- Haskell-Cafe - Haskell.org

ANN: Elerea 2.x.x + question about AOP in Haskell
by Patai Gergely 06 Aug '10

06 Aug '10

Hello all, I silently updated Elerea [1] to version 2 about half a month ago, which deprecated the old interface in favour of the 'experimental' branch of version 1. The most important change besides this swap is the addition of the clocked variant of the library, which allows the programmer to easily freeze parts of the data-flow network, a feature similar to the clock system of Lucid Synchrone. Also, as of version 2.1.0 (today's update), the parametric variant was revised and simplified, which is reflected in the types of the combinators. Feel free to play with the new features! On to my question. There are three main variants of the library: a basic one and two others that extend it in some way. The extensions are orthogonal -- on the level of denotational semantics they could be represented as monad transformers (at least I strongly believe so, but I haven't verified yet). This means that there should be a fourth variant that combines the two extensions. Also, it might easily be the case that there will be more independent extensions in the future. This smells like a perfect opportunity to turn to aspect oriented programming techniques, but unfortunately I couldn't find a sensible way to exploit the similarities (of which there are plenty!) and share the core logic of the library across these slightly distinct variants. So the question: how does one go about doing aspect oriented programming in Haskell? Any nice examples in the wild? Gergely [1] http://hackage.haskell.org/package/elerea -- http://www.fastmail.fm - Does exactly what it says on the tin

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hsb2hs preprocessor looking for a maintainer
by Joachim Breitner 06 Aug '10

06 Aug '10

Hi, out of a discussion in haskell-devscripts, John MacFarlane wrote a very nice tool, called hsb2hs, that allows you to include any binary (or text) file as a constant in your program. This can be useful in various instances, e.g. when creating programs that should not depend on data files installed in the right location, or larger, multi-line strings that you’d like to edit on their own, not as a long string constant in the Haskell file (I once wished I had this when including some CSS in a CGI script written in Haskell). The current code is on http://github.com/jgm/hsb2hs and you can read more about it in the subthread starting at http://lists.debian.org/debian-haskell/2010/08/msg00013.html Unfortunately, John says that he does not have time to develop it further, and I probably don’t either. Is there anyone here interested in picking up the projects? Possible TODOs involve the optimization I suggested in http://lists.debian.org/debian-haskell/2010/08/msg00015.html and making Cabal support .hsb files directly (just as with .y etc. files). Greetings, Joachim -- Joachim "nomeata" Breitner mail: mail(a)joachim-breitner.de | ICQ# 74513189 | GPG-Key: 4743206C JID: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/ Debian Developer: nomeata(a)debian.org

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Why is toRational a method of Real?
by Omari Norman 06 Aug '10

06 Aug '10

Why is toRational a method of Real? I thought that real numbers need not be rational, such as the square root of two. Wouldn't it make more sense to have some sort of Rational typeclass with this method? Thanks. --Omari

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Problem with System.Timeout in Windows
by Adolfo Builes 05 Aug '10

05 Aug '10

Hi All: I have been trying to use System.timeout in windows, but for some reason it doesn't work, a concrete example is: import Network.HTTP import System.Timeout main = do rsp <- timeout 10000000 $ simpleHTTP $ getRequest "http://10.1.2.3" case rsp of Just rsp -> print $ show rsp Nothing -> error "Nothing" In this case I would expect to receive "Nothing" after 10 seconds, but it ignores the timeout function. I'm compiling with -threaded, but it does not work either, I'm using ghc 6.12.2. Does someone know what could be going wrong ? Thanks !

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hGetContents: resource exhausted
by Lally Singh 05 Aug '10

05 Aug '10

Hey all, This is on OpenSolaris. Simple attempts to build cabal packages give me this error, and I don't know what it means. Here's an example: [07/25 18:51::lally@sol type-level]$ runghc Setup.hs configure Configuring type-level-0.2.4... Setup.hs: fd:8: hGetContents: resource exhausted (Resource temporarily unavailable) Setup.hs: fd:8: resource exhausted Does anyone know what this means? I'm trying to just get the llvm bindings installed (requiring mtl & type-level). Cheers, -Lally

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can Haskell do everyting as we want?
by Qi Qi 05 Aug '10

05 Aug '10

As more I learn haskell, I am more interested in this function programming language. I am intended to more focus on haskell than other languages like python, Java, or C++. But I am still wonder whether haskell can do everyting as other languages do, such as python, perl, Java and C++. Is there anyone happen to come into any tasks that haskell is not able to achieve? Thanks. -- Qi Qi

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Playing with ATs again
by Andrew Coppin 05 Aug '10

05 Aug '10

OK, so if you do something like class Container c where type Element c :: * then we now have a clean and concise way to discover what type of element any given container holds. (Regardless of whether it's element type is parametric, hard-coded, class-constrained or anything else.) I really like this! Now suppose that instead of a container type with an associated element type, what we want is several pairs of types having a one-to-one relationship, and we want to be able to traverse that relationship in either direction. What's the best way to do that? (I tried just having two mirror image classes like the one above. But that doesn't work; there's no guarantee that going one way and then going back the other way will definitely take you back to where you started. I tried to make that a constraint, and got an error message that basically says "this feature is not implemented". Great. Still, there's probably a better way to do this.)

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ANN: blaze-builder 0.1
by Jasper Van der Jeugt 05 Aug '10

05 Aug '10

Dear all, I have just released a package called blaze-builder on Hackage. It is basically a builder monoid supporting fast string concatenation. It was originally written for the blaze-html package, but because we thought it could be useful elsewhere as well, we are releasing it as a separate package. Here is a blogpost with more information: http://jaspervdj.be/posts/2010-08-05-blaze-builder.html And here is the Hackage link: http://hackage.haskell.org/package/blaze-builder All feedback is welcome, Kind regards, Jasper Van der Jeugt

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subversion with haskell
by Andrew U. Frank 05 Aug '10

05 Aug '10

hackage contains a package Hs2SVN HsSVN seems to do what i need, but unfortunately, i cannot install it - i have the error : checking for stdint.h... yes checking for unistd.h... yes checking svn_error.h usability... no checking svn_error.h presence... no checking for svn_error.h... no configure: error: SVN headers are required. Hint: EXTRA_CPPFLAGS cabal: Error: some packages failed to install: HsSVN-0.4.3 failed during the configure step. The exception was: ExitFailure 1 i installed all development packages i could see related to svn - which one is really needed? can you clarify and point me in the right direction? (and put the information in the package cabal file so it is copied to hackage) thank you andrew

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packages on hackage: how to download files which are not included in the darcs inventory
by Andrew U. Frank 05 Aug '10

05 Aug '10

i try to use jsmw. i see that there are multiple example files in a directory. this directory is not included in darcs and does not download automatically. is there an easy way to download directories (i can get the files individually with curl - but this is slow and errorprone)? help is appreciated! andrew

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ANNOUNCE: uu-parsinglib version 2.5.3; extended with merging combinators
by S. Doaitse Swierstra 05 Aug '10

05 Aug '10

I have uploaded a new version of the uu-parsinglib. It contains, besides the extension of the abstract interpretation part and the fixing of some very subtle corner cases in that part, some nice new functionality: The call of the parser: -- run ((,,,) `pMerge` (pSome pa <||> pMany pb <||> pOne pc <||> pNatural `pOpt` 5)) "babc45a" results in: -- > Result: (["a", "a"],["b","b"],"c",45) and -- run ((,,,) `pMerge` (pSome pa <||> pMany pb <||> pOne pc <||> pNatural `pOpt` 5)) "bbc" results in -- > Result: (["a"],["b","b"],"c",5) -- > Correcting steps: -- > Inserted 'a' at position (0,3) expecting one of ['0'..'9', 'b', 'a'] -- pSome means that the number of elements occurring in the input should be >= 1 pMany >= 0 pOne = 1 pOpt <= 1 The implementation can be found in the Derived module, and is remarkably simple. Doaitse

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ANNOUNCE: hierarchical-clustering and gsc-weighting
by Felipe Lessa 04 Aug '10

04 Aug '10

Hello! I'm pleased to announce the release of two new packages: http://hackage.haskell.org/package/hierarchical-clustering http://hackage.haskell.org/package/gsc-weighting 'hierarchical-clustering' provides a function to create a dendrogram from a list of items and a distance function between them. The most common linkage types are available: single linkage, complete linkage and UPGMA. An item can be anything, for example a DNA sequence, so this may used to create a phylogenetic tree. Or it may be used with the 'gsc-weighting' package to assign weights to the items. Weights are assigned such that close items get smaller weight than distance items, meaning that the weights try to avoid the over-representation of some closely related items. The package name come from the authors of the algorithm, Gerstein, Sonnhammer and Chothia. Again, this may be used for DNA or protein sequences. Cheers! -- Felipe.

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Maybe to Either -- is there a better way?
by Tom Davies 04 Aug '10

04 Aug '10

I find it convenient sometimes to convert a Maybe value to an Either thus (excuse the syntax, it's CAL, not Haskell): maybeToEither :: a -> Maybe b -> Either a b; maybeToEither errorValue = maybe (Left errorValue) (\x -> Right x); but that seemingly obvious function isn't in Hoogle, AFAICT, so perhaps there's some other approach? Thanks, Tom

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Re: Laziness question
by Janis Voigtländer 04 Aug '10

04 Aug '10

Hi, I am late to reply in this thread, but as I see Stefan has already made what (also from my view) are the main points: - Putting seq in a type class makes type signatures more verbose, which one may consider okay or not. In the past (and, as it seems, again in every iteration of the language development process since then) the majority of the language design decision makers have considered this verbosity non-okay enough, so that they decided to have a fully polymorhpic seq. - Even if putting seq in a type class, problems with parametricity do not simply vanish. The question is what instances there will be for that class. (For example, if there is not instance at all, then no seq-related problems of *any* nature can exist...) - The Haskell 1.3 "solution" was to, among others, have a class instance for functions. As we show in the paper Stefan mentioned, that is not a solution. Some statements claimed by parametricity will then still be wrong due to seq. - If there is no class instance for function types, then those problems go away, of course. But it is doubtful whether that would be a viable solution. Quite a few programs would be rejected as a consequence. (Say, you want to use the strict version of foldl. That will lead to a type class constraint on one of the type variables. Now suppose you actually want to fold in a higher-order fashion, like when expressing efficient reverse via foldr. That would not anymore be possible for the strict version of foldl, as it would require the type-class-constrained variable to be instantiated with a function type.) Two more specific answers to Nicolas' comments: > Actually my point is that if we do not use any polymorphic primitive to > implement a family of seq functions then it cannot be flawed. Indeed > if you read type classes as a way to implicitly pass and pick > functions > then it cannot add troubles. Completely correct. But the point is that without using any polymorphic primitive you won't be able to implement a member of that family for the case of function types (which you do not consider a big restriction, but others do). > However I absolutely do not buy their argument using as example a function > f :: Eval (a -> Int) => (a -> Int) -> (a -> Int) -> Int. They consider as > an issue the fact that the type does not tell us on which argument seq is > used. I think it is fine we may want a more precise type for it to get more > properties out of it but it is not flawed. > As much as we don't know where (==) is used given a function of type > ∀ a. Eq a => [a] -> [a]. I fear you do not buy our argument since you did not fully understand what our argument is, which in all probability is our fault in not explaining it enough. The point is not that we dislike per se that one doesn't know from the type signature how/where exactly methods from a type class are used. In your example ∀ a. Eq a => [a] -> [a] it's alright that we don't know more about where (==) is used. But for a function of type f :: Eval (a -> Int) => (a -> Int) -> (a -> Int) -> Int, in connection with trying to find out whether uses of seq are harmful or not, it is absolutely *essential* to know on which of the two functions (a -> Int) seq is used. The type class approach cannot tell that. Hence, a type class approach is unsuitable for trying to prevent seq from doing parametricity-damage while still allowing to write all the Haskell programs one could before (including ones that use seq on functions). That is the flaw of the type class approach to controlling seq. It is of course no flaw of using type classes in Haskell for other things, and we certainly did not meant to imply such a thing. Best regards, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

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Software architecture
by Charles-Pierre Astolfi 04 Aug '10

04 Aug '10

Hey there, I'm searching for software designs in Haskell ; for example, I have a pretty good ideo of how I would arrange my modules/classes (in ocaml/(java/c++)) and how they would all fit together to create, say, a website aspirator. But I don't have any clue of the right way to do it with Haskell. I don't need a solution for this example, I'd just like to see how to manage non-trivial code. I haven't found any pointers on the interwebs. On an unrelated note, what is the simplest way to get the llvm bitcode? I understand I can compile myself ghc but it there an easier way? Thanks a lot! -- Cp

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Re: [Haskell-cafe] Preview the new haddock look and take a short survey
by Ben 04 Aug '10

04 Aug '10

Great work! I'm sure you're already aware of http://sphinx.pocoo.org/ which is used to generate the Python docs. A lot of good ideas there. One thing which would be great would be to integrate their javascript in-browser text search engine. Obviously not a priority but it might be nice. Best, B > Message: 14 > Date: Wed, 4 Aug 2010 15:56:58 +0100 > From: Thomas Schilling <nominolo(a)googlemail.com> > Subject: Re: [Haskell-cafe] Preview the new haddock look and take a > short survey > To: aditya siram <aditya.siram(a)gmail.com> > Cc: haddock <haddock(a)projects.haskell.org>, haskell-cafe(a)haskell.org > Message-ID: > <AANLkTincL6oUsUJ6hFdi+3-UaO2bqmrX-p8dvRzYME2-(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > On 4 August 2010 15:44, aditya siram <aditya.siram(a)gmail.com> wrote: >> I really like the color scheme and the Javadoc looking frames. >> >> One suggestion I can make is to have the index show all the functions with >> type signatures without having to pick a letter. A lot of times I'll be >> looking for a function of a certain signature as opposed to a name. Indeed >> an index of type signatures would great! I remember wishing I had this when >> trying the understand the Parsec package. >> >> -deech > > Wouldn't hoogle be better for this kind of use case? The index can > become very large already. > > More direct hoogle/hayoo integration (at least on Hackage) sounds like > a worthwhile goal, though. Noted. > >> >> On Wed, Aug 4, 2010 at 8:55 AM, Yitzchak Gale <gale(a)sefer.org> wrote: >>> >>> Mark Lentczner wrote: >>> > The Haddock team... >>> > Please take a look, and then give us your feedback >>> >>> Very very nice. I took the survey, but here are some comments >>> I left out. >>> >>> I like the idea of the Snappy style the best, but there are two >>> serious problems with it, at least in my browser (Safari): >>> >>> 1. The black on dark blue of the "Snap Packages" title makes it >>> nearly unreadable for me. >>> 2. The wide fonts stretch things out so far on my screen that the >>> page becomes almost unusable. >>> >>> The other styles are fine, I would use them instead. >>> >>> Here is a comment I'll repeat from the survey because of its >>> importance: Please add a "collapse all" button for the tree on >>> the contents page. For me, that is perhaps the most urgent >>> thing missing in all of Haddock. It would make that tree so >>> much more usable. >>> >>> Thanks for the great work, >>> Yitz >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > > > > -- > If it looks like a duck, and quacks like a duck, we have at least to > consider the possibility that we have a small aquatic bird of the > family Anatidae on our hands. >

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Arrays and image processing
by kirstin penelope rhys 04 Aug '10

04 Aug '10

Hi! I need to do some image processing for an algorithm. It involves two (well, three) different kinds of arrays. The result array I'm happy to leave as a Data.Array for now, since it tends to be bulk updated, and involves Either with tuples of tuples, and it's easy enough to switch to mutable without a significant impact. So far, using a mutable result array hasn't resulted in a significant speed difference, since it's not directly involved in the inner loops. But the static/immutable input arrays, the source images, are more of a present concern, since they are so directly involved with the inner loops, and are expected to be rather large. Right now, the algorithm is stable enough that it's time to move beyond black and white, which were well enough served by UArray and some ugly optimized inner functions which look rather like C loops, sigh. But now I need a fast multidimensional array which can handle a tuple of primitive types. My options, as far an I can see, are: 1) Add an instance for UArray (Int,Int) (Word16, Word16, Word16) and/or UArray (Int,Int) (Word8, Word8, Word8) 2) Add a multidimensional wrapper for Data.Vector 3) Use Data.Array.Repa. While in the currently released form, as opposed to the paper, it sadly lacks support for pairs, it's not too hard to coerce it into accepting :*:. Though, that seems to involve, for color in this case, three separate lookups. As a note, Repa is really quite lovely, but in my case/opinion, not quite there yet. I need folds/maps from one type to another unfortunately. On the one hand, I might be able to rewrite the statistical functions in a numerically stable integral and repa friendly form with a bit of thought. Though, that sort of feels like I'm programming in APL instead of Haskell, it shouldn't be so headache inducing. On the other hand, I don't really need parallelism, as my development laptop only has one cpu. Though the algorithm is eventually going to be translated to C and OpenCL for use on an embedded device. Still, this now involves even more delving into DPH. 4) Use DPH directly. While I don't really need data parallelism--for the development at least--this seems like the most accommodating option for my needs. But I worry about the stability and correctness. I really like the automatic loop fusion that Repa provides. It really goes a long way toward making the code simple and nice. I'm tempted to just start recoding the algorithm in C. It has to be done eventually, and it's kind of annoying that something which is trivial in C is such a headache here... Thanks for any help/pointers. Cheers, kirstin -- The end move in politics is always to pick up a gun. -- Buckminster Fuller

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Re: [Haskell-cafe] can Haskell do everyting as we want?
by Ivan Lazar Miljenovic 04 Aug '10

04 Aug '10

Marc Weber <marco-oweber(a)gmx.de> writes: > Excerpts from Ivan Lazar Miljenovic's message of Wed Aug 04 12:37:29 +0200 2010: >> functionality in Emacs. > > I know - I patched the py backend for scion. I'm talking about: > > node.getParent().getParent().<tab>Attributes["value"] > > Or (let's talk about a haskell example): > > pkgVersion $ package $ packageDescription > > It feels unnatural to complete this. How should this be done? > I know that you can define your own .dot. operator, so that you can > write it in (let's call it Java style): > > packageDescription .dot. package .dot. pkgVersion I don't get the point of this ".dot." operator (which isn't even a valid operator). > Without completion I have to get the type of packageDescription, then > follow the structure until I find Version. Using tags and Vim that's > fast. Still completion is a lot faster. Not really following your example here: you want something to write your code for you and read your mind to know what that code should be? My understanding of tab-completion in IDEs for Java, etc. is that it just displayed every single possible class method for a particular object value, and then did some kind of matching based upon what you typed to narrow down the list, not that it was type-based. > I'm not talking about the universal "put everything in scope" completion > thing - eg which Vim provides for some languages. I'm talking about > really helpful context sensitive completion. I know its pretty hard to > get this hard for Haskell - because type is determined by things you > use. But the example I gave above is one which could be implemented > easily. How is it easy? How would this mythical completer know which function to use? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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design question: decision tree from "Programming Collective Intelligence"
by Daniel Lyons 04 Aug '10

04 Aug '10

Hi, I'm having a little trouble figuring out precisely how to port the decision tree code from the book "Programming Collective Intelligence." You can see the code here: http://code.google.com/p/memothing/source/browse/trunk/PCI/ch7/treepredict.… The design issue is that this code depends on dynamic typing pretty heavily. He has a table of heterogenous data. Each column has the same type, but that's implicit, not explicit. He depends on all of the values supporting "==", which he uses to get a list of distinct values from each column. These values are used by the divide set function which takes a column and a value as parameters. Based on the type of the value, he chooses a different partitioning function; for strings, it's just equality again, but for numeric types he uses >=. The decision tree nodes then collect not just the left and right branches but also the column number and value on which the split was performed. I have thought about this for several days and can't seem to come to a design that I like, so I'm wondering how others would approach the problem. I guess you could implement it as a list of lists of some data type that is either a string or numeric, but this feels a bit like a cop-out. How would you support creating a decision tree with different types of data? I imagine possibilities using existential quantification, SYB, Data.Data and other stuff, but I don't know how to make it work. I wonder if there is an obvious functional design hiding in here that doesn't depend on any fancy stuff, but I'm blinded to it by having read and understood the Python version of the code. Anybody have any suggestions? I'd greatly appreciate any help. Thanks, — Daniel Lyons

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Re: Type Families: deleting from HList
by oleg＠okmij.org 04 Aug '10

04 Aug '10

Serguey Zefirov wrote: > Is it possible to delete an element from heterogenous list using type > families alone? > > I can do it using multiparameter type classes: > class Del a list result > instance Del a (a,list) list > instance Del a list list' => Del a (a',list) list' > instance Del a () () I'm afraid you'll find that class quite difficult to use: lots of type annotations will be required. For example, if we define del :: Del a li lo => a -> li -> lo del = undefined and write test1 = del True ('a',(True,())) the compiler will complain because it doesn't know which instance to choose. BTW, since GHC checks for overlapping lazily, the fact that GHC accepts a set of instance declarations does not tell that the instances `work'. The problems may lurk, becoming apparent when we define values that attempt to use the instances. In order for something like Del to be useful, functional dependencies are needed. It is easy to see that Del uses overlapping instances. That answers the question: since type families do not support overlapping instances, one cannot re-write Del using type families alone. One may ask is it possible to define some sort of type inequality using type families. And the answer is yes, recently discussed at http://www.haskell.org/pipermail/haskell-cafe/2010-July/080473.html

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Error Monad and strings
by Gerald Gutierrez 04 Aug '10

04 Aug '10

Reading the Control.Monad.Error documentation, I see that the Error class has noMsg and strMsg as its only two functions. Now, I understand that you can define your own Error instances such as in example 1 of the documentation, so why the need to always support strings via noMsg/strMsg ? What uses these? And if in my code, I will never throw an error with a string, am I supposed to implement these functions and then ignore them?

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Installation problem with latest version of haskell platform
by Pieter Laeremans 03 Aug '10

03 Aug '10

Hi, When I try to install haskell platform 64 bit on a x86_64 debian etch I get the following error after : "make install": /usr/bin/install -c -m 755 -d "/usr/local/lib/ghc-6.12.3/package.conf.d" "/usr/local/lib/ghc-6.12.3/ghc-pkg" --force --global-conf "/usr/local/lib/ghc-6.12.3/package.conf.d" update libffi/package.conf.install /usr/local/lib/ghc-6.12.3/ghc-pkg: error while loading shared libraries: libtinfo.so.5: cannot open shared object file: No such file or directory make[1]: *** [install_packages] Error 127 make: *** [install] Error 2 I have no good strategy to locate the needed library. A quick google search yielded ncurses, but I have already installed it, and it doesn't contain that binary. Furthermore configure doesn't yield any errors. And a dependency on ncurses does seem quite strange doesn't it ? thans in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

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ANNOUNCE: approximate-equality 1.0 -- Newtype wrappers for approximate equality
by Gregory Crosswhite 03 Aug '10

03 Aug '10

I am pleased to announce the release of the package "approximate-equality", which provides newtype wrappers that allow one to effectively override the equality operator of a value so that it is /approximate/ rather than /exact/. The wrappers use type annotations to specify the tolerance; the 'Digits' type constructor has been provided for specifying the tolerance using type-level natural numbers. Instances for all of the classes in the numerical hierarchy have been provided for the wrappers, so the wrapped values can mostly be used in the same way as the original values. (In fact, most of the time one doesn't even have to wrap the starting values, since expressions such as (1+sqrt 2/3) are automatically wrapped thanks to the 'fromIntegral' method of the 'Num' typeclass.) The motivation behind this package is that there are times when working with floating point numbers that one would like the equality operator to check for approximate equality rather than exact equality. For example, in one of my own projects I have a walker that spits out a list of coordinate values that I keep track of, and since floating point arithmetic is inexact it will often arrive at the same point from two different paths and thus obtain slightly different values for the coordinates of that point. I could have just written a function to do approximate matching of the point and be done with it, but then I can't leverage pre-built data structures such as Data.Set and Data.Map which use (==) for determining whether a key is a member. This package is compatible with Haskell 2010, as it only uses the EmptyDataDecls extension, and the only package dependency is type-level-natural-numbers which itself is Haskell 2010. Any feedback from the community is, of course, very welcome. Cheers, Greg

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ANN: scan-0.1.0.3, a Haskell style scanner
by Christian Maeder 03 Aug '10

03 Aug '10

Dear Haskell friends, I like to announce a Haskell style scanner at http://hackage.haskell.org/package/scan documented under http://projects.haskell.org/style-scanner/ It's best used in conjunction with hlint http://community.haskell.org/~ndm/hlint/ and gives many suggestions regarding the file format. Feedback is welcome. Thanks Christian (Discussions should only be posted to either haskell-cafe@ or beginners@)

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problem installing pivotal
by rustom 03 Aug '10

03 Aug '10

I tried to install pivotal According to the site I must do: ghc --make Main -fglasgow-exts -package plugins I get cant satisfy package plugins I believe the more general problem is that one has to choose between 'direct' cabal installs and using debian's packages. At least this is the same situation Ive seen before in python, ruby etc. What do I do? So this is really 2 questions: 1. How to remove this error and install pivotal 2. Is it better to install haskell-platform or is it better to have a minimal install of ghc and then use hackage/cabal [Or is this a type- wrong question :-) ]

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RE: what's the best environment for haskell work? (Rustom Mody)
by Luc TAESCH 03 Aug '10

03 Aug '10

On Sat, Jul 31, 2010 at 12:07 PM, Rustom Mody <rustompmody(a)gmail.com> wrote: > Do most people who work with haskell use emacs/vi/eclipse or something > else?? > > I used Emacs, as I used to in other langauages.( mainly on Linux) When switching to Mac Ox, I Re tried Leksah at 0.8 , and Now Am sticking to it. what I found of value: - In a Nutshell, increased focus. not only time, but focus. changes in my behavior , mainly : -when exploring unknown libraries, I found the one click access to libraries ( search Metadata) , then one click to read the source is easy (easier means then I do it more often). - the right click "type this ..." that seemlessly toogles betwwen ghi-ghci , is cool to paste types once a part of my code solidify - session saving means I find the same windows opened at the same place I left them the day before or a while ago... more focus again. ( when working in parallele on different things) ( Note : I have a 27 ' screen)

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Re: (kein Betreff)
by Janis Voigtländer 03 Aug '10

03 Aug '10

Stefan Holdermans wrote: >> The point of this discussion is that the Eval constraint needs to be >> on one >> of the functions. So I tried to specify that (x -> Int) and (y -> >> Int) are >> different types despite x and y being the same type, because one of >> them has >> an Eval constraint. This may be a shortcoming of Haskell (or System Fc?) >> types, although it may be doable with a newtype. > > That was kind of what my thinking out loud was getting at. If you want x > -> Int and y -> Int to be different types even if x and y actually are > the same type, then apparently you want x -> Int and y -> Int to be > built from different function-space constructors, say -> and ->*, > yielding x -> Int and y ->* Int. Replacing equals for equals again, you > get x -> Int and x ->* Int. So, basically, we are annotating function > types, what is IIRC exactly what Janis and David are doing. (I hope > Janis corrects me if I'm wrong here). Wrong only in that David's name is Daniel. :-) Other than that, I agree. Best, Janis. -- Jun.-Prof. Dr. Janis Voigtländer http://www.iai.uni-bonn.de/~jv/ mailto:jv@iai.uni-bonn.de

2 1

Fw: Re: [Haskell-cafe] Re: Announce snm-0.0.2: The Simple Nice-Looking Manual Generator
by Benjamin L. Russell 03 Aug '10

03 Aug '10

The following message was sent to me, but was apparently intended for this thread, so I am forwarding it to the Haskell-Cafe Mailing List: On Mon, Aug. 2, 2010 at 11:44 PM, Gordon Sommers <gordon.sommers(a)gmail.com> wrote: > This looks pretty neat! The website would benefit from one or two > "full-size" examples I think. A good example is the classgen > documentation -- they have a series of pages on the specific features, > all with small examples, but then they also have some full examples > like so: http://classgen.sourceforge.net/docs/ex1.html, which provide > a great starting place. > On Thu, Jul 29, 2010 at 5:45 PM, Benjamin L. Russell <DekuDekuplex(a)yahoo.com> wrote: > > Johnny Morrice <spoon(a)killersmurf.com> writes: > > [...] > > > > snm allows you to write clean, web-friendly reports, user guides and > > manuals without having to edit fickle html. > Interesting project! Unlike some users, I happen to enjoy writing > documentation. Perhaps you should also announce this project on the main Haskell > mailing list. > > [...] > > > > Read the snm user guide here: > > http://www.killersmurf.com/static/snm_help.html > > 2. What snm does not do and is not > > > > [...] > > > > snm is not an adult activity. > Hmm ... the term "adult activity" could mean many different things > ... what one did you mean? > Thank you for the many examples in your user guide; they help clarify > usage. > -- Benjamin L. Russell > -- > Benjamin L. Russell / DekuDekuplex at Yahoo dot com > http://dekudekuplex.wordpress.com/ > Translator/Interpreter / Mobile: +011 81 80-3603-6725 > "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^ > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

ICFP 2010: Call for participation
by Wouter Swierstra 03 Aug '10

03 Aug '10

===================================================================== Call for Participation The 15th ACM SIGPLAN International Conference on Functional Programming (ICFP 2010) http://www.icfpconference.org/icfp2010/ Baltimore, Maryland September 25 – October 2 ===================================================================== ICFP 2010 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/icfp2010/program.html * Invited speakers: - Mike Gordon ML: Metalanguage or Object Language? - Matthias Felleisen TeachScheme!: A Checkpoint - Guy Blelloch Functional Parallel Algorithms Schedule including related events: * September 25: Workshop on Mechanizing Metatheory (WMM) Workshop on High-Level Parallel Programming and Applications (HLPP) * September 26: Workshop on ML Workshop on Generic Programming (WGP) * September 27-29: ICFP 2010 * September 30: Haskell Symposium Erlang Workshop * October 1: Commercial Users of Functional Programming – Day 1 (CUFP Tutorials) Haskell Implementors' Workshop * October 2: Commercial Users of Functional Programming – Day 2 (CUFP Talks) Registration information: * Registration link: https://regmaster3.com/2010conf/ICFP10/register.php Local arrangements (including travel and accommodation): * http://www.icfpconference.org/icfp2010/local.html * Conference reservation/rate deadline: September 1st Conference organizers: * General Chair: Paul Hudak, Yale University * Program Chair: Stephanie Weirich, University of Pennsylvania * Local Arrangements Chair: Michael Hicks, University of Maryland * Workshop Co-Chairs: Derek Dreyer, Max Planck Institute for Software Systems Christopher Stone, Harvey Mudd College * Programming Contest Chair: Johannes Waldmann, Hochschule für Technik, Wirtschaft und Kultur, Leipzig * Video Chair: Scott Smith, Johns Hopkins University * Publicity Chair: Wouter Swierstra, Vector Fabrics =====================================================================

1 0

new Cabal user question -- installing to Haskell Platform on Windows network drive?
by Peter Schmitz 03 Aug '10

03 Aug '10

I have recently installed the Haskell Platform (for the first time) to a MS Windows network drive; e.g.: H:\aaa\bbb\Haskell Platform\2010.1.0.0\ I did so without admin privs. It has ghc-6.12.1 I need to not install to C:. I would like to install and use Gtk2Hs and Glade on the Platform also. I have used Gtk2Hs and Glade in Haskell (with just ghc, not the Platform), but I have never used Cabal or the Haskell Platform. I see at http://www.haskell.org/gtk2hs/ that Gtk2Hs now comes in a Cabal package ("Gtk2Hs 0.11.0 released"). The info there says you can just install "the Gtk+ libraries", and then do: cabal install gtk2hs-buildtools cabal install gtk I interpret "the Gtk+ libraries" to mean "gtk: the base GUI library" package listed at http://www.haskell.org/gtk2hs/download/ (is that correct?). That is a link to http://hackage.haskell.org/package/gtk which has "gtk-0.11.0.tar.gz (Cabal source package)", which I downloaded and unpacked to a temp directory which now contains "gtk-0.11.0". I've reviewed the Cabal documentation (that came with the Platform) and I'm having a little trouble determining exactly what to do next -- the exact commands to use for my non-C: / network drive installation. (I have not used Cabal before.) I would prefer to do this without admin privs, if possible. ** If anyone could help me out here with a step-by-step, I would appreciate it. Hopefully the procedure for glade will then be similar. Thanks (very much) in advance. -- Peter Schmitz

4 9

Reverse unification question
by Martijn van Steenbergen 03 Aug '10

03 Aug '10

Dear café, Given: > instance Category C > y :: forall r. C r (A -> r) I am looking for the types of x and z such that: > x . y :: forall r. C r r > y . z :: forall r. C r r Can you help me find such types? I suspect only one of them exists. Less importantly, at least to me at this moment: how do I solve problems like these in general? Thank you, Martijn.

3 2

Microsoft's Singularity Project and Haskell
by Vasili I. Galchin 02 Aug '10

02 Aug '10

Hello, In the latest ACM CACM is a paper on Singularity. Here also is an overview: http://lambda-the-ultimate.org/node/1081. I haven't finished the CACM paper yet but I only mention of languages like C# and F#. Singularity is predicated around providing a safe environment. IMO Haskell is even better than their languages. My $.02. Regards, Vasili

9 11

Fwd: Bug in Parsec.Token
by Derek Elkins 02 Aug '10

02 Aug '10

This is a forward of a message from March 4th. ---------- Forwarded message ---------- From: Derek Elkins <derek.a.elkins(a)gmail.com> Date: Thu, Mar 4, 2010 at 9:43 PM Subject: Re: Bug in Parsec.Token To: Don Stewart <dons(a)galois.com> Cc: Greg Fitzgerald <garious(a)gmail.com>, Antoine Latter <aslatter(a)gmail.com>, "Sittampalam, Ganesh" <ganesh.sittampalam(a)credit-suisse.com>, Ian Lynagh <igloo(a)earth.li>, libraries(a)haskell.org I'm not subscribed to libraries, so this won't go there. One of the first benchmarks against Parsec 3.0.0 was John MacFarlane's here: http://www.haskell.org/pipermail/haskell-cafe/2008-March/040258.html In it, he found Parsec 3.0.0 about 2x slower for his benchmark. I can't recreate his benchmark, but I suspect it is a variant of one he describes here:http://code.google.com/p/pandoc/wiki/Benchmarks I decided to do a similar benchmark. I used Parsec 2.1.0.1, Parsec 3.0.1, and Parsec 3.1.0. Of particular note, building all three required -only- changing which library pandoc depended on. No change to the source was necessary. All tests in pandoc's test suite passed for all versions. Doing that benchmark with a different input file, this file [http://wpcal.firetree.net/wp-content/plugins/PHP%20Markdown%201.0.1k/PHP%20…] concatenated to itself 32 times to produce a 730KB markdown file, I get the following times for the last three of four runs. Parsec 2.1.0.1 derek@derek-laptop:~/temp/pandoc-1.3/dist/build/pandoc$ time ./pandoc-2.1.0.1 --strict t.text > /dev/null real 0m9.863s user 0m7.792s sys 0m0.160s real 0m9.756s user 0m7.792s sys 0m0.132s real 0m10.123s user 0m7.976s sys 0m0.168s Parsec 3.0.1 derek@derek-laptop:~/temp/pandoc-1.3/dist/build/pandoc$ time ./pandoc-3.0.1 --strict t.text > /dev/null real 0m22.008s user 0m17.445s sys 0m0.324s real 0m21.789s user 0m17.433s sys 0m0.160s real 0m21.754s user 0m17.677s sys 0m0.168s Parsec 3.1.0 derek@derek-laptop:~/temp/pandoc-1.3/dist/build/pandoc$ time ./pandoc-3.1.0 --strict t.text > /dev/null real 0m10.708s user 0m8.201s sys 0m0.168s real 0m11.078s user 0m8.401s sys 0m0.232s real 0m10.797s user 0m8.513s sys 0m0.224s These results recreate the approximate 2x slowdown that John originally mentioned between Parsec 2.1.0.1 and Parsec 3.0. It also demonstrates that Parsec 3.1.0 is significantly faster than 3.0.1 but still a little bit slower than Parsec 2.1.0.1. On Thu, Mar 4, 2010 at 4:39 PM, Don Stewart <dons(a)galois.com> wrote: > derek.a.elkins: >> Who is going to maintain "Parsec 4"? >> >> I'm completely against this. If people absolutely must have exactly >> Parsec 2's implementation we can simply copy it into Parsec 3, and the >> "compatibility" layer, in that case, will simply -be- Parsec 2. I've >> considered this as a temporary solution for the performance issues >> just so people could move to Parsec 3 dependencies, but that should >> not be necessary now, and even then I considered it a much less than >> ideal solution. >> >> If the community wants to freeze on Parsec 2, then I have no problem >> renaming the package, otherwise I think it is both unnecessary and a >> waste of effort. >> > > The problem is the ongoing lack of confidence in Parsec 3's performance. > The new release goes some way to addressing this, but I think this has > gone unaddressed for too long. > > Can someone address the lingering concern with benchmarks against parsec 2? > > -- Don >

1 0

ANNOUNCE: parsec2 - a fork or parsec circa 2.1
by Antoine Latter 02 Aug '10

02 Aug '10

Hello, I would like to announce the parsec2 package, which is a maintained fork of the parsec library as of version 2.1.0.1. This project is for folks who would like to use the simpler interface and fewer extensions relative to parsec-3.0+, but don't want to rely on an old version of a package on hackage. Parsec is designed from scratch as an industrial-strength parser library. It is simple, safe, well documented (on the package homepage), has extensive libraries and good error messages, and is also fast. Changes relative to parsec-2.1.0.1: * Added haddocks done for parsec-3 * in Parsec.Token, sort names after accounting for case sensitivity, not before from: Greg Fitzgerald When I say that I'm maintaining the parsec2 package, that means I am accepting bugfix patches and doing work to ensure that the package builds on the current version of GHC. Patches are welcome to support building on any other Haskell implementations. Any inovations in the field of parser combinators will be happening in other venues. Documentation patches are also welcome. I'm not quite sure what to do about the fact that both parsec2 and mainline parsec provide modules of the same name. If there's motion towards using the parsec2 package, we might be able to deprecate and then retire the compatibility layer provided by parsec-3.*. Links: On Hackage: http://hackage.haskell.org/package/parsec2-1.0.0 Source repository: darcs get http://code.haskell.org/parsec2/parsec2 Report a bug: http://trac.haskell.org/parsec2/newticket Antoine

5 6

ANNOUNCE: Takusen 0.8.6
by Jason Dagit 02 Aug '10

02 Aug '10

Hello, The Takusen team would like to announce the latest release of Takusen, 0.8.6. This is primarily a bug fix and test suite enhancement release. The most notable new feature is limited support for string encodings with ODBC. The full list of changes is included at the at the end of this announcement. = Interested in Takusen development? = Takusen is looking for a new long term maintainer. I have agreed to fill the role of maintainer for now, but we are seeking an enthusiastic individual with spare time and a desire to lead Takusen development. = How to get it = This release is available on hackage: cabal update && cabal install Takusen The source code is available on code.haskell.org: darcs get http://code.haskell.org/takusen = New features = - Alistair Bayley: * Database/PostgreSQL/PGFunctions.lhs: show instance for UUID prints Word64s in hex. * Database/PostgreSQL/Enumerator.lhs: add UUID and marshaling functions. * Database/PostgreSQL/PGFunctions.lhs: add UUID and marshalling functions. * Database/ODBC/OdbcFunctions.hsc: add support for different String encodings. New functions to marshal to/from various encodings (Latin1, UTF8, UTF16), and bind/get functions changed to use these. - Daniel Corson * binary data with postgres = Bug fixes = - Alistair Bayley: * Database/ODBC/OdbcFunctions.hsc: fix bug in mkBindBufferForStorable for Nothing (null) case: pass -1 (sqlNullData) as value size. * Database/ODBC/OdbcFunctions.hsc: use sqlNullData in bindParamString Nothing case, rather than -1. A bit more descriptive. * Database/ODBC/Enumerator.lhs: store bind buffers in stmt object, so we retain reference to them for lifetime of statement. Destroy with statement (well, lose the reference). Should fix bind errors found by Jason Dagit. * Database/ODBC/Enumerator.lhs: Oracle only supports two transaction isolation levels (like Postgres). String output bind parameters have max size 8000 (we use 7999 because module OdbcFunctions adds one to the size). * Database/ODBC/OdbcFunctions.hsc: string parameters have different SQL data types when binding columns (SQL_CHAR) and parameters (SQL_VARCHAR). Oracle only supports two transaction isolation levels. * Database/PostgreSQL/PGFunctions.lhs: fix byteaUnesc and add byteaEsc. = Refactoring = - Jason Dagit: * update urls in cabal file - Alistair Bayley: * Takusen.cabal: fixed QuickCheck version spec. * Takusen.cabal: bump version to 0.8.6. * Database/ODBC/OdbcFunctions.hsc: makeUtcTimeBuffer: pass struct size as both buffer size and data size in call to mkBindBuffer. * Database/ODBC/OdbcFunctions.hsc: mkBindBufferForStorable calls mkBindBuffer (reduces duplicated code). * Database/ODBC/Enumerator.lhs: add instance for EnvInquiry to change session char encoding. * Database/ODBC/Enumerator.lhs: add comments to beginTransaction. * Database/Util.lhs: print "printArrayContents", to match function name. * Database/PostgreSQL/Enumerator.lhs: expose byteaEsc and byteaUnesc. = New tests and changes to tests = - Alistair Bayley: * Database/ODBC/Test/OdbcFunctions.lhs: added testBindDouble to test Nothing (null) case for Storable types. * Database/ODBC/Test/OdbcFunctions.lhs: split transaction isolation level tests so there is one test per level. String marshaling tests use 0x10FF40 as max unicode codepoint, because that keeps Oracle happy. Max size for String parameter buffer is 7999 (SQL Server restriction). Don't bury errors raised by tests; print, but continue. Fix fixture cleanup bug in testBindOutput (dropped wrong procedure). * Database/ODBC/Test/Enumerator.lhs: suffix xxx to bindOutput test expected value. * Database/PostgreSQL/Test/PGFunctions.lhs: tests for UUID. * Database/PostgreSQL/Test/Enumerator.lhs: round-trip test for UUID. * Database/PostgreSQL/Test/PGFunctions.lhs: test select of UUID value. * Database/ODBC/Test/OdbcFunctions.lhs: set client charset to UTF8 when postgresql. * Database/Test/Enumerator.lhs: add order-bys to tests with unions. * Database/PostgreSQL/Test/PGFunctions.lhs: add order-by to union test. * Database/ODBC/Test/Enumerator.lhs: set char encoding to UTF8. inquire InfoDbmsName already returns lowercase. * Takusen.cabal: add random to build-depends for tests. * Database/Test/Enumerator.lhs: make test fixtures more friendly to MS Access. * Database/ODBC/Test/OdbcFunctions.lhs: tests modified for MS Access (date tests), plus use new char-encoding aware functions. * Database/ODBC/Test/Enumerator.lhs: change boundary dates test to not use union. Union seems to make MS Access choke. * Database/PostgreSQL/Test/PGFunctions.lhs: add tests for bytea, including QuickCheck roundtrip. * Database/PostgreSQL/Test/Enumerator.lhs: add bytea bind and select test.

9 18

Laziness question
by michael rice 02 Aug '10

02 Aug '10

From: http://en.wikibooks.org/wiki/Haskell/Laziness Given two functions of one parameter, f and g, we say f is stricter than g if f x evaluates x to a deeper level than g x Exercises 1. Which is the stricter function? f x = length [head x] g x = length (tail x) Prelude> let f x = length [head x] Prelude> let g x = length (tail x) Prelude> f undefined 1 Prelude> g undefined *** Exception: Prelude.undefined Prelude> So, g is stricter than f? Wouldn't both functions need to evaluate x to the same level, *thunk* : *thunk* to insure listhood? f x = length [head *thunk* : *thunk*] g x = length (tail *thunk* : *thunk*) Michael

13 20

Cabal LD errors
by wren ng thornton 02 Aug '10

02 Aug '10

So I'm getting some weird linking errors from cabal-install when doing `cabal configure && cabal build` ld warning: atom sorting error for _postazm0zi2zi0_DataziMonoidziOrdziArgmax_Max_closure_tbl and _postazm0zi2zi0_DataziMonoidziOrdziArgmax_Min_closure_tbl in dist/build/Data/Monoid/Ord/Argmax.o ld warning: atom sorting error for _postazm0zi2zi0_DataziMonoidziOrdziArgmax_Max_closure_tbl and _postazm0zi2zi0_DataziMonoidziOrdziArgmax_Min_closure_tbl in dist/build/Data/Monoid/Ord/Argmax.o But cabal doesn't quit with an error code or anything. Doing `ghc --make Data/Monoid/Ord/Argmax.hs` works just fine without warnings. Anyone know what could cause them, or how to fix it? -- Live well, ~wren

3 3

Chart package segfaults when rendering to window
by briand＠aracnet.com 02 Aug '10

02 Aug '10

Seems to be ok rendering to png files. I was wondering if anybody has been using Chart and may have seen the same thing. I'm running ghc 6.12.1 and the latest and greatest gtk2hs :-) Thanks, Brian p.s. the amplitude modulation

4 4

Anyone here from New Zealand?
by Tim Matthews 01 Aug '10

01 Aug '10

Any of the haskellers here from NZ? Are you using haskell in production, internally within your company or just outside of work in your own time? In Aus they've got hackathons<http://www.haskell.org/haskellwiki/AusHac2010>and user groups <http://www.haskell.org/haskellwiki/User_groups#Australia>. Anything even remotely similar here in New Zealand? Thanks Tim Matthews

5 5

Problem loading hxt
by David Place 01 Aug '10

01 Aug '10

Hello: I am trying to load hxt into my Haskell Platform 2010.2.0.0 on OSX. I get the following bizarre comment: David-Places-Mac-Mini:dev2 davidplace$ cabal install hxt Resolving dependencies... cabal: dependencies conflict: ghc-6.12.3 requires directory ==1.0.1.1 however directory-1.0.1.1 was excluded because ghc-6.12.3 requires directory ==1.0.1.2 I see that I have the older directory in my global package list and the newer one in my local list. Help? Thanks. David-Places-Mac-Mini:dev2 davidplace$ ghc-pkg list /Library/Frameworks/GHC.framework/Versions/612/usr/lib/ghc-6.12.3/package.conf.d Cabal-1.8.0.6 GLUT-2.1.2.1 HTTP-4000.0.9 HUnit-1.2.2.1 OpenGL-2.2.3.0 QuickCheck-2.1.1.1 array-0.3.0.1 base-3.0.3.2 base-4.2.0.2 bin-package-db-0.0.0.0 bytestring-0.9.1.7 cgi-3001.1.7.3 containers-0.3.0.0 deepseq-1.1.0.0 directory-1.0.1.1 dph-base-0.4.0 dph-par-0.4.0 dph-prim-interface-0.4.0 dph-prim-par-0.4.0 dph-prim-seq-0.4.0 dph-seq-0.4.0 extensible-exceptions-0.1.1.1 ffi-1.0 fgl-5.4.2.3 filepath-1.1.0.4 ghc-6.12.3 ghc-binary-0.5.0.2 ghc-prim-0.2.0.0 haskell-src-1.0.1.3 haskell98-1.0.1.1 hpc-0.5.0.5 html-1.0.1.2 integer-gmp-0.2.0.1 mtl-1.1.0.2 network-2.2.1.7 old-locale-1.0.0.2 old-time-1.0.0.5 parallel-2.2.0.1 parsec-2.1.0.1 pretty-1.0.1.1 process-1.0.1.3 random-1.0.0.2 regex-base-0.93.2 regex-compat-0.93.1 regex-posix-0.94.2 rts-1.0 stm-2.1.2.1 syb-0.1.0.2 template-haskell-2.4.0.1 time-1.1.4 unix-2.4.0.2 xhtml-3000.2.0.1 zlib-0.5.2.0 /Users/davidplace/.ghc/i386-darwin-6.12.3/package.conf.d Cabal-1.8.0.6 binary-0.5.0.2 directory-1.0.1.2 fingertree-0.0.1.0 haskell98-1.0.1.1 process-1.0.1.3 random-1.0.0.2 time-1.2.0.3 uu-parsinglib-2.4.2 uu-parsinglib-2.4.4 uu-parsinglib-2.4.5 uu-parsinglib-2.5.0 uulib-0.9.12

2 1

minimizing jitter while maximizing throughput
by Claude Heiland-Allen 01 Aug '10

01 Aug '10

Greeting, Jitter vs Throughput ==================== Scenario -------- I have the following scenario: CPU with [C] cores concurrent program the 1 main thread uses OpenGL for animated visual output [W] worker threads uses FFI to lengthy numerical computations with the following desires : (J) minimize jitter : the 1 main thread needs to be responsive (T) maximize throughput: idle CPU time is a waste of time The problem is, I want both (J) and (T)! Benchmarks ---------- Some rough benchmarks from my 'production' implementation[1] where: jitter = stddev of actual period (target period = 40ms) idle = C - (real time) / (user+sys time) (cores used elsewhere) (but for example Xorg will use some time for the OpenGL output, etc) C W jitter idle 2 0 1.5ms 168% threaded RTS 2 1 1.9ms 65% " " 2 2 9.4ms 46% " " 2 3 13.3ms 37% " " For comparison, a (very) minimal GLUT program gives: 0.3ms threaded RTS 0.5ms non-threaded RTS Picking (J) over (T), then setting W=1 when C=2 gives best jitter Picking (T) over (J), then setting W=2 when C=2 gives best throughput Question -------- What is the best way to re-structure the program to have both low jitter and high throughput? Options I am considering: 1. worker threads estimate the time needed to complete each job, and don't start a job if it is likely to break the deadline (bonus points if just those worker Haskell threads running on the same OS thread as the main Haskell thread need to pause) 2. change the foreign code to break jobs into smaller pieces, for example perhaps something like: worker :: IO (IO Bool) -> IO () worker getJob = forever $ do job <- getJob whileM_ job yield -- [2] instead of worker :: IO (IO ()) -> IO () worker = forever . join 3. re-implement the foreign code in Haskell instead of C and hope that GHC makes the Haskell go as fast as GCC makes the C go 5. wait (for a long time) for a new RTS with: full pre-emption (including interrupting/resuming foreign code) user settable thread priorities (1) is "a fun challenge" (there may be PhDs awarded for less, I imagine) (2) isn't quite trivial, some scope for tuning subjob chunk size (3) is boring translation but could lead to interesting benchmarks even (especially?) if it fails to be as fast as C Which would you pick? Links ----- [1] http://hackage.haskell.org/package/mandulia [2] http://hackage.haskell.org/packages/archive/monad-loops/latest/doc/html/Con… Thanks for any insight and advice, Claude -- http://claudiusmaximus.goto10.org

1 0

dear traversable
by Ben 01 Aug '10

01 Aug '10

dear traversable geniuses -- i am looking for "better" implementations of unzipMap :: M.Map a (b, c) -> (M.Map a b, M.Map a c) unzipMap m = (M.map fst m, M.map snd m) unliftMap :: (Ord a) => M.Map a (b -> c) -> M.Map a b -> M.Map a c unliftMap mf ma = M.mapWithKey (\k v -> mf M.! k $ v) ma the first is obviously inefficient as it traverses the map twice. the second just seems like it is some kind of fmap. any ideas? ben

7 14

Type Families: deleting from HList.
by Serguey Zefirov 01 Aug '10

01 Aug '10

Is it possible to delete an element from heterogenous list using type families alone? I can do it using multiparameter type classes: class Del a list result instance Del a (a,list) list instance Del a list list' => Del a (a',list) list' instance Del a () () I tried to express the same using type families: type family Del a list type instance Del a () = () type instance Del a (a,list) = list type instance Del a (a',list) = (a',Del a list) to no avail. I got "conflicting family instance declarations" error. I tried to express that with associated type synonyms that contains both multiparameter type classes and type families: class HDel a list where type Del a list instance HDel a () where type Del a () = () instance HDel a (a,list) where type Del a (a,list) = list instance HDel a list => HDel a (a',list) where type Del a (a',list) = (a',Del a list) And I once again got "conflicting family instance declaration". It looks like right now there is no way to express type equality or inequality with type families, even when they are combined with multiparameter type classes. What is the right way to express something like that? Only MPTC?

1 0

Constructor question
by michael rice 31 Jul '10

31 Jul '10

From: Data.Complex data (RealFloat a) => Complex a = !a :+ !a What's the purpose of the exclamation marks? Michael

3 6

Monad transformers, design
by Tony Morris 31 Jul '10

31 Jul '10

Hello I have a question regarding monad transformers and how to design an API with a transformer. I have a narrowed code example of the question. Please see the questions in the comments below. import Data.Monoid import Control.Monad -- Suppose some data type newtype Inter a = Inter (Int -> a) -- and a monad transformer for that data type. newtype InterT m a = InterT (m (Inter a)) -- It's easy to implement this type-class instance (Monoid a) => Monoid (Inter a) where mempty = Inter (const mempty) Inter a `mappend` Inter b = Inter (a `mappend` b) -- and for the transformer too by lifting into the monad instance (Monad m, Monoid a) => Monoid (InterT m a) where mempty = InterT (return mempty) InterT a `mappend` InterT b = InterT (liftM2 mappend a b) -- But what about this type-class? class Ints a where ints :: a -> Int -> Int -- Seems easy enough instance (Integral a) => Ints (Inter a) where ints (Inter a) n = fromIntegral (a n) -- OH NO! {- instance (Monad m, Integral a) => Ints (InterT m a) where ints (InterT a) n = error "OH NO!" -} -- We could try this class Copointed f where copoint :: f a -> a -- but it seems rather impractical instance (Copointed m, Integral a) => Ints (InterT m a) where ints (InterT a) = ints (copoint a) {- So it seems that for some type-classes it is possible to implement for both the data type and the transformer, but not all type-classes. Is there a general approach to this problem? -} -- Tony Morris http://tmorris.net/

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using the orc library
by Günther Schmidt 31 Jul '10

31 Jul '10

Hello, I'd like to download 1,000 web pages with up to 6 six concurrent downloads at a time. How can I express such a thread limit within the orc EDSL? Günther

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Re: Re: Can we come out of a monad?
by Jason Catena 31 Jul '10

31 Jul '10

On Jul 30, 11:17 am, Anton van Straaten wrote: > Prelude> :m Control.Monad.State > Prelude Control.Monad.State> let addToState :: Int -> State Int (); > addToState x = do s <- get; put (s+x) > Prelude Control.Monad.State> let mAdd4 = addToState 4 > Prelude Control.Monad.State> :t mAdd4 > m :: State Int () > Prelude Control.Monad.State> let s = execState mAdd4 2 > Prelude Control.Monad.State> :t s > s :: Int > Prelude Control.Monad.State> s > 6 By this example State doesn't seem to give you anything more than a closure would, since it doesn't act like much of an accumulator (by, for example, storing 6 as its new internal value). Could you use State for something like storing the latest two values of a Fibonacci series? For example, each time you call it, it generates the next term, discards the oldest term, and stores the newly-generated term? And could you then use this Fibonacci State monad in a lazy computation, to grab for example the first twenty even Fibonacci numbers, without computing and storing the series beyond what the filter asks for? We can generate Fibonacci series double-recursively in a lazy computation. Would it be more or less efficient to use a Fibonacci State monad instead? Would the State implementation provide a larger range before it blew the stack (which tail-recursion should prevent), or became too slow for impatient people? Would Haskell memoize already-generated values in either case? Could we write a general memoizer across both the recursive and State implementations, or must we write a specific one to each case? Jason Catena

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Announce type-level-natural-number-1.0: Simple, Haskell 2010-compatible type level natural numbers
by Gregory Crosswhite 31 Jul '10

31 Jul '10

Hey everyone, I am pleased to announce the release of the "type-level-natural-number" package, which implements natural numbers at the type level. Although there are many packages on Hackage that implement type level natural numbers, this package is distinguished by its simplicity: it does not offer any operations other than predecessor, successor, and conversion to Int, and so the only Haskell extension it needs is EmptyDataDecls. Thus, this package is compatible with Haskell 2010. The envisioned use case is for annotating types with natural numbers, so that extra functionality such as addition and subtraction of natural numbers is unnecessary. Nothing is stopping someone from implementing this functionality, and in fact I may do so myself; however, since such functionality is not needed merely for using these numbers, I decided to leave it out in order to avoid having to use non-Haskell 2010 extensions (especially UndecidableInstances). In particular, a major motivation behind designing the package in this way is to make it more appealing as a standard means for representing natural number annotations in order to promote interoperability between libraries. I welcome any feedback that the community has to offer. Cheers, Gregory Crosswhite

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Database.CouchDB broken?
by Moritz Ulrich 31 Jul '10

31 Jul '10

Hello, I'm currently learning Haskell and I want to write a small tool to collect some data in a CouchDB-Database Sadly, the Database.CouchDB module from hackage (and from git) seems broken. It looks like a bug deep in the JSON handling of the lib. Some examples can be found in this gist: http://gist.github.com/475323 ('test' is a database with two simple documents, the doc with the id '8e9112011580882422393f6291000f7d' exists) I filed an issue, but the maintainer hasn't responded in 5 days. Is there anything I missed? Thanks in advance! -- Moritz Ulrich Programmer, Student, Almost normal Guy http://www.google.com/profiles/ulrich.moritz

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Definition of List type?
by michael rice 31 Jul '10

31 Jul '10

From: Data.Maybe Description The Maybe type, and associated operations. From: Data.List Description Operations on lists. One description has the type and associated operations, the other only has the operations. Where can I find the type definition for List, and why isn't it in Data.List? Michael

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