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Re: [Haskell-cafe] [Agda] Defining subtraction for naturals
by wren ng thornton 17 Mar '11

17 Mar '11

On 3/17/11 4:22 PM, Martin Escardo wrote: > On 17/03/11 18:35, wren ng thornton wrote: >> (2) Use saturating subtraction, i.e. if the result would drop below zero >> then return zero; > > This is what people working with quantales do. > > Subtraction y-z, when it exists, is the solution in s to the equation > s+z =y. > > Truncated subtraction y - z is the supremum of the set of solutions s to > the inequality s+z <= y, when this supremum exists. (In your example, > when z>y, the set of solutions is empty, and the empty set has supremum > zero.) > > This amounts to saying that the truncated subtraction function (-) - z > is the right adjoint to the addition function (-) + z. > > So (2) is very natural. Thanks! That's exactly the kind of thing I was looking for. I'd been needlessly thinking of it as a hack :) -- Live well, ~wren

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X11 package bug: XClientMessageEvent long data
by Dylan Alex Simon 17 Mar '11

17 Mar '11

Does anyone know the current maintenance status of the X11 package? I emailed Spencer Janssen a number of months ago and never heard back. So, I'll put this here in case any one else runs into it or can get it to the right place. This is a proposed bug fix for a problem I ran into using xmonad client messages to send remote commands on 64LP architectures (i.e., amd64), wherein the C X11 library and Haskell's disagree about the size of client message arguments. Tue Nov 16 23:41:49 EST 2010 Dylan Simon <dylan(a)dylex.net> * change XClientMessageEvent long data The XClientMessageEvent.data.l field is actually a long, not an int, so it must be interpreted as such, even though format is set to 32 in this case. Ostensibly this is an Xlib bug, but it is unlikely to be fixed there. diff -rN -u old-src/Graphics/X11/Xlib/Extras.hsc new-src/Graphics/X11/Xlib/Extras.hsc --- old-src/Graphics/X11/Xlib/Extras.hsc 2011-03-15 22:40:39.687844812 -0400 +++ new-src/Graphics/X11/Xlib/Extras.hsc 2011-03-15 22:40:39.724522814 -0400 @@ -601,7 +601,7 @@ 16 -> do a <- peekArray 10 datPtr return $ map fromIntegral (a::[Word16]) 32 -> do a <- peekArray 5 datPtr - return $ map fromIntegral (a::[Word32]) + return $ map fromIntegral (a::[CLong]) _ -> error "X11.Extras.clientMessage: illegal value" return $ ClientMessageEvent { ev_event_type = type_

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How large is the Haskell community ?
by Aaron Gray 17 Mar '11

17 Mar '11

I was wondering if anyone had an idea or estimate as to how large the Haskell community is ? Aaron

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Haskell report minutiae
by wren ng thornton 17 Mar '11

17 Mar '11

So I have a rules-lawyering question about how specified the overflow behavior is for the types Int and Word. * In the Haskell 2010 report it seems to be specified that (sections 18.1, 23.1): "All arithmetic is performed modulo 2^n, where n is the number of bits in the type." And this seems to apply to Int and Word as well as Int8, Word8, Int16, Word16, Int32, Word32, Int64, Word64. * However, in the Haskell 98 report ---and this verbiage is retained in the same section of H2010--- there is no such guarantee (section 6.4): "The results of exceptional conditions (such as overflow or underflow) on the fixed-precision numeric types are undefined; an implementation may choose error (_|_, semantically), a truncated value, or a special value such as infinity, indefinite, etc." * However, the FFI addendum to H98 does specify modulo behavior for the Int8, Word8, Int16, Word16, Int32, Word32, Int64, Word64 types (section 5.3): "There is, however, the additional constraint that all arithmetic on the fixed-sized types is performed modulo 2^n." But it is unclear whether this also applies to Int and Word. So when it comes to interpreting the report(s) and ensuring proper implementation, which is it? Are Int and Word arithmetic modular, or not? Did the specification change between H98 and H98+FFI or between H98+FFI and H2010? -- Live well, ~wren

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Weird warnings from recent GHC snapshot
by Michael Vanier 17 Mar '11

17 Mar '11

I run haskell on Mac OS X (Snow Leopard). After upgrading my Xcode installation to 4.0 I had a tricky time getting ghc working again; the version bundled with the Haskell Platform no longer works and I had to compile a recent snapshot (ghc-7.1.20110315) from source. This worked fine, but now when I compile my code I keep getting really weird warnings. For instance: ld: warning: could not create compact unwind for .LFB3: non-standard register 5 being saved in prolog and SpecConstr Function `$w$j{v s3hL} [lid]' has one call pattern, but the limit is 0 Use -fspec-constr-count=n to set the bound Use -dppr-debug to see specialisations SpecConstr Function `$w$j{v s3ic} [lid]' has two call patterns, but the limit is 1 Use -fspec-constr-count=n to set the bound Use -dppr-debug to see specialisations The code seems to work, but this is quite irritating. Is there anything that can be done about these warnings? Thanks, Mike

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Lazy evaluation and tail-recursion
by Yves Parès 17 Mar '11

17 Mar '11

Hello, A question recently popped into my mind: does lazy evaluation reduce the need to "proper" tail-recursion? I mean, for instance : fmap f [] = [] fmap f (x:xs) = f x : fmap f xs Here fmap is not tail-recursive, but thanks to the fact that operator (:) is lazy, I think that it may still run in constant space/time, am I right?

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Data.Time.Calendar.Day does not seem to have an instance for Read
by C K Kashyap 17 Mar '11

17 Mar '11

Hi, I was wondering if this is a defect - Prelude> import Data.Time.Calendar Prelude Data.Time.Calendar> read "2011-10-10" :: Day <interactive>:1:1: No instance for (Read Day) arising from a use of `read' Possible fix: add an instance declaration for (Read Day) In the expression: read "2011-10-10" :: Day In an equation for `it': it = read "2011-10-10" :: Day Prelude Data.Time.Calendar> The documentation says that Day has an instance for Read. Regards, Kashyap

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Data.ByteString.Lazy.ByteString vs Data.ByteString.Lazy.Internal.ByteString
by C K Kashyap 17 Mar '11

17 Mar '11

Hi, I noticed that even though I declare the type of a function in my code as Data.ByteString.Lazy.ByteString ... when I check it out in ghci using :t, it shows this - Data.ByteString.Lazy.Internal.ByteString Is this expected? Regards, Kashyap

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Type trickery
by Andrew Coppin 17 Mar '11

17 Mar '11

The well-known ST monad uses an ingenious hack to make it impossible for distinct ST computations to interact with each other. Is there a way to do something similar so that I can create "cursors" that reference a (mutable) container, and then write a function that takes two cursor arguments which are statically guaranteed to refer to the same container?

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Haskell Weekly News: Issue 173
by Daniel Santa Cruz 17 Mar '11

17 Mar '11

Welcome to issue 173 of the HWN, a newsletter covering developments in the [1]Haskell community. This release covers the week of March 06-12, 2011. For a (easier to read) HTML version goto: http://bit.ly/gQCQHf Announcements Bas van Dijk [2]annouced updates to regions (0.9), regions-mtl (0.3.1.5), and regions-monadstf (0.3.1.5) Don Stewart [3]gave us reason to celebrate with the announcement of the new Haskell Platform 2011.2, the "single, standard Haskell distribution for everyone." Most of you have probably already heard/gotten this new release, but in case you haven't... Remi Turk [4]annouced the first release of cinvoke, "a binding to the C library cinvoke, allowing functions to be loaded and called whose names and types are not known before run-time." Quotes of the Week * kmc: [After discussing monads, containers, and tortillas] therefore the key difference between a container and a monad is delicious carne asada * edwardk: [you mean #haskell has a 2 phD/nick average ?] clearly this is why SPJ stays away, he doesn't want to dilute that metric. * byorgey: The magic of printf is that it is well-typed but you need not actually know what the types are. * monochrom: How do I extract the IO out of IO String? * kmc: ghc -XTwitter -ddump-tweets * xplat: because, you know, you have to write a quicksort so differently in java than any other language. first you create an AbstractSequenceSubrangeFactory interface, and then you create the implementation for ArrayList ... Top Reddit Stories * The Haskell Platform 2011.2 is out!: From (hackage.haskell.org.nyud.net) scored 80 with 34 comments. Read on [5]reddit or the [6]original post. * Umm... yes... you can now write new GHC compiler passes in Coq (!!?): From (cs.berkeley.edu) scored 55 with 16 comments. Read on [7]reddit or the [8]original post. * The Type Technology Tree (mapping Type Extensions in GHC): From (blog.ezyang.com) scored 53 with 5 comments. Read on [9]reddit or the [10]original post. * a message from Tokyo: From (haskell.org) scored 47 with 9 comments. Read on [11]reddit or the [12]original post. * Haskellâs Niche: Hard Problems: From (cdsmith.wordpress.com) scored 43 with 16 comments. Read on [13]reddit or the [14]original post. * Functional flocks : modelling and visualizing flocking behavior in Haskell: From (syntacticsalt.com) scored 21 with 0 comments. Read on [15]reddit or the [16]original post. * Tech Talk Video: Faster Persistent Data Structures Through Hashing: From (corp.galois.com) scored 20 with 5 comments. Read on [17]reddit or the [18]original post. * Recursion is a low-level operation: From (blog.fogus.me), scored 18 with 25 comments. Read on [19]reddit or the [20]original post. * Yesod 1.0 Roadmap: From (docs.yesodweb.com) scored 17 with 9 comments. Read on [21]reddit or the [22]original post. * Type inference weirdness: From (self.haskell), scored 13 with 26 comments. Read on [23]reddit. Top StackOverflow Answers * [25]haskell beginner - recursive recursion votes: 10 This can be improved substantially by using a different algorithm: The smallest number which can be divided by a set of numbers (in this case, the set is [1..10]) is the least common multiple of those numbers. Haskell even has a least common multiple function (lcm) built-in which you can use: Prelude> foldl lcm 1 [1..10] 2520 If you prefer not to use ... * [26]Reasoning laziness votes: 10 I believe the problem is that Data.Vector is not strict in its elements. So although your reasoning is right, when accumulating the histogram your thunks looks like: <1+(1+(1+0)) (1+(1+0)) 0 0 (1+(1+(1+(1+0)))) ... > Rather than <3 2 0 0 4 ...> And only when you print are those sums computed. I don't see a strict accum function in the ... * [27]What are some compelling use cases of infinite data structures? votes: 13 Here are two examples, one big and one small: Why Functional Programming Matters by John Hughes has a good example, of a chess game. The move tree for a chess game is not actually infinite, but its big enough that it might as well be infinite (call it "near-infinite"). In a strict language you can't actually treat it as a tree, because there isn't enough ... * [28]Computational cost of applicative style votes: 14 I really suspect they'll be compiled down to the same thing in most cases. I did a tiny test, import Control.Applicative test1 :: (b -> b -> c) -> (a -> b) -> (a -> b) -> a -> c test1 bracket takeConn putConn pool = bracket (takeConn pool) (putConn pool) test2 :: (b -> b -> c) -> (a -> b) -> (a -> b) -> a -> ... Top StackOverflow Questions * what does "exists" mean in Haskell type system? (votes: 20, answers: 4) [29]read * Is there a book which teaches about compilers using Haskell? (votes: 12, answers: 3) [30]read * Reasoning laziness (votes: 10, answers: 1) [31]read * What are some compelling use cases of infinite data structures? (votes: 10, answers: 5) [32]read * The difference between +1 and -1 (votes: 9, answers: 3) [33]read About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. I'm in dire need of finding good "quotes of the week". If you happen to come across any, please don't hesitate to send it along. Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://article.gmane.org/gmane.comp.lang.haskell.general/18554 3. http://article.gmane.org/gmane.comp.lang.haskell.platform/1519 4. http://article.gmane.org/gmane.comp.lang.haskell.general/18543 5. http://www.reddit.com/r/haskell/comments/g1g0a/the_haskell_platform_20112_i… 6. http://hackage.haskell.org.nyud.net/platform/?2011-2 7. http://www.reddit.com/r/haskell/comments/fze1q/umm_yes_you_can_now_write_ne… 8. http://www.cs.berkeley.edu/~megacz/coq-in-ghc/ 9. http://www.reddit.com/r/haskell/comments/fz0ve/the_type_technology_tree_map… 10. http://blog.ezyang.com/2011/03/type-tech-tree/ 11. http://www.reddit.com/r/haskell/comments/g2m0t/a_message_from_tokyo/ 12. http://www.haskell.org/pipermail/glasgow-haskell-users/2011-March/020166.ht… 13. http://www.reddit.com/r/haskell/comments/g382d/haskells_niche_hard_problems/ 14. http://cdsmith.wordpress.com/2011/03/13/haskells-niche-hard-problems/ 15. http://www.reddit.com/r/haskell/comments/g2v75/functional_flocks_modelling_… 16. http://syntacticsalt.com/2011/03/10/functional-flocks/ 17. http://www.reddit.com/r/haskell/comments/g030j/tech_talk_video_faster_persi… 18. http://corp.galois.com/blog/2011/3/8/tech-talk-video-faster-persistent-data… 19. http://www.reddit.com/r/haskell/comments/g1ajf/recursion_is_a_lowlevel_oper… 20. http://blog.fogus.me/2011/03/09/recursion-is-a-low-level-operation/ 21. http://www.reddit.com/r/haskell/comments/fywf1/yesod_10_roadmap/ 22. http://docs.yesodweb.com/blog/road-to-yesod-1-0 23. http://www.reddit.com/r/haskell/comments/g2aiv/type_inference_weirdness/ 25. http://stackoverflow.com/questions/5250012/haskell-beginner-recursive-recur… 26. http://stackoverflow.com/questions/5274700/reasoning-laziness/5275489#52754… 27. http://stackoverflow.com/questions/5284458/what-are-some-compelling-use-cas… 28. http://stackoverflow.com/questions/5230043/computational-cost-of-applicativ… 29. http://stackoverflow.com/questions/5235116/what-does-exists-mean-in-haskell… 30. http://stackoverflow.com/questions/5234398/is-there-a-book-which-teaches-ab… 31. http://stackoverflow.com/questions/5274700/reasoning-laziness 32. http://stackoverflow.com/questions/5284458/what-are-some-compelling-use-cas… 33. http://stackoverflow.com/questions/5210182/the-difference-between-1-and-1

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