August 2011 - Haskell-Cafe

library-profiling default
by Tom Doris 07 Aug '11

07 Aug '11

Hi Is there a good reason that the default for library-profiling in .cabal/config is set to False? It seems a lot of people hit the problem of trying to profile for the first time, finding it doesn't work because profiling libraries haven't been installed, then they have to walk the dependencies reinstalling everything. Is there a major cost or problem with just defaulting this to True? Apologies if this is answered elsewhere, I saw various discussions on why it is difficult to automatically build required libs with profiling on demand, but nothing that discussed changing the default so that they are always built. Tom

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ANN: Netwire AFRP library
by Ertugrul Soeylemez 07 Aug '11

07 Aug '11

Hi there, after a few weeks of development time, I have released the first official version of my arrowized FRP library called Netwire: http://hackage.haskell.org/package/netwire I have written it with networking applications in mind, but it is a general purpose library, so it can be used in all of the classic fields like gaming, animation, user interfaces and others. Its basic idea is the same as in Yampa, but its internal structure is much simpler and hence it's easier to extend. Also like in most of my libraries I have specifically decided to expose the internals, so you can extend Netwire without having to fork. Features not found in Yampa/Animas: * switching by ArrowChoice, * suspended signal transformers, * signal inhibition, * combination through ArrowZero and ArrowPlus, * impure signal transformers. There exists a wiki page, which should give you a quickstart: http://haskell.org/haskellwiki/Netwire I would be grateful for any constructive feedback. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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trouble using the aeson package
by anonymous 07 Aug '11

07 Aug '11

I recently installed the aeson package using cabal-install without any trouble: ❲~❳ uname -rspi; cabal -V; ghc -V; ghc-pkg list | grep "aeson\|double-conversion" Linux 2.6.38-10-generic x86_64 x86_64 cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library The Glorious Glasgow Haskell Compilation System, version 7.0.3 aeson-0.3.2.9 double-conversion-0.2.0.1 However when I try to use it I receive this error message: λ> let f = Data.Aeson.String Loading package double-conversion-0.2.0.1 ... can't load .so/.DLL for: stdc++ (libstdc++.so: cannot open shared object file: No such file or directory) This is the version I have installed: ❲~❳ aptitude search stdc++ | grep ^i i libstdc++6 - The GNU Standard C++ Library v3 i A libstdc++6-4.4-dev - The GNU Standard C++ Library v3 (developme i libstdc++6-4.5-dev - The GNU Standard C++ Library v3 (developme ❲~❳ aptitude show libstdc++6-4.4-dev | head -n 4 Package: libstdc++6-4.4-dev State: installed Automatically installed: yes Version: 4.4.5-15ubuntu1

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Prelude> read "1234" :: String -> "*** Exception: Prelude.read: no parse
by michael rice 06 Aug '11

06 Aug '11

Prelude> read "1234" :: Int1234Prelude> read "1234" :: String"*** Exception: Prelude.read: no parse Why? Michael

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Re: [Haskell-cafe] Deriving instances with GADTs
by Daniel Schüssler 05 Aug '11

05 Aug '11

Hi, you can declare an instance instance Show (OrderType s o) where ... this makes sense since it means "for every type s and o, OrderType s o is showable", which is vacuously true if (s,o) isn't in {Buy,Sell} x {Market, Limit} (because in that case, there /is/ no non-bottom value of type 'OrderType s o'). Obviously, this will make the newtype's 'deriving' clause work. Furthermore, you can use {-# LANGUAGE StandaloneDeriving #-} and write deriving instance Show (OrderType s o) (Standalone deriving works differently from a 'deriving' clause; the former generates the code regardless of how weird the type is and lets the typechecker decide. For your GADT, this succeeds). Cheers, Daniel Schüssler On 2011-August-04 Thursday 08:57:46 Tim Cowlishaw wrote: > Hi all, > > I've been writing a DSL to describe securities orders, and after a lot > of help from the kind folk of this list and #haskell have come up with > the following implementation, using generalised algebraic data types: > > https://gist.github.com/1124621 > > Elsewhere in my application, I make use of the order type defined > therein in the following newtype declaration: > > newtype OrderListLevel s = OrderListLevel {orders :: [Order s Limit]} > deriving (Eq, Show) > > However, the 'deriving' clause here fails: > > src/Simulation/OrderList.hs:9:82: > No instance for (Eq (Order s Limit)) > arising from the 'deriving' clause of a data type declaration > at src/Simulation/OrderList.hs:9:82-83 > > src/Simulation/OrderList.hs:9:86: > No instance for (Show (Order s Limit)) > arising from the 'deriving' clause of a data type declaration > at src/Simulation/OrderList.hs:9:86-89 > > > > I don't fully understand this - the error is correct that there is no > instance of either Eq or Show for (Order s Limit), however, instances > are defined for Order Buy Limit and Order Sell Limit, and since these > are the only possible types that a value can be constructed with (the > type constructor is 'closed' over these types in some sense I guess), > it seems to me that this should provide enough information to derive > the Eq and Show instances. Am I making unreasonable expectations of > ghci's instance-deriving mechanism here, or missing something obvious? > > Many thanks in advance, > > Tim > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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weird type signature in Arrow Notation
by bob zhang 04 Aug '11

04 Aug '11

hi, all testB :: (ArrowChoice t1, Num a1, Num a) => (a -> a1 -> t2) -> t1 a t3 -> t1 a1 t3 -> t1 (a, a1) t testB f g h = proc (x,y) -> do if (f x y)then g -< x + 1 else h -< y + 2 it's very strange that the type of _f_ is (a->a1->t2) which I thought should be a -> a1 -> Bool, btw, is there any way to get the output of preprocessing using -XArrow extensions, Thanks a lot best, bob

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One place available at CamHac
by Simon Marlow 04 Aug '11

04 Aug '11

Hi Folks, There is a place available at CamHac next Friday-Sunday (12-14 August). First come first served! For instructions on how to register, see the wiki page: http://www.haskell.org/haskellwiki/CamHac#Registration Also a reminder, if you have already registered but won't be able to make it, please let us know so that we can reallocate your place. Cheers, Simon

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Deriving instances with GADTs
by Tim Cowlishaw 04 Aug '11

04 Aug '11

Hi all, I've been writing a DSL to describe securities orders, and after a lot of help from the kind folk of this list and #haskell have come up with the following implementation, using generalised algebraic data types: https://gist.github.com/1124621 Elsewhere in my application, I make use of the order type defined therein in the following newtype declaration: newtype OrderListLevel s = OrderListLevel {orders :: [Order s Limit]} deriving (Eq, Show) However, the 'deriving' clause here fails: src/Simulation/OrderList.hs:9:82: No instance for (Eq (Order s Limit)) arising from the 'deriving' clause of a data type declaration at src/Simulation/OrderList.hs:9:82-83 src/Simulation/OrderList.hs:9:86: No instance for (Show (Order s Limit)) arising from the 'deriving' clause of a data type declaration at src/Simulation/OrderList.hs:9:86-89 I don't fully understand this - the error is correct that there is no instance of either Eq or Show for (Order s Limit), however, instances are defined for Order Buy Limit and Order Sell Limit, and since these are the only possible types that a value can be constructed with (the type constructor is 'closed' over these types in some sense I guess), it seems to me that this should provide enough information to derive the Eq and Show instances. Am I making unreasonable expectations of ghci's instance-deriving mechanism here, or missing something obvious? Many thanks in advance, Tim

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Haskell Weekly News: Issue 193
by Daniel Santa Cruz 03 Aug '11

03 Aug '11

Welcome to issue 193 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of July 24 to 30, 2011. [1] http://goo.gl/8hDku You can find an HTML rendition of this issue at: http://contemplatecode.blogspot.com/2011/08/haskell-weekly-news-issue-193.h… Announcements Wouter Swierstra issued a call for participation to the 16th ACM SIGPLAN International Conference on Function Programming (ICFP 2011) to be held in Tokyo, Japon on September 19-21, 2011. [2] http://goo.gl/ob6Tq New and Updated Projects These projects were announced in the mailing lists: * syntactic (Emil Axelsson): Update to version 0.5 [3] http://goo.gl/sKutg * Yi (Jeff Wheeler): Update to version 0.6.4.0 [4] http://goo.gl/Tr2Ns Quotes of the Week * sipa: it can optimize non-terminating programs into programs that output "<<loop>>" and quit; that's an optimization of infinity% ! * fazzone: People generally seem to react the same way to "My first assembly was x86" as "My dog died" -- "Oh my god, I'm so sorry" * [danharaj] I hate myself [edwardk] happy to help * bos: the number of people who grok rank-K types is inversely proportional to ackermann(K) * copumpkin: holy shit, I'm unsafeCoerce * quicksilver: edwardk had this brilliant idea of how he could stress-test our package architecture * chrisdone: monoid sounds like a name for a hairless alien species that communicates telepathically from star trek * [copumpkin] I basically translated repa to agda and made it all safe n shit. also all slow n shit [thoughtpolice] safe n shit, like, type systems yo * theorbtwo: Well, that's one way to respond to ghc having overly-long error messages. Just replace all error messges with "that's the stupidist shit I've ever heard". * acowley: I'm 101% sure I never would have figured it out. I was told all hope was lost, then simonmar swung in on his web shooters and figured it out * quicksilver: agda is an alluring trap for haskell programmers who hope to write better code. instead they end up writing no code. Top Reddit Stories * my humble rant about the Haskell learning curve Domain: willowbend.cx, Score: 66, Comments: 134 On Reddit: [5] http://goo.gl/g1buw Original: [6] http://goo.gl/3xDxl * GHC 7.2.* release candidates are now available Domain: haskell.org, Score: 28, Comments: 17 On Reddit: [7] http://goo.gl/0C0gx Original: [8] http://goo.gl/cvec5 * oleg compares iteratee with python's yield Domain: haskell.org, Score: 27, Comments: 12 On Reddit: [9] http://goo.gl/8YQKq Original: [10] http://goo.gl/BafGo * Report from HacPDX-II about progress on the new Hackage server Domain: well-typed.com, Score: 26, Comments: 0 On Reddit: [11] http://goo.gl/vBr7N Original: [12] http://goo.gl/QXy9a * Arrow = Category + Applicative? (Part I) Domain: cdsmith.wordpress.com, Score: 26, Comments: 34 On Reddit: [13] http://goo.gl/Zs5Yg Original: [14] http://goo.gl/AsfL1 * Profunctors in Haskell - A Neighborhood of Infinity Domain: blog.sigfpe.com, Score: 19, Comments: 11 On Reddit: [15] http://goo.gl/58CbB Original: [16] http://goo.gl/IKI9H * BlockedIndefinitelyOnMVar : Inside 1712B Domain: blog.ezyang.com, Score: 19, Comments: 3 On Reddit: [17] http://goo.gl/3Xtn0 Original: [18] http://goo.gl/4osPH * The Great Yesod Reorganization of 2011 Domain: yesodweb.com, Score: 18, Comments: 8 On Reddit: [19] http://goo.gl/ZGNoa Original: [20] http://goo.gl/V4buP * Is there a Haskell code formatter? Domain: stackoverflow.com, Score: 16, Comments: 32 On Reddit: [21] http://goo.gl/lQOX3 Original: [22] http://goo.gl/AthoZ * Building a Rate of Return System in Haskell – Part 3, Annualized Returns Domain: blog.virtucal.com, Score: 9, Comments: 7 On Reddit: [23] http://goo.gl/0QGPB Original: [24] http://goo.gl/R6am5 Top StackOverflow Questions * Is there a Haskell code formatter? votes: 18, answers: 1 Read on SO: [25] http://goo.gl/AthoZ * Help in understanding pointfree code votes: 17, answers: 1 Read on SO: [26] http://goo.gl/Ryy5c * Haskell: What is Weak Head Normal Form? votes: 17, answers: 5 Read on SO: [27] http://goo.gl/3CHAB * Understanding GHC assembly output votes: 13, answers: 1 Read on SO: [28] http://goo.gl/ujlve * What are Haskell's monad transformers in categorical terms? votes: 13, answers: 1 Read on SO: [29] http://goo.gl/vMPdO * Managing a stateful computation system in Haskell votes: 12, answers: 1 Read on SO: [30] http://goo.gl/HABkL * Am I using reactive-banana right? votes: 11, answers: 1 Read on SO: [31] http://goo.gl/7jiKr * Is there an object-identity-based, thread-safe memoization library somewhere? votes: 11, answers: 2 Read on SO: [32] http://goo.gl/7vGvl * Is there a practical way of using natural numbers in Haskell? votes: 11, answers: 2 Read on SO: [33] http://goo.gl/hJBK9 * Current state of integrating unit tests with Haskell's Cabal? votes: 10, answers: 1 Read on SO: [34] http://goo.gl/nm8Zc About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://permalink.gmane.org/gmane.comp.science.types.announce/2446 3. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/91427 4. http://permalink.gmane.org/gmane.editors.yi.devel/5550 5. http://willowbend.cx/2011/07/22/whats-wrong-with-learning-haskell/ 6. http://www.reddit.com/r/haskell/comments/j0say/my_humble_rant_about_the_has… 7. http://www.haskell.org/ghc/dist/7.2.1-rc1/ 8. http://www.reddit.com/r/haskell/comments/j3yly/ghc_72_release_candidates_ar… 9. http://www.haskell.org/pipermail/haskell-cafe/2011-July/094384.html 10. http://www.reddit.com/r/haskell/comments/j32c4/oleg_compares_iteratee_with_… 11. http://www.well-typed.com/blog/57 12. http://www.reddit.com/r/haskell/comments/iznjs/report_from_hacpdxii_about_p… 13. http://cdsmith.wordpress.com/2011/07/30/arrow-category-applicative-part-i/ 14. http://www.reddit.com/r/haskell/comments/j4h6a/arrow_category_applicative_p… 15. http://blog.sigfpe.com/2011/07/profunctors-in-haskell.html 16. http://www.reddit.com/r/haskell/comments/iyima/profunctors_in_haskell_a_nei… 17. http://blog.ezyang.com/2011/07/blockedindefinitelyonmvar/ 18. http://www.reddit.com/r/haskell/comments/j1ld6/blockedindefinitelyonmvar_in… 19. http://www.yesodweb.com/blog/2011/07/yesod-reorg 20. http://www.reddit.com/r/haskell/comments/iz2ew/the_great_yesod_reorganizati… 21. http://stackoverflow.com/questions/6870148/is-there-a-haskell-code-formatter 22. http://www.reddit.com/r/haskell/comments/j31f4/is_there_a_haskell_code_form… 23. http://blog.virtucal.com/cyclical/2011/7/28/building-a-rate-of-return-syste… 24. http://www.reddit.com/r/haskell/comments/j2gyp/building_a_rate_of_return_sy… 25. http://stackoverflow.com/questions/6870148/is-there-a-haskell-code-formatter 26. http://stackoverflow.com/questions/6797247/help-in-understanding-pointfree-… 27. http://stackoverflow.com/questions/6872898/haskell-what-is-weak-head-normal… 28. http://stackoverflow.com/questions/6808867/understanding-ghc-assembly-output 29. http://stackoverflow.com/questions/6854303/what-are-haskells-monad-transfor… 30. http://stackoverflow.com/questions/6835777/managing-a-stateful-computation-… 31. http://stackoverflow.com/questions/6812533/am-i-using-reactive-banana-right 32. http://stackoverflow.com/questions/6878193/is-there-an-object-identity-base… 33. http://stackoverflow.com/questions/6884220/is-there-a-practical-way-of-usin… 34. http://stackoverflow.com/questions/6787231/current-state-of-integrating-uni…

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ANNOUNCE: vector-algorithms 0.5.2
by Dan Doel 03 Aug '11

03 Aug '11

Greetings, Following some work at hac-phi, I've finally put together a new release of vector-algorithms. It should now be available via hackage, or you can pull from code.haskell.org if you prefer: hackage: http://hackage.haskell.org/package/vector-algorithms/ latest: darcs get http://code.haskell.org/~dolio/vector-algorithms/ The highlights of the new release are as follows: - Some bit rot in the test suite has been fixed - Some strictness has been added to the merge sort to improve unboxing on current GHCs - A couple out-of-bounds errors have been caught and fixed - An entirely new sort---American flag sort---has been added to the line-up * American flag sort is an in-place bucket sort, which only uses a constant amount of auxiliary heap space (determined by the element type). And unlike a typical radix sort, it is actually suitable for sorting arrays of variable-length (O(1) lookup) strings. To this end, there is a (strict) ByteString instance for the relevant class, and possibly more such instances to come. Bug reports, patches, requests, etc. can be sent to me at this address. Enjoy, -- Dan

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