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Prelude> read "1234" :: String -> "*** Exception: Prelude.read: no parse
by michael rice 07 Aug '11

07 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 05 Aug '11

05 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 04 Aug '11

04 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 04 Aug '11

04 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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Fwd: shootout
by Ben 03 Aug '11

03 Aug '11

hello cafe-istas -- for those of you who are into these things, a lot of the shootout programs are suffering from "make errors" and thus do not have benchmarks. http://shootout.alioth.debian.org/u64q/haskell.php best, ben Begin forwarded message: > From: Don Stewart <dons00(a)gmail.com> > Date: July 30, 2011 9:52:12 AM PDT > To: Ben <midfield(a)gmail.com> > Cc: Don Bruce Stewart <dons(a)cse.unsw.edu.au> > Subject: Re: shootout > > Best to bring this up on haskell-cafe@ > > On Sun, Jul 31, 2011 at 12:34 AM, Ben <midfield(a)gmail.com> wrote: >> FYI, a lot of the haskell programs on the shootout suffer from "make errors" and thus do not have benchmarks. >> >> best, ben

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Difference between class and instance contexts
by Patrick Browne 03 Aug '11

03 Aug '11

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Fractional Part
by Ata Jafari 03 Aug '11

03 Aug '11

Hi everyone, I'm totally new to Haskell and functional programming. I try to solve some problems from Proejct Euler with Haskell so that I can improve myself in functional programming. In the first step I want to write a little code that can give me only the decimal part of a float. For instance: if the number is (0.123) I want to obtain only the (123) part to do some processes on that. (I mean the result of this code must be an integer.) I don't expect any complete code, any suggestion and help will be appreciated. Thanks Ata

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