what events are comming up in the community? Come and see the [2]Haskell Community Calendar. Send new events to dstcruz(a)gmail.com Michael Snoyman [3]announced and asked for advice on [4]Haskellers.com. If you are into Haskell, by all means, please stop by and make a profile on the site! A new version of the venerable Haskell XML Toolbox (version 9.0.0) [5]was announced by Uwe Schmidt. A package for tls, native TLS/SSL protocol implementation [6]was announced by Vincent Hanquez. Interesting Threads on Haskell-Cafe Petr Pudlak Asked if there are any suggested/recommended Haskell coding conventions? Perhaps something like Java has? [7]see thread caseyh commented of difficulties with GUIs working under Windows. The thread included talk about the state of Haskell GUI programming in general. [8]see thread Don Steward and Duncan Coutts presented a status report on the Haskell distribution infrastructure: Hackage, Haskell Platform and Cabal, during the Haskell Implementors Workshop held in Baltimore. You can find the [9]slides and the [10]video of the presentation. Christopher Done [11]annouced that he has been doing some refactoring on Haskell web development entries on the Wiki. Max Bolingbroke [12]told us about a proposed haskell feature: lambda-case/lambda-if. There seems to be interest from multiple people to see this feature. Some modifications to the original syntax were also proposed in the thread. Top Reddit Stories * Learn You a Haskell for Great Good! The Book! Domain: amazon.com Score: 72, Comments: 28 On Reddit: http://bit.ly/dp07AZ Original: http://amzn.to/b1JYWM * LYAH: For A Few Monads More Domain: learnyouahaskell.com Score: 64, Comments: 25 On Reddit: http://bit.ly/bLQMsD Original: http://bit.ly/dpkD0R * Real World Experiences with Long Running Haskell Servers? Domain: self.haskell Score: 34, Comments: 25 On Reddit: http://bit.ly/9NTcLo * Want professional work in Haskell? Don't forget to register your skills Domain: haskellers.com Score: 28, Comments: 21 On Reddit: http://bit.ly/9bqlhG Original: http://www.haskellers.com/ * Enumerators Tutorial Part 3: Enumeratee Domain: docs.yesodweb.com Score: 25, Comments: 1 On Reddit: http://bit.ly/d3RPP0 Original: http://bit.ly/dwEOWO * Scheduling Lazy Evaluation on Multicore : Simon Marlow : Video Domain: vimeo.com Score: 24, Comments: 5 On Reddit: http://bit.ly/cGXeFJ Original: http://vimeo.com/15573590 * Scrap Your Zippers : Michael Adams : Video Domain: vimeo.com Score: 23, Comments: On Reddit: http://bit.ly/aco7b6 Original: http://www.vimeo.com/15481513 * [The Haskell Platform] libraries process is a trackless mire Domain: haskell.org Score: 21, Comments: 7 On Reddit: http://bit.ly/cby0zw Original: http://bit.ly/a62xg0 * Angel: a daemon that runs and monitors other processes, developed by Bump Domain: github.com Score: 19, Comments: On Reddit: http://bit.ly/9P8Ccm Original: http://github.com/jamwt/Angel#readme * The HTML purification manifesto (Inside 245s) Domain: blog.ezyang.com Score: 15, Comments: On Reddit: http://bit.ly/a6Bu4N Original: http://bit.ly/duCoSz Top StackOverflow Questions * haskell regex substitution (votes: 8, answers: 1) http://bit.ly/b34nPB * What is the Haskell response to Node.js? (votes: 6, answers: 2) http://bit.ly/9YKRPj * Conventions for Stability field of Cabal packages (votes: 5, answers: 2) http://bit.ly/cxDksd * is there a lazy way to write the minus function (remove items from a list)? (votes: 4, answers: 3) http://bit.ly/c85gi9 * "maybe"-like function for Bool and List? (votes: 3, answers: 3) http://bit.ly/biWCUl Quotes of the Week * Cale: makes a fruit salad with some Abelian grapes and some Banananach spaces. * Cale: If you don't feel at sea most of the time that you're studying something, you're probably not learning fast enough. * jamy: windows Keyboard doesn't found. To rescan - press any key * kmc: we put the fun in -fundecidable * sipa: asking for experts before asking your questions prevents non-experts who know the answer as well from answering * monochrom: but yes there is hope that haskell 2039 becomes haskell 1971, all is not lost. Now or Updates Hackage Packages * anansi 0.2.1 - Simple literate programming preprocessor (JohnMillikin) * archlinux 0.3.3 - Support for working with Arch Linux packages (DonaldStewart) * aterm 0.1.0.0 - serialisation for Haskell values with sharing support (ChristianMaeder) * authenticate 0.6.6.1 - Authentication methods for Haskell web applications. (MichaelSnoyman) * bff 0.3.1.1 - Bidirectionalization for Free! (POPL'09) (DanielSeidel) * bidispec 0.1.3 - Specification of generators and parsers (StefanWehr) * bidispec-extras 0.1 - Extra helper functions for bidirectional specifications (StefanWehr) * bindings-apr 0.1 - Low level bindings to Apache Portable Runtime (APR) (MasatakeDaimon) * bindings-apr-util 0.1 - Low level bindings to Apache Portable Runtime Utility (APR Utility) (MasatakeDaimon) * bindings-libzip 0.1 - Low level bindings to libzip. (SergeyAstanin) * cabal2arch 0.7.4 - Create Arch Linux packages from Cabal packages (DonaldStewart) * certificate 0.3 - Certificates and Key Reader/Writer (VincentHanquez) * collada-output 0.2 - Generate animated 3d objects in COLLADA (TillmannVogt) * Crypto 4.2.2 - Collects together existing Haskell cryptographic functions into a package (CreightonHogg) * cuda 0.2.2 - FFI binding to the CUDA interface for programming NVIDIA GPUs (TrevorMcDonell) * dataenc 0.13.0.4 - Data encoding library (MagnusTherning) * deepseq 1.1.0.1 - Fully evaluate data structures (SimonMarlow) * ecu 0.0.4 - Tools for automotive ECU development. (TomHawkins) * enumerator 0.4.1 - Implementation of Oleg Kiselyov's left-fold enumerators (JohnMillikin) * equivalence 0.2.1 - Maintaining an equivalence relation implemented as union-find using STT. (PatrickBahr) * exception-monads-fd 0.2 - Exception monad transformer instances for monads-fd classes. (GeoffreyMainland) * exception-monads-tf 0.2 - Exception monad transformer instances for monads-tf classes. (GeoffreyMainland) * exception-mtl 0.2 - Type classes and monads for unchecked extensible exceptions. (GeoffreyMainland) * exception-transformers 0.2 - Type classes and monads for unchecked extensible exceptions. (GeoffreyMainland) * explicit-iomodes-text 0.1.0.2 - Extends explicit-iomodes with Text operations (BasVanDijk) * extemp 0.0.1 - automated printing for extemp speakers (SamAnklesaria) * fibon 0.1.0 - A reworking of the classic nofib benchmark suite (DavidPeixotto) * free-theorems-counterexamples 0.3.0.1 - Automatically Generating Counterexamples to Naive Free Theorems (DanielSeidel) * Fungi 1.0.4 - An interpreter for Funge-98 programming languages, including Befunge. (ThomasEding) * hakismet 0.1 - Akismet spam protection library (NilsSchweinsberg) * haskelldb 2.0 - A library of combinators for generating and executing SQL statements. (JustinBailey) * haskelldb-hdbc 2.0 - HaskellDB support for HDBC. (JustinBailey) * haskelldb-hdbc-odbc 2.0 - HaskellDB support for the HDBC ODBC driver. (JustinBailey) * haskelldb-hdbc-postgresql 2.0 - HaskellDB support for the HDBC PostgreSQL driver. (JustinBailey) * haskelldb-hdbc-sqlite3 2.0 - HaskellDB support for the HDBC SQLite driver. (JustinBailey) * haskelldb-th 2.0 - Template Haskell utilities for HaskellDB. (JustinBailey) * haskell-qrencode 1.0.2 - Haskell bindings for libqrencode. (JamesSanders) * haskell-src-exts 1.9.4 - Manipulating Haskell source: abstract syntax, lexer, parser, and pretty-printer (NiklasBroberg) * haskell-src-meta 0.2 - Parse source to template-haskell abstract syntax. (BenMillwood) * haskoon 0.3.1.1 - Web Application Abstraction (StefanWehr) * haskoon-httpspec 0.5.0.1 - Integrating HttpSpec with Haskoon (StefanWehr) * haskoon-salvia 0.4.0.2 - Integrating HttpSpec with Haskoon (StefanWehr) * hothasktags 0.1.1 - Generates ctags for Haskell, incorporating import lists and qualified imports (LukePalmer) * hsc3-process 0.3.1 - Create and control scsynth processes (StefanKersten) * HTF 0.5.0.0 - The Haskell Test Framework (StefanWehr) * httpspec 0.3.0.1 - Specification of HTTP request/response generators and parsers (StefanWehr) * hums 0.3.1 - Haskell UPnP Media Server (BardurArantsson) * hws 1.1.0.1 - Simple Haskell Web Server (SimonMarlow) * hxt-extras 0.4 - Extra functions for HXT (TonyMorris) * hxthelper 0.2.2 - Helper functions for HXT (StefanWehr) * language-c-comments 0.1 - Extracting comments from C code (MatthewSottile) * libpq 0.4.0 - libpq binding for Haskell (GrantMonroe) * LibZip 0.2.0 - Bindings to libzip, a library for manipulating zip archives. (SergeyAstanin) * loli 2010.10.9 - A minimum web dev DSL in Haskell (JinjingWang) * mainland-pretty 0.1.0.1 - Pretty printing designed for printing source code. (GeoffreyMainland) * memscript 0.0.1.0 - Command line utility for memorizing scriptures or any other text (KiYungAhn) * multiset 0.2.1 - The Data.MultiSet container type (TwanVanLaarhoven) * network-bytestring 0.1.3.1 - Fast, memory-efficient, low-level networking (JohanTibell) * parallel 3.1.0.1 - Parallel programming library (SimonMarlow) * persistent-sqlite 0.2.2.1 - Backend for the persistent library using sqlite3. (MichaelSnoyman) * plot 0.1.2 - A plotting library, exportable as eps/pdf/svg/png or renderable with gtk (VivianMcPhail) * polyseq 0.1.2.1 - Taming Selective Strictness (DanielSeidel) * proc 0.0.8 - Parse process information for Linux (AndyStewart) * pureMD5 2.1.0.1 - A Haskell-only implementation of the MD5 digest (hash) algorithm. (ThomasDuBuisson) * random-extras 0.18 - Additional functions for random values. (AristidBreitkreuz) * safer-file-handles-text 0.1.0.3 - Extends safer-file-handles with Text operations (BasVanDijk) * shaker 0.4.3 - simple and interactive command-line build tool (AnthoninBonnefoy) * smartGroup 0.2.1 - group strings by words in common (SamAnklesaria) * spata 2010.10.10 - brainless form validation (JinjingWang) * stream-monad 0.3 - Simple, Fair and Terminating Backtracking Monad (SebastianFischer) * tls 0.2 - TLS/SSL protocol native implementation (Server and Client) (VincentHanquez) * tree-monad 0.3 - Non-Determinism Monad for Tree Search (SebastianFischer) * uni-events 2.2.0.0 - Event handling for the uniform workbench (ChristianMaeder) * uni-graphs 2.2.0.0 - Graphs (ChristianMaeder) * uni-htk 2.2.0.0 - Graphical User Interface for Haskell Programs (ChristianMaeder) * uni-posixutil 2.2.0.0 - Posix utilities for the uniform workbench (ChristianMaeder) * uni-reactor 2.2.0.0 - Reactors for the uniform workbench (ChristianMaeder) * uni-uDrawGraph 2.2.0.0 - Graphs binding (ChristianMaeder) * uni-util 2.2.0.0 - Utilities for the uniform workbench (ChristianMaeder) * usb 0.6.0.2 - Communicate with USB devices (BasVanDijk) * wumpus-basic 0.10.0 - Common drawing utilities built on wumpus-core. (StephenTetley) * wumpus-core 0.35.0 - Pure Haskell PostScript and SVG generation. (StephenTetley) * wumpus-microprint 0.10.0 - Microprints - "greek-text" pictures. (StephenTetley) * wumpus-tree 0.9.0 - Drawing trees (StephenTetley) * xilinx-lava 5.0.1.4 - The Lava system for Xilinx FPGA design with layout combinators. (SatnamSingh) * xsd 0.3.3 - XML Schema data structures (TonyMorris) * yesod-auth 0.1.1 - Authentication for Yesod. (MichaelSnoyman) About the Haskell Weekly News New editions are posted to [13]the Haskell mailing list as well as to [14]the Haskell Sequence and [15]Planet Haskell. [16]RSS is also available, and headlines appear on [17]haskell.org. To help create new editions of this newsletter, please send stories to dstcruz * at * gmail * dot * com. The code used to produce this version of the newsletter is not yet publicly available, as it is a complete hack designed to get things started again. Until next time, Daniel Santa Cruz References 1. http://twitterfall.com/ 2. https://sites.google.com/site/haskellcommunitycalendar/home 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81530 4. http://haskellers.com/ 5. http://osdir.com/ml/haskell-cafe@haskell.org/2010-10/msg00286.html 6. http://osdir.com/ml/haskell-cafe@haskell.org/2010-10/msg00252.html 7. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81083 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81225 9. http://donsbot.wordpress.com/2010/10/01/hackage-cabal-and-the-haskell-platf… 10. http://www.vimeo.com/15462768 11. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81345 12. http://article.gmane.org/gmane.comp.lang.haskell.cafe/81366 13. http://www.haskell.org/mailman/listinfo/haskell 14. http://sequence.complete.org/ 15. http://planet.haskell.org/ 16. http://sequence.complete.org/node/feed 17. http://haskell.org/

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How to fix "undefined reference" error with getDataDir?
by Andy Stewart 13 Oct '10

13 Oct '10

Hi all, I have two package A and B, and B depend A. I use below code snippets in package A: ------------------------------> code start <------------------------------ ... import Paths_manatee_ircclient import System.FilePath ... dir <- getDataDir let imagePath imageName = dir </> "icons" </> (imageName ++ ".png") ... ------------------------------> code end <------------------------------ Anyway, package A compile fine, but when i compile package B, i got below error: ------------------------------> error start <------------------------------ Linking dist/build/manatee/manatee ... /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `s6sP_info': (.text+0x3bea): undefined reference to `manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_getDataDir3_closure' /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `s6sP_info': (.text+0x3bf0): undefined reference to `manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_getDataDir2_closure' /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `s6Uf_info': (.text+0x560f): undefined reference to `__stginit_manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_' /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `r5Mp_closure': (.data+0x9d8): undefined reference to `manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_getDataDir3_closure' /home/andy/.cabal/lib/manatee-ircclient-0.0.1/ghc-6.12.3/libHSmanatee-ircclient-0.0.1.a(Smile.o): In function `r5Mp_closure': (.data+0x9e0): undefined reference to `manateezmircclientzm0zi0zi1_Pathszumanateezuircclient_getDataDir2_closure' collect2: ld returned 1 exit status cabal: Error: some packages failed to install: manatee-0.0.1 failed during the building phase. The exception was: ExitFailure 1 ------------------------------> error end <------------------------------ How to fix above error? Thanks! -- Andy Below is .cabal file for package A: ------------------------------> A.cabal start <------------------------------ name: manatee-ircclient version: 0.0.1 Cabal-Version: >= 1.6 license: GPL-3 license-file: LICENSE copyright: (c) 2009 ~ 2010 Andy Stewart synopsis: IRC client extension for Manatee. description: manatee-ircclient is IRC client extension for Manatee (Haskell/Gtk+ Integrated Live Environment) author: Andy Stewart maintainer: Andy Stewart <lazycat.manatee(a)gmail.com> stability: provisional category: Development, Other tested-with: GHC==6.12.3 build-type: Simple extra-source-files: Setup.lhs data-dir: icons data-files: angry.png confused.png crying.png embarrassed.png inlove.png kiss.png sleepy.png sad.png laugh.png smile.png surprised.png tired.png tongue.png whistling.png wink.png Library build-depends: base >= 4 && < 5, manatee-core >= 0.0.1, dbus-client >= 0.3 && < 0.4, stm >= 2.1.2.0, containers >= 0.3.0.0, gtk-serialized-event >= 0.11.0, gtk >= 0.11.0, text >= 0.7.1.0, bytestring >= 0.9.1.5, dbus-core, template-haskell, gtksourceview2 >= 0.11.0, unix >= 2.4.0.0, network, groom, fastirc >= 0.2.0, split >= 0.1.2, nano-md5 >= 0.1.2, filepath, regex-posix >= 0.94.1, array >= 0.3.0.0, GoogleTranslate >= 0.0.3, utf8-string, mtl, Cabal exposed-modules: Manatee.Extension.IrcClient Manatee.Extension.IrcClient.Types Manatee.Extension.IrcClient.DBus Manatee.Extension.IrcClient.Daemon Manatee.Extension.IrcClient.HighlightNick Manatee.Extension.IrcClient.IrcBuffer Manatee.Extension.IrcClient.IrcView Manatee.Extension.IrcClient.Smile other-modules: extensions: ghc-options: -O -fwarn-unused-matches -fwarn-unused-binds -fwarn-unused-imports -fwarn-overlapping-patterns -fwarn-duplicate-exports -threaded -fwarn-unrecognised-pragmas -fwarn-hi-shadowing Executable manatee-irc-daemon main-is: Main.hs ghc-options: -threaded ------------------------------> A.cabal end <------------------------------

2 2

Re: Re-order type
by André Batista Martins 13 Oct '10

13 Oct '10

Might have not been clear, but i will try illustrate . f:: a-> b -> c -> (b,(c,a)) f1 :: c -> a -> d input type: A B C ---------- | f | | _____ | output (B,(C,A)) C A ---------- | f1 | | _____ | output D If i want compose f and f1, i need to do a correct input to f1 from the output of f. So i want one function to convert the output of "f" to input off "f!". In this case, we do f1 fst (snd (t,(t1,t2))) snd (snd (t,(t1,t2))) But i want do this automaticaly, for type of any two function. I search for the "glue". I don't have any concern about what the function does, i only have interess on input and output types. Cheers, André No dia 9 de Outubro de 2010 22:38, André Batista Martins < andre_bm(a)netcabo.pt> escreveu: > Hello, > exists any algorithm to determine how terms can be changed to safisty the > type of one function? > > > example: > > f:: a-> b -> c -> (b,c,a) > > f1 :: c -> a -> d > > In my first function "f" i want assign the output "c" and "a" for to input > of function "f1". > I searched for any solution, but i didn't find any anything. > > One clue i have found is "minimal edit distance algorithm" for 2 strings. > Perhaps if i convert de output type of "f" to one string, and de input of > "f1" to another string and then use this algorithm , i will get one "dirty" > solution... > > I'm open to any sugestion. >

13 18

Things I would like to see in protocol-buffers
by Mads Lindstrøm 13 Oct '10

13 Oct '10

Hi I have been trying to use protocol-buffers[2], and I have some ideas which would have improved the protocol-buffers package for my usage (and maybe for others): * hprotoc should have an option to generate messages with [Char], Int and [] in stead of Utf8, Int32 and Seq. While some people may need the speed of bytestrings, for others they are just cumbersome. And if you need to convert bytestring to [Char] anyway (maybe some other library requires [Char]) you really do not gain any performance anyway. * Integration with System.IO.Handle, so that I would have a simpler interface like: readDelimitedMessage :: System.IO.Handle -> IO MessageType writeDelimitedMessage :: System.IO.Handle -> MessageType -> IO () * Union types. A common protocol buffers idiom for union types is[3]: message Foo { enum FooType { SOME_FOO_MESSAGE = 0; ANOTHER_FOO_MESSAGE = 1; } message SomeFoo { ... } message AnotherFoo { ... } required FooType fooType = 1; optional SomeFoo someFoo = 2; optional AnotherFoo anotherFoo = 3; } While the message can (statically) contain both someFoo and anotherFoo, it would be a runtime error. In reality it is a union type, that must either contain someFoo or antoherFoo. It would be nice if one could mark in the .proto file that hprotec should see Foo as a union type. Hprotoc should then generate the Foo type as: data Foo = SomeFoo ... | AnotherFoo ... Maybe the custom option mechanism[1] can be used in the .proto file, to implement this feature. What do people think of these ideas? Regards, Mads Lindstrøm [1] http://code.google.com/apis/protocolbuffers/docs/proto.html#options [2] http://hackage.haskell.org/package/protocol-buffers [3] http://code.google.com/apis/protocolbuffers/docs/techniques.html#union

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Re: [Haskell-cafe] Finite but not fixed length...
by Permjacov Evgeniy 13 Oct '10

13 Oct '10

infinite value is value, that have no upper bound (see infinity definition). So, you have to provide upper bound at compile time. Tree example provides such bound. On 10/13/2010 03:27 PM, Eugene Kirpichov wrote: > Again, the question is not how to arrange that all non-bottom values > are finite: this can easily be done using strictness, as in your List > example. > > The trick is to reject infinite values in compile time. How can *this* be done? > > 13 октября 2010 г. 15:26 пользователь Permjacov Evgeniy > <permeakra(a)gmail.com> написал: >> On 10/13/2010 03:09 PM, Eugene Kirpichov wrote: >> >> 1-st scenario: If you have, for example, 2-3 tree, it definitly has a >> root. If you construct tree from list and then match over root, the >> entire tree (and entire source list) will be forced. And on every >> update, 2-3 tree's root is reconstructed in functional setting. So, if >> you'll try to build 2-3 tree from infinite list, it will fail in process >> due insuffisient memory. >> Of course, you can make the same with >> data List a = Cons a (!List a) | Nil >> >> second scenario >> data Node a = Nil | One a | Two a a >> and so Node (Node (Node (Node a))) has at most 2^4 = 16 elements. with >> some triks you'll be able to set upper bound in runtime. >> >>> I don't see how. Could you elaborate? >>> >>> 13 октября 2010 г. 14:46 пользователь Permjacov Evgeniy >>> <permeakra(a)gmail.com> написал: >>>> On 10/13/2010 12:33 PM, Eugene Kirpichov wrote: >>>> I think, tree-like structure, used as sequence (like fingertrees), will >>>> do the work. >>>>> but it's not so easy to make infinite lists fail to typecheck... >>>>> That's what I'm wondering about. >>>>> >>>>> 2010/10/13 Miguel Mitrofanov <miguelimo38(a)yandex.ru>: >>>>>> hdList :: List a n -> Maybe a >>>>>> hdList Nil = Nothing >>>>>> hdList (Cons a _) = Just a >>>>>> >>>>>> hd :: FiniteList a -> Maybe a >>>>>> hd (FL as) = hdList as >>>>>> >>>>>> *Finite> hd ones >>>>>> >>>>>> this hangs, so, my guess is that ones = _|_ >>>>>> >>>>>> >>>>>> 13.10.2010 12:13, Eugene Kirpichov пишет: >>>>>>> {-# LANGUAGE ExistentialQuantification, GADTs, EmptyDataDecls #-} >>>>>>> module Finite where >>>>>>> >>>>>>> data Zero >>>>>>> data Succ a >>>>>>> >>>>>>> class Finite a where >>>>>>> >>>>>>> instance Finite Zero >>>>>>> instance (Finite a) => Finite (Succ a) >>>>>>> >>>>>>> data List a n where >>>>>>> Nil :: List a Zero >>>>>>> Cons :: (Finite n) => a -> List a n -> List a (Succ n) >>>>>>> >>>>>>> data FiniteList a where >>>>>>> FL :: (Finite n) => List a n -> FiniteList a >>>>>>> >>>>>>> nil :: FiniteList a >>>>>>> nil = FL Nil >>>>>>> >>>>>>> cons :: a -> FiniteList a -> FiniteList a >>>>>>> cons a (FL x) = FL (Cons a x) >>>>>>> >>>>>>> list123 = cons 1 (cons 2 (cons 3 nil)) >>>>>>> >>>>>>> ones = cons 1 ones -- typechecks ok >>>>>> _______________________________________________ >>>>>> Haskell-Cafe mailing list >>>>>> Haskell-Cafe(a)haskell.org >>>>>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>>>>> >>> >> > >

1 0

Desired behaviour of rounding etc.
by Daniel Fischer 13 Oct '10

13 Oct '10

The methods of the RealFrac class produce garbage when the value lies outside the range of the target type, e.g. Prelude GHC.Float> truncate 1.234e11 :: Int -- 32-bits -1154051584 and, in the case of truncate, different garbage when the rewrite rule fires: Prelude GHC.Float> double2Int 1.234e11 -2147483648 I'm currently working on faster implementations of properFraction, truncate, round, ceiling and floor for Float and Double, so I'd like to know - does it matter at all what garbage is returned in the above case? - if it does, what is the desired behaviour (at least for Int, I can't cater for all possibilities)? On a related note, in my benchmarks, truncFloatGen :: Integral a => Float -> a truncFloatGen = fromInteger . truncFloatInteger truncFloatInteger :: Float -> Integer truncFloatInteger x = case decodeFloat x of (m,e) | e == 0 -> m | e < 0 -> let s = -e in if m < 0 then - ((-m) `shiftR` s) else m `shiftR` s | otherwise -> m `shiftL` e is more than twice as fast as GHC.Float.float2Int, the corresponding for Double almost twice as fast as double2Int. Can anybody confirm that the above is faster than float2Int on other machines/architectures? Cheers, Daniel

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Re: Polyvariadic functions operating with a monoid
by oleg＠okmij.org 13 Oct '10

13 Oct '10

Sorry, I'm still catching up. I'm replying to first few messages. > instance Show a => Monoidable a [String] where > toMonoid a = [show a] > > main = putStrLn $ unwrap $ polyToMonoid [] True () (Just (5::Int)) > fails to compile. The error message points to the first problem: > No instances for (Monoidable Bool [a], > Monoidable () [a], > ... The presence of the type variable 'a' means that the type checker doesn't know list of what elements you want (in other words, the context is not specific enough to instantiate the type variable a). Thus, we need to explicitly tell that we wish a list of strings: > test3 = putStrLn $ unwrap $polyToMonoid ([]::[String]) True () (Just (5::Int)) Now we get a different error, which points to the real problem this time: the expression `unwrap ....' appears as an argument to putStrLn. That means that we are required to produce a String as a monoid. Yet we specified ([]::[String]) as mempty, which is unsuitable as mempty for the String monoid. If we desire the [String] monoid as the result, we need to change the context. For example, > test3 = mapM_ putStrLn $ unwrap $ > polyToMonoid ([]::[String]) True () (Just (5::Int)) > Another example that also fails to compile (but I cannot see why): > main = putStrLn $ show $ unwrap $ polyToMonoid (0::Int) (1::Int) > (2::Int) (3::Int) > No instance for (PolyVariadic Int (WMonoid m)) > arising from a use of `polyToMonoid' The error message is informative, mentioning the type variable, m. Whenever that happens, we know that we put a bounded polymorphic expression in the context that is not specific enough. We need some type annotations. In our case, the function 'show' can show values of many types. The type checker does not know that we wish an Int monoid specifically. So, we have to specialize the show function: > test4 = putStrLn $ (show :: Int -> String) $ > unwrap $ polyToMonoid (0::Int) (1::Int) (2::Int) (3::Int) At this point one may wonder if this is all worth it. There are too many annotations. Fortunately, if you are not afraid of one more extension, the annotations can be avoided. Your example would be accepted as it was written, see test3 and test4 below. > {-# LANGUAGE TypeSynonymInstances #-} > {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, TypeFamilies #-} > > module M where > > import Data.Monoid > > newtype WMonoid m = WMonoid{unwrap :: m} > > class Monoid m => Monoidable a m where > toMonoid :: a -> m > > class Monoid m => PolyVariadic m p where > polyToMonoid :: m -> p > > instance (Monoid m', m' ~ m) => PolyVariadic m (WMonoid m') where > polyToMonoid acc = WMonoid acc > > instance (Monoidable a m, PolyVariadic m r) => PolyVariadic m (a->r) where > polyToMonoid acc = \a -> polyToMonoid (acc `mappend` toMonoid a) > > instance Show a => Monoidable a String where > toMonoid = show > > instance Show a => Monoidable a [String] where > toMonoid a = [show a] > > test2 = putStrLn $ unwrap $ polyToMonoid "" True () (Just (5::Int)) > > test3 = mapM_ putStrLn $ unwrap $ polyToMonoid [] True () (Just (5::Int)) > > instance Monoid Int where > mappend = (+) > mempty = 0 > > instance Monoidable Int Int where > toMonoid = id > > test4 = putStrLn $ show $ > unwrap $ polyToMonoid (0::Int) (1::Int) (2::Int) (3::Int) P.S. Indeed, "polyToMonoid' = unwrap . polyToMonoid" does not do what one wishes to. One should regard `unwrap' as a sort of terminator of the argument list.

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Windows binary for latest darcs head
by Aaron Gray 12 Oct '10

12 Oct '10

Does anyone have a Windows binary for the latest GHC darcs head or at least October 8th they could send me. Many thanks in advance, Aaron

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