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• Is there a good way to QuickCheck Happstack.State methods?

  - by Paul Kuliniewicz

  I have a set of Happstack.State MACID methods that I want to test using QuickCheck, but I'm having trouble figuring out the most elegant way to accomplish that. The problems I'm running into are: The only way to evaluate an Ev monad computation is in the IO monad via query or update. There's no way to create a purely in-memory MACID store; this is by design. Therefore, running things in the IO monad means there are temporary files to clean up after each test. There's no way to initialize a new MACID store except with the initialValue for the state; it can't be generated via Arbitrary unless I expose an access method that replaces the state wholesale. Working around all of the above means writing methods that only use features of MonadReader or MonadState (and running the test inside Reader or State instead of Ev. This means forgoing the use of getRandom or getEventClockTime and the like inside the method definitions. The only options I can see are: Run the methods in a throw-away on-disk MACID store, cleaning up after each test and settling for starting from initialValue each time. Write the methods to have most of the code run in a MonadReader or MonadState (which is more easily testable), and rely on a small amount of non-QuickCheck-able glue around it that calls getRandom or getEventClockTime as necessary. Is there a better solution that I'm overlooking?

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• Haskell: monadic takeWhile?

  - by Mark Rushakoff

  I have some functions written in C that I call from Haskell. These functions return IO (CInt). Sometimes I want to run all of the functions regardless of what any of them return, and this is easy. For sake of example code, this is the general idea of what's happening currently: Prelude> let f x = print x >> return x Prelude> mapM_ f [0..5] 0 1 2 3 4 5 Prelude> I get my desired side effects, and I don't care about the results. But now I need to stop execution immediately after the first item that doesn't return my desired result. Let's say a return value of 4 or higher requires execution to stop - then what I want to do is this: Prelude> takeWhile (<4) $ mapM f [0..5] Which gives me this error: <interactive:1:22: Couldn't match expected type `[b]' against inferred type `IO a' In the first argument of `mapM', namely `f' In the second argument of `($)', namely `mapM f ([0 .. 5])' In the expression: takeWhile (< 4) $ mapM f ([0 .. 5]) And that makes sense to me - the result is still contained in the IO monad, and I can't just compare two values contained in the IO monad. I know this is precisely the purpose of monads -- chaining results together and discarding operations when a certain condition is met -- but is there an easy way to "wrap up" the IO monad in this case to stop executing the chain upon a condition of my choosing, without writing an instance of MonadPlus? Can I just "unlift" the values from f, for the purposes of the takeWhile? Is this a solution where functors fit? Functors haven't "clicked" with me yet, but I sort of have the impression that this might be a good situation to use them. Update: @sth has the closest answer to what I want - in fact, that's almost exactly what I was going for, but I'd still like to see whether there is a standard solution that isn't explicitly recursive -- this is Haskell, after all! Looking back on how I worded my question, now I can see that I wasn't clear enough about my desired behavior. The f function I used above for an example was merely an example. The real functions are written in C and used exclusively for their side effects. I can't use @Tom's suggestion of mapM_ f (takeWhile (&lt;4) [0..5]) because I have no idea whether any input will really result in success or failure until executed. I don't actually care about the returned list, either -- I just want to call the C functions until either the list is exhausted or the first C function returns a failure code. In C-style pseudocode, my behavior would be: do { result = function_with_side_effects(input_list[index++]); } while (result == success && index < max_index); So again, @sth's answer performs the exact behavior that I want, except that the results may (should?) be discarded. A dropWhileM_ function would be equivalent for my purposes. Why isn't there a function like that or takeWhileM_ in Control.Monad? I see that there was a similar discussion on a mailing list, but it appears that nothing has come of that.

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• Nhibernate Criteria Query with Join

  - by John Peters

  I am looking to do the following using an NHibernate Criteria Query I have "Product"s which has 0 to Many "Media"s A product can be associated with 1 to Many ProductCategories These use a table in the middled to create the join ProductCategories Id Title ProductsProductCategories ProductCategoryId ProductId Products Id Title ProductMedias ProductId MediaId Medias Id MediaType I need to implement a criteria query to return All Products in a ProductCategory and the top 1 associated Media or no media if none exists. So although for example a "T Shirt" may have 10 Medias associated, my result should be something similar to this Product.Id Product.Title MediaId 1 T Shirt 21 2 Shoes Null 3 Hat 43 I have tried the following solutions using JoinType.LeftOuterJoin 1) productCriteria.SetResultTransformer(Transformers.DistinctRootEntity); This hasnt worked as the transform is done code side and as I have .SetFirstResult() and .SetMaxResults() for paging purposes it wont work. 2) .SetProjection( Projections.Distinct( Projections.ProjectionList() .Add(Projections.Alias(Projections.Property("Id"), "Id")) ... .SetResultTransformer(Transformers.AliasToBean()); This hasn't worked as I cannot seem to populate a value for Medias.Id in the projections. (Similar to http://stackoverflow.com/questions/1036116/nhibernate-criteria-api-projections) Any help would be greatly appreciated

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• Seeking Functional Programming Lexicon

  - by Randall Schulz

  Hi, Knowing the argot of a field helps me a lot, especially since it allows me to converse intelligently with those who know a lot more than I, so I would like to find a good lexicon of Functional Programming terms. E.g., I repeatedly encounter these: Functor, Arrow, Category, Kleisli, Monad, Monoid, a veritable zoo of Morphisms, etc. I also notice many of these appear with prefixes such as "covariant", "co-", "endo-" etc. Of these, I can say I actually understand Monoid and Covariant and sort of get Monad, but the rest are still gibberish to me. (Note that I don't mean this list as exhaustive and I'm not looking to have these defined or described for me here, I'm looking for learning resources.) Can someone point me towards an FP lexicon? It need not be on-line, as long as it's possible to find it (and it's not a rare volume for which I'd have to pay many tens of dollars).

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• Performance of looping over an Unboxed array in Haskell

  - by Joey Adams

  First of all, it's great. However, I came across a situation where my benchmarks turned up weird results. I am new to Haskell, and this is first time I've gotten my hands dirty with mutable arrays and Monads. The code below is based on this example. I wrote a generic monadic for function that takes numbers and a step function rather than a range (like forM_ does). I compared using my generic for function (Loop A) against embedding an equivalent recursive function (Loop B). Having Loop A is noticeably faster than having Loop B. Weirder, having both Loop A and B together is faster than having Loop B by itself (but slightly slower than Loop A by itself). Some possible explanations I can think of for the discrepancies. Note that these are just guesses: Something I haven't learned yet about how Haskell extracts results from monadic functions. Loop B faults the array in a less cache efficient manner than Loop A. Why? I made a dumb mistake; Loop A and Loop B are actually different. Note that in all 3 cases of having either or both Loop A and Loop B, the program produces the same output. Here is the code. I tested it with ghc -O2 for.hs using GHC version 6.10.4 . import Control.Monad import Control.Monad.ST import Data.Array.IArray import Data.Array.MArray import Data.Array.ST import Data.Array.Unboxed for :: (Num a, Ord a, Monad m) => a -> a -> (a -> a) -> (a -> m b) -> m () for start end step f = loop start where loop i | i <= end = do f i loop (step i) | otherwise = return () primesToNA :: Int -> UArray Int Bool primesToNA n = runSTUArray $ do a <- newArray (2,n) True :: ST s (STUArray s Int Bool) let sr = floor . (sqrt::Double->Double) . fromIntegral $ n+1 -- Loop A for 4 n (+ 2) $ \j -> writeArray a j False -- Loop B let f i | i <= n = do writeArray a i False f (i+2) | otherwise = return () in f 4 forM_ [3,5..sr] $ \i -> do si <- readArray a i when si $ forM_ [i*i,i*i+i+i..n] $ \j -> writeArray a j False return a primesTo :: Int -> [Int] primesTo n = [i | (i,p) <- assocs . primesToNA $ n, p] main = print $ primesTo 30000000

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• How and why is ap defined as liftM2 id in Haskell

  - by luke_randall

  Whilst trying to better understand Applicative, I looked at the definition of <*, which tends to be defined as ap, which in turn is defined as: ap :: (Monad m) => m (a -> b) -> m a -> m b ap = liftM2 id Looking at the type signatures for liftM2 and id, namely: liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r id :: a -> a I fail to understand how just by passing in id, the relevant part of the type signature seems to transform from (a1 -> a2 -> r) -> m a1 to m (a -> b). What am I missing here?

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• Linking/Combining Type Classes in Haskell

  - by thegravian

  Say I have two type classes defined as follows that are identical in function but different in names: class Monad m where (>>=) :: m a -> (a -> m b) -> m b return :: a -> m a class PhantomMonad p where pbind :: p a -> (a -> p b) -> p b preturn :: a -> p b Is there a way to tie these two classes together so something that is an instance of PhantomMonad will automatically be an instance of Monad, or will instances for each class have to be explicitly written? Any insight would be most appreciated, thanks!

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• Haskell: 'No instance for' arising from a trivial usage of Regex library

  - by artemave

  Following the (accepted) answer from this question, I am expecting the following to work: Prelude Text.Regex.Posix Text.Regex.Base.RegexLike Text.Regex.Posix.String> makeRegex ".*" (makeRegex is a shortcut for makeRegexOpts with predefined options) However, it doesn't: <interactive>:1:0: No instance for (RegexMaker regex compOpt execOpt [Char]) arising from a use of `makeRegex' at <interactive>:1:0-13 Possible fix: add an instance declaration for (RegexMaker regex compOpt execOpt [Char]) In the expression: makeRegex ".*" In the definition of `it': it = makeRegex ".*" Prelude Text.Regex.Posix Text.Regex.Base.RegexLike Text.Regex.Posix.String> make Regex ".*"::Regex <interactive>:1:0: No instance for (RegexMaker Regex compOpt execOpt [Char]) arising from a use of `makeRegex' at <interactive>:1:0-13 Possible fix: add an instance declaration for (RegexMaker Regex compOpt execOpt [Char]) In the expression: makeRegex ".*" :: Regex In the definition of `it': it = makeRegex ".*" :: Regex And I really don't understand why. EDIT Haskell Platform 2009.02.02 (GHC 6.10.4) on Windows EDIT2 Prelude Text.Regex.Base.RegexLike Text.Regex.Posix.String> :i RegexMaker class (RegexOptions regex compOpt execOpt) => RegexMaker regex compOpt execOpt source | regex -> compOpt execOpt, compOpt -> regex execOpt, execOpt -> regex compOpt where makeRegex :: source -> regex makeRegexOpts :: compOpt -> execOpt -> source -> regex makeRegexM :: (Monad m) => source -> m regex makeRegexOptsM :: (Monad m) => compOpt -> execOpt -> source -> m regex -- Defined in Text.Regex.Base.RegexLike

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• NHibernate returning duplicate object in child collections when using Fetch

  - by UpTheCreek

  When doing a query like this (using Nhibernate 2.1.2): ICriteria criteria = session.CreateCriteria<MyRootType>() .SetFetchMode("ChildCollection1", FetchMode.Eager) .SetFetchMode("ChildCollection2", FetchMode.Eager) .Add(Restrictions.IdEq(id)); I am getting multiple duplicate objects in some cartesian fashion. E.g. if ChildCollection1 has 3 elements, and ChildColection2 has 2 elements then I get results with each element in ChildColection1 one duplicated, and each element in ChildColection2 triplicated! This was a bit of a WTF moment for me... So how to do this correctly? Is using SetFetchMode like this only supported when specifying one collection? Am I just using it wrong (I've seen some references to results transformers, but imagined this would be simplier). Is this something that's different in NH3? Update: As per Felice's suggestion, I tried using the DistinctRootEntity transformer, but this is still returning duplicates. Code: ICriteria criteria = session.CreateCriteria<MyRootType>() .SetFetchMode("ChildCollection1", FetchMode.Eager) .SetFetchMode("ChildCollection2", FetchMode.Eager) .Add(Restrictions.IdEq(id)); criteria.SetResultTransformer(Transformers.DistinctRootEntity); return criteria.UniqueResult<MyRootType>();

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• How can I generate the Rowland prime sequence idiomatically in J?

  - by Gregory Higley

  If you're not familiar with the Rowland prime sequence, you can find out about it here. I've created an ugly, procedural monadic verb in J to generate the first n terms in this sequence, as follows: rowland =: monad define result =. 0 $ 0 t =. 1 $ 7 while. (# result) < y do. a =. {: t n =. 1 + # t t =. t , a + n +. a d =. | -/ _2 {. t if. d > 1 do. result =. ~. result , d end. end. result ) This works perfectly, and it indeed generates the first n terms in the sequence. (By n terms, I mean the first n distinct primes.) Here is the output of rowland 20: 5 3 11 23 47 101 7 13 233 467 941 1889 3779 7559 15131 53 30323 60647 121403 242807 My question is, how can I write this in more idiomatic J? I don't have a clue, although I did write the following function to find the differences between each successive number in a list of numbers, which is one of the required steps. Here it is, although it too could probably be refactored by a more experienced J programmer than I: diffs =: monad : '}: (|@({.-2&{.) , $:@}.) ^: (1 < #) y'

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• Are monads Writer m and Either e categorically dual?

  - by sdcvvc

  I noticed there is a dual relation between Writer m and Either e monads. If m is a monoid, then unit :: () -> m join :: (m,m) -> m can be used to form a monad: return is composition: a -> ((),a) -> (m,a) join is composition: (m,(m,a)) -> ((m,m),a) -> (m,a) The dual of () is Void (empty type), the dual of product is coproduct. Every type e can be given "comonoid" structure: unit :: Void -> e join :: Either e e -> e in the obvious way. Now, return is composition: a -> Either Void a -> Either e a join is composition: Either e (Either e a) -> Either (Either e e) a -> Either e a and this is the Either e monad. The arrows follow exactly the same pattern. Question: Is it possible to write a single generic code that will be able to perform both as Either e and as Writer m depending on the monoid given?

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• How do I define a monadic function to work on a list in J?

  - by Gregory Higley

  Let's say I have the following J expression: # 3 ((|=0:)#]) 1+i.1000 This counts the number of numbers between 1 and 1000 that are evenly divisible by 3. (Now, before anyone points out that there's an easier way to do this, this question is about the syntax of J, and not mathematics.) Let's say I define a monadic function for this, as follows: f =: monad define # y ((|=0:)#]) 1+i.1000 ) This works great with a single argument, e.g., f 4 250 If I pass a list in, I get a length error: f 1 2 3 |length error: f Now, I completely understand why I get the length error. When you substitute the list 1 2 3 for the y argument of the monad, you get: # 1 2 3 ((|=0:)#]) 1+i.1000 If you know anything about J, it's pretty clear why the length error is occurring. So, I don't need an explanation of that. I want to define the function such that when I pass a list, it returns a list, e.g., f 1 2 3 1000 500 333 How can I either (a) redefine this function to take a list and return a list or (b) get the function to work on a list as-is without being redefined, perhaps using some adverb or other technique?

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• Developer’s Life – Summary of Superhero Articles

  - by Pinal Dave

  Earlier this year, I wrote an article series where I talked about developer’s life and compared it with Superhero. I have got amazing response to this series and I have been receiving quite a lots of email suggesting that I should write more blog post about them. Currently I am not planning to write more blog post but I will soon continue another series. In this blog post, I have summarized the entire series. Let me know if you want me to write about any superhero. I will see what I can do about that hero. Developer’s Life – Every Developer is a Captain America Captain America was first created as a comic book character in the 1940’s as a way to boost morale during World War II.  Aimed at a children’s audience, his legacy faded away when the war ended.  However, he has recently has a major reboot to become a popular movie character that deals with modern issues. Developer’s Life – Every Developer is the Incredible Hulk The Incredible Hulk is possibly one of the scariest superheroes out there.  All superheroes are meant to be “out of this world” and awe-inspiring, but I think most people will agree with I say The Hulk takes this to the next level.  He is the result of an industrial accident, which is scary enough in it’s own right.  Plus, when mild-mannered Bruce Banner is angered, he goes completely out-of-control and transforms into a destructive monster that he cannot control and has no memories of. Developer’s Life – Every Developer is a Wonder Woman We have focused a lot lately on this “superhero series.”  I love fantasy books and movies, and I feel like there is a lot to be learned from them.  As I am writing this series, though, I have noticed that every super hero I write about is a man.  So today, I would like to talk about the major female super hero – Wonder Woman. Developer’s Life – Every Developer is a Harry Potter Harry Potter might not be a superhero in the traditional sense, but I believe he still has a lot to teach us and show us about life as a developer.  If you have been living under a rock for the last 17 years, you might not know that Harry Potter is the main character in an extremely popular series of books and movies documenting the education and tribulation of a young wizard (and his friends). Developer’s Life – Every Developer is Like Transformers Transformers may not be superheroes – they don’t wear capes, they don’t have amazing powers outside of their size and folding ability, they’re not even human (technically).  Part of their enduring popularity is that while we are enjoying over-the-top movies, we are learning about good leadership and strong personal skills. Developer’s Life – Every Developer is a Iron Man Iron Man is another superhero who is not naturally “super,” but relies on his brain (and money) to turn him into a fighting machine.  While traditional superheroes are still popular, a three-movie franchise and incorporation into the new Avengers series shows that Iron Man is popular enough on his own. Developer’s Life – Every Developer is a Sherlock Holmes I have been thinking a lot about how developers are like super heroes, and I have written two blog posts now comparing them to Spiderman and Superman.  I have a lot of love and respect for developers, and I hope that they are enjoying these articles, and others are learning a little bit about the profession.  There is another fictional character who, while not technically asuper hero, is very powerful, and I also think stands as a good example of a developer. That character is Sherlock Holmes.  Sherlock Holmes is a British detective, first made popular at the turn of the 19thcentury by author Sir Arthur Conan Doyle.  The original Sherlock Holmes was a brilliant detective who could solve the most mind-boggling crime through simple observations and deduction. Developer’s Life – Every Developer is a Chhota Bheem Chhota Bheem is a cartoon character that is extremely popular where I live.  He is my daughter’s favorite characters.  I like to say that children love Chhota Bheem more than their parents – it is lucky for us he is not real!  Children love Chhota Bheem because he is the absolute “good guy.”  He is smart, loyal, and strong.  He and his friends live in Dholakpur and fight off their many enemies – and always win – in every episode.  In each episode, they learn something about friendship, bravery, and being kind to others.  Chhota Bheem is a good role model for children, and I think that he is a good role model for developers are well. Developer’s Life – Every Developer is a Batman Batman is one of the darkest superheroes in the fantasy canon.  He does not come to his powers through any sort of magical coincidence or radioactive insect, but through a lot of psychological scarring caused by witnessing the death of his parents.  Despite his dark back story, he possesses a lot of admirable abilities that I feel bear comparison to developers. Developer’s Life – Every Developer is a Superman I enjoyed comparing developers to Spiderman so much, that I have decided to continue the trend and encourage some of my favorite people (developers) with another favorite superhero – Superman.  Superman is probably the most famous superhero – and one of the most inspiring. Developer’s Life – Every Developer is a Spiderman I have to admit, Spiderman is my favorite superhero.  The most recent movie recently was released in theaters, so it has been at the front of my mind for some time. Spiderman was my favorite superhero even before the latest movie came out, but of course I took my whole family to see the movie as soon as I could!  Every one of us loved it, including my daughter.  We all left the movie thinking how great it would be to be Spiderman.  So, with that in mind, I started thinking about how we are like Spiderman in our everyday lives, especially developers. I would like to know which Superhero is your favorite hero! Reference: Pinal Dave (http://blog.SQLAuthority.com)Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL Tagged: Developer, Superhero

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• How I understood monads, part 1/2: sleepless and self-loathing in Seattle

  For some time now, I had been noticing some interest for monads, mostly in the form of unintelligible (to me) blog posts and comments saying oh, yeah, thats a monad about random stuff as if it were absolutely obvious and if I didnt know what they were talking about, I was probably an uneducated idiot, ignorant about the simplest and most fundamental concepts of functional programming. Fair enough, I am pretty much exactly that. Being the kind of guy who can spend eight years in college just to...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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• How to Save Tweet Links for Later Reading from Your Desktop and Phone

  - by Zainul Franciscus

  Have you come across a lot of interesting links from Twitter, but you don’t have the time to read all of them? Today we’ll show you how to read these links later from your desktop and phone. Organizing links from Twitter can be a troublesome, but these tools will reduce the effort greatly Latest Features How-To Geek ETC The How-To Geek Valentine’s Day Gift Guide Inspire Geek Love with These Hilarious Geek Valentines RGB? CMYK? Alpha? What Are Image Channels and What Do They Mean? How to Recover that Photo, Picture or File You Deleted Accidentally How To Colorize Black and White Vintage Photographs in Photoshop How To Get SSH Command-Line Access to Windows 7 Using Cygwin View the Cars of Tomorrow Through the Eyes of the Past [Historical Video] Add Romance to Your Desktop with These Two Valentine’s Day Themes for Windows 7 Gmail’s Priority Inbox Now Available for Mobile Web Browsers Touchpad Blocker Locks Down Your Touchpad While Typing Arrival of the Viking Fleet Wallpaper A History of Vintage Transformers [Infographic]

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• Desktop Fun: TRON and TRON Legacy Customization Set

  - by Asian Angel

  Are you a Program or a User? Choose your destiny while bringing the battle for the Grid and freedom to your desktop with our TRON and TRON Legacy Customization Set Latest Features How-To Geek ETC Inspire Geek Love with These Hilarious Geek Valentines RGB? CMYK? Alpha? What Are Image Channels and What Do They Mean? How to Recover that Photo, Picture or File You Deleted Accidentally How To Colorize Black and White Vintage Photographs in Photoshop How To Get SSH Command-Line Access to Windows 7 Using Cygwin The How-To Geek Video Guide to Using Windows 7 Speech Recognition A History of Vintage Transformers [Infographic] Amazon Finally Adds Real Page Numbers to the Kindle Now You Can Print Google Docs and Gmail through Google Cloud Print AppBrain Enables Direct-to-Phone Installation Again Build a DIY Clapper to Hone Your Electronics Chops How to Kid Proof Your Computer’s Power and Reset Buttons

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• Ask the Readers: Browser Wars – Which One will be Victorious in 2011?

  - by Asian Angel

  With each passing week it seems like the browser wars are becoming more fierce as all of the participants add new features and release versions more often. This week we would like to know which browser or browsers you think will be victorious in 2011 Latest Features How-To Geek ETC How to Change the Default Application for Android Tasks Stop Believing TV’s Lies: The Real Truth About "Enhancing" Images The How-To Geek Valentine’s Day Gift Guide Inspire Geek Love with These Hilarious Geek Valentines RGB? CMYK? Alpha? What Are Image Channels and What Do They Mean? How to Recover that Photo, Picture or File You Deleted Accidentally Now Together and Complete – McBain: The Movie [Simpsons Video] Be Creative by Using Hex and RGB Codes for Crayola Crayon Colors on Your Next Web or Art Project [Geek Fun] Flash Updates; Finally Supports Full Screen Video on Multiple Monitors 22 Ways to Recycle an Altoids Mint Tin Make Your Desktop Go Native with the Tribal Arts Theme for Windows 7 A History of Vintage Transformers: Decepticons Edition [Infographic]

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• Paramount Pictures Movies Dynamic Theme for Windows 7

  - by Asian Angel

  Would you like to have all of the latest movie wallpapers from Paramount Pictures delivered straight to your desktop? Then this is the theme you are looking for. This dynamic RSS-fed theme brings you the latest wallpapers from movies such as Thor, True Grit, Kung Fu Panda 2, Super 8, and Transformers 3. Download the Paramount Pictures Movies Dynamic Theme [Windows 7 Personalization Gallery] You can learn more about Microsoft’s dynamic themes here: RSS-fed dynamic themes FAQ How To Easily Access Your Home Network From Anywhere With DDNSHow To Recover After Your Email Password Is CompromisedHow to Clean Your Filthy Keyboard in the Dishwasher (Without Ruining it)

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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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• Hibernate aliastobean

  - by cometta

  Query query = getHibernateTemplate().getSessionFactory().getCurrentSession().createSQLQuery( "select proj_employee.employee_no as employeeNo, ... .setResultTransformer(Transformers.aliasToBean(User.class)); Inside User.class does the property employeNo need to be in capital letter? private String EMPLOYEENO //get/set for EMPLOYEENO if i changed to small letter, it doesnt work. can anyone explain why must be in capital letter?

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• Haskell: Left-biased/short-circuiting function

  - by user2967411

  Two classes ago, our professor presented to us a Parser module. Here is the code: module Parser (Parser,parser,runParser,satisfy,char,string,many,many1,(+++)) where import Data.Char import Control.Monad import Control.Monad.State type Parser = StateT String [] runParser :: Parser a -> String -> [(a,String)] runParser = runStateT parser :: (String -> [(a,String)]) -> Parser a parser = StateT satisfy :: (Char -> Bool) -> Parser Char satisfy f = parser $ \s -> case s of [] -> [] a:as -> [(a,as) | f a] char :: Char -> Parser Char char = satisfy . (==) alpha,digit :: Parser Char alpha = satisfy isAlpha digit = satisfy isDigit string :: String -> Parser String string = mapM char infixr 5 +++ (+++) :: Parser a -> Parser a -> Parser a (+++) = mplus many, many1 :: Parser a -> Parser [a] many p = return [] +++ many1 p many1 p = liftM2 (:) p (many p) Today he gave us an assignment to introduce "a left-biased, or short-circuiting version of (+++)", called (<++). His hint was for us to consider the original implementation of (+++). When he first introduced +++ to us, this was the code he wrote, which I am going to call the original implementation: infixr 5 +++ (+++) :: Parser a -> Parser a -> Parser a p +++ q = Parser $ \s -> runParser p s ++ runParser q s I have been having tons of trouble since we were introduced to parsing and so it continues. I have tried/am considering two approaches. 1) Use the "original" implementation, as in p +++ q = Parser $ \s - runParser p s ++ runParser q s 2) Use the final implementation, as in (+++) = mplus Here are my questions: 1) The module will not compile if I use the original implementation. The error: Not in scope: data constructor 'Parser'. It compiles fine using (+++) = mplus. What is wrong with using the original implementation that is avoided by using the final implementation? 2) How do I check if the first Parser returns anything? Is something like (not (isNothing (Parser $ \s - runParser p s) on the right track? It seems like it should be easy but I have no idea. 3) Once I figure out how to check if the first Parser returns anything, if I am to base my code on the final implementation, would it be as easy as this?: -- if p returns something then p <++ q = mplus (Parser $ \s -> runParser p s) mzero -- else (<++) = mplus Best, Jeff

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• What do you call this functional language feature?

  - by Jimmy

  ok, embarrassing enough, I posted code that I need explained. Specifically, it first chains absolute value and subtraction together, then tacks on a sort, all the while not having to mention parameters and arguments at all, because of the presense of "adverbs" that can join these functions "verbs" What (non-APL-type) languages support this kind of no-arguments function composition (I have the vague idea it ties in strongly to the concepts of monad/dyad and rank, but its hard to get a particularly easy-to-understand picture just from reading Wikipedia) and what do I call this concept?

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• functional dependencies vs type families

  - by mhwombat

  I'm developing a framework for running experiments with artificial life, and I'm trying to use type families instead of functional dependencies. Type families seems to be the preferred approach among Haskellers, but I've run into a situation where functional dependencies seem like a better fit. Am I missing a trick? Here's the design using type families. (This code compiles OK.) {-# LANGUAGE TypeFamilies, FlexibleContexts #-} import Control.Monad.State (StateT) class Agent a where agentId :: a -> String liveALittle :: Universe u => a -> StateT u IO a -- plus other functions class Universe u where type MyAgent u :: * withAgent :: (MyAgent u -> StateT u IO (MyAgent u)) -> String -> StateT u IO () -- plus other functions data SimpleUniverse = SimpleUniverse { mainDir :: FilePath -- plus other fields } defaultWithAgent :: (MyAgent u -> StateT u IO (MyAgent u)) -> String -> StateT u IO () defaultWithAgent = undefined -- stub -- plus default implementations for other functions -- -- In order to use my framework, the user will need to create a typeclass -- that implements the Agent class... -- data Bug = Bug String deriving (Show, Eq) instance Agent Bug where agentId (Bug s) = s liveALittle bug = return bug -- stub -- -- .. and they'll also need to make SimpleUniverse an instance of Universe -- for their agent type. -- instance Universe SimpleUniverse where type MyAgent SimpleUniverse = Bug withAgent = defaultWithAgent -- boilerplate -- plus similar boilerplate for other functions Is there a way to avoid forcing my users to write those last two lines of boilerplate? Compare with the version using fundeps, below, which seems to make things simpler for my users. (The use of UndecideableInstances may be a red flag.) (This code also compiles OK.) {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances, UndecidableInstances #-} import Control.Monad.State (StateT) class Agent a where agentId :: a -> String liveALittle :: Universe u a => a -> StateT u IO a -- plus other functions class Universe u a | u -> a where withAgent :: Agent a => (a -> StateT u IO a) -> String -> StateT u IO () -- plus other functions data SimpleUniverse = SimpleUniverse { mainDir :: FilePath -- plus other fields } instance Universe SimpleUniverse a where withAgent = undefined -- stub -- plus implementations for other functions -- -- In order to use my framework, the user will need to create a typeclass -- that implements the Agent class... -- data Bug = Bug String deriving (Show, Eq) instance Agent Bug where agentId (Bug s) = s liveALittle bug = return bug -- stub -- -- And now my users only have to write stuff like... -- u :: SimpleUniverse u = SimpleUniverse "mydir"

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• Avoiding explicit recursion in Haskell

  - by Travis Brown

  The following simple function applies a given monadic function iteratively until it hits a Nothing, at which point it returns the last non-Nothing value. It does what I need, and I understand how it works. lastJustM :: (Monad m) => (a -> m (Maybe a)) -> a -> m a lastJustM g x = g x >>= maybe (return x) (lastJustM g) As part of my self-education in Haskell I'm trying to avoid explicit recursion (or at least understand how to) whenever I can. It seems like there should be a simple non-explicitly recursive solution in this case, but I'm having trouble figuring it out. I don't want something like a monadic version of takeWhile, since it could be expensive to collect all the pre-Nothing values, and I don't care about them anyway. I checked Hoogle for the signature and nothing shows up. The m (Maybe a) bit makes me think a monad transformer might be useful here, but I don't really have the intuitions I'd need to come up with the details (yet). It's probably either embarrassingly easy to do this or embarrassingly easy to see why it can't or shouldn't be done, but this wouldn't be the first time I've used self-embarrassment as a pedagogical strategy. Background: Here's a simplified working example for context: suppose we're interested in random walks in the unit square, but we only care about points of exit. We have the following step function: randomStep :: (Floating a, Ord a, Random a) => a -> (a, a) -> State StdGen (Maybe (a, a)) randomStep s (x, y) = do (a, gen') <- randomR (0, 2 * pi) <$> get put gen' let (x', y') = (x + s * cos a, y + s * sin a) if x' < 0 || x' > 1 || y' < 0 || y' > 1 then return Nothing else return $ Just (x', y') Something like evalState (lastJustM (randomStep 0.01) (0.5, 0.5)) <$> newStdGen will give us a new data point.

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• Use Shakespeare-text and external file

  - by Adam

  How can I convert the below example to use an external file instead of the embedded lazy text quasi quotes? {-# LANGUAGE QuasiQuotes, OverloadedStrings #-} import Text.Shakespeare.Text import qualified Data.Text.Lazy.IO as TLIO import Data.Text (Text) import Control.Monad (forM_) data Item = Item { itemName :: Text , itemQty :: Int } items :: [Item] items = [ Item "apples" 5 , Item "bananas" 10 ] main :: IO () main = forM_ items $ \item -> TLIO.putStrLn [lt|You have #{show $ itemQty item} #{itemName item}.|] This is from the yesod online book.

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