- Haskell-Cafe - Haskell.org

Why the reluctance to introduce the Functor requirement on Monad?
by Arlen Cuss 26 Jul '11

26 Jul '11

Hi cafe! I feel a bit like I'm speaking out of turn for bringing this up -- and I'm sure it must have been brought up many times before -- but I hope there can be something fruitful had from a discussion. In my travels I've read several people with much better grasp of Haskell than I have mention -- with a sad sigh of resignation -- that functions like liftM and return abound because some Monads don't state their fulfillment of Functor (or Applicative, but that's even more recent), and thus we can't use fmap/<$> or pure. I understand a motivation might be that code would break if the former lot were removed, but surely they could shifted to the latter (and the former simply be defined as the latter). It might be a very large effort, I suppose, to comb through the standard libraries and make everything compile again, but is there something else I'm surely missing? Cheers, A

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ICFP 2011: Call for participation
by Wouter Swierstra 26 Jul '11

26 Jul '11

===================================================================== Call for Participation The 16th ACM SIGPLAN International Conference on Functional Programming (ICFP 2011) http://www.icfpconference.org/icfp2011/ Tokyo, Japan September 19-21, 2011 ===================================================================== ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. The conference covers the entire spectrum of work, from practice to theory, including its peripheries. * Program: http://www.icfpconference.org/icfp2011/program.html * Registration link: https://regmaster3.com/2011conf/ICFP11/register.php * Local arrangements (including travel and accommodation): http://www.biglab.org/icfp11local/index.html Schedule including related events: September 18 Workshop on Generic Programming (WGP) Workshop on High-Level Parallel Programming and Applications (HLPP) Workshop on ML September 19-21 ICFP September 22 Commercial Users of Functional Programming – Day 1 (CUFP Tutorials) Haskell Symposium September 23 Commercial Users of Functional Programming – Day 2 (CUFP Tutorials) Erlang Workshop Haskell Implementors' Workshop September 24 Commercial Users of Functional Programming – Day 3 (CUFP Talks) Continuation Workshop Conference organizers: * General Co-Chairs: Manuel Chakravarty, University of New South Wales Zhenjiang Hu, National Institute of Informatics * Program Chair: Olivier Danvy, Aarhus University * Local Arrangements Chair: Soichiro Hidaka, National Institute of Informatics * Workshop Co-Chairs: Gabriele Keller, University of New South Wales Derek Dreyer, MPI-SWS * Programming Contest Chair: Eijiro Sumii, Tohoku University * Publicity Chair: Wouter Swierstra, Radboud Universiteit Nijmegen =====================================================================

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More QuickCheck Questions
by Mark Spezzano 26 Jul '11

26 Jul '11

Hi all, I still don't understand exactly how the QuickCheck "sized" function is meant to work, and what it's useful for. Please explain! Also, I think that class CoArbitrary can help to generate random *functions *(as opposed to values; but tell me if I'm wrong) but I don't see how this could be useful. Also what does shrink do? Example code of these would be tremendously helpful understanding these concepts. Thanks in advance, Mark Spezzano

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deriving ListLike instance
by bob zhang 26 Jul '11

26 Jul '11

Hi, all, newtype Stream a = Stream [a] I wanna derive ListLike [a] a automatically, but did not find a solution, I tried deriving instance ListLile (Stream a) a -- does not work Thank you.

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How to select last inserted record from Table Using Database.HSQL.MySQL
by Sergiy Nazarenko 25 Jul '11

25 Jul '11

Hi, everyone! trycon <- connect "mysql" "bigtables" "vasya" "*******" stmt' <- query trycon "INSERT INTO mytable (user,time,host,) VALUES ("Vasya","2011.07.30 11.59","foo")" I am beginner to use HSQL and I have a problem. I need to know row ID after INSERT into table. fetch stmt' returned False What should I do? Cheers!

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Slightly tangential topic about functional programming (specifically, Lambda-Calculus)
by Haskell Lippy 25 Jul '11

25 Jul '11

Hi everybody - I'm just starting to learn Haskell, and I figured it would be productive to first review my Lambda-Calculus that I picked up in college. Actually, we only touched upon it in college, so I'm effective learning it all over again for the first time. Anyway, I'm working my way through a textbook, and questions come up periodically, of course. My question - does anybody here belong to a forum, mailing list, or any other medium through which I might be able to engage some knowledgeable sources on Lambda-Calculus, and functional programming in general? (By the way, been hacking in Haskell for only two weeks and I'm already falling in love with it!) Thanks, and I look forward to 'chatting' with you all on more on-topic subjects in the future! Shaun shaunlippy(a)gmail.com

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Re: [Haskell-cafe] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by Thomas Schilling 25 Jul '11

25 Jul '11

> So is the change taking effect? > >> We were approaching consensus for the addition of of: >> infixr 6 <> > >> (<>) :: Monoid m => m -> m -> m >> (<>) = mappend >> and a matching change for (<+>) in the pretty package. > >> It was also suggested to make (<>) a method of Monoid and insert the following default definitions: >> (<>) = mappend >> mappend = (<>) > > > And how about the following: > (<<>>) = mconcat > mconcat = (<<>>) > > And "mempty" could be "id". The problems with this are: - mconcat is not a binary operator, so using <<>> would bring little advantage. You'd always have to use it in its bracketed form, for example. - Changing mempty to id could break a lot of existing code (e.g. due to typeclass ambiguity). This is the main reason. It's also not clear to me that this is a better name (it's just shorter). The current naming of Monoid methods is modelled after list operations, because list is a (the?) free monoid. However, in many cases "zero" or "one" would be an equally good name. I think (<>) is fairly uncontroversial because: 1. It's short 2. It's abstract. i.e., no intended pronunciation and it looks like LaTeX's \diamond operator 3. Changing it would be compatible with most existing libraries. For this reason, I think a larger change would have to come with a larger library re-organization. Johan Tibell suggested something like that a while ago: instead of lots of little cuts (backwards incompatible changes), a working group of activists should redesign a whole new (incompatible) alternative set of core libraries. > > > On Sun, Jul 24, 2011 at 11:39 AM, Thomas Schilling > <nominolo(a)googlemail.com> wrote: >> Yes, this has sort-of been agreed upon in a GHC ticket about a year >> ago: http://hackage.haskell.org/trac/ghc/ticket/3339 >> >> I had a patch in Darcs, but then came the switch to Git. I ported it >> to Git, but didn't iron out all the issues. That was quite a while >> ago so it's currently a bit bitrotten. I don't think it's enough time >> to get it into 7.2, but since that's largely an unstable release, it >> wouldn't hurt until 7.4. I'll probably work on it at CamHac in a few >> weeks. > > > > -- > -- > Regards, > KC > -- Push the envelope. Watch it bend.

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Web application design advice
by Sævar Berg 25 Jul '11

25 Jul '11

Hello, Café. I am really not fond of web development, but being able to do it in my favorite language is interesting, so I've been spending some time taking a look at happstack. I decided to start a concrete project so that I could work towards some goal in my learning process, and I have decided on writing an inventory management system for this small computer workshop I work in as a volunteer. We probably have several metric tonnes of various computer equipment; interntal hardware(RAM, hard drives, graphics card, processors and so on) to peripherals and even non-computer related stuff, and it would be good to be able to catalog it and know what exactly we have and so on. Anyway, my question is about the design itself. My original idea for the database was to create a data type, something like data Hardware = Ram { ramType :: RamType , ramSize :: Int ... } data RamType = SDRAM | DDR | DDR2 | DDR3 .. The obvious advantage to this is type safety and thus having an ORM database, so to speak(Please forgive me if my web dev lingo is off here, this is *really *not my forté). However, a big disadvantage is that we might want to add new 'type' of hardware to the database and this would require me to stop the server, modify the haskell code to add a new type constructor and so on. On top of that, since a graphics card has different attributes than RAM, I will need to dynamically generate the forms required to add new hardware to the database or to search for hardware in the database. Due to this, I had another idea. I would create a state type that would merely represent groups of items using a map of some kind, where the group name would be a string. Maybe something like this: data HardwareGroup = HardwareGroup { groupName :: String -- Valid key values for the map , groupValues :: [String] , groupItems :: [Data.Map String String] } Which means I can create a new group like this: type Hardware = [HardwareGroups] hardwareDatabase :: Hardware hardwareDatabase = [] newGroup = HardwareGroup "RAM" ["ramSize", "ramSpeed", "ramType", "ramName"] Data.Map.empty -- Nothing if some value in map isn't a valid part of addToGroup :: HardwareGroup -> Data.Map String String -> Maybe HardwareGroup addToGroup (HardwareGroup name values items) m = case (any (flip notElem values) $ keys m) of True -> return Nothing _ -> return $ HardwareGroup name values (m : items) addToGroup newGroup (fromList [("ramSize", "512"), ("ramSpeed", "400 Mhz"), ("ramType", "DDR"), ("ramName", "Kingston")]) The code may not be correct. I wrote it in this email and didn't test it, but I assume you guys get what I'm going for here. This will enable me to dynamically generate forms in an easy manner and add to / search in the database, without having to think about some types being strings and some types being ints or something entirely different. The reason I've come here asking this question and not some web design mailing list is because I'm writing this in Haskell, and I was wondering if you guys could see some disadvantages to either design that I may not have foreseen, or even come up with other suggestions. I would just really appreciate any hints, tips, pros or cons to either design. Am I losing something big by sacrificing type safety? Or is the first idea unwieldy enough to justify the sacrifices? Thanks a lot in advance guys.

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Re: [Haskell-cafe] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by wren ng thornton 25 Jul '11

25 Jul '11

On 7/24/11 10:09 PM, Sebastian Fischer wrote: >> >> because list is a (the?) free monoid. > > Yes, all free monoids are isomorphic (to lists). > > Sebastian For completeness... The free monoid over a set S is the set of all finite sequences of elements drawn from S. Often this is written with the Kleene star: S^*. Finite lists are a particular representation for the free monoid (namely with all operations associated to the right), though this representation may not be "isomorphic enough" for certain analyses (e.g., algorithmic complexity). Note that the free semigroup over a set S is the set of all finite non-empty sequences of elements drawn from S. We can name this by S^+. Other than the fact that S^+ does not contain the empty sequence, the other important difference is that, while both S^* and S^+ are both closed under the operation, only S^* is co-closed under the operation (i.e., not all elements of S^+ can be decomposed into two elements of S^+ joined by the operator). -- Live well, ~wren

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Re: [Haskell-cafe] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by Maciej Wos 25 Jul '11

25 Jul '11

Personally, I have nothing against mempty (although I agree that mid makes more sense), but I don't like mappend at all. I wonder what happened to the idea of using <> instead of mappend (that's what I always do). I think a <> b <> c looks so much better than a `mappend` b `mappend` c and it solves the name problem altogether. -- Maciej On Jul 24, 2011 3:42 AM, "KC" <kc1956(a)gmail.com> wrote:

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Re: [Haskell-cafe] [Haskell-beginners] For class Monoid; better names than mempty & mappend might have been: mid (mident) & mbinop
by wren ng thornton 25 Jul '11

25 Jul '11

On 7/24/11 2:29 PM, Julian Porter wrote: > On 24 Jul 2011, at 19:19, KC wrote: >> A lot of math books use "+" or "x" enclosed in a circle to indicate >> that the usual meaning of "+" nor "x" is intended for the meaning of >> the binary operation. > > Er no. Both symbols have extremely precise meanings. $\oplus$ is the direct sum of two modules and $\otimes$ is their tensor product. Well, they mean that if you're off in module/vector land; but they don't mean that everywhere. Another place they're used frequently is for semirings: with $\oplus$ the associative monoid and $\otimes$ the other monoid. While the tourism board for semirings is less well funded than the one for vectors, it's a beautiful locale with rich history. Natural language processing, graph theory, Boolean algebras, and related logics are nothing to scoff at. -- Live well, ~wren

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Re: [Haskell-cafe] ANNOUNCE: Haskell in the Cloud (http://quid2.org)
by Pasqualino "Titto" Assini 25 Jul '11

25 Jul '11

Hi David, no there is no direct relationship to distributed haskell. Quid2's focus is not so much on extending Haskell itself and its runtime to work in a distributed fashion but rather to provide a global naming system for functional values. Very similar to the Web, whose main innovation is to provides a global naming system for all kind of resources that previously had only local names (/dir/myfile versus http://ww.myserver.com/myfile) A description of Quid2 architecture will soon be available and it will be clearer how is meant to work. It will be far from complete, a working functional web will only emerge if many different ideas converge in a coherent whole. Best, titto > On 22 July 2011 15:57, David Barbour <dmbarbour(a)gmail.com> wrote: >> Any relationship to distributed haskell? >> http://www.macs.hw.ac.uk/~dsg/gdh/ >> >> On Fri, Jul 22, 2011 at 3:00 AM, Pasqualino "Titto" Assini >> <tittoassini(a)gmail.com> wrote: >>> >>> Fellow Haskeller, >>> >>> Has your strongly typed, quick checked, formally verified, Oleg blessed, >>> higher order monadic code become a little bit too predictable? >>> >>> Are you feeling a bit bored, emotionally drained and disillusioned with >>> perfection? >>> >>> Longing for the days when Men were Men (and Women were Women for that >>> matter) and squashed bugs out of Assembler code with just their bare hands >>> and a line editor? >>> >>> Enter Quid2 [1]: the half baked, barely tested, totally unsafe and >>> spectacularly unoptimised Haskell in the Cloud system. >>> >>> A delightful blend of Haskell crispiness and JavaScript fluffiness, Quid2 >>> guarantees hours and hours of guilt free hacking enjoyment. >>> >>> Let a soft lamb(a)da rhythm [2] take you over as you gingerly step onto >>> the Haskell Cloud, it's time to party as if it were 1991 all over again! >>> >>> Yours truly, >>> >>> titto >>> >>> [1] http://quid2.org >>> >>> [2] http://www.youtube.com/watch?v=i8mz9uOvFQA >>> >>> -- >>> Dr. Pasqualino "Titto" Assini >>> http://quid2.org/ >>> >>> >>> >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> >> > > > > -- > Dr. Pasqualino "Titto" Assini > http://quid2.org/ > -- Dr. Pasqualino "Titto" Assini http://quid2.org/

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ANNOUNCE: Haskell in the Cloud (http://quid2.org)
by Pasqualino "Titto" Assini 25 Jul '11

25 Jul '11

Fellow Haskeller, Has your strongly typed, quick checked, formally verified, Oleg blessed, higher order monadic code become a little bit too predictable? Are you feeling a bit bored, emotionally drained and disillusioned with perfection? Longing for the days when Men were Men (and Women were Women for that matter) and squashed bugs out of Assembler code with just their bare hands and a line editor? Enter Quid2 <http://quid2.org> [1]: the half baked, barely tested, totally unsafe and spectacularly unoptimised Haskell in the Cloud system. A delightful blend of Haskell crispiness and JavaScript fluffiness, Quid2 guarantees hours and hours of guilt free hacking enjoyment. Let a soft lamb(a)da rhythm <http://www.youtube.com/watch?v=i8mz9uOvFQA> [2] take you over as you gingerly step onto the Haskell Cloud, it's time to party as if it were 1991 all over again! Yours truly, titto [1] http://quid2.org [2] http://www.youtube.com/watch?v=i8mz9uOvFQA -- Dr. Pasqualino "Titto" Assini http://quid2.org/

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ANN: shelltestrunner 1.0
by Simon Michael 24 Jul '11

24 Jul '11

I'm pleased to announce the 1.0 release of shelltestrunner! Home page: http://joyful.com/repos/shelltestrunner Install: $ cabal install shelltestrunner shelltestrunner tests command-line programs or arbitrary shell commands. It reads simple declarative tests specifying a command, some input, and the expected output, error output and exit status. Tests can be run selectively, in parallel, with a timeout, in color, and/or with differences highlighted. Projects using it include hledger, yesod, and berp. shelltestrunner is free software released under GPLv3+. I started shelltestrunner two years ago, inspired by John Wiegley's ledger tests. John Macfarlane, Bernie Pope and Trygve Laugst?l have contributed code. The hackage page shows the libraries it relies on - most notably, Max Bolingbroke's test-framework. The site uses hakyll, pandoc & hamlet. New in 1.0: * New home page/docs * The `>>>=` field is now required; you may need to add it to your existing tests * Input and expected output can now contain lines beginning with `#` * Multiple tests in a file may now have whitespace between them * The `-i/--implicit` option has been dropped * New `-d/--diff` option shows test failures as a unified diff when possible, including line numbers to help locate the problem * New `-x/--exclude` option skips certain test files (eg platform-specific ones) * Passing arguments through to test-framework is now more robust * Fixed: parsing could fail when input contained left angle brackets * Fixed: some test files generated an extra blank test at the end

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Cloud Haskell
by Tom Murphy 24 Jul '11

24 Jul '11

Is anyone using Cloud Haskell yet? I'm really excited by the possibilities. Tom

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Re: [Haskell-cafe] Monad for binary tree data structure
by wren ng thornton 23 Jul '11

23 Jul '11

On 7/23/11 1:31 AM, Александр wrote: > Hello, > > I built binary tree with: > > data Tree a = Empty > | Node a (Tree a) (Tree a) > deriving (Eq, Ord, Read, Show) > > How can i make Monad type class instance for this tree? And can i make it on not? Of course you can. One way of thinking about monads is that it's just performing substitution, aka tree grafting. In this case, because the "variables" being substituted for are located within the tree, we need to make sure to distribute the substructure of the original tree over the result of substituting for the variable. Thus, we can define: data Tree a = Empty | Node a (Tree a) (Tree a) deriving (Eq, Ord, Read, Show) -- This one should be obvious. instance Functor Tree where fmap f Empty = Empty fmap f (Node x l r) = Node (f x) (fmap f l) (fmap f r) -- | The tree @graft gl gr t@ is the result of grafting @gl@ in -- place of all left @Empty@ leaves and @gr@ in place of all -- right @Empty@ leaves; with the empty tree mapped to itself. graft :: Tree a -> Tree a -> Tree a -> Tree a graft gl gr = go where go Empty = Empty go (Node x Empty Empty) = Node x gl gr go (Node x Empty r ) = Node x gl (go r) go (Node x l Empty) = Node x (go l) gr go (Node x l r ) = Node x (go l) (go r) instance Monad Tree where return x = Node x Empty Empty Empty >>= _ = Empty Node x l r >>= f = graft (l >>= f) (r >>= f) (f x) And we can verify the correctness by using QuickCheck or lazy SmallCheck: import Control.Monad ((<=<)) import Test.QuickCheck instance Arbitrary a => Arbitrary (Tree a) where arbitrary = do b <- elements [True,False] if b then return Empty else do x <- arbitrary l <- arbitrary r <- arbitrary return (Node x l r) -- | Extensional function equality. (f === g) x = f x == g x prop_mapIdentity = fmap id === id prop_mapCompose f g = (fmap f . fmap g) === fmap (f . g) prop_leftUnit f = (return <=< f) === f prop_rightUnit f = (f <=< return) === f prop_assoc f g h = (f <=< (g <=< h)) === ((f <=< g) <=< h) Of course, there are other monads as well. In particular, for any semigroup on A there is a monad for Tree A. For these instances we would want to define a merging function: -- | The first argument is an associative operation for resolving -- conflicts in merging. Non-conflicts are resolved by assuming -- an identity element for the operation, thus lifting it into a -- monoid. merge :: (a -> a -> a) -> Tree a -> Tree a -> Tree a merge _ Empty tr = tr merge _ tl Empty = tl merge f (Node x xl xr) (Node y yl yr) = Node (f x y) (merge f xl yl) (merge f xr yr) Of course, there are only two semigroups which are parametric in A, hence there are only two more Monad instances we can define this way. newtype Tree2 a = T2 (Tree a) deriving (Eq, Ord, Read, Show) instance Monad Tree2 where return x = T2 $ Node x Empty Empty T2 Empty >>= _ = T2 Empty T2(Node x xl xr) >>= f = case f x of T2 Empty -> T2 Empty T2 (Node y yl yr) -> T2 $ Node y (merge' yl (T2 xl >>= f)) (merge' yr (T2 xr >>= f)) where merge' xs (T2 ys) = merge const xs ys instance Arbitrary a => Arbitrary (Tree2 a) where arbitrary = arbitrary >>= (return . T2) And we can similarly define Tree3 with (flip const) in place of const. All the other monads formed this way (with any semigroup operation in place of const) would require using a type class for restricted monads since we need to restrict the element type to support the semigroup operation we want to use. -- Live well, ~wren

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Re: [Haskell-cafe] question regarding the $ apply operator
by wren ng thornton 23 Jul '11

23 Jul '11

On 7/23/11 9:37 AM, Ting Lei wrote: > > I know the Reverse Polish is not a couple of hundred years old. > I have an impression of reading something about people writing natural > deduction systems using only dots in place of parenthesis. And it is > said that it was "natural" in those pre-historic times. The dot notation was used in Principia Mathematica (1910--1913), where differing numbers of dots represented differing levels of grouping (N.B., not quite the same as depth of grouping, i.e. count of parentheses, as indicated by expressions such as p . = . q V r where, because the right dot is required for grouping the disjunct, we must also introduce the left dot in order to ensure that the p, the equality symbol, and the disjunctive expression are all on the same level). The dots were also used to represent logical conjunction. However parentheses, brackets, and braces do show up occasionally without much explanation as to their meaning. Whitehead and Russell claim to have taken the dot notation from Peano. In fact, the dot notation is the origin of the period in the current notation for binding forms like lambda, Pi, Sigma, forall, and exists; as well as, IIRC, the origin of the colon for typing expressions and set comprehensions. The PM notation for quantifications were (x) . P (exists x) . P and more-modern philosophical texts retain this notation sans the period. It is by far the most baffling notation IMO. The other popular notation in modern philosophical texts is to replace the period by bracketing, leading to forall x[P] exists x[P] Note that an apparent variant latter with the forall elided, is actually a notation for expressing an open term P (N.B., not an abstraction) which dates back to Frege. -- Live well, ~wren

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OpenGL vs OpenGLRaw
by Yves Parès 23 Jul '11

23 Jul '11

Hello Café, Where do you people stand on using OpenGLRaw instead of the higher-level layer? I saw that the ports of the nehe tutorial use directly OpenGLRaw, and I wondered why that choice had been made.

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Reactive Programming in Machine Learning
by bob zhang 23 Jul '11

23 Jul '11

Hi all, I am doing a survey on combining Functional Reactive Programming and Machine Learning. Has anyone did relevant research on this topic? Any discussion or link is appreciable. Best,bob

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Monad for binary tree data structure
by Александр 23 Jul '11

23 Jul '11

Hello, I built binary tree with: data Tree a = Empty | Node a (Tree a) (Tree a) deriving (Eq, Ord, Read, Show) How can i make Monad type class instance for this tree? And can i make it on not? i try: instance Monad Tree where return x = Node x Empty Empty Empty >>= f = Empty (Node x Empty Empty) >>= f = f x But i can't make (>>=) for Node x left right. Thank you.

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question regarding the $ apply operator
by Ting Lei 23 Jul '11

23 Jul '11

Hi I have a naive question regarding the basic use of the $ operator, and I am confused why certain times it doesn't seem to work. e.g. The following works: applySkip i f ls = (take i) ls ++ f (drop i ls) But the following doesn't: applySkip i f ls = (take i) ls ++ f $ drop i ls which gives me an error: The first argument of ($) takes one argument, but its type `[a0]' has none In the expression: (take i) ls ++ f $ drop i ls In an equation for `applySkip': applySkip i f ls = (take i) ls ++ f $ drop i ls FYI, I am using GHC 7.0.3 for this. Thanks in advance for comments. Ting

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Github support for cabal files
by Joris Putcuyps 23 Jul '11

23 Jul '11

Hello everybody I'm looking for a way to avoid having two files with almost the same information on github: README.markdown and package.cabal. As far as I can tell, a new github README parser[1] can be written, but it only supports README files with different extensions and requires either ruby or a script which converts stdin into html. Another solution would be that haddock supports a README file, just like github, as the index.html of the cabal package. Github houses a lot of cabal packages so more people could benefit from this solution. Greetings Joris [1]: https://github.com/github/markup

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ghc on arm status
by Sergey Mironov 23 Jul '11

23 Jul '11

Hi. I was searching for info about building ghc on ARM arch. I already know about [1] approach, and also saw debian binaries [2], but I am afraid I have to compile ghc by myself this time, since our system uses incompatible libc, so my question is addressed to gentoo users and sympathetic :) Did anybody see any working (currently or in the past) dev-lang/ghc arm ebuild? Thanks, Sergey [1] - http://hackage.haskell.org/trac/ghc/wiki/ArmLinuxGhc [2] - http://packages.debian.org/sid/ghc6

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Re: [Haskell-cafe] Monad for binary tree data structure
by Александр 23 Jul '11

23 Jul '11

Hello, >1) Do you really need a Monad instance for this? Only for training purposes. 2) One possibility is just have it being (Node x _ _) >>= f = f x I've already tried to do so, but i get only 1 element. Look. I have a function fillTree that's fill this binary tree. let a = fillTree 1 Empty a >>= \x -> return (x * 2) I got: Node 2 Empty Empty But i have tree with 100 elements. Thank you. Thank you.

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ANNOUNCE: dbus-core 0.9
by John Millikin 23 Jul '11

23 Jul '11

================================================ D-Bus implementation for Haskell (dbus-core_0.9) ================================================ This is the first significant release to dbus-core in a while, and it contains some significant improvements to both the API and implementation. Users are advised to upgrade, as the 0.8 branch will no longer be receiving improvements or corrections. * Release announcement: https://john-millikin.com/releases/dbus-core/0.9/ * API documentation: http://hackage.haskell.org/package/dbus-core * Source code (literate PDF): https://john-millikin.com/downloads/dbus-core_0.9.pdf * Source code (tarball): https://john-millikin.com/downloads/dbus-core_0.9.tar.gz Changes in release 0.9 ====================== The biggest change in this release is that dbus-client has been merged into dbus-core. The original purpose of dbus-client (as a very high-level, often-updated support library) never materialized, and the split packaging caused issues with dependencies and versioning. The parts of dbus-client that were commonly used were moved into the DBus.Client and DBus.Client.Simple modules. Users are highly encouraged to use these modules in their applications. Other changes and improvements include: * The Array, Dictionary, and Structure types are no longer needed for most users. You can cast containers directly to corresponding standard types, such as Vector and Map. * Smart constructors for magic strings have been renamed so they're easier to read and type; for example, mkBusName is now just busName. * Support for custom transports and authentication mechanisms. * Removed some seldom-used modules, such as MatchRule and NameReservation. Their functionality has been merged into the new client modules. * Performance improvements for marshaling and unmarshaling messages. How to use the new simple client API ==================================== The new API makes calling and exporting methods very easy; take a look at these examples! Getting a list of connected clients ----------------------------------- {-# LANGUAGE OverloadedStrings #-} module Main (main) where import Data.List (sort) import DBus.Client.Simple main :: IO () main = do client <- connectSession -- Request a list of connected clients from the bus bus <- proxy client "org.freedesktop.DBus" "/org/freedesktop/DBus" reply <- call bus "org.freedesktop.DBus" "ListNames" [] -- org.freedesktop.DBus.ListNames returns a single value, which is -- a list of names. let Just names = fromVariant (reply !! 0) -- Print each name on a line, sorted so reserved names are below -- temporary names. mapM_ putStrLn (sort names) Exporting methods ----------------- {-# LANGUAGE OverloadedStrings #-} module Main (main) where import Control.Concurrent (threadDelay) import Control.Monad (forever) import DBus.Client.Simple onEcho :: String -> IO String onEcho str = do putStrLn str return str onDivide :: Double -> Double -> IO Double onDivide x y = if y == 0 then throwError "com.example.DivisionByZero" "Divided by zero" [] else return (x / y) main :: IO () main = do client <- connectSession -- Request a unique name on the bus. If the name is already -- in use, continue without it. requestName client "com.example.ExportExample" [] -- Export an example object at "/divide" export client "/divide" [ method "com.example.Echo" "Echo" onEcho , method "com.example.Math" "Divide" onDivide ] -- Wait forever for method calls forever (threadDelay 50000)

1 0

Simple GADTs, type families and type classes combination with type error.
by Serguey Zefirov 22 Jul '11

22 Jul '11

Why does GHC complains on the code below ? (I'll explain in a second a requirement to do just so) I get errors with ghc 6.12.1 and 7.0.2. --------------------------------------------------------------------------------------------------------------------- {-# LANGUAGE GADTs, TypeFamilies #-} class CPU cpu where type CPUFunc cpu data Expr cpu where EVar :: String -> Expr cpu EFunc :: CPU cpu => CPUFunc cpu -> Expr cpu class CPU cpu => FuncVars cpu where funcVars :: CPUFunc cpu -> [String] exprVars :: FuncVars cpu => Expr cpu -> [String] exprVars (EVar v) = [v] -- an offending line: exprVars (EFunc f) = funcVars f --------------------------------------------------------------------------------------------------------------------- I tried to split creation and analysis constraints. CPU required for creation of expressions, FuncVars required for analysis. It all looks nice but didn't work. (In our real code EVar is slightly more complicated, featuring "Var cpu" argument) It looks like GHC cannot relate parameters "inside" and "outside" of GADT constructor. Not that I hesitate to add a method to a CPU class, but I think it is not the right thing to do. So if I can split my task into two classes, I will feel better.

4 6

Wrong Answer in SPOJ 6044. Minimum Diameter Circle
by mukesh tiwari 22 Jul '11

22 Jul '11

Hello all This is my first post to Haskell-cafe so i am not aware of protocols here and pardon me for my stupidity . I am trying to solve this problem [ https://www.spoj.pl/problems/QCJ4 ] but getting wrong answer . I implemented the algorithm by Pr. Chrystal described here [ http://www.personal.kent.edu/~rmuhamma/Compgeometry/MyCG/CG-Applets/Center/… ] . I tested my convex hull code at this site and its accepted for problem [ http://www.spoj.pl/problems/GARDENHU ] so i think its correct but i am not sure if i have implemented Chrystal's algorithm correctly . Could some please tell me if i have implemented the algorithm correctly and if its possible then why i am getting wrong answer for this problem . In case of indentation problem [ http://ideone.com/perhE ] Thank you Mukesh Tiwari import Data.List import qualified Data.Sequence as DS import Text.Printf data Point a = P a a deriving ( Show , Ord , Eq ) data Turn = S | L | R deriving ( Show , Eq , Ord , Enum ) -- straight left right --start of convex hull http://en.wikipedia.org/wiki/Graham_scan compPoint :: ( Num a , Ord a ) => Point a -> Point a -> Ordering compPoint ( P x1 y1 ) ( P x2 y2 ) | compare x1 x2 == EQ = compare y1 y2 | otherwise = compare x1 x2 findMinx :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] findMinx xs = sortBy ( \x y -> compPoint x y ) xs compAngle ::(Num a , Ord a ) => Point a -> Point a -> Point a -> Ordering compAngle ( P x1 y1 ) ( P x2 y2 ) ( P x0 y0 ) = compare ( ( y1 - y0 ) * ( x2 - x0 ) ) ( ( y2 - y0) * ( x1 - x0 ) ) sortByangle :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] sortByangle (z:xs) = z : sortBy ( \x y -> compAngle x y z ) xs convexHull ::( Num a , Ord a ) => [ Point a ] -> [ Point a ] convexHull [ P x0 y0 ] = [ P x0 y0 ] convexHull xs = reverse . findHull [y,x] $ ys where (x:y:ys) = sortByangle.findMinx $ xs findTurn :: ( Num a , Ord a , Eq a ) => Point a -> Point a -> Point a - > Turn findTurn ( P x0 y0 ) ( P x1 y1 ) ( P x2 y2 ) | ( y1 - y0 ) * ( x2- x0 ) < ( y2 - y0 ) * ( x1 - x0 ) = L | ( y1 - y0 ) * ( x2- x0 ) == ( y2 - y0 ) * ( x1 - x0 ) = S | otherwise = R findHull :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] -> [ Point a ] findHull [x] ( z : ys ) = findHull [ z , x ] ys --incase of second point on line from x to z findHull xs [] = xs findHull ( y : x : xs ) ( z:ys ) | findTurn x y z == R = findHull ( x : xs ) ( z:ys ) | findTurn x y z == S = findHull ( x : xs ) ( z:ys ) | otherwise = findHull ( z : y : x : xs ) ys --end of convex hull --start of finding point algorithm http://www.personal.kent.edu/~rmuhamma/Compgeometry/MyCG/CG-Applets/Center/… Applet’s Algorithm findAngle :: ( Num a , Ord a , Floating a ) => Point a -> Point a -> Point a -> ( Point a , Point a , Point a , a ) findAngle u@(P x0 y0 ) v@(P x1 y1 ) t@(P x2 y2) | u == t || v == t = ( u , v , t , 10 * pi ) -- two points are same so set the angle more than pi | otherwise = ( u , v, t , ang ) where ang = acos ( ( b + c - a ) / ( 2 * sb * sc ) ) where b = ( x0 - x2 ) ^ 2 + ( y0 - y2 ) ^ 2 c = ( x1 - x2 ) ^ 2 + ( y1 - y2 ) ^ 2 a = ( x0 - x1 ) ^ 2 + ( y0 - y1 ) ^ 2 sb = sqrt b sc = sqrt c findPoints :: ( Num a , Ord a , Floating a ) => Point a -> Point a -> [ Point a ] -> ( Point a , Point a , Point a , a ) findPoints u v xs | 2 * theta >= pi = ( a , b , t , theta ) | and [ 2 * alpha <= pi , 2 * beta <= pi ] = ( a , b , t , theta ) | otherwise = if 2 * alpha > pi then findPoints v t xs else findPoints u t xs where ( a , b , t , theta ) = minimumBy ( \(_,_,_, t1 ) ( _ , _ , _ ,t2 ) - > compare t1 t2 ) . map ( findAngle u v ) $ xs ( _ , _ , _ , alpha ) = findAngle v t u --angle between v u t angle subtended at u by v t ( _ , _ , _ , beta ) = findAngle u t v -- angle between u v t angle subtended at v by u t --end of finding three points algorithm --find the circle through three points http://paulbourke.net/geometry/circlefrom3/ circlePoints :: ( Num a , Ord a , Floating a ) => Point a -> Point a - > Point a -> ( Point a , a ) --( center , radius ) circlePoints u@(P x1 y1 ) v@(P x2 y2 ) t@(P x3 y3 ) | x2 == x1 = circlePoints u t v | x3 == x2 = circlePoints v u t | otherwise = ( P x y , 2 * r ) where m1 = ( y2 - y1 ) / ( x2 - x1 ) m2 = ( y3 - y2 ) / ( x3 - x2 ) x = ( m1 * m2 * ( y1 - y3 ) + m2 * ( x1 + x2 ) - m1 * ( x2 + x3 ) ) / ( 2 * ( m2 - m1 ) ) y = if y2 /= y1 then ( ( x1 + x2 - 2 * x ) / 2 * m1 ) + ( ( y1 + y2 ) / 2.0 ) else ( ( x2 + x3 - 2 * x ) / 2 * m2 ) + ( ( y2 + y3 ) / 2.0 ) r = sqrt $ ( x - x1 ) ^2 + ( y - y1 ) ^ 2 --end of circle through three points --start of SPOJ code format::(Num a , Ord a ) => [[a]] -> [Point a] format xs = map (\[x0 , y0] -> P x0 y0 ) xs readInt ::( Num a , Read a ) => String -> a readInt = read solve :: ( Num a , Ord a , Floating a ) => [ Point a ] -> ( Point a , Point a , Point a , a ) solve [ P x0 y0 ] = ( P x0 y0 , P x0 y0 , P x0 y0 , 0 ) --in case of one point solve [ P x0 y0 , P x1 y1 ] = ( P x0 y0 , P x0 y0 , P x0 y0 , sqrt $ ( x0 - x1 ) ^ 2 + ( y0 - y1 ) ^2 ) -- in case of two points the solve xs = findPoints x y t where t@( x : y : ys ) = convexHull xs final :: ( Num a , Ord a , Floating a ) => ( Point a , Point a , Point a , a ) -> a final ( u , v , t , w ) | w == 0 = 0 | and [ u == v , v == t ] = w | otherwise = r where ( P x y , r ) = circlePoints u v t main = interact $ ( printf "%.2f\n" :: Double -> String ) . final . solve . convexHull . format . map ( map readInt . words ) . tail . lines

1 0

Haskell Parallel Digest 4
by Nicolas Wu 22 Jul '11

22 Jul '11

Parallel Haskell Digest ======================= Edition 4 2011-07-22 http://www.well-typed.com/blog/55 Hello Haskellers! It's time for the fourth edition of the Parallel Haskell Digest, bringing you a summary of the news and discussions of parallelism and concurrency in Haskell. The digest is made possible by the [Parallel GHC Project][0]. News ---------------------------------------------------------------------- * The Monad.Reader: special issue on parallelism and concurrency In the words of [The Monad Reader][6] itself: > Whether you're an established academic or have only just > started learning Haskell, if you have something to say, please > consider writing an article for The Monad.Reader! Issue 19 > will be a special issue focusing on articles related to > parallelism and concurrency, construed broadly. The submission > deadline for Issue 19 will be: Tuesday, August 16. * Threadscope Implementor's Summit The Threadscope implementor's summit was held this month at Microsoft Research, Cambridge. The summit brought together developers who are currently working with Threadscope, whether that be hacking on generating the events that are emitted by GHC for analysis in Threadscope, using the event trace that is produced for detailed profiling information, or working on improving Threadscope itself to provide better tools for parallel profile analysis. The meeting was full of ideas, and covered topics such as: adding extensions to the current eventlog format to enable additional information to be tagged to events; improving the visualisation of information in Threadscope; formalising the transitions of thread states into a finite state machine; and matching up executed code with corresponding source locations. With all this food for thought, we should expect plenty of interesting work in this area. Word of the Month ---------------------------------------------------------------------- In this issue we have two words of the month: *par* and *pseq*. Haskell provides two annotations, `par` and `pseq`, that allow the programmer to give hints to the compiler about where there are opportunities to exploit parallelism. While these annotations are not typically used directly by programmers, it is useful to understand them because they are the underlying mechanism for higher level tools such as "parallel strategies" and the Eval monad. The two combinators have the following signatures: par :: a -> b -> b pseq :: a -> b -> b While their signatures are the same, they are used to annotate different things. The `par` combinator hints to the Haskell implementation that it might be beneficial to evaluate the first argument in parallel. However, since Haskell does not impose an evaluation order, we also need `pseq`, which instructs the compiler to ensure that its first argument is evaluated before the second. Let's take a look at an example inspired by [Parallel Performance Tuning for Haskell][24] by Jones, Marlow, and Singh, which illustrates this nicely. Suppose you're interested in the sum of two expensive computations. To keep things simple, we'll use a naive implementation of `fib` (the point here isn't to have an efficient computation, I'm trying to show an *expensive* one): fib :: Int -> Int fib 0 = 0 fib 1 = 1 fib n = fib (n-1) + fib (n-2) For a second expensive computation, we'll calculate the negafibonacci number, which works on negative numbers: negafib :: Int -> Int negafib 0 = 0 negafib (-1) = 1 negafib n = nfib (n+2) - nfib (n+1) The sum of these two can be calculated by the following sequential function: sumfib :: Int -> Int sumfib n = x + y where x = fib n y = negafib (-n) There's obvious room for improvement here when we have two cores: we simply calculate the expensive computations on separate cores. Annotating the code above is a fairly simple process. We first use `par` to annotate the fact that `x` must be calculated in parallel with the rest of the computation. Second, we ensure that `y` gets computed before `x + y` by annotating with `pseq`. The result is as follows: import Control.Parallel (par, pseq) psumfib :: Int -> Int psumfib n = x `par` (y `pseq` x + y) where x = fib n y = negafib (-n) We can write a simple program that outputs the result of running this computation with the following `main` function: main :: IO () main = putStrLn . show . sumfib $ 37 We should hope for the parallel version to work twice as fast, since the two expensive functions should take about the same time to compute. Here's the output of compiling and running the sequential version of the program: $ ghc -rtsopts Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... $ time ./Main real 0m6.113s user 0m6.090s sys 0m0.010s Now replacing `sumfib` with `psumfib` produces the following results: $ ghc -rtsopts -threaded Main.hs [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... $ time ./Main +RTS -N2 real 0m3.402s user 0m6.660s sys 0m0.040s This is obviously a very trivial example, but the point is that annotations provide a powerful way of expressing parallel algorithms. It's interesting to note that for this simple program, the timings for the parallel version on a single core performs as well as the single core version compiled without threading. While annotations are a simple mechanism for expressing where parallelism might be exploited in a program, beware that there are a number of pitfalls to using this technique: all that glitters is not gold! The main difficulty in using `par` and `pseq` directly is that you really need to have a clear understanding of evaluation order. In particular, unless you understand what laziness is doing to the evaluation order, then you might find that the computations you're sparking off with `par` might not occur when you expected they should. Then there are all the general difficulties that you face with parallel programming, like getting the right granularity of work to do in parallel. Using `par` is quite lightweight so can be used to exploit reasonably fine grained parallelism, but it is certainly not free. Finally, parallel performance suffers when there are too many garbage collections, so keeping this to a minimum by either using more efficient data structures or increasing available memory, becomes an important factor. Nevertheless, it's well worth having a play with `par` and `pseq`. The next step after that is to look at parallel strategies. Strategies is a layer of abstraction on top of `par` and `pseq`. You might like to to read [Seq no more: Better Strategies for Parallel Haskell][1], by Simon Marlow et al. which describes Strategies and the Eval monad. It's all available in the [parallel library][2] on Hackage. More recently, the `Par` monad has also been introduced as yet another way of describing parallel evaluations. These key topics will no doubt feature in a future word of the month, so stay tuned! Parallel GHC Project Update ---------------------------------------------------------------------- The Parallel GHC Project is an MSR-funded project to push the real-world use of parallel Haskell. The aim is to demonstrate that parallel Haskell can be employed successfully in industrial projects. This month we're having a guest column from the team over at Los Alamos National Laboratory, one of the partners involved in the project (you can see the full details in report LA-UR 11-0341). They have been working on writing Monte Carlo physics simulations in Haskell, which has given them high levels of parallelism, along with useful tools for abstraction. So, without further ado, over to Michael Buksas from LANL: Our goal is to build highly efficient Monte Carlo physics simulations using parallel Haskell. We're focusing on SMP performance though some combination of explicit threading and pure parallel annotations. The Monte Carlo approach involves randomly sampling the space of solutions to generate data which contributes to the solution. For these physical problems, our samples are the tracks of particles as they move through space, interacting with a physical material as they go. Data collected from each particle trajectory is then combined into information needed to compute the solution. For example, the detailed information about the particle's interaction with the material is collected into a collective effect on the material properties. To date, we have a code base which includes two approaches to the problem. One is a specific and parallel-tuned application code targeting relativistic neutrino transport in stellar atmospheres. The other is building a more general environment for creating specific applications, such as this one. We recently presented to our colleagues in LANL some preliminary results on the parallel performance of the targeted application code. To give a sense of the approach to parallelization in this code, consider these high-level functions from an earlier serial version: main :: IO () main = do (n, rest) <- parseCL let tally = runMany infMesh simpleMat n writeTally "tally" tally runMany :: Mesh -> Material -> Word32 -> RNG -> Tally runMany msh mat ntot rng = let ps = genParticles ntot msh rng tallies = map (runParticle msh mat) $ ps in foldl' merge emptyTally tallies And consider the following changes for the parallel version: main :: IO () main = do (n,sz) <- parseCL let tally = feed infMesh simpleMat n sz prand writeTally "tally" tally feed :: Mesh -> Material -> Word32 -> Word32 -> RNG -> Tally feed msh mat ntot chunkSz rng | ntot <= chunkSz = runMany msh mat ntot rng | otherwise = t `par` (ts `pseq` (merge t ts)) where t = runMany msh mat chunkSz g1 ts = feed msh mat (ntot - chunkSz) chunkSz g2 (g1,g2) = split g We've wrapped function `runMany` in `feed`, which partitions the collection of generated particles into groups of size `chunkSz`, and issues these particles to `runMany` in parallel. With this simple change, we seeing upwards of 80% utilization of up to 8 cores, for a performance improvement greater than a factor of 6. We believe that performance can be further improved with different strategies for breaking down the work, and looking for additional parallelization opportunities in the collection of results. Our other branch of development is focused on finding useful abstractions and high-level functions to support programming a variety of Monte Carlo problems of this kind. We have identified a few such useful abstractions, and implemented them as type classes and type families. For example, `Space` is a general term for the physical space and imposed symmetries in which we can perform a simulation. We express this as follows: class Space s where type Position s :: * type Direction s :: * stream :: s -> Distance -> s position :: s -> Position s direction :: s -> Direction s make :: Position s -> Direction s -> s and implement specific spaces, such as one with the symmetry of the unit sphere: instance Space Spherical1D where type Position Spherical1D = Radius type Direction Spherical1D = Normalized Vector2 stream (Vector2 x y) (Distance d) = Vector2 (x+d) y position s = Radius $ vmag s direction s = normalize s make (Radius pos) dir = pos *| (normalized_value dir) This allows the specific space data types to be used in a variety of contexts. Using ordinary parametric polymorphism is also effective: -- | Stream a single particle: stream :: (p -> (e,p)) -- ^ Function to produce each step. Comes from a model. -> (e -> Bool) -- ^ Check for terminal events to stop streaming -> p -- ^ Initial particle -> [(e, p)] -- ^ Resulting list of events and particle states. stream stepper continue p = next p where next p = let (e, p') = stepper p in (e, p') : if continue e then next p' else [] The above is our high-level routine function for generating a history from a single particle, recorded as a list of (event, particle) pairs, where the event and particle data types are provided for each problem. Blogs, Papers, and Packages ---------------------------------------------------------------------- * [Fun with parallel monad comprehensions (19 July 2011)][3] This blog post by Thomas Petricek featured in the Monad Reader 18, and covers some of the interesting things that can be achieved with monad comprehensions when viewed from a parallel perspective. Along the way, he deals with examples such as the parallel composition of parsers. * [Parallelizing a nonogram solver (05 July 2011)][4] Jasper Van der Jeugt detailed his implementation of a parallel nonogram solver. Nonograms also go by the name of Paint Sudoku: the aim is to colour in a grid where a list of numbers is given for each row and column and these numbers indicate consecutive runs of filled-in squares in the corresponding row or column. For large puzzles, grids that are 20x20, Jasper reports that on a dual core machine his a parallel algorithm reduces execution by 37.9% compared to its sequential counterpart. * [The IVar monad (29 June)][5] Edward Z. Yang has written a series of posts discussing IVars, which are immutable variables which are a write-once, read-many (these are particularly handy for communicating results from a child process to its parent). Edward's post outlines the difficulties involved in defining a monad for IVars. * [Map reduce as a monad (05 July 2011)][7] Julian Porter wrote an article for The Monad Reader 18 about how MapReduce could be expressed as a monad. The MapReduce framework finds its roots in functional programming, and this is an interesting take on the problem. Mailing list discussions --------------------------------------------------------------------- * [NVIDIA's CUDA and Haskell (5 July 2011)][11] Vasili Galchin was wondering whether or not there had been any efforts to build bridges between NVIDIA's CUDA and Haskell. Don Stewart was quick to respond with a number of links to active work in the area: * [Direct access to CUDA][20] * [CUDA in Haskell][21] * [Direct access to OpenCL][22] * [The accelarate package][23] Trevor McDonell noted that the accelerate package was best accessed from the source repository on github, and that the CUDA bindings hadn't yet been tested or updated for the latest toolkit release. * [Unbelievable parallel speedup (3 June 2011)][12] While reading Simon Marlow's [tutorial][19] on parallel and concurrent programming, John Ramsdell reported some remarkable (slightly superlinear!) performance gains for one of his programs. Thomas Schilling guessed that this was due to the large variance in the figures reported, but went on to describe how it might be possible to obvserve such performance boosts due to reduced local cache misses when using several cores. Without more information about the program in question, it's difficult to do any kind of diagnosis, but nevertheless, it's great to hear about good results from a happy Haskeller! * [Automatic Reference Counting (2 July 2011)][13] After hearing about the new static analysis tools in Clang that does automatic reference counting (ARC), Thomas Davie was wondering if some compiler gurus might be able to comment on the applicability of this kind of analysis to Haskell, as an alternative to garbage collection. This led to an enlightening discussion about reference counting versus garbage collection. * [Haskell on NUMA (16 June 2011)][14] Michael Lesniak was wondering what the state of parallel performance of Haskell on Non-Uniform Memory Access (NUMA) machines was like, since he's having problems and can't find much useful information online. Nobody seems to have answered this one, are there any suggestions? * [Parallel compilation and execution? (26 May 2011)][15] Michael Rice was trying to figure out how to compile and run a simple program that outputs the result of a parallel fibonacci algorithm. After a quick reminder to use `pseq` rather than `seq` to force sequential evaluation, Daniel Fischer suggested that recompilation might be required, and that passing `--fforce-recomp` would be a good way to ensure that this occurred. Michael was also keen to know whether Control.Parallel was comparable to OpenMP. Alex Mason gave a detailed reply and gave an example of parallel mergesort as a means of comparison. * [parMap doesn't work fine (12 May 2011)][16] After just starting out with parallel computations in Haskell, Grigory Sarnitskiy ran into troubles making parMap work with lazy structures. To resolve these issues, Brandon Moore pointed to using `rdeepseq`, and Maciej Piechotka suggested `deepseq`. * [efficient parallel foldMap for lists/sequences (17 June 2011)][17] Sebastian Fischer re-posted his question about efficient parallel `foldMap` for lists to the parallel mailing list. In essence he was seeking an efficient implementation of `foldMap`, where a list is folded into a single value before a map is applied to the result. Johannes Waldmann advised against using ordinary lists, and mentioned that he was using `Data.Vector` instead. Additionally, he recommended switching to a sequential fold once a parallel fold had been used to a certain depth. Christopher Brown further confirmed that it was a good idea to spark off computations when the granularity is high enough to make it worthwhile, and also mentioned that it was best to spark computations that were evaluated to normal form. * [Wanted: parallel Haskell tutorial/talk/demonstration in Leipzig, Germany, October 7 (8 July 2011)][18] Johannes Waldmann is looking for volunteers who might be able to present at their local Haskell Workshop, and welcomes submissions on parallel and distributed computing using Haskell. The submission deadline is 20 August. Stack Overflow ---------------------------------------------------------------------- * [How to write nested loop problem using parallel strategies in Haskell][8] * [How to measure sequential and parallel runtimes of Haskell program][9] * [Poor performance / lockup with STM][10] Help and Feedback ---------------------------------------------------------------------- If you'd like to make an announcement in the next Haskell Parallel Digest, then get in touch with me, Nicolas Wu, at <parallel(a)well-typed.com>. Please feel free to leave any comments and feedback! [0]: http://www.haskell.org/haskellwiki/Parallel_GHC_Project [1]: http://research.microsoft.com/apps/pubs/default.aspx?id=138042 [2]: http://hackage.haskell.org/package/parallel [3]: http://tomasp.net/blog/comprefun.aspx [4]: http://jaspervdj.be/posts/2011-07-05-parallelizing-a-nonogram-solver.html [5]: http://blog.ezyang.com/2011/06/the-iva-monad/ [6]: http://themonadreader.wordpress.com/2011/07/11/call-for-copy-issue-19-speci… [7]: http://themonadreader.files.wordpress.com/2011/07/issue18.pdf [8]: http://stackoverflow.com/questions/6444716/how-to-write-nested-loop-problem… [9]: http://stackoverflow.com/questions/6623316/how-to-measure-sequential-and-pa… [10]: http://stackoverflow.com/questions/6439925/poor-performance-lockup-with-stm [11]: http://www.haskell.org/pipermail/haskell-cafe/2011-July/093751.html [12]: http://www.haskell.org/pipermail/haskell-cafe/2011-June/092661.html [13]: http://www.haskell.org/pipermail/haskell-cafe/2011-July/093689.html [14]: http://www.haskell.org/pipermail/haskell-cafe/2011-June/093231.html [15]: http://www.haskell.org/pipermail/haskell-cafe/2011-May/092389.html [16]: http://www.haskell.org/pipermail/haskell-cafe/2011-May/091843.html [17]: http://groups.google.com/group/parallel-haskell/t/b34d796e6672fd6b [18]: http://groups.google.com/group/parallel-haskell/t/f5f0946d0780b59b [19]: http://community.haskell.org/~simonmar/par-tutorial.pdf [20]: http://hackage.haskell.org/package/cuda [21]: http://hackage.haskell.org/package/language-c-quote [22]: http://hackage.haskell.org/package/OpenCLRaw [23]: http://hackage.haskell.org/package/accelerate [24]: http://community.haskell.org/~simonmar/papers/threadscope.pdf

1 0

Grammar hacker in Haskell
by sharadha Raghunathan 22 Jul '11

22 Jul '11

Hi all, I am new to Haskell, I need to write a grammar hacker in Haskell; that would find the first set of grammar, follow set of grammar, determine if it is in LL or not and generate a LR table. Can someone help me with the same? Any reply would be much appreciated. -- Sharadha Raghunathan

3 3

Introspection
by Patrick Browne 22 Jul '11

22 Jul '11

Is it possible to access type information and perform either of the following in a Haskell program? 1) Find the type/class of a variable e.g. a type predicate: is y of-type A, or is y of-Class A 2) Assert the type of a variable e.g. if y.length > 100 then y is of type big. Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

5 6

Haskell Weekly News: Issue 191
by Daniel Santa Cruz 22 Jul '11

22 Jul '11

Welcome to issue 191 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of July 10 to 16, 2011. [1] http://haskell.org/ You can find the HTML (and mobile) version of the issue at: http://contemplatecode.blogspot.com/2011/07/haskell-weekly-news-issue-191.h… Announcements Brent Yorgey put out a call for articles for the next Monad.Reader, which will be focused on parallelism and concurrency. [2] http://goo.gl/Qh6na Quotes of the Week * PenguinOfDoom: Being enlightened gentlemen, we split all programming languages into two groups, sucks and doesn't-suck and put all of them into the first group. * cmccann: beta reducing the chainsaws can be tricky * kmc: edwardk measures API lifetime in hours * _Ray_: the difference in mathematical/CS knowledge of the average haskeller, and the average PHPer, is likely similar to that of the average PHPer and a banana * quicksilver: [on LYAH] by the time I read it I knew everything it was trying to teach, so I just thought "Yay! Cute caterpillar". * edwardk: shapr is the only person i know of who just decides to drive across the country with a car full of machetes * copumpkin is now known as contrapumpkin. contrapumpkin is now known as yoda. yoda is now known as coyoda. + Eduard_Munteanu: Yonoda + coedwardk: i keep reading coyoda as coyoneda + monochrom: I read it as toyota + Eduard_Munteanu: Mmm, the monad is strong in this one... + monochrom: rumours actually say that it should be toyoda but the japaneses want it easier on americans, so toyota Top Reddit Stories * The Haskell School of Music - new book being written by Paul Hudak Domain: plucky.cs.yale.edu, Score: 66, Comments: 9 On Reddit: [3] http://goo.gl/9Jjrd Original: [4] http://goo.gl/r54xG * Confusion between (.) and ($) operators Domain: self.haskell, Score: 41, Comments: 27 On Reddit: [5] http://goo.gl/eFqRj Original: [6] http://goo.gl/eFqRj * Fitter, happier, more productive UTF-8 decoding Domain: serpentine.com, Score: 35, Comments: 6 On Reddit: [7] http://goo.gl/o2REx Original: [8] http://goo.gl/dKB04 * Static Single Assignment for Functional Programmers Domain: wingolog.org, Score: 30, Comments: On Reddit: [9] http://goo.gl/13sSP Original: [10] http://goo.gl/90MEH * [GSoC] Text/UTF-8: Initial results Domain: jaspervdj.be, Score: 29, Comments: 14 On Reddit: [11] http://goo.gl/nJ4pJ Original: [12] http://goo.gl/uPkBl * Data Driven Programming in haskell - live screencast. What are your tips/suggestions? Domain: entirelysubjective.com, Score: 29, Comments: 17 On Reddit: [13] http://goo.gl/kIiX8 Original: [14] http://goo.gl/28k6p * Ur/Web is hiring: web metaprogramming with class Domain: haskell.org, Score: 28, Comments: 11 On Reddit: [15] http://goo.gl/nWf5S Original: [16] http://goo.gl/l9HhI * Monads in C++. Bartosz Milewski's Programming Cafe Domain: bartoszmilewski.wordpress.com, Score: 23, Comments: 0 On Reddit: [17] http://goo.gl/IJnVa Original: [18] http://goo.gl/Bq4Lq * The Typeable class is changing Domain: haskell.org, Score: 22, Comments: 2 On Reddit: [19] http://goo.gl/x1InO Original: [20] http://goo.gl/E2jVK * Reverse Engineering of Compiled Haskell Domain: self.haskell, Score: 18, Comments: 22 On Reddit: [21] http://goo.gl/GCrfN Original: [22] http://goo.gl/GCrfN Top StackOverflow Questions * Haskell - How to avoid typing the same context over and over again? votes: 18, answers: 3 Read on SO: [23] http://goo.gl/0h1qF * Haskell iteratee: simple worked example of stripping trailing whitespace votes: 14, answers: 1 Read on SO: [24] http://goo.gl/bYujx * Parametrised data type in Scala votes: 12, answers: 2 Read on SO: [25] http://goo.gl/4NyZw * How to organize Haskell modules with instances: stick to data type vs type class? votes: 10, answers: 1 Read on SO: [26] http://goo.gl/Tivoh * On improving Haskell's performance compared to C in fibonacci micro-benchmark votes: 10, answers: 3 Read on SO: [27] http://goo.gl/q5O8Q * Constructing a tuple (or list) from already-existing objects - what is the cost? votes: 9, answers: 2 Read on SO: [28] http://goo.gl/UuKft * Writing A Function Polymorphic In A Type Family votes: 9, answers: 1 Read on SO: [29] http://goo.gl/vPpoH * Haskell Lazy ByteString + read/write progress function votes: 9, answers: 1 Read on SO: [30] http://goo.gl/LfNEN 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.lang.haskell.general/18844 3. http://www.reddit.com/r/haskell/comments/in1my/the_haskell_school_of_music_… 4. http://plucky.cs.yale.edu/cs431/reading.htm 5. http://www.reddit.com/r/haskell/comments/ipfok/confusion_between_and_operat… 6. http://www.reddit.com/r/haskell/comments/ipfok/confusion_between_and_operat… 7. http://www.reddit.com/r/haskell/comments/im82t/fitter_happier_more_producti… 8. http://www.serpentine.com/blog/2011/07/11/fitter-happier-more-productive-ut… 9. http://www.reddit.com/r/haskell/comments/ip2de/static_single_assignment_for… 10. http://wingolog.org/archives/2011/07/12/static-single-assignment-for-functi… 11. http://www.reddit.com/r/haskell/comments/ilif5/gsoc_textutf8_initial_result… 12. http://jaspervdj.be/posts/2011-07-10-text-utf8-initial-results.html 13. http://www.reddit.com/r/haskell/comments/insp1/data_driven_programming_in_h… 14. http://entirelysubjective.com/programming/data-driven-programming-haskell-1/ 15. http://www.reddit.com/r/haskell/comments/ips56/urweb_is_hiring_web_metaprog… 16. http://www.haskell.org/pipermail/haskell-cafe/2011-July/093994.html 17. http://www.reddit.com/r/haskell/comments/imzaj/monads_in_c_bartosz_milewski… 18. http://bartoszmilewski.wordpress.com/2011/07/11/monads-in-c/ 19. http://www.reddit.com/r/haskell/comments/imm0j/the_typeable_class_is_changi… 20. http://www.haskell.org/pipermail/haskell-cafe/2011-July/093947.html 21. http://www.reddit.com/r/haskell/comments/iqz0l/reverse_engineering_of_compi… 22. http://www.reddit.com/r/haskell/comments/iqz0l/reverse_engineering_of_compi… 23. http://stackoverflow.com/questions/6706248/haskell-how-to-avoid-typing-the-… 24. http://stackoverflow.com/questions/6643495/haskell-iteratee-simple-worked-e… 25. http://stackoverflow.com/questions/6694269/parametrised-data-type-in-scala 26. http://stackoverflow.com/questions/6641105/how-to-organize-haskell-modules-… 27. http://stackoverflow.com/questions/6716315/on-improving-haskells-performanc… 28. http://stackoverflow.com/questions/6655902/constructing-a-tuple-or-list-fro… 29. http://stackoverflow.com/questions/6663547/writing-a-function-polymorphic-i… 30. http://stackoverflow.com/questions/6668716/haskell-lazy-bytestring-read-wri…

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Network.CGI strangeness
by Alexander Solla 21 Jul '11

21 Jul '11

Hi Everybody, I'm having some strange issues with Network.CGI's pathInfo action, or else some other strangeness with pattern matching. My "mainCGI" CGI action is: mainCGI :: CGI CGIResult mainCGI = pathInfo >>= outputSlotTHtml . packagesPage outputSlotTHtml is irrelevant to my problem. packagesPage generates a SlotT IO Html -- Slot semantics are essentially irrelevant to my problem. But packagesPage calls a function called pathCrossRef which is having some problems: packagesPage :: WebPath -> SlotT IO Html packagesPage wp = do infos <- getPackageInfos . pathCrossRef $ wp ... pathCrossRef :: String -> String pathCrossRef "/working/" = "/development/code/haskell/working/" pathCrossRef _ = "/development/code/haskell/packages/" The intention is that when the cgi program is called with "/working/" as an argument, the pathCrossRef should point at "/development/.../working/", and when called with "/packages/" (or anything else) as an argument, pathCrossRef should point to "/development/.../packages/". But it is not working! When I visit http://localhost/working/, I end up using the "packages" cross ref. I have even printed the cgi argument to the Apache log, and the argument is "/working/". If I get rid of the wildcard pattern, I get a "missing pattern" error when I visit http://localhost/working/. Can anybody help? Thanks, Alex

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ANN: boomerang and web-routes-boomerang
by Jeremy Shaw 21 Jul '11

21 Jul '11

Hello, I am pleased to announce the release of two new libraries: boomerang and web-routes-boomerang. boomerang is a library for general purpose, invertible parsing and pretty printing. It provides combinators which allow you to specify a grammar once and automatically extract a parser and pretty-printer from it. This library was refactored, with permission, from the Zwaluw library created by Sjoerd Visscher and Martijn van Steenbergen. hackage: http://hackage.haskell.org/package/boomerang quick intro: http://hackage.haskell.org/packages/archive/boomerang/1.0.0/doc/html/Text-B… web-route-boomerang provides glue code for using boomerang with web-routes. web-routes is a framework-independent, type-safe, url routing library. hackage: http://hackage.haskell.org/package/web-routes-boomerang tutorial: http://happstack.com/docs/crashcourse/WebRoutes.html#web-routes-boomerang - jeremy p.s. boomerang is similar in purpose to the inveritble-syntax library: http://hackage.haskell.org/package/invertible-syntax The libraries take different approaches and are both worth considering.

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Using SAX
by Larry Lee 21 Jul '11

21 Jul '11

I'm new to Haskell and am trying to use the SAX module to parse an XML file. I'm using the following code: module FPHParser where import Data.Text as Text import Data.Maybe import Text.XML.LibXML.SAX as SAX import System.IO f :: IO (Parser IO) f = do let g = do putStrLn ("g called.") return True let h msg = do putStrLn (Text.unpack msg) return False p <- SAX.newParserIO (Just (Text.pack "test.xml")) SAX.setCallback p SAX.reportError h SAX.setCallback p SAX.parsedBeginDocument g SAX.parseComplete p return p and am getting the following error when I parse a file with the following contents: <?xml version="1.0"?> <html/> [This is the error] "Extra content at the end of the document" This error seems to be coming from the LibSAX library, which is a C library. But I think that my error is in my code, as the file is valid XML. Any help would be greatly appreciated!

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Make Show instance
by Александр 21 Jul '11

21 Jul '11

Hello, I have binary tree, with every leaf tuple - (k,v): data Tree k v = EmptyTree | Node (k, v) (Tree k v) (Tree k v) How can i make Show Instance for (Tree Int Int) ? I try: instance Show (Tree k v) where show EmptyTree = show "Empty" show (Node (Int, Int) left right) = .. But get error: Not in scope: data constructor `Int' I have a function: fillTree :: Int -> Tree Int Int -> Tree Int Int fillTree 1000000 tree = tree fillTree x tree = let a = treeInsert (x, x) EmptyTree in fillTree (x + 1) a If i execute it: No instance for (Show (Tree Int Int)) arising from a use of `print' How can i make Show instance for my Tree? Thank you.

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Helsinki Haskellers?
by Dean Herington 21 Jul '11

21 Jul '11

Hi, Cafe, My wife, a biologist, will attend a conference outside Helsinki in mid-September. I've decided to accompany her on the trip to Finland. Are there any Haskellers in the area who might be interested in an informal meetup? I'd also welcome any suggestions on what to see, do, eat, etc. We'll have 3-4 days after her conference. We especially enjoy nature (hiking, biking, boating), music (folk/ethnic, jazz, classical), and dance (performance and participatory). And I love Haskell, of course. As my inquiry is only weakly Haskell-related, I suggest that replies be sent to me rather than the list. Thanks. Dean (from North Carolina)

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pointer equality
by Nikhil A. Patil 21 Jul '11

21 Jul '11

Hi, Is there any way of getting the following code to immediately return True without performing the element-by-element comparison? Essentially this boils down to checking whether pointers are equal before comparing the contents. > main = print $ f == f > where f = [1..10^9] Thanks!! nikhil

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haskell zlip read position
by rahul 21 Jul '11

21 Jul '11

Hi Guys, I am trying to parse a binary stream with the format for one entry [headers, zlib compressed content] , with multiple entries. I can use the Zlib library to get the content for the first entry after the headers, but I cannot find a way to get the offset to start parsing for the second entry. Is there a way I can get this information out of ZLib? or is there a better approach to doing this? Any pointers would be very much appreciated. Regards, Rahul

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Network.CGI and SQL Queries
by william murphy 21 Jul '11

21 Jul '11

Hello there, There's an odd error every time I try to run this code: import Network.CGI import Text.XHtml import qualified Text.Html as H import Database.HDBC import Database.HDBC.Sqlite3 import System.Random ... sqlReaderIO = do handle <- connectSqlite3 "table2.db" scoresSql <- quickQuery' handle "SELECT * FROM pachinkoScores ORDER BY score, name" [] let scores = map (map (\x -> fromSql x :: String)) scoresSql return (show scores) ... cgiTMain = do ... scoresPassed <- sqlReaderIO output . renderHtml $ (inputForm n ss) maybe (inputForm n ss) (\mT -> page "Loading..." (visual t v a mT ss) n v a (t + 1) mT) maxT main = runCGI $ handleErrors cgiTMain When I remove the "scoresPassed <- sqlReaderIO" line, the code run smoothly. Nothing else references "scoresPassed". The ghc, however, gives me this error when the line is included: tankwSql.hs:144:15: No instance for (MonadCGI IO) arising from a use of `output' Possible fix: add an instance declaration for (MonadCGI IO) In the first argument of `(.)', namely `output' In the expression: output . renderHtml In the expression: output . renderHtml $ inputForm n tankwSql.hs:147:30: Couldn't match expected type `CGIT m0 CGIResult' with actual type `IO CGIResult' In the first argument of `handleErrors', namely `cgiTMain' In the second argument of `($)', namely `handleErrors cgiTMain' In the expression: runCGI $ handleErrors cgiTMain Thanks, Will

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Re: [Haskell-cafe] pointer equality
by wren ng thornton 20 Jul '11

20 Jul '11

On 7/20/11 1:22 PM, Chris Smith wrote: > If the latter, then it seems this would be a > pretty serious garbage collector bug, and that it would be impossible > that such a bug wouldn't also break other code that doesn't use pointer > equality at all. After all, we've got a running user thread, which if > it were to force those thunks now they would necessarily be observably > equal, but if it doesn't and waits until later they may be different? Not at all. Some amount of time passes between obtaining the pointer of x and obtaining the pointer of y in order to test whether they're equal. Given that time passes between these two steps, it's possible for GC to happen after getting one of the pointers and then when returning to user code we get the other pointer. It so happens that GC will never happen at that point on current versions of GHC, but that's not to say it couldn't (as it mustn't if the API specified no false positives). I can't think of any plausible optimization that would lead to a design which allows false positives, but I'm not sure pointer equality should really be offered in the first place. That's what StableNames are for. Pointer equality may be a reasonable implementation of StableName equality on some systems, but I really think that the two ideas should be kept separate. -- Live well, ~wren

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Re: [Haskell-cafe] pointer equality
by wren ng thornton 20 Jul '11

20 Jul '11

On 7/20/11 5:08 PM, Paul Johnson wrote: > I would have thought that the compiler, as a matter of optimisation, > could insert a check to see if (==) is comparing an object with itself. > The only way I can see this breaking is with perverse instances of Eq > that would return False for "f == f". Like the instance for Double (where f = NaN)? -- Live well, ~wren

1 0

NetSNMP; FreeBSD 8.1
by Комар Максим 20 Jul '11

20 Jul '11

I want to install package NetSNMP: > cabal install NetSNMP Resolving dependencies... Configuring NetSNMP-0.1.6... cabal: Missing dependency on a foreign library: * Missing C library: netsnmp This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: NetSNMP-0.1.6 failed during the configure step. The exception was: ExitFailure 1 Package net-mgmt/net-snmp already installed: > pkg_info | grep snmp net-snmp-5.5_4 An extendable SNMP implementation With options --extra-include-dirs= and --extra-lib-dirs= result is same: > cabal install NetSNMP --extra-include-dirs=/usr/local/include > --extra-lib-dirs=/usr/local/lib Resolving dependencies... Configuring NetSNMP-0.1.6... cabal: Missing dependency on a foreign library: * Missing C library: netsnmp This problem can usually be solved by installing the system package that provides this library (you may need the "-dev" version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: NetSNMP-0.1.6 failed during the configure step. The exception was: ExitFailure 1 How to install NetSNMP package?

3 3

Haskell fgl example
by Rohit Agrawalla 20 Jul '11

20 Jul '11

I am new to haskell and the haskell fgl (functional graph library). l am looking for some examples of the haskell fgl. Would appreciate any pointers regarding the same. -- Rohit

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Haskell Implementors Workshop talk proposals due this Friday!
by Ben Lippmeier 20 Jul '11

20 Jul '11

Call for Talks ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2011 Tokyo, Japan, September 23rd, 2011 The workshop will be held in conjunction with ICFP 2011 http://www.icfpconference.org/icfp2011/ Important dates Proposal Deadline: 22nd July 2011 Notification: 8th August 2011 Workshop: 23rd September 2011 The Haskell Implementors' Workshop is to be held alongside ICFP 2011 this year in Tokyo, Japan. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. Scope and target audience ------------------------- It is important to distinguish the Haskell Implementors' Workshop from the Haskell Symposium which is also co-located with ICFP 2011. The Haskell Symposium is for the publication of Haskell-related research. In contrast, the Haskell Implementors' Workshop will have no proceedings -- although we will aim to make talk videos, slides and presented data available with the consent of the speakers. In the Haskell Implementors' Workshop we hope to study the underlying technology. We want to bring together anyone interested in the nitty gritty details necessary to turn a text file into a deployed product. Having said that, members of the wider Haskell community are more than welcome to attend the workshop -- we need your feedback to keep the Haskell ecosystem thriving. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimisation and benchmarking * Virtual machines and run-time systems * Libraries and Tools for development or deployment Talks ----- At this stage we would like to invite proposals from potential speakers for a relatively short talk. We are aiming for 20 min talks with 10 mins for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Please submit a talk title and abstract of no more than 200 words to benl(a)cse.unsw.edu.au We will also have a lightning talks session which will be organised on the day. These talks will be 2-10 minutes, depending on available time. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. Organisers ---------- * Rebekah Leslie (Portland State University) * Ben Lippmeier - co-chair (University of New South Wales) * Andres Loeh (Well-Typed LLP) * Oleg Lobachev (University of Marburg) * Neil Mitchell - co-chair (Standard Chartered) * Dimitrios Vytiniotis (Microsoft Research)

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Re: [Haskell-cafe] pointer equality
by David Barbour 20 Jul '11

20 Jul '11

On Tue, Jul 19, 2011 at 11:14 PM, yi huang <yi.codeplayer(a)gmail.com> wrote: > 2011/7/20 Eugene Kirpichov <ekirpichov(a)gmail.com> > >> reallyUnsafePointerEq#, and it really is as unsafe as it sounds :) >> >> Why is it so unsafe? i can't find any documentation on it. > I think always compare pointer first is a good optimization. > Any number of conditions could make it fail, but most especially GC. It is almost always better to explicitly model a property when you want it. Uniqueness based on construction, for example, can be modeled with a monad and protected by an ADT. > module Unique ( HasUnique(..), Unique, newUnique, openUnique) where > class HasUnique m uid where > newUniqueID :: m uid > > data Unique uid a = U !uid a > > newUnique :: (HasUnique m uid, Monad m) => a -> m (Unique uid a) > newUnique a = newUniqueID >>= \ uid -> return (U uid a) > > openUnique :: (Unique m a) -> a > openUnique (U _ x) = x > > instance (Eq uid) => Eq (Unique uid a) > instance (Ord uid) => Ord (Unique uid a)

1 0

derive + quickCheck
by bob zhang 19 Jul '11

19 Jul '11

Hi, all, I found derive + quickCheck very useful but I came across some problems. I used derive to derive instance of Arbitrary immeditaely, but sometimes the sample is non-terminating, which I mean the result is very very big. I used samples <- take 10 <$> sample' in ghci to test the result, it's non-terminating.. another problem is that $(derive makeArbitrary ''JValue) uses reify, so I can not see the generated code, any better way to have a look at the generated code in ghci? my sample code {-# LANGUAGE FlexibleInstances ,MultiParamTypeClasses ,GeneralizedNewtypeDeriving ,FunctionalDependencies ,TypeSynonymInstances ,TemplateHaskell #-} module JsonParse where import Text.ParserCombinators.Parsec import Text.Parsec.String() import Control.Applicative hiding ( (<|>) , many, optional ) import Control.Monad import Test.QuickCheck -- unGen, sample -- import Language import Data.DeriveTH import Data.Binary import Data.Derive.Arbitrary import Language.Haskell.TH import Language.Haskell.TH.Syntax data JValue = JString String | JNumber Double | JBool Bool | JNull | JObject [(String, JValue)] -- | JArray [JValue] -- deriving (Eq,Ord,Show) $(derive makeArbitrary ''JValue)

2 3

partial inheritance
by Patrick Browne 19 Jul '11

19 Jul '11

Hi, Is it possible to model partial inheritance using Haskell type classes? For example, if the class Bird has a flying method can we represent Penguins as a sub-class of Bird without a flying method? Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Parsing cabal files to calculate average number of dependencies
by Gwern Branwen 19 Jul '11

19 Jul '11

Athas on #haskell wondered how many dependencies the average Haskell package had. I commented that it seemed like some fairly simple scripting to find out, and as these things tend to go, I wound up doing a complete solution myself. First, we get most/all of Hackage locally to examine, as tarballs: for package in `cabal list | grep '\*' | tr -d '\*'`; do cabal fetch $package; done Then we cd .cabal/packages/hackage.haskell.org Now we can run a command which extracts the .cabal file from each tarball to standard output: find . -name "*.tar.gz" -exec tar --wildcards "*.cabal" -Oxf {} \; We could grep for 'build-depends' or something, but that gives unreliable dirty results. (>80k items, resulting in a hard to believe 87k total deps and an average of 27 deps.) So instead, we use the Cabal library and write a program to parse Cabal files & spit out the dependencies, and we feed each .cabal into that: find . -name "*.tar.gz" -exec sh -c 'tar --wildcards "*.cabal" -Oxf {} | runhaskell ~/deps.hs' \; And what is deps.hs? Turns out to be surprisingly easy to parse a String, extract the Library and Executable AST, and grab the [Dependency] field, and then print it out (code is not particularly clean): import Distribution.Package import Distribution.PackageDescription import Distribution.PackageDescription.Parse main :: IO () main = do cbl <- getContents let desc = parsePackageDescription cbl case desc of ParseFailed _ -> return () ParseOk _ d -> putStr $ unlines $ map show $ map (\(Dependency x _) -> x) $ extractDeps d extractDeps :: GenericPackageDescription -> [Dependency] extractDeps d = ldeps ++ edeps where ldeps = case (condLibrary d) of Nothing -> [] Just c -> condTreeConstraints c edeps = concat $ map (condTreeConstraints . snd) $ condExecutables d So what are the results? (The output of one run is attached.) I get 18,134 dependencies, having run on 3,137 files, or 5.8 dependencies per package. -- gwern http://www.gwern.net

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ANN: cabal-dev 0.8
by Rogan Creswick 19 Jul '11

19 Jul '11

We're happy to announce the release of cabal-dev 0.8! This version is available on hackage now, and contains many bug fixes and improvements, as outlined in the full release notes below. --Rogan cabal-dev release 0.8 ================================================== The 0.8 release of `cabal-dev` fixes many of the bugs that have been reported by `cabal-dev` users. It also provides more flexible, easier configuration and improves tool support for working within the `cabal-dev` sandbox (ghc-pkg suport and better ghci support). Upgrading is recommended for all users. Please report any bugs or submit enhancement requests to <https://github.com/creswick/cabal-dev/issues>. Much of the work on this release (and previous releases) of `cabal-dev` was funded by Galois, Inc. <http://www.galois.com/> About cabal-dev ================================================== `cabal-dev` is a Haskell build tool that augments Cabal and cabal-install to make it convenient to work with a sandboxed set of packages. It limits the effect of your build on unrelated parts of your system. Among other things, `cabal-dev` satisfies the following needs: * Avoid conflicts between libraries by working only with the set of packages that are needed for the current project. * Enable depending on a patched dependency without that patched dependency accidentally being used by another project's build. * Enable the simultaneous development of multiple interdependent packages without the development versions leaking to other project build environments. Significant effort has been made to ensure that `cabal-dev` works across platforms (Mac, Windows, and Linux) and with the versions of GHC that are most commonly in use (6.10-7.0). What's new ================================================== Details can be found on github at <https://github.com/creswick/cabal-dev>. Bugs fixed: * `cabal-dev` no longer passes `--verbose` to cabal-install commands that do not accept it. <https://github.com/creswick/cabal-dev/issues/18> * `cabal-dev` now recognizes and passes through short flags to cabal-install. In particular, this bug frequently came up when attempting to pass a flag (`-f`) to `cabal-dev install`. <https://github.com/creswick/cabal-dev/issues/6> * `cabal-dev` now supports all cabal-install commands, including `test`. <https://github.com/creswick/cabal-dev/issues/10> * We also made changes that should address inconsistencies with the remote repository (Hackage) cache location, but those issues remain open until we have confirmation from end users (you!) that these changes actually fix those issues. <https://github.com/creswick/cabal-dev/issues/15> and <https://github.com/creswick/cabal-dev/issues/14> Enhancements and new features: * `cabal-dev ghci` now invokes ghci with the exact same set of flags that would be used in a cabal-install build (using a trick copied from Leksah). This means that it will now have the appropriate package visibility, language settings, and source directories set when it is invoked. Note that in order for `cabal-dev ghci` to work, all dependencies must already be installed in the sandbox, and the package must already be configured. * `cabal-dev buildopts` will print the set of build options that would be passed to GHC when invoking `cabal build`. You can then use these flags for other tools that invoke GHC (e.g. the doctest package <http://hackage.haskell.org/package/doctest>). * `cabal-dev ghc-pkg` will invoke `ghc-pkg` with the appropriate flags to examine or alter the package database inside the `cabal-dev` sandbox. * `cabal-dev` now has more flexible support for cabal-install configuration: * By default `cabal-dev` will now use any settings from your cabal-install configuration file, except those that must be overridden in order to sandbox the build. This makes it more convenient to e.g. use an alternate remote repository with `cabal-dev`. * `cabal-dev` now supports merging of additional cabal-install configuration settings, to make it easy to work with multiple cabal-install configurations (`--extra-config-file=...`). * `cabal-dev` now has a flag to pass explicit options to cabal-install in case there are any conflicts between `cabal-dev` flags (`--cabal-install-arg`). * `cabal-dev` now has a flag to specify a particular cabal-install executable to use (`--with-cabal-install=...`).

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I confused myself with generics, and now I confused ghc too
by Ari Rahikkala 19 Jul '11

19 Jul '11

I want to implement generic comparison on top of syb-with-class, and based on looking at syb, it seems I could base it on something like gzipWithQ from syb's Data.Generics.Twins. However, there's no Data.Generics.SYB.WithClass.Twins. So I set out to see if I can write one myself (well, one with just enough stuff to support gzipWithQ anyway) Here's what I've got so far: http://hpaste.org/49168 The problem is that GHCi seems to be somewhat confused, or at least confusing, about the type of gzipWithQ here. When I ask with :t, it reports the type as: gzipWithQ :: (Data ctx a2, Data ctx a3, Data ctx a) => Proxy ctx -> (forall a0. Data ctx a0 => a0 -> forall a1. Data ctx a1 => a1 -> r) -> a2 -> a3 -> [r] If I actually replace gzipWithQ's annotated type with this, GHCi will reject the program. And in any case with GenericQ being defined as "type GenericQ ctx r = Data ctx a => a -> r", isn't this closer to gzipWithQ's type with the type synonyms expanded? gzipWithQ :: forall ctx a r. Data ctx a => Proxy ctx -> (Data ctx a0 => a0 -> (Data ctx a1 => a1 -> r)) -> (Data ctx a2 => a2 -> (Data ctx a3 => a3 -> [r])) (GHCi accepts this as gzipWithQ's type so I assume it's the same one. I assume those class constraints after the => come with implicit foralls? Because it seems to mean the same thing if I put foralls in there) I have no idea how to even start figuring out how to make gzipWithQ and its type work. I only got this far by copying code from Data.Generics.Twins and passing down the proxy parameter and/or context type parameter (is it even possible for this approach to work?). What I can tell is that GHCi will still change the type like that even if gzipWithQ = undefined, so I'll have to change the type and not just the body of the function. Is this the kind of thing that all those newtypes in Data.Generics.Twins are for? Should I read up on type theory?

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