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Haskell Weekly News: Issue 233
by Daniel Santa Cruz 27 Jun '12

27 Jun '12

Welcome to issue 233 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of June 17 to 23, 2012. Quotes of the Week * ciaranm: if it's ugly, it's imperative. if it's incomprehensible, it's functional. * xplat: when life gives you lemons, make lemonads Top Reddit Stories * What are everyone's favorite "aha" moments in learning Haskell? Domain: self.haskell, Score: 93, Comments: 156 On Reddit: [1] http://goo.gl/8YSyY Original: [2] http://goo.gl/8YSyY * Unit testing isn't enough. You need static typing too. Domain: evanfarrer.blogspot.ca, Score: 82, Comments: 22 On Reddit: [3] http://goo.gl/b73gf Original: [4] http://goo.gl/JF4ND * Snap Framework 0.9 Released Domain: snapframework.com, Score: 60, Comments: 7 On Reddit: [5] http://goo.gl/7fP9D Original: [6] http://goo.gl/7SoU6 * Haskell’s record system is a cruel joke Domain: labs.scrive.com, Score: 52, Comments: 100 On Reddit: [7] http://goo.gl/XKIch Original: [8] http://goo.gl/wOvyc * Polymorphic Update with van Laarhoven Lenses Domain: r6.ca, Score: 51, Comments: 9 On Reddit: [9] http://goo.gl/qvpPg Original: [10] http://goo.gl/34TbY * Building a blog with Snap and Heist Domain: bonus500.github.com, Score: 45, Comments: 30 On Reddit: [11] http://goo.gl/UDHCs Original: [12] http://goo.gl/eo9a3 * Totality vs. Turing completeness (McBride)[PDF, 35 slides, ±] Domain: personal.cis.strath.ac.uk, Score: 42, Comments: 22 On Reddit: [13] http://goo.gl/qua0n Original: [14] http://goo.gl/8auaP * GHC's imprecise (non-deterministic) "error" Domain: stackoverflow.com, Score: 41, Comments: On Reddit: [15] http://goo.gl/6WQWx Original: [16] http://goo.gl/MOCeT * Hackage comments — a simple user script Domain: chrisdone.com, Score: 39, Comments: 17 On Reddit: [17] http://goo.gl/t9oBH Original: [18] http://goo.gl/CmXuB * A Notation for Comonads (Orchard, Mycroft) [pdf] Domain: cl.cam.ac.uk, Score: 38, Comments: 48 On Reddit: [19] http://goo.gl/S4lz9 Original: [20] http://goo.gl/uw8ZY * Taming list syntax in Haskell Domain: labs.scrive.com, Score: 36, Comments: 29 On Reddit: [21] http://goo.gl/YLTZL Original: [22] http://goo.gl/UJpcB * Shell scripting with type-safety using Haskell « Linux-Magazin Online Domain: linux-magazin.de, Score: 36, Comments: 32 On Reddit: [23] http://goo.gl/yEMQ0 Original: [24] http://goo.gl/U3zO5 * Roy, A Statically Typed, Functional Language for Javascript. The Functional Web [PDF] Domain: brianmckenna.org, Score: 32, Comments: 9 On Reddit: [25] http://goo.gl/SBhec Original: [26] http://goo.gl/v0iH2 * [ANN] GenCheck - a generalized property-based testing framework Domain: permalink.gmane.org, Score: 30, Comments: On Reddit: [27] http://goo.gl/GCUGJ Original: [28] http://goo.gl/c2Y0s Top StackOverflow Questions * How do exceptions in Haskell work? votes: 53, answers: 1 Read on SO: [29] http://goo.gl/MOCeT * Has anyone ever compiled a list of the imports needed to avoid the “not polymorphic enough” definitions in Haskell's standard libraries? votes: 19, answers: 1 Read on SO: [30] http://goo.gl/kyXde * Why does adding a polymorphic type signature degrade performance? votes: 13, answers: 2 Read on SO: [31] http://goo.gl/Lf18k * To write or not to write `module Main where` in Haskell votes: 10, answers: 1 Read on SO: [32] http://goo.gl/IDSOx * Why aren't FingerTrees used enough to have a stable implementation? votes: 10, answers: 3 Read on SO: [33] http://goo.gl/wzw4e * What is the difference between PArray and [::] in Data Parallel Haskell? votes: 9, answers: 0 Read on SO: [34] http://goo.gl/FpUJX * Why discarded values are () instead of ⊥ in Haskell? votes: 8, answers: 5 Read on SO: [35] http://goo.gl/jIK26 * Is it possible to embed Haskell in a C library opaquely? votes: 8, answers: 1 Read on SO: [36] http://goo.gl/3H5tu * NoMonomorphismRestriction helps preserve sharing? votes: 8, answers: 2 Read on SO: [37] http://goo.gl/FDdeA * Why doesn't Haskell/GHC support record name overloading votes: 7, answers: 2 Read on SO: [38] http://goo.gl/NsHKM Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/vi425/what_are_everyones_favorite_… 2. http://www.reddit.com/r/haskell/comments/vi425/what_are_everyones_favorite_… 3. http://evanfarrer.blogspot.ca/2012/06/unit-testing-isnt-enough-you-need.html 4. http://www.reddit.com/r/haskell/comments/vbwrv/unit_testing_isnt_enough_you… 5. http://snapframework.com/blog/2012/06/17/snap-0.9-released 6. http://www.reddit.com/r/haskell/comments/v779m/snap_framework_09_released/ 7. http://labs.scrive.com/2012/06/haskell-lacks-a-serious-record-system/ 8. http://www.reddit.com/r/haskell/comments/vdg55/haskells_record_system_is_a_… 9. http://r6.ca/blog/20120623T104901Z.html 10. http://www.reddit.com/r/haskell/comments/vhbf4/polymorphic_update_with_van_… 11. http://bonus500.github.com/sc2blog/ 12. http://www.reddit.com/r/haskell/comments/vg3q2/building_a_blog_with_snap_an… 13. https://personal.cis.strath.ac.uk/conor.mcbride/pub/Totality.pdf 14. http://www.reddit.com/r/haskell/comments/vazfp/totality_vs_turing_completen… 15. http://stackoverflow.com/questions/11070690/how-do-exceptions-in-haskell-wo… 16. http://www.reddit.com/r/haskell/comments/v6sec/ghcs_imprecise_nondeterminis… 17. http://chrisdone.com/hackage-comments/ 18. http://www.reddit.com/r/haskell/comments/v8gvc/hackage_comments_a_simple_us… 19. http://www.cl.cam.ac.uk/~dao29/drafts/codo-notation-orchard12.pdf 20. http://www.reddit.com/r/haskell/comments/v6ik6/a_notation_for_comonads_orch… 21. http://labs.scrive.com/2012/06/taming-list-syntax-in-haskell/ 22. http://www.reddit.com/r/haskell/comments/v9ilw/taming_list_syntax_in_haskel… 23. http://www.linux-magazin.de/Online-Artikel/Shell-scripting-with-type-safety… 24. http://www.reddit.com/r/haskell/comments/vdrt0/shell_scripting_with_typesaf… 25. http://brianmckenna.org/files/IC-Roy.pdf 26. http://www.reddit.com/r/haskell/comments/vhkgf/roy_a_statically_typed_funct… 27. http://permalink.gmane.org/gmane.comp.lang.haskell.general/19371 28. http://www.reddit.com/r/haskell/comments/va5ao/ann_gencheck_a_generalized_p… 29. http://stackoverflow.com/questions/11070690/how-do-exceptions-in-haskell-wo… 30. http://stackoverflow.com/questions/11076062/has-anyone-ever-compiled-a-list… 31. http://stackoverflow.com/questions/11145503/why-does-adding-a-polymorphic-t… 32. http://stackoverflow.com/questions/11112371/to-write-or-not-to-write-module… 33. http://stackoverflow.com/questions/11151732/why-arent-fingertrees-used-enou… 34. http://stackoverflow.com/questions/11103707/what-is-the-difference-between-… 35. http://stackoverflow.com/questions/11076938/why-discarded-values-are-instea… 36. http://stackoverflow.com/questions/11137443/is-it-possible-to-embed-haskell… 37. http://stackoverflow.com/questions/11145904/nomonomorphismrestriction-helps… 38. http://stackoverflow.com/questions/11100322/why-doesnt-haskell-ghc-support-…

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ANNOUNCE : Leksah 0.12.1.2 (fixes some metadata issues)
by Hamish Mackenzie 27 Jun '12

27 Jun '12

This release has an important bug fix for the metadata download. When metadata was downloaded using libcurl it was not treated as binary data. If you used one of our binary installers or if you built Leksah with the -flibcurl flag then it is likely you have bad metadata files. To fix this please install Leksah 0.12.1.2, then delete your ~/.leksah-0.12/metadata folder to force Leksah to download the metadata files again. The first time you run leksah after doing this it will take a while for leksah to update the metadata (you can track its progress by looking in the metadata directory). If you choose to generate your metadata locally or you used wget to download (the default if you cabal installed leksah) then your metadata should be ok. There are some other bug fixes in this release including one for metadata generation for users with lots of .cabal files (some lazy IO was leaving file handles open). Source is in Hackage and https://github.com/leksah Binary Installers Use "ghc --version" to work out which one you need. OS X http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.3.dmg http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.4.dmg http://leksah.org/packages/leksah-0.12.1.2-ghc-7.4.1.dmg Windows http://leksah.org/packages/leksah-0.12.1.2-ghc-6.12.3.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.3.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.0.4.exe http://leksah.org/packages/leksah-0.12.1.2-ghc-7.4.1.exe

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cabal doens't forget old dependencies
by Sjoerd Visscher 27 Jun '12

27 Jun '12

Hi all, I tried to install reactive-banana. This failed due to a dependency conflict, and then I noticed there was a newer version of reactive-banana. So I did cabal update, and tried to install again. But whatever I do, cabal keeps trying to install fclabels, but fclabels is no longer a dependency of reactive-banana-0.6.0.0! How can I let cabal forget this dependency? Thanks, -- Sjoerd Visscher https://github.com/sjoerdvisscher/blog

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Unambiguous choice implementation
by Bartosz Milewski 27 Jun '12

27 Jun '12

I'm reading Conal Elliot's paper, Push-Pull FRP. At some point he needs an unambiguous choice operator, essentially to implement select: a future that waits for one of its future arguments to fire. His implementation of unamb creates two threads racing on a shared MVar. By his own admission, this is very inefficient. My question is, is there a better implementation? -- [:Bartosz:]

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 106, Issue 38
by John Lato 26 Jun '12

26 Jun '12

> Message: 12 > Date: Wed, 27 Jun 2012 00:19:30 +0200 > From: Tillmann Rendel <rendel(a)informatik.uni-marburg.de> > Subject: Re: [Haskell-cafe] Martin Odersky on "What's wrong with > Monads" > Cc: haskell-cafe(a)haskell.org > Message-ID: <4FEA3572.5060200(a)informatik.uni-marburg.de> > Content-Type: text/plain; charset=ISO-8859-1; format=flowed > > Hi, > > MightyByte wrote: >> Of course every line of your program that uses a Foo will change if you switch >> to IO Foo instead. > > But we often have to also change lines that don't use Foo at all. For > example, here is the type of binary trees of integers: > > data Tree = Leaf Integer | Branch (Tree Integer) (Tree Integer) > > A function to add up all integers in a tree: > > amount:: Tree -> Integer > amount (Leaf x) = x > amount (Branch t1 t2) = amountt1 + amountt2 > > All fine so far. Now, consider the following additional requirement: "If > the command-line flag --multiply is set, the function amount computes > the product instead of the sum." > > In a language with implicit side effects, it is easy to implement this. > We just change the third line of the amount function to check whether to > call (+) or (*). In particular, we would not touch the other two lines. > > How would you implement this requirement in Haskell without changing the > line "amount (Leaf x) = x"? Why would you do that even in an imperative language? The program logic turns into a spaghetti mess, and it's much harder to test. I would write Tree like this: data Tree a = Leaf a | Branch (Tree a) ( Tree a) deriving (Foldable, Show) and instead of an "amount" function I would use a fold. If you like, you can use "ala" from Control.Newtype *Main Data.Foldable Data.Monoid> let t1 = Branch (Leaf 1) (Branch (Leaf 4) (Leaf 5)) :: Tree Int *Main Data.Foldable Data.Monoid> ala Sum foldMap t1 10 *Main Data.Foldable Data.Monoid> ala Product foldMap t1 20 Now the amount calculation can be something like amount :: Num a => Tree a -> IO a amount tree = multFlag >>= \b -> if b then ala Product foldMap tree else ala Sum foldMap tree although I probably wouldn't actually write it unless it was called in more than one place. There are other ways to write it too; the important part is that checking the configuration is completely separate from the tree traversal. Plus, if it changes again (now there's another flag that says to ignore values == n or something, you can use the same fold, just change the function that's passed to it (or the monoid instance if you're using that). Plus, this type of code is much simpler to debug, test, and maintain than imperative-style magic functions. Cheers, John

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Re: [Haskell-cafe] StableNames and monadic functions
by Ismael Figueroa Palet 26 Jun '12

26 Jun '12

Thanks Lorenzo, I'm cc'ing the list with your response also: As you point out, when you do some kind of "let-binding", using the where clause, or explicit let as in: main :: IO () main = do let f1 = (successor :: Int -> State Int Int) let f2 = (successor :: Int -> Maybe Int) b2 <- eq f2 f2 b1 <- eq f1 f1 print (show b1 ++ " " ++ show b2) The behavior is as expected. I guess the binding triggers some internal optimization or gives more information to the type checker; but I'm still not clear why it is required to be done this way -- having to let-bind every function is kind of awkward. I know the details of StableNames are probably implementation-dependent, but I'm still wondering about how to detect / restrict this situation. Thanks 2012/6/26 Lorenzo Bolla <lbolla(a)gmail.com> > From StableName docs: > >> The reverse is not necessarily true: if two stable names are not equal, >> then the objects they name may still be equal. > > > This version works as expected: > > import System.Mem.StableName > import Control.Monad.State > > eq :: a -> b -> IO Bool > eq a b = do > pa <- makeStableName a > pb <- makeStableName b > return (hashStableName pa == hashStableName pb) > > successor :: (Num a, Monad m) => a -> m a > successor n = return (n+1) > > -- main :: IO () > -- main = do > -- b2 <- eq (successor :: Int -> State Int Int) (successor :: Int > -> State Int Int) > -- b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> > Maybe Int) > -- print (show b1 ++ " " ++ show b2) > > main :: IO () > main = do > b2 <- eq f2 f2 > b1 <- eq f1 f1 > print (show b1 ++ " " ++ show b2) > where f1 = (successor :: Int -> Maybe Int) > f2 = (successor :: Int -> State Int Int) > > > > hth, > L. > > > > > On Tue, Jun 26, 2012 at 1:15 PM, Ismael Figueroa Palet < > ifigueroap(a)gmail.com> wrote: > >> I'm using StableNames to have a notion of function equality, and I'm >> running into problems when using monadic functions. >> >> Consider the code below, file Test.hs >> >> import System.Mem.StableName >> import Control.Monad.State >> >> eq :: a -> b -> IO Bool >> eq a b = do >> pa <- makeStableName a >> pb <- makeStableName b >> return (hashStableName pa == hashStableName pb) >> >> successor :: (Num a, Monad m) => a -> m a >> successor n = return (n+1) >> >> main :: IO () >> main = do >> b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> >> Maybe Int) >> b2 <- eq (successor :: Int -> State Int Int) (successor :: Int -> >> State Int Int) >> print (show b1 ++ " " ++ show b2) >> >> Running the code into ghci the result is "False False". There is some old >> post saying that this is due to the dictionary-passing style for >> typeclasses, and compiling with optimizations improves the situation. >> >> Compiling with ghc --make -O Tests.hs and running the program, the result >> is "True True", which is what I expect. >> However, if I change main to be like the following: >> >> main :: IO () >> main = do >> b2 <- eq (successor :: Int -> State Int Int) (successor :: Int -> >> State Int Int) >> b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> >> Maybe Int) >> print (show b1 ++ " " ++ show b2) >> >> i.e. just changing the sequential order, and then compiling again with >> the same command, I get "True False", which is very confusing for me. >> Similar situations happens when using the state monad transformer, and >> manually built variations of it. >> >> It sounds the problem is with hidden closures created somewhere that do >> not point to the same memory locations, so StableNames yields false for >> that cases, but it is not clear to me under what circumstances this >> situation happens. Is there other way to get some approximation of function >> equality? or a way to "configure" the behavior of StableNames in presence >> of class constraints? >> >> I'm using the latests Haskell Platform on OS X Lion, btw. >> >> Thanks >> >> -- >> Ismael >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > -- Ismael

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HXT: Replace an element with its text
by Michael Orlitzky 26 Jun '12

26 Jun '12

I would like to replace, <body><a href="#">foo</a></body> with, <body>foo</body> using HXT. So far, the closest I've come is to parse the HTML and apply the following stuff: is_link :: (ArrowXml a) => a XmlTree XmlTree is_link = hasName "a" replace_links_with_their_text :: (ArrowXml a) => a XmlTree XmlTree replace_links_with_their_text = processTopDown $ (getText >>> mkText) `when` is_link Unfortunately, this just removes the "a" element and its text entirely. The other-closest solution is, replace_links_with_their_text :: (ArrowXml a) => a XmlTree XmlTree replace_links_with_their_text = processTopDown $ (txt "foo") `when` is_link Of course, I don't want to hard-code the value "foo", and I can't figure out a way to feed the element's text back into 'txt'. Anyone tried this before?

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StableNames and monadic functions
by Ismael Figueroa Palet 26 Jun '12

26 Jun '12

I'm using StableNames to have a notion of function equality, and I'm running into problems when using monadic functions. Consider the code below, file Test.hs import System.Mem.StableName import Control.Monad.State eq :: a -> b -> IO Bool eq a b = do pa <- makeStableName a pb <- makeStableName b return (hashStableName pa == hashStableName pb) successor :: (Num a, Monad m) => a -> m a successor n = return (n+1) main :: IO () main = do b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> Maybe Int) b2 <- eq (successor :: Int -> State Int Int) (successor :: Int -> State Int Int) print (show b1 ++ " " ++ show b2) Running the code into ghci the result is "False False". There is some old post saying that this is due to the dictionary-passing style for typeclasses, and compiling with optimizations improves the situation. Compiling with ghc --make -O Tests.hs and running the program, the result is "True True", which is what I expect. However, if I change main to be like the following: main :: IO () main = do b2 <- eq (successor :: Int -> State Int Int) (successor :: Int -> State Int Int) b1 <- eq (successor :: Int -> Maybe Int) (successor :: Int -> Maybe Int) print (show b1 ++ " " ++ show b2) i.e. just changing the sequential order, and then compiling again with the same command, I get "True False", which is very confusing for me. Similar situations happens when using the state monad transformer, and manually built variations of it. It sounds the problem is with hidden closures created somewhere that do not point to the same memory locations, so StableNames yields false for that cases, but it is not clear to me under what circumstances this situation happens. Is there other way to get some approximation of function equality? or a way to "configure" the behavior of StableNames in presence of class constraints? I'm using the latests Haskell Platform on OS X Lion, btw. Thanks -- Ismael

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Hackage 2 maintainership
by Ben Gamari 25 Jun '12

25 Jun '12

Hey all, Those of you who follow the Haskell subreddit no doubt saw today's post regarding the status of Hackage 2. As has been said many times in the past, the primary blocker at this point to the adoption of Hackage 2 appears to be the lack of an administrator. It seems to me this is a poor reason for this effort to be held up. Having taken a bit of time to consider, I would be willing to put in some effort to get things moving and would be willing to maintain the haskell.org Hackage 2.0 instance going forward if necessary. I currently have a running installation on my personal machine and things seem to be working as they should. On the whole, installation was quite trivial, so it seems likely that the project is indeed at a point where it can take real use (although a "logout" option in the web interface would make testing a bit easier). That being said, it would in my opinion be silly to proceed without fixing the Hackage trac. It was taken down earlier this year due to spamming[1] and it seems the recovery project has been orphaned. I would be willing to help with this effort, but it seems like the someone more familiar with the haskel.org infrastructure might be better equipped to handle the situation. It seems that this process will go something like this, 1) Bring Hackage trac back from the dead 2) Bring up a Hackage 2 instance along-side the existing hackage.haskell.org 3) Enlist testers 4) Let things simmer for a few weeks/months ensuring nothing explodes 5) After it's agreed that things are stable, eventually swap the Hackage 1 and 2 instances This will surely be a non-trivial process but I would be willing to move things forward. Cheers, - Ben [1] http://www.haskell.org/pipermail/cabal-devel/2012-January/008427.html

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Munich Haskell Meeting
by Heinrich Hördegen 25 Jun '12

25 Jun '12

Dear all, once again, we want to invite you to our monthly Munich Haskell Meeting at Thu, 28th of June, at 19h30. This time, the venue will be Max Emanuell Brauerei in Munich (http://www.max-emanuel-brauerei.de/) Be aware of that change! Thursday will also be the day of a local football play, at least for those living in Europe. Therefore, I want to reserve a table on Wednesday in order to get a table at all. If you plan to join, please go to: http://www.haskell-munich.de/dates and click the button. Until then, have fun. Heinrich

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