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Haskell Weekly News: Issue 256
by Daniel Santa Cruz 31 Jan '13

31 Jan '13

Welcome to issue 256 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the weeks of January 20 to 26, 2013. Quotes of the Week * elliott: cmccann: unfortunately it is too perfect an abstraction to be useful. * SamanthaD: shachaf: you're one of those dirty imperative communists who want the state to dictate everything! * monochrom: I refuse camel case and mark zuckerberg. same level. not negotiable. * mauke: a newtype is like an existing type but wearing glasses and a fake mustache and a sign saying "you've never seen me before" Top Reddit Stories * Taking magic out of GHC or: Tracing compilation by transformation (intro to Core transformations, inlining,.. Domain: ics.p.lodz.pl, Score: 59, Comments: 2 On Reddit: [1] http://goo.gl/lJmsb Original: [2] http://goo.gl/IbJ5O * Introduction to Haskell IO Domain: haskellforall.com, Score: 57, Comments: 26 On Reddit: [3] http://goo.gl/fPky4 Original: [4] http://goo.gl/Xymbr * The longest type signature I have ever seen (package Nussinov78) Domain: i.imgur.com, Score: 48, Comments: 26 On Reddit: [5] http://goo.gl/ePwNs Original: [6] http://goo.gl/B74yW * There is now a patch for the GHC runtime system which implements a scheduler which can handle threads with priorities. Domain: hackage.haskell.org, Score: 47, Comments: 12 On Reddit: [7] http://goo.gl/22XgH Original: [8] http://goo.gl/8iC8w * Success story: novice haskeller moves console cursor with “netwire” FRP library Domain: danbst.wordpress.com, Score: 43, Comments: 8 On Reddit: [9] http://goo.gl/dm8ga Original: [10] http://goo.gl/Ue7pK * Haskell as pseudo code Domain: arxiv.org, Score: 33, Comments: 2 On Reddit: [11] http://goo.gl/JL8Mf Original: [12] http://goo.gl/5D4zV * Generalizing do notation Domain: self.haskell, Score: 28, Comments: 19 On Reddit: [13] http://goo.gl/teYAD Original: [14] http://goo.gl/teYAD * Tutorial: Up-front Unit Testing in Haskell Domain: github.com, Score: 26, Comments: 21 On Reddit: [15] http://goo.gl/kjzYJ Original: [16] http://goo.gl/oB2jg * The Design of a Practical Proof Checker for a Lazy Functional Language [PDF] Domain: people.cs.missouri.edu, Score: 26, Comments: On Reddit: [17] http://goo.gl/sj5Aj Original: [18] http://goo.gl/eEUOj * Introduction to Haskell, Lecture 2 is Live Domain: shuklan.com, Score: 24, Comments: 4 On Reddit: [19] http://goo.gl/1nUVv Original: [20] http://goo.gl/rCbfx * Hugs in your browser Domain: tryhaskell.org, Score: 24, Comments: 21 On Reddit: [21] http://goo.gl/zNtex Original: [22] http://goo.gl/wMPPW * Free functors - the reason Free and operational are not equivalent Domain: self.haskell, Score: 24, Comments: 12 On Reddit: [23] http://goo.gl/E7qe7 Original: [24] http://goo.gl/E7qe7 * Scribbling, smudging and smearing with Haskell Domain: self.haskell, Score: 22, Comments: 108 On Reddit: [25] http://goo.gl/KBw0V Original: [26] http://goo.gl/KBw0V * Touch and Keyboard API (announcing Elm 0.7.1) Domain: elm-lang.org, Score: 20, Comments: 3 On Reddit: [27] http://goo.gl/OhFaH Original: [28] http://goo.gl/rV9bU * (re)implementing the generalised Functor strength function using lenses. Domain: statusfailed.com, Score: 20, Comments: 3 On Reddit: [29] http://goo.gl/gEYHr Original: [30] http://goo.gl/TDX2A Top StackOverflow Questions * Can using UndecidableInstances pragma locally have global consequences on compilation termination? votes: 20, answers: 1 Read on SO: [31] http://goo.gl/PcQF1 * Why doesn't GHC Haskell support overloaded record parameter names? votes: 14, answers: 1 Read on SO: [32] http://goo.gl/BJ71T * How much does it cost for Haskell FFI to go into C and back? votes: 14, answers: 2 Read on SO: [33] http://goo.gl/kUCKn * Compile unsafe Haskell votes: 13, answers: 1 Read on SO: [34] http://goo.gl/nGaCX * Why GADT/existential data constructors cannot be used in lazy patterns? votes: 12, answers: 2 Read on SO: [35] http://goo.gl/zhpCh * Correct design for Haskell exception handling votes: 11, answers: 2 Read on SO: [36] http://goo.gl/zeWoH * How to use phase control of inlining in haskell? votes: 10, answers: 0 Read on SO: [37] http://goo.gl/dyuGI * Looking for resources that help testing a Haskell implementation for standard conformance votes: 9, answers: 1 Read on SO: [38] http://goo.gl/DL7vY * Generate a C struct based on a complex Haskell type votes: 8, answers: 1 Read on SO: [39] http://goo.gl/7E8OT * How to structure a Haskell project? votes: 7, answers: 1 Read on SO: [40] http://goo.gl/J4GuA * Difference between two functions creating a singleton list votes: 7, answers: 3 Read on SO: [41] http://goo.gl/ZXzw0 Until next time, Daniel Santa Cruz References 1. http://ics.p.lodz.pl/~stolarek/blog/2013/01/taking-magic-out-of-ghc-or-trac… 2. http://www.reddit.com/r/haskell/comments/17bu54/taking_magic_out_of_ghc_or_… 3. http://www.haskellforall.com/2013/01/introduction-to-haskell-io.html 4. http://www.reddit.com/r/haskell/comments/17712e/introduction_to_haskell_io/ 5. http://i.imgur.com/Zv9JH9A.png 6. http://www.reddit.com/r/haskell/comments/178jwh/the_longest_type_signature_… 7. http://hackage.haskell.org/trac/ghc/ticket/7606 8. http://www.reddit.com/r/haskell/comments/1725g0/there_is_now_a_patch_for_th… 9. http://danbst.wordpress.com/2013/01/23/novice-netwire-user/ 10. http://www.reddit.com/r/haskell/comments/173cxi/success_story_novice_haskel… 11. http://arxiv.org/abs/1301.5076 12. http://www.reddit.com/r/haskell/comments/176xk0/haskell_as_pseudo_code/ 13. http://www.reddit.com/r/haskell/comments/16zrsp/generalizing_do_notation/ 14. http://www.reddit.com/r/haskell/comments/16zrsp/generalizing_do_notation/ 15. https://github.com/kazu-yamamoto/unit-test-example/blob/master/markdown/en/… 16. http://www.reddit.com/r/haskell/comments/1750i9/tutorial_upfront_unit_testi… 17. http://people.cs.missouri.edu/~harrisonwl/papers/tfp2012.pdf 18. http://www.reddit.com/r/haskell/comments/179367/the_design_of_a_practical_p… 19. http://shuklan.com/haskell 20. http://www.reddit.com/r/haskell/comments/172h5q/introduction_to_haskell_lec… 21. http://tryhaskell.org/hugs/ 22. http://www.reddit.com/r/haskell/comments/177a56/hugs_in_your_browser/ 23. http://www.reddit.com/r/haskell/comments/17a33g/free_functors_the_reason_fr… 24. http://www.reddit.com/r/haskell/comments/17a33g/free_functors_the_reason_fr… 25. http://www.reddit.com/r/haskell/comments/175mxw/scribbling_smudging_and_sme… 26. http://www.reddit.com/r/haskell/comments/175mxw/scribbling_smudging_and_sme… 27. http://elm-lang.org/blog/announce/version-0.7.1.elm 28. http://www.reddit.com/r/haskell/comments/179nwl/touch_and_keyboard_api_anno… 29. http://statusfailed.com/blog/2013/01/26/haskells-strength-generalising-with… 30. http://www.reddit.com/r/haskell/comments/17b5ak/reimplementing_the_generali… 31. http://stackoverflow.com/questions/14476230/can-using-undecidableinstances-… 32. http://stackoverflow.com/questions/14422169/why-doesnt-ghc-haskell-support-… 33. http://stackoverflow.com/questions/14519905/how-much-does-it-cost-for-haske… 34. http://stackoverflow.com/questions/14429966/compile-unsafe-haskell 35. http://stackoverflow.com/questions/14538255/why-gadt-existential-data-const… 36. http://stackoverflow.com/questions/14538294/correct-design-for-haskell-exce… 37. http://stackoverflow.com/questions/14446368/how-to-use-phase-control-of-inl… 38. http://stackoverflow.com/questions/14484569/looking-for-resources-that-help… 39. http://stackoverflow.com/questions/14500582/generate-a-c-struct-based-on-a-… 40. http://stackoverflow.com/questions/14421178/how-to-structure-a-haskell-proj… 41. http://stackoverflow.com/questions/14530555/difference-between-two-function… 42. https://plus.google.com/105107667630152149014/about

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TIPS: To Insure Package Sanity
by Albert Y. C. Lai 31 Jan '13

31 Jan '13

If you possess multiple instances (may be different versions, may be same version different builds) of a package, life can be hard and confusing. The problems are explained in my http://www.vex.net/~trebla/haskell/sicp.xhtml and faced by many people regularly. (Just read this mailing list.) cabal-install has a mechanism for not adding more instances. It is available since version 0.14.0, or earlier. It is just little known. It is also a bit manual. You have to give 1 instruction for each package. If you want to say it for n packages, you have to give n instructions. Also, if you give such an instruction for a package you do not already have, there is a problem: now you can't install that package. To insure package sanity, add these lines to your $HOME/.cabal/config, one line per package you want to protect. (Cannot merge into one line.) constraint: array installed constraint: bytestring installed constraint: Cabal installed constraint: containers installed ... Generally, do it for every package that comes with GHC, and every package that comes with the Haskell Platform if you have it, plus every package that you want stable. (You may omit packages that cannot possibly be on hackage, e.g., integer-gmp.) Remember to revise those lines whenever you switch GHC versions or Haskell Platform versions, since their package lists change. Alternatively, if you want even more manual work (in exchange for more fine-tuning perhaps?): every time you use "cabal install": cabal install --constraint="array installed" --constraint="bytestring installed" --constraint="Cabal installed" --constraint="containers installed" ...

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Package conflicts using cabal-dev
by Arnaud Bailly 30 Jan '13

30 Jan '13

Hello, I am running into a strange issue that reminds me of Java's classloader black magic havoc :-) My code compiles fine using cabal-dev install, but when I try to compile an individual file with ghc, I got the following error: $ ghc -package-conf cabal-dev/packages-7.4.1.conf YakGraph.hs [1 of 1] Compiling YakGraph ( YakGraph.hs, YakGraph.o ) YakGraph.hs:13:30: Couldn't match expected type `Data.Text.Lazy.Internal.Text' with actual type `text-0.11.2.0:Data.Text.Lazy.Internal.Text' In the first argument of `unpack', namely `s' In the expression: unpack s In an equation for `fromGraphId': fromGraphId (Str s) = unpack s YakGraph.hs:24:57: Couldn't match expected type `Data.Text.Lazy.Internal.Text' with actual type `text-0.11.2.0:Data.Text.Lazy.Internal.Text' In the first argument of `unpack', namely `name' In the expression: unpack name In a case alternative: (Just (Str name)) : _ -> unpack name The code leading to this error is here: https://gist.github.com/4677191 Any insights on this would be appreciated. Arnaud

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Heads up: planned removal of String instances from HTTP package
by Ganesh Sittampalam 30 Jan '13

30 Jan '13

Hi, tl;dr: I'm planning on removing the String instances from the HTTP package. This is likely to break code. Obviously it will involve a major version bump. The basic reason is that this instance is rather broken in itself. A String ought to represent Unicode data, but the HTTP wire format is bytes, and HTTP makes no attempt to handle encoding. This was discussed on the libraries@ list a while back, but I'm happy to discuss further if there's a general feeling that this is a bad thing to do: http://www.haskell.org/pipermail/libraries/2012-September/018426.html I will probably upload the new version in a week or two. Cheers, Ganesh

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Darcs Hacking Sprint #8 (February 15-17, Paris)
by Guillaume Hoffmann 30 Jan '13

30 Jan '13

Dear hackers, the Darcs project is glad to announce that the next Hacking Sprint will take place in Paris on February 15-17, at the IRILL (near Place d'Italie). Please check out the details at: http://darcs.net/Sprints/2013-02 Here are three things to know 1. Everybody is welcome to join us. We'd love to have you, whatever your Haskell or Darcs hacking experience. Also, if you've got a wacky idea for the future of version control, or a cool use for the Darcs library, you should join us too :-) 2. Please let us know if you're attending. Just add your name to http://wiki.darcs.net/Sprints/2013-02 and you're good to go. You can also send us an email. 3. We can reimburse travel costs (within reason!). Let us know if you'd like a reimbursement, and save your receipts. Many thanks to everybody who participated in our fundraising drives or who gave money on the side. Thanks also to the Software Freedom Conservancy for making fundraising and reimbursements so painless! If you can't join us in person, but you'd like to cheer us on, say hello at http://darcs.net/Donations ! Looking forward to seeing you! Guillaume

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Handling exceptions or gracefully releasing resources
by Thiago Negri 30 Jan '13

30 Jan '13

`Control.Exception.bracket` is a nice function to acquire and release a resource in a small context. But, how should I handle resources that are hold for a long time? Should I put `Control.Exception.finally` on every single line of my finalizers? What exceptions may occur on an IO operation? Every IO function has the risk of throwing an exception? Thanks, Thiago.

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Uniplate and rewriting with different types
by Chris Mears 29 Jan '13

29 Jan '13

Hi all, I have a question about the Uniplate library, regarding rewriting with transformations that have different types. With the following type, and transformation functions: data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) t1,t2,t3 :: Even -> Maybe Even t1 (EvenOne (OddOne x)) = Just $ EvenOne (OddZero x) t1 x = Nothing t2 (EvenOne (OddZero x)) = Just $ EvenZero (OddOne x) t2 x = Nothing t3 (EvenZero (OddOne x)) = Just $ EvenZero (OddZero x) t3 x = Nothing it is easy to combine the transformations into a single transformation, because they all have the same type. The result can then be passed to the Uniplate's "rewriteBi" function: allts x = t1 x `mplus` t2 x `mplus` t3 x example = OddOne (EvenOne (OddOne (EvenOne (OddOne Nil)))) go = rewriteBi allts example But if one of the transformations has a different type, you can't do it this way. For instance, redefine t2 to have a different type: t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing and you are stuck because the functions of different types can't be combined into a single transformation. My question is: is there a good way to combine the transformation functions if they have different types? I have come up with a possible solution (see below), but I am not sure that it is the right approach, and it is probably inefficient. Chris Mears {-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE ScopedTypeVariables #-} import Control.Monad import Control.Monad.State import Data.Generics import Data.Generics.Uniplate.Data data Odd = OddOne Even | OddZero Even deriving (Data,Typeable,Show) data Even = EvenOne Odd | EvenZero Odd | Nil deriving (Data,Typeable,Show) t1 (EvenOne (OddOne x)) = Just $ EvenOne (OddZero x) t1 x = Nothing t2 :: Odd -> Maybe Odd t2 (OddZero (EvenOne x)) = Just $ OddZero (EvenZero x) t2 x = Nothing t3 (EvenZero (OddOne x)) = Just $ EvenZero (OddZero x) t3 x = Nothing -- The transformers are wrapped in an existential type. data WrappedTransformer from = forall to. (Data to, Biplate from to) => WrappedTransformer (to -> Maybe to) -- Apply a single transformation, and return "Just x" if the -- transformation fired (where x is the result of the rewriting), and -- "Nothing" if no transformation fired. rewriteBiCheck :: Biplate from to => (to -> Maybe to) -> from -> Maybe from rewriteBiCheck f e = case runState (rewriteBiM check e) False of (e', True) -> Just e' (_, False) -> Nothing where check x = case f x of Nothing -> return Nothing Just y -> put True >> return (Just y) -- Apply a list of wrapped transformations until no more -- transformations can be applied. rewriteBiList :: forall from. [WrappedTransformer from] -> from -> from rewriteBiList transformers m = go transformers m where go :: [WrappedTransformer from] -> from -> from go [] m = m go ((WrappedTransformer t):ts) m = case rewriteBiCheck t m of Just m' -> go transformers m' Nothing -> go ts m -- Test case. example = OddOne (EvenOne (OddOne (EvenOne (OddOne Nil)))) go = rewriteBiList [ WrappedTransformer t1 , WrappedTransformer t2 , WrappedTransformer t3 ] example

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catamorphisms and attribute grammars
by Petr P 29 Jan '13

29 Jan '13

Dear Haskellers, I read some stuff about attribute grammars recently [1] and how UUAGC [2] can be used for code generation. I felt like this should be possible inside Haskell too so I did some experiments and I realized that indeed catamorphisms can be represented in such a way that they can be combined together and all run in a single pass over a data structure. In fact, they form an applicative functor. [1] http://www.haskell.org/haskellwiki/Attribute_grammar [2] Utrecht University Attribute Grammar Compiler To give an example, let's say we want to compute the average value of a binary tree. If we compute a sum first and then count the elements, the whole tree is retained in memory (and moreover, deforestation won't happen). So it's desirable to compute both values at once during a single pass: -- Count nodes in a tree. count' :: (Num i) => CataBase (BinTree a) i count' = ... -- Sums all nodes in a tree. sum' :: (Num n) => CataBase (BinTree n) n sum' = ... -- Computes the average value of a tree. avg' :: (Fractional b) => CataBase (BinTree b) b avg' = (/) <$> sum' <*> count' Then we can compute the average in a single pass like runHylo avg' treeAnamorphism seed My experiments together with the example are available at https://github .com/ppetr/recursion-attributes I wonder, is there an existing library that expresses this idea? Best regards, Petr Pudlak

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quotRem and divMod
by Artyom Kazak 29 Jan '13

29 Jan '13

Hi! I’ve always thought that `quotRem` is faster than `quot` + `rem`, since both `quot` and `rem` are just "wrappers" that compute both the quotient and the remainder and then just throw one out. However, today I looked into the implementation of `quotRem` for `Int32` and found out that it’s not true: quotRem x@(I32# x#) y@(I32# y#) | y == 0 = divZeroError | x == minBound && y == (-1) = overflowError | otherwise = (I32# (narrow32Int# (x# `quotInt#` y#)), I32# (narrow32Int# (x# `remInt#` y#))) Why? The `DIV` instruction computes both, doesn’t it? And yet it’s being performed twice here. Couldn’t one of the experts clarify this bit?

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Mac os x (Intel, 10.6.8) problem compiling yesod-core-1.1.7.1
by jean-christophe mincke 28 Jan '13

28 Jan '13

Hello GHC version : 7.4.2 When I do a cabal-dev instal yesod-core, I get the following error: Loading package blaze-builder-conduit-0.5.0.3 ... linking ... done. Loading package hashable-1.2.0.5 ... linking ... ghc: lookupSymbol failed in resolveImports /Users/V3/windev/Haskell/Z/cabal-dev//lib/HShashable-1.2.0.5.o: unknown symbol `_hashable_siphash24_sse2' ghc: unable to load package `hashable-1.2.0.5' Has anyone encountered the same problem? Thank you J-C

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