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  • Haskell's cabal dependency problem with happy

    - by wirrbel
    I have problems installing ghc-mod on my linux machine. cabal worries about "happy" not being available in versione = 1.17: $ cabal install ghc-mod Resolving dependencies... [1 of 1] Compiling Main ( /tmp/haskell-src-exts-1.14.0-1357/haskell-src-exts-1.14.0/Setup.hs, /tmp/haskell-src-exts-1.14.0-1357/haskell-src-exts-1.14.0/dist/setup/Main.o ) Linking /tmp/haskell-src-exts-1.14.0-1357/haskell-src-exts-1.14.0/dist/setup/setup ... Configuring haskell-src-exts-1.14.0... setup: The program happy version =1.17 is required but it could not be found. Failed to install haskell-src-exts-1.14.0 cabal: Error: some packages failed to install: ghc-mod-3.1.3 depends on haskell-src-exts-1.14.0 which failed to install. haskell-src-exts-1.14.0 failed during the configure step. The exception was: ExitFailure 1 hlint-1.8.53 depends on haskell-src-exts-1.14.0 which failed to install. However, it even is installed in v. 1.19, as you can see here: $ cabal install happy Resolving dependencies... [1 of 1] Compiling Main ( /tmp/happy-1.19.0-1124/happy-1.19.0/Setup.lhs, /tmp/happy-1.19.0-1124/happy-1.19.0/dist/setup/Main.o ) Linking /tmp/happy-1.19.0-1124/happy-1.19.0/dist/setup/setup ... Configuring happy-1.19.0... Building happy-1.19.0... Preprocessing executable 'happy' for happy-1.19.0... [ 1 of 18] Compiling NameSet ( src/NameSet.hs, dist/build/happy/happy-tmp/NameSet.o ) [ 2 of 18] Compiling Target ( src/Target.lhs, dist/build/happy/happy-tmp/Target.o ) [ 3 of 18] Compiling AbsSyn ( src/AbsSyn.lhs, dist/build/happy/happy-tmp/AbsSyn.o ) [ 4 of 18] Compiling ParamRules ( src/ParamRules.hs, dist/build/happy/happy-tmp/ParamRules.o ) [ 5 of 18] Compiling GenUtils ( src/GenUtils.lhs, dist/build/happy/happy-tmp/GenUtils.o ) [ 6 of 18] Compiling ParseMonad ( src/ParseMonad.lhs, dist/build/happy/happy-tmp/ParseMonad.o ) [ 7 of 18] Compiling Lexer ( src/Lexer.lhs, dist/build/happy/happy-tmp/Lexer.o ) [ 8 of 18] Compiling Parser ( dist/build/happy/happy-tmp/Parser.hs, dist/build/happy/happy-tmp/Parser.o ) [ 9 of 18] Compiling AttrGrammar ( src/AttrGrammar.lhs, dist/build/happy/happy-tmp/AttrGrammar.o ) [10 of 18] Compiling AttrGrammarParser ( dist/build/happy/happy-tmp/AttrGrammarParser.hs, dist/build/happy/happy-tmp/AttrGrammarParser.o ) [11 of 18] Compiling Grammar ( src/Grammar.lhs, dist/build/happy/happy-tmp/Grammar.o ) [12 of 18] Compiling First ( src/First.lhs, dist/build/happy/happy-tmp/First.o ) [13 of 18] Compiling LALR ( src/LALR.lhs, dist/build/happy/happy-tmp/LALR.o ) [14 of 18] Compiling Paths_happy ( dist/build/autogen/Paths_happy.hs, dist/build/happy/happy-tmp/Paths_happy.o ) [15 of 18] Compiling ProduceCode ( src/ProduceCode.lhs, dist/build/happy/happy-tmp/ProduceCode.o ) [16 of 18] Compiling ProduceGLRCode ( src/ProduceGLRCode.lhs, dist/build/happy/happy-tmp/ProduceGLRCode.o ) [17 of 18] Compiling Info ( src/Info.lhs, dist/build/happy/happy-tmp/Info.o ) [18 of 18] Compiling Main ( src/Main.lhs, dist/build/happy/happy-tmp/Main.o ) Linking dist/build/happy/happy ... Installing executable(s) in /home/hope/.cabal/bin Installed happy-1.19.0 Any ideas? cabal-install version 1.16.0.2 using version 1.16.0 of the Cabal library

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  • Explore Historic Sites from the Comfort of Your Desktop with Google’s ‘World Wonders Project’

    - by Asian Angel
    Have you always wanted to explore historic sites across the world but lack the extra time and/or funds to do so? Then take heart! Now you can visit historic sites to your heart’s content from home with Google’s ‘World Wonders Project’. Note: The screenshot shown above is from the ‘Archaeological Areas of Pompei’ site. You can explore exotic locations such as Pompei, the Palace and Park of Versailles, Shark Bay, the Tenryu-ji-Temple in Ancient Kyoto, and more. The World Wonders Project Homepage The World Wonders Project YouTube Channel HTG Explains: Learn How Websites Are Tracking You Online Here’s How to Download Windows 8 Release Preview Right Now HTG Explains: Why Linux Doesn’t Need Defragmenting

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  • Haskell vs Erlang for web services

    - by Zachary K
    I am looking to start an experimental project using a functional language and am trying to decide beween Erlang and Haskell, and both have some points that I really like. I like Haskell's strong type system and purity. I have a feeling it will make it easier to write really reliable code. And I think that the power of haskell will make some of what I want to do much easier. On the minus side I get the feeling that some of the Frameworks for doing web stuff on Haskell such as Yesod are not as advanced as their Erlang counter parts. I rather like the Erlang approach to threads and to fault tollerence. I have a feeling that the scalability of Erlang could be a major plus. Which leeds to to my question, what has people's exerience been in implementing web application backends in both Haskell and Erlang. Are there packages for Haskell to provide some of the lightweight threads and actors that one has in Erlang?

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  • 14+ Real Estate WordPress Themes

    - by Aditi
    If you are looking for a great WordPress real estate theme. Below is a list of some of the best wordpress real estate themes, so you can find one, which is the best suited for you and be at par with increasing industry demands in real estates business.We have covered only the best themes available. The Themes are flexible & can be used by anybody in real estate business. If you are realtor, agent, appraiser or realty these can be modified as per your use. Estate It is an immensely powerful and simple to manage business theme. It offers advanced SEO control, clean code and styling modification features. It has new “Properties” management facility when installed – proving it’s far more than just a WordPress theme. It offers flexible page templates, an advanced search facility that allows you to drill down into properties based on very specific criteria, Google Maps integration and smart property images management. It is a complete web solution. It also has IDX functionality due to dsIDXpress plugin integration, which allows multi-listing services. Price: $200 View Demo Download ElegantEstate It makes your WordPress blog into a full-feature real estate website. The theme makes browsing your listings easy, and adds special integration features for property info, photos, Google Maps and more. Help increase sales by establishing an elegant and professional online presence today. It has opera compatibility, Netscape compatibility, Safari compatibility, WordPress 3.0 compatibility. It comes with five color schemes, threaded comments, optional blog-style structure, Gravatar ready, firefox compatible, IE8 + IE7 + IE6 compatible, advertisement ready, widget ready sidebars, theme options page, custom thumbnail images, PSD files, valid XHTML + CSS, smooth table less design, ePanel theme options, page templates, complete localization and many more features. Price: $39 (Package includes more than 55 themes) View Demo Download Open House Open House is fully compatible with WordPress 3.0+ and a highly customizable Real Estate WordPress theme. It has Google Maps Integration with Street View. It has a professional look for Agents and Realtors both. It is best suited for all markets and countries with theme localization, translation and internationalization. It provides for English, Spanish and Portuguese language files in the Developer Package. It has custom scripts, which makes it easy to add/delete/modify listings. It also includes photo gallery with a lightbox effect, gorgeous photo fade animations and automatic Google Maps integration. The theme can be used as a single or multi-agent website with individual Agent-Realtor pages with listings and biography information, Agent photo uploader, financing calculator.There is Multi Category search for potential customers to locate the house they want. Price: $39.95 essential | $69.95 standard | $99.95 premium View Demo Download Residence Real Estate It is a WordPress 3.0+ compatible stunning real estate theme. It has a dynamic real estate framework management module for easy edit-delete-add more features options, which makes this theme super easy to customize to the market needs. It allows you to add your own labels and values in your own language and switch the theme to your own language with English and Spanish files included with the ability to add your own language. It offers Multi-Category search with breadcrumb filtered results, easy photo gallery management with drag-drop sorting of images. It allows you to build your own multi-category search section menu with custom labels-choices and unlimited dropdown menus. They have been presented in a professional module with search results in breadcrumb navigation. Price: $39.95 essential | $69.95 standard | $99.95 premium View Demo Download Smooth Smooth is a WordPress Real Estate theme. It is a complete theme, which comes with Multi Category Search, Google Maps Integration, Agent Photo and Logo uploader that offers a professional and extremely affordable solution for Realtors and Agents to showcase their properties with ease. You can add your listings with the extremely easy and flexible Dynamic Real Estate Framework, edit-add-modify-delete all features, labels and values within the WordPress administration and upload unlimited photos to your galleries with latest WordPress 3.0+ features. It is a complete solution for real estate sites. Price: $39.95 essential | $69.95 standard | $99.95 premium View Demo Download Homeowners It is another WordPress Real Estate theme, which is a fast loading optimized theme with Google Maps Integration, fully compatible with WordPress 3.0 features and all Real Estate markets. It has a professional clean look and it is full of features extremely easy to modify. It also provides for 12 new styles provided. English, Spanish and Portuguese language files are provided in the Developer Package. Homeowners WordPress Real Estate features custom scripts that make add/delete/modify listings an easy task with an included photo gallery with a lightbox effect and automatic Google Map integration with street view (New) Agents will have access only to their own listings and add the listing management for their account making this theme an ideal affordable solution for Realtors and Real Estate agencies. The theme can be used as a single or multi-agent website with individual Agent-Realtor pages with listings and biography information, Agent photo uploader, financing calculator. Multi category search has also been provided. Price: $39.95 essential | $69.95 standard | $99.95 premium View Demo Download Real Agent Real Estate This theme is a WordPress 3.0+ compatible clean grid based real estate theme. It has a dynamic real estate framework management module for easy edit-delete-add more features options. It is easy to customize according to market. It allows you to add your own labels and values in your own language switch the theme to your own language with English and Spanish files included with the ability to add your own language. Multi-Category search with breadcrumb filtered results, easy photo gallery management with drag-drop sorting of images. You can upload property photos in bulk with the native WordPress uploader and the new image editing and resizing options in WordPress 3.0+. The theme features 5 different color styles, blue, black, red, green and purple with professional layouts, logo and agent photo uploaders. This theme is best suited for individual or multiple agents both. Price: $39.95 essential | $69.95 standard | $99.95 premium View Demo Download Agent Press The AgentPress theme is an ideal solution for real estate agents. It offers multiple page templates that can be used to create a complete real estate website. You can create from single property templates to a custom homepage easily with it. It is compatible to WordPress 3.0 and 3.1. It has custom background/header, property template, 6 layout options, fixed width, threaded comments and many more features. Price: $99.95 View Demo Download Real Estate It is one of the best Real Estate themes. It offers single click auto install of the site, Allow user to pay & submit properties on your site, Multi-agent site with profiles, Strategically built real estate site with professional design, User dashboard to edit/renew their submissions, Auto generated Google Maps and Image Slideshows and many more unique features. Once the users search property as per their criteria, the properties are listed with all the necessary parameters that let them select the property of their choice. Users can also add the property to favorite so they can check the property later from their member area dashboard. Admin may display different sidebar on this page and add widgets of their choice. This theme is full of custom, dynamic widgets such as top agents, finance calculator, user login; advertise blocks, testimonials and so on. There is a property details page where users can see the actual property. The agent details is displayed with the full contact details and appropriate links so the visitor can get all info about the property being sold, seller and may contact them by filling out a simple form. The email will be sent directly to the person who listed the property. Price: $89.95 Single | $159.95 Developer View Demo Download Broker Real Estate It is also a WordPress 3.0+ compatible real estate theme. It has a featured property slideshow, dynamic real estate framework management module for easy edit-delete-add more features. You can add your own labels and values in your own language. It offers multi-category search with breadcrumb-filtered results, easy photo gallery management with drag-drop sorting of images. You can also build your own multi-category search section menu with custom labels-choices and unlimited dropdown menus. Price: $39.95 essential | $69.95 standard | $99.95 premium View Demo Download Decasa It has custom search panel that lets your user easily browse your properties by keyword search or category select drop downs. It offers the property exposé, which is a user-friendly overview over the most important details of each real estate object. You can easily add this data through a post settings meta box on the post edit screen. You can easily create a real estate image gallery. Its theme options panel makes it easy to make the basic theme settings. It supports the new WordPress post thumbnail feature. When uploading an image file the theme will automatically create all the necessary image size. You can also create your own custom menu easily and fast with drag and drop without touching any code. Price: 39 € View Demo Download RealtorPress A real estate premium WordPress theme from PremiumPress. Versatile WordPress Theme that can be used by individual agents or real estate companies. The theme allows you to easily add property listings via the custom backend admin area or import CSV spreadsheets. It features customisable search options, Google maps integration, real estate data custom field creator, image management tools and more. Price: $79 | Premium Collection: $259 (all PremiumPress themes) View Demo Download Related posts:21+ WordPress Photo Blog & Portfolio Themes 14+ WordPress Portfolio Themes Professional WordPress Business Themes

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  • What's the progress on Haskell records?

    - by mmh
    Recently I stumbled once again on the issues of Haskells records, in particular the uniqueness of field names (it's a pain ...) I already read A proposal for records in Haskell from SPJ and Greg Morrisett but it's last update was 2003. Another paper Lightweight Extensible Records for Haskell from SPJ and Mark Jones is even older: It's from a Haskell workshop in 1999. Now I wonder if the process of giving Haskell new records made any progress. Does anybody know something about it or can point me to some further reading ?

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  • ICC Cricket World Cup 2011- Free Online Live Streaming, Mobile Apps, TV and Radio Guide

    - by Kavitha
    The ICC Cricket World Cup 2011 will be hosted jointly by Bangladesh, India and Sri Lanka. This 10th edition of World Cup is held between 19 February-2 April 2011. The World Cup drive will be starting in Dhaka on 19 February with the inaugural match between India and Bangladesh. The 43 days long ICC World Cup Cricket 2011 event will host 49 matches, day matches starting as early as 9.30am IST and day-night matches starting at 2.30pm IST. Here is our guide to follow 2011 ICC Cricket World Cup live on your computers, televisions,mobiles and radios Free Live Streaming On The Web (Official & Unofficial) http://espnstar.com will live stream all the matches of World Cup 2011 and they will be available in HD quality as they are the official broadcasters of World Cup 2011 cricket event. This is the first time ever a world cup cricket event is streamed online officially. If you are not able to access the official live streaming of Cricket World Cup due to regional restrictions, point your browser to any of the following unofficial live streams on the web. NOTE: MAKE SURE THAT YOUR ANTIVIRUS and ANTIMALWARE software are up and running before opening any of these sites. crictime.com - this site offers 6 live streaming servers that offer World Cup 2011 Cricket matches streams. Don’t mind the ads that are displayed left,right and center and just enjoy the cricket. Web pages dedicated for the world cup streaming are already live and you can bookmark them for your reference. cricfire.com/live-cricket: cricfire   gathers cricket live streams available around the web and provides them for easy access. Also they provide links for watching highlights and other post match analysis shows. Other sites that provide live streaming videos extracover.net webcric.com Searching for Unofficial Streams On Live Video Streaming Sites One of the best ways to find the unofficial streams is look for live streaming feeds on popular video streaming websites. We can be assured that these sites does not spread malware and spammy ads as they are well established. Here are the queries that you can use to search the popular sites FreedoCast  http://freedocast.com/search.aspx?go=cricket%20world%20cup Justin.tv      http://www.justin.tv/search?q=cricket+world+cup Ustream.tv  http://www.ustream.tv/discovery/live/all?q=cricket%20world%20cup TV Channels That Telecast Cricket World Cup Live Even though web is the place where we spend most of our time for entertainment, TVs are still popular for watching sports events. Mostly 90% of us are going to follow this cricket world cup matches on television sets. Here is the list of TV channels that paid whooping amounts of money for broadcasting rights and going to telecast live cricket Afghanistan – Ariana Television Network: Lemar TV Australia – Nine Network, Fox Sports Bangladesh – Bangladesh Television Canada – Asian Television Network China – ESPN Star Sports Europe (Except UK & Ireland) – Eurosport2 Fiji – Fiji TV India – ESPN Star Sports, Star Cricket, DD National (mostly India matches alone) Ireland – Zee Cafe Jamaica – Television Jamaica Middle East – Arab Radio and Television Network Nepal – ESPN Star Sports New Zealand – Sky Sport Pacific Islands – Sky Pacific Pakistan – GEO Super, Pakistan Television Corporation Pan-Africa – South African Broadcasting Corporation Singapore – Star Cricket South Africa – Supersport, Sabc3 Sport Sri Lanka – Sri Lanka Rupavahini Corporation United Kingdom – Sky Sports HD USA – Willow Cricket, DirecTV, Dish Network West Indies – Caribbean Media Corporation Radio Stations That Provide Live Commentary Don’t we listen to radio? Yes we still listen to radios, especially when we are on the go. Radios are part of our mobiles as well as music players like iPods. Here are the stations that you can tune into for catching live cricket commentary Australia – ABC Local Radio Bangladesh – Bangladesh Betar Canada , Central America – EchoStar India – All India Radio Pakistan, United Arab Emirates – Hum FM Sri Lanka – FM Derana United Kingdom, Ireland – BBC Radio West Indies – Caribbean Media Corporation Watch World Cup Cricket On Your Mobile This section is for Indian users. 3G rollout is happening at very high pace in all part of the India and most of the metros and towns are able to access 3G services. With 3G on your mobile you will be able to watch live ICC world cricket on your Reliance Mobiles and you can read more about it here. Top 10 Cricket Websites Check out our earlier post on top 10 cricket web sites for information. This article titled,ICC Cricket World Cup 2011- Free Online Live Streaming, Mobile Apps, TV and Radio Guide, was originally published at Tech Dreams. Grab our rss feed or fan us on Facebook to get updates from us.

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  • What kind of things are easy in Haskell and hard in Scala, and vice versa?

    - by Daniel C. Sobral
    There has been some intermingling of Scala and Haskell communities, and I have noticed now and then people commenting on stuff that's supposed to be easy in Haskell and hard and Scala. Less often (maybe because I read Scala questions, not Haskell ones), I see someone mentioning that something in Scala is easier than in Haskell. So. I'd like to know from people who are knowledgeable in both what kind of things are easy in Haskell and hard in Scala, and, conversely, what kind of things are easy in Scala and hard in Haskell.

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  • How to implement early exit / return in Haskell?

    - by Giorgio
    I am porting a Java application to Haskell. The main method of the Java application follows the pattern: public static void main(String [] args) { if (args.length == 0) { System.out.println("Invalid number of arguments."); System.exit(1); } SomeDataType d = getData(arg[0]); if (!dataOk(d)) { System.out.println("Could not read input data."); System.exit(1); } SomeDataType r = processData(d); if (!resultOk(r)) { System.out.println("Processing failed."); System.exit(1); } ... } So I have different steps and after each step I can either exit with an error code, or continue to the following step. My attempt at porting this to Haskell goes as follows: main :: IO () main = do a <- getArgs if ((length args) == 0) then do putStrLn "Invalid number of arguments." exitWith (ExitFailure 1) else do -- The rest of the main function goes here. With this solution, I will have lots of nested if-then-else (one for each exit point of the original Java code). Is there a more elegant / idiomatic way of implementing this pattern in Haskell? In general, what is a Haskell idiomatic way to implement an early exit / return as used in an imperative language like Java?

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  • Real Time BI in the Real World

    - by tobin.gilman(at)oracle.com
    Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman","serif";} Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman","serif"; mso-fareast-font-family:"Times New Roman";} One of my favorite BI offerings from Oracle is a solution called Oracle Real Time Decisions.  Whenever I mention this product in customer meetings, eyes light up.  There are some fascinating examples of customers using it to up-sell, cross-sell, increase customer retention, and reduce risk in real time, with off the charts return on investment. I plan to share some of those stories in a future blog.  In this post however, I want to share some far more common real time analytics use case scenarios that are being addressed with widely deployed Oracle BI and data integration technologies Not all real time BI applications require continuous learning, predictive modeling, and data mining.  Many simply require the ability to integrate, aggregate, and access information that is current (typically within in few minutes or a few seconds).  The use cases are infinite.  A few I've seen: ·         Purchasing agents need to match demand against available inventory ·         Manufacturing planners need to monitor current parts and material against scheduled build plans ·         Airline agents need to match ticket demand against flight schedules, ·         Human resources managers need to track the status of global hiring requisitions against current headcount authorizations...you get the idea. One way of doing this is to run reports or federated queries directly against transactional systems.  That approach can be viable if you only need to access simple data sets on rare occasions.  High volume and complex queries can quickly bog down performance of mission critical transactional systems.  There is an architecturally simple way of solving the problem, and it's being applied by real companies around the world to solve real needs in real time.    Cbeyond is an Atlanta, GA based  provider of voice, data and mobile business applications delivers.  They deliver real time information to its call center agents  as they are interacting with their customers. The data they need resides in production CRM and other transactional systems, but  instead or reporting directly off the those systems, data is first moved to an operational data store (ODS).  Rather than running data intensive, time consuming, and performance degrading batch ETL routines to populate the ODS, Cbeyond uses Oracle Golden Gate software to incrementally capture and move only the changed records from log files of the transactional systems every few minutes.  There is no impact on transactional system performance, and the information needed by call center representatives is up to date.  Oracle Business Intelligence software presents the information to services reps in a rich, visual, and highly interactive format. Avea is similar to Cbeyond.  They are a telecommunications company who integrates billing and customer information in an ODS that is accessed by their call center agents in real time using Oracle Golden Gate and Oracle Business Intelligence.  They've taken it a step further by using the ODS to feed a data warehouse.  The operational data store provides the current information needed by call center agents during "in flight" customer interactions.  The data warehouse is used for more sophisticated analysis of historical data.  For maximum performance, both the ODS and data warehouse run on the Oracle Exadata Database Machine. These are practical illustrations of companies addressing real time reporting and analysis needs using established business intelligence/data warehousing methodologies and tools common to many IT departments.  If real time BI could benefit your organization, you may be already be closer than you thought to having the pieces in place to solving the problem.    Give us a shout if you are interested in learning more or if you have an interesting use or approach to real-time BI.

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  • Type checking and recursive types (Writing the Y combinator in Haskell/Ocaml)

    - by beta
    When explaining the Y combinator in the context of Haskell, it's usually noted that the straight-forward implementation won't type-check in Haskell because of its recursive type. For example, from Rosettacode [1]: The obvious definition of the Y combinator in Haskell canot be used because it contains an infinite recursive type (a = a -> b). Defining a data type (Mu) allows this recursion to be broken. newtype Mu a = Roll { unroll :: Mu a -> a } fix :: (a -> a) -> a fix = \f -> (\x -> f (unroll x x)) $ Roll (\x -> f (unroll x x)) And indeed, the “obvious” definition does not type check: ?> let fix f g = (\x -> \a -> f (x x) a) (\x -> \a -> f (x x) a) g <interactive>:10:33: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 Expected type: t2 -> t0 -> t1 Actual type: (t2 -> t0 -> t1) -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a <interactive>:10:57: Occurs check: cannot construct the infinite type: t2 = t2 -> t0 -> t1 In the first argument of `x', namely `x' In the first argument of `f', namely `(x x)' In the expression: f (x x) a (0.01 secs, 1033328 bytes) The same limitation exists in Ocaml: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; Error: This expression has type 'a -> 'b but an expression was expected of type 'a The type variable 'a occurs inside 'a -> 'b However, in Ocaml, one can allow recursive types by passing in the -rectypes switch: -rectypes Allow arbitrary recursive types during type-checking. By default, only recursive types where the recursion goes through an object type are supported. By using -rectypes, everything works: utop # let fix f g = (fun x a -> f (x x) a) (fun x a -> f (x x) a) g;; val fix : (('a -> 'b) -> 'a -> 'b) -> 'a -> 'b = <fun> utop # let fact_improver partial n = if n = 0 then 1 else n*partial (n-1);; val fact_improver : (int -> int) -> int -> int = <fun> utop # (fix fact_improver) 5;; - : int = 120 Being curious about type systems and type inference, this raises some questions I'm still not able to answer. First, how does the type checker come up with the type t2 = t2 -> t0 -> t1? Having come up with that type, I guess the problem is that the type (t2) refers to itself on the right side? Second, and perhaps most interesting, what is the reason for the Haskell/Ocaml type systems to disallow this? I guess there is a good reason since Ocaml also will not allow it by default even if it can deal with recursive types if given the -rectypes switch. If these are really big topics, I'd appreciate pointers to relevant literature. [1] http://rosettacode.org/wiki/Y_combinator#Haskell

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  • Multithreaded Haskell

    - by Ingdas
    Hi I'm learning Haskell and I'd like to write some multithreaded programs now to see the performance gains from that in a functional language. I can find some references to it on the internet but never a proper introduction to it. Can anyone point me to a guide which is understandable for someone who knows the syntax fairly well, but is not a wizard in Haskell?

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  • Are comonads a good fit for modeling the Wumpus world?

    - by Tim Stewart
    I'm trying to find some practical applications of a comonad and I thought I'd try to see if I could represent the classical Wumpus world as a comonad. I'd like to use this code to allow the Wumpus to move left and right through the world and clean up dirty tiles and avoid pits. It seems that the only comonad function that's useful is extract (to get the current tile) and that moving around and cleaning tiles would not use be able to make use of extend or duplicate. I'm not sure comonads are a good fit but I've seen a talk (Dominic Orchard: A Notation for Comonads) where comonads were used to model a cursor in a two-dimensional matrix. If a comonad is a good way of representing the Wumpus world, could you please show where my code is wrong? If it's wrong, could you please suggest a simple application of comonads? module Wumpus where -- Incomplete model of a world inhabited by a Wumpus who likes a nice -- tidy world but does not like falling in pits. import Control.Comonad -- The Wumpus world is made up of tiles that can be in one of three -- states. data Tile = Clean | Dirty | Pit deriving (Show, Eq) -- The Wumpus world is a one dimensional array partitioned into three -- values: the tiles to the left of the Wumpus, the tile occupied by -- the Wumpus, and the tiles to the right of the Wumpus. data World a = World [a] a [a] deriving (Show, Eq) -- Applies a function to every tile in the world instance Functor World where fmap f (World as b cs) = World (fmap f as) (f b) (fmap f cs) -- The Wumpus world is a Comonad instance Comonad World where -- get the part of the world the Wumpus currently occupies extract (World _ b _) = b -- not sure what this means in the Wumpus world. This type checks -- but does not make sense to me. extend f w@(World as b cs) = World (map world as) (f w) (map world cs) where world v = f (World [] v []) -- returns a world in which the Wumpus has either 1) moved one tile to -- the left or 2) stayed in the same place if the Wumpus could not move -- to the left. moveLeft :: World a -> World a moveLeft w@(World [] _ _) = w moveLeft (World as b cs) = World (init as) (last as) (b:cs) -- returns a world in which the Wumpus has either 1) moved one tile to -- the right or 2) stayed in the same place if the Wumpus could not move -- to the right. moveRight :: World a -> World a moveRight w@(World _ _ []) = w moveRight (World as b cs) = World (as ++ [b]) (head cs) (tail cs) initWorld = World [Dirty, Clean, Dirty] Dirty [Clean, Dirty, Pit] -- cleans the current tile cleanTile :: Tile -> Tile cleanTile Dirty = Clean cleanTile t = t Thanks!

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  • Inspirational software for end-users written in Haskell?

    - by Lenny222
    I think great technology ist invisible. Besides the usual suspects (GHC, Xmonad, proprietary trading software) what great examples are there for end-user software written in Haskell? I think good examples are FreeArc, Hledger and "Nikki And The Robots". Do you have more examples (full blown GUI apps, small CLI tools, etc)? Edit: For example i am fascinated by Wings3D, because while written in Erlang, users can not tell that. It just works. Among Haskell's weak spots are cross-platform GUIs. There are not many GUI apps written in Haskkel in general and most of them are no easy to use, install or even compile. What are good examples to learn from how to make hard things look easy?

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  • in memory datastore in haskell

    - by Simon
    I want to implement an in memory datastore for a web service in Haskell. I want to run transactions in the stm monad. When I google hash table steam Haskell I only get this: Data. BTree. HashTable. STM. The module name and complexities suggest that this is implemented as a tree. I would think that an array would be more efficient for mutable hash tables. Is there a reason to avoid using an array for an STM hashtable? Do I gain anything with this stem hash table or should I just use a steam ref to an IntMap?

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  • Learning Haskell: How to remove an item from a List in Haskell

    - by BM
    Trying to learn Haskell. I am trying to write a simple function to remove a number from a list without using built-in function (delete...I think). For the sake of simplicity, let's assume that the input parameter is an Integer and the list is an Integer list. Here is the code I have, Please tell me what's wrong with the following code areTheySame :: Int -> Int-> [Int] areTheySame x y | x == y = [] | otherwise = [y] removeItem :: Int -> [Int] -> [Int] removeItem x (y:ys) = areTheySame x y : removeItem x ys

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  • Problem with do construct in haskell

    - by Anonymous Coward
    Hello everyone I'm trying to learn Haskell and want to write a small program which prints the content of a file to the screen. When I load it into GHCi I get the following error: The last statement in a 'do' construct must be an expression I know this question has be asked already here: Haskell — “The last statement in a 'do' construct must be an expression”. Even though my code is very similar I still can't figure out the problem. If anyone could point out the problem to me I'd be very thankful. module Main (main) where import System.IO import System(getArgs) main :: IO() main = do args <- getArgs inh <- openFile $ ReadMode head args printFile inh hClose inh printFile :: Handle -> IO () printFile handle = do end <- hIsEOF handle if end then return () else do line <- hGetLine handle putStrLn line printFile handle

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  • Preferred way to do locales in the Haskell Platform

    - by user355466
    The Haskell platform includes two obsolete libraries, old-time and old-locale. For old-time, it also includes the preferred alternative (namely time), but I can't figure out what the recommended alternative for old-locale is. Is this simply a shortcoming of the Platform as of now (version 2010.1.0.0), or something I've overlooked?

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  • Multiple actions upon a case statement in Haskell

    - by Schroedinger
    One last question for the evening, I'm building the main input function of my Haskell program and I have to check for the args that are brought in so I use args <- getArgs case length args of 0 -> putStrLn "No Arguments, exiting" otherwise -> { other methods here} Is there an intelligent way of setting up other methods, or is it in my best interest to write a function that the other case is thrown to within the main? Or is there an even better solution to the issue of cases. I've just got to take in one name.

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  • Haskell web frameworks survey

    - by Phuc Nguyen
    There are several web frameworks for Haskell like Happstack, Snap, and Yesod, and probably a few more. In what aspects do they differ from each other? For example: features (e.g. server only, or also client scripting, easy support for different kinds of database) maturity (e.g. stability, documentation quality) scalability (e.g. performance, handy abstraction) main targets Also, what are examples of real-world sites / web apps using these frameworks? Many thanks.

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  • Real Estate SEO For Your Real Estate Business

    The online real estate industry is flourishing and while there are a lot of deals that happen online, there are also a lot of prospective investors looking to make worthy investments. This opens up a very big opportunity for real estate business owners as they can get a huge audience to market their offers.

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  • Real Estate SEO For Your Real Estate Business

    The online real estate industry is flourishing and while there are a lot of deals that happen online, there are also a lot of prospective investors looking to make worthy investments. This opens up a very big opportunity for real estate business owners as they can get a huge audience to market their offers.

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  • Is Haskell worth learning?

    - by Jason K
    I am looking at this question primarily from a career point of view, so I hope you answer it accordingly. I am fairly proficient with Python, can write C++ and I am a final year student of computer science engineering I am looking to learn Haskell because I have heard a lot about it. My question is: apart from learning it because of all the good I have heard about it, is it any good for my career? Is it used in the industry? I am curious to learn it but unless it helps me somehow in my career, I am not willing to make that change at this stage. Looking for some personal experiences here. Thanks!

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  • How to Achieve Real-Time Data Protection and Availabilty....For Real

    - by JoeMeeks
    There is a class of business and mission critical applications where downtime or data loss have substantial negative impact on revenue, customer service, reputation, cost, etc. Because the Oracle Database is used extensively to provide reliable performance and availability for this class of application, it also provides an integrated set of capabilities for real-time data protection and availability. Active Data Guard, depicted in the figure below, is the cornerstone for accomplishing these objectives because it provides the absolute best real-time data protection and availability for the Oracle Database. This is a bold statement, but it is supported by the facts. It isn’t so much that alternative solutions are bad, it’s just that their architectures prevent them from achieving the same levels of data protection, availability, simplicity, and asset utilization provided by Active Data Guard. Let’s explore further. Backups are the most popular method used to protect data and are an essential best practice for every database. Not surprisingly, Oracle Recovery Manager (RMAN) is one of the most commonly used features of the Oracle Database. But comparing Active Data Guard to backups is like comparing apples to motorcycles. Active Data Guard uses a hot (open read-only), synchronized copy of the production database to provide real-time data protection and HA. In contrast, a restore from backup takes time and often has many moving parts - people, processes, software and systems – that can create a level of uncertainty during an outage that critical applications can’t afford. This is why backups play a secondary role for your most critical databases by complementing real-time solutions that can provide both data protection and availability. Before Data Guard, enterprises used storage remote-mirroring for real-time data protection and availability. Remote-mirroring is a sophisticated storage technology promoted as a generic infrastructure solution that makes a simple promise – whatever is written to a primary volume will also be written to the mirrored volume at a remote site. Keeping this promise is also what causes data loss and downtime when the data written to primary volumes is corrupt – the same corruption is faithfully mirrored to the remote volume making both copies unusable. This happens because remote-mirroring is a generic process. It has no  intrinsic knowledge of Oracle data structures to enable advanced protection, nor can it perform independent Oracle validation BEFORE changes are applied to the remote copy. There is also nothing to prevent human error (e.g. a storage admin accidentally deleting critical files) from also impacting the remote mirrored copy. Remote-mirroring tricks users by creating a false impression that there are two separate copies of the Oracle Database. In truth; while remote-mirroring maintains two copies of the data on different volumes, both are part of a single closely coupled system. Not only will remote-mirroring propagate corruptions and administrative errors, but the changes applied to the mirrored volume are a result of the same Oracle code path that applied the change to the source volume. There is no isolation, either from a storage mirroring perspective or from an Oracle software perspective.  Bottom line, storage remote-mirroring lacks both the smarts and isolation level necessary to provide true data protection. Active Data Guard offers much more than storage remote-mirroring when your objective is protecting your enterprise from downtime and data loss. Like remote-mirroring, an Active Data Guard replica is an exact block for block copy of the primary. Unlike remote-mirroring, an Active Data Guard replica is NOT a tightly coupled copy of the source volumes - it is a completely independent Oracle Database. Active Data Guard’s inherent knowledge of Oracle data block and redo structures enables a separate Oracle Database using a different Oracle code path than the primary to use the full complement of Oracle data validation methods before changes are applied to the synchronized copy. These include: physical check sum, logical intra-block checking, lost write validation, and automatic block repair. The figure below illustrates the stark difference between the knowledge that remote-mirroring can discern from an Oracle data block and what Active Data Guard can discern. An Active Data Guard standby also provides a range of additional services enabled by the fact that it is a running Oracle Database - not just a mirrored copy of data files. An Active Data Guard standby database can be open read-only while it is synchronizing with the primary. This enables read-only workloads to be offloaded from the primary system and run on the active standby - boosting performance by utilizing all assets. An Active Data Guard standby can also be used to implement many types of system and database maintenance in rolling fashion. Maintenance and upgrades are first implemented on the standby while production runs unaffected at the primary. After the primary and standby are synchronized and all changes have been validated, the production workload is quickly switched to the standby. The only downtime is the time required for user connections to transfer from one system to the next. These capabilities further expand the expectations of availability offered by a data protection solution beyond what is possible to do using storage remote-mirroring. So don’t be fooled by appearances.  Storage remote-mirroring and Active Data Guard replication may look similar on the surface - but the devil is in the details. Only Active Data Guard has the smarts, the isolation, and the simplicity, to provide the best data protection and availability for the Oracle Database. Stay tuned for future blog posts that dive into the many differences between storage remote-mirroring and Active Data Guard along the dimensions of data protection, data availability, cost, asset utilization and return on investment. For additional information on Active Data Guard, see: Active Data Guard Technical White Paper Active Data Guard vs Storage Remote-Mirroring Active Data Guard Home Page on the Oracle Technology Network

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  • Haskell: Best tools to validate textual input?

    - by Ana
    In Haskell, there are a few different options to "parsing text". I know of Alex & Happy, Parsec and Attoparsec. Probably some others. I'd like to put together a library where the user can input pieces of a URL (scheme e.g. HTTP, hostname, username, port, path, query, etc.) I'd like to validate the pieces according to the ABNF specified in RFC 3986. In other words, I'd like to put together a set of functions such as: validateScheme :: String -> Bool validateUsername :: String -> Bool validatePassword :: String -> Bool validateAuthority :: String -> Bool validatePath :: String -> Bool validateQuery :: String -> Bool What is the most appropriate tool to use to write these functions? Alex's regexps is very concise, but it's a tokenizer and doesn't straightforwardly allow you to parse using specific rules, so it's not quite what I'm looking for, but perhaps it can be wrangled into doing this easily. I've written Parsec code that does some of the above, but it looks very different from the original ABNF and unnecessarily long. So, there must be an easier and/or more appropriate way. Recommendations?

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