- Haskell-Cafe - Haskell.org

Canonical Graphviz code
by Ivan Miljenovic 07 Jul '10

07 Jul '10

Graphviz (http://graphviz.org/) has the option to convert provided Dot code for visualising a graph into a canonical form. For example, take the sample Dot code: ---- | digraph foo { | a [color=blue]; | subgraph cluster_bar { | a [label="hi!"] | b [color=blue, label="bye!"]; | b -> c; | } | a -> b; | a -> c; | c -> d; | d [color=red] `---- Calling "dot -Tcanon" on this produces: ,---- | digraph foo { | node [label="\N"]; | subgraph cluster_bar { | a [label="hi!", color=blue]; | b [label="bye!", color=blue]; | b -> c; | a -> b; | a -> c; | } | d [color=red]; | c -> d; | } `---- Note in particular the following: * The addition of the top-level "node [label="\N"]" attribute; this happens automatically even if all nodes have their own custom label. * The two `a' nodes are merged into the inner-most cluster/subgraph that they are defined in (since they represent the same node). * The attributes for `b' are re-ordered. * The "a -> b" and "a -> c" edges are shifted into the cluster, since all of the related nodes are in that cluster (note that the `c' node is implicitly defined there since an edge involving it is defined there). * The "c -> d" edge is shifted to being _after_ the definition of the `d' node (in fact, in each grouping all nodes are defined before edges). I've recently thought up a way that I can duplicate this functionality (in terms of what it does, not necessarily the actual output) in my graphviz library (http://hackage.haskell.org/package/graphviz) however I'm not sure how closely to follow what Graphviz does. What would the community prefer out of the following (including a mixture, such as options 2 and 3): 1) Match Graphviz's output as much as possible (note that the ordering of the attributes won't happen this way, since I'll be sorting them alphabetically rather than working out Graphviz's arcane rules). 1a) As with 1), but don't worry about defining extra attributes such as "node [label="\N"]". 2) Explicitly define nodes that are only mentioned in edges (e.g. actually define a `c' node, even if it doesn't have any extra attributes). Note that this already happens if such an edge is specified inside a different cluster than the already known node is in; in that case then the stand-alone node is defined in the cluster its edge is defined in, and the actual edge is moved into the outer context that combines the two clusters. 2a) As with 2), but define all edges in the overall graph rather than grouping them internally inside clusters, etc. 3) Group common attributes together as much as possible (e.g. inside the bar cluster, define another subgroup which has a top-level attribute "node [color=blue]" and define the `a' and `b' nodes in there). Not sure how well this will work with edges though, and is probably not possible in conjunction with option 2a). This is a bit tricky and subtle: if `a' and `b' are already in such a subgraph, but an edge specifying one of them is defined _before_ the subgraph is defined, then that node will have an explicit color attribute defined of "" (i.e. empty string), though I would classify this as a bug. 4) As an alternate to option 3), remove all explicit top-level node and edge attributes and move them all to the affected node and edge definitions themselves; this may require option 2). I'm also willing to hear and consider other possible variants; however, I'm only going to code (at most) one. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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ANNOUNCE: ghc-gc-tune: a tool for analyzing the impact of GC flags
by Don Stewart 07 Jul '10

07 Jul '10

Inspired by a comment by Simon Marlow on Stack Overflow, about the time and space tradeoffs we make with garbage collection, particularly with a generational GCs, I wrote a small program, ghc-gc-tune, to traverse the garbage collector variable space, to see the relationship between settings and program performance. Given a program, it will show you an (optionally interactive) graph of how -A and -H flags to the garbage collector affect performance. http://donsbot.wordpress.com/2010/07/05/ghc-gc-tune-tuning-haskell-gc-setti… Feedback and patches welcome! -- Don

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CGI differences FastCGI
by Jonas Fager 06 Jul '10

06 Jul '10

Hi. This is probably "I must be blind department" I have a very basic strip down echo cgi that runs with apache2. When I try to modfi it to be a FastCGI instead and run it with spawn-fcgi with nginx It don't behave as expected. I suppose that the line that don't get any data is mn <- getInputFPS "file" when running under FastGCI. So is the code wrong? or have I missed something with the setup? I can run the example of fastcgi provided in http://mult.ifario.us/p/wiring-haskell-into-a-fastcgi-web-server with no problem. Description: Ubuntu 10.04 LTS 2.6.32-22-generic-pae #36-Ubuntu SMP The Glorious Glasgow Haskell Compilation System, version 6.12.1 +++++ import Control.Concurrent import Network.CGI -- import Network.FastCGI import Text.Html import qualified Data.ByteString.Lazy as BS import qualified Data.ByteString.Lazy.Char8 as BS8 import Control.Monad (liftM) import Data.Maybe (fromJust) import Char import Data.List import System.Time import qualified Data.ByteString.Char8 as C import Text.JSON.Generic import Text.JSON insertthis fb = do let f = BS8.unpack fb return (f) fileForm = form ! [method "post", enctype "multipart/form-data"] << [textfield "file", submit "" "Upload"] saveFile cont = do r<-liftIO $ insertthis cont return $ paragraph << ("Inserted " ++ (show r)) page t b = header << thetitle << t +++ body << b test :: CGI CGIResult test = do setHeader "Content-Type" "text/html; charset=utf-8" mn <- getInputFPS "file" h <- maybe (return fileForm) saveFile mn output $ renderHtml $ page "Upload" h main = runCGI $ handleErrors test --main= runFastCGI $ handleErrors test +++++

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Subtype polymorphism in Haskell
by Simon Courtenage 06 Jul '10

06 Jul '10

Hi, I am porting a C++ program to Haskell. My current task is to take a class hierarchy and produce something equivalent in Haskell, but I don't seem to be able to get a grip on how type classes and instances contribute to the solution. Can anyone enlighten me? Roughly, the class hierarchy in C++ is of the form class A { public: virtual int do_x(int,int) = 0; }; class B { public: int do_x(int x,int y) { ...} }; class C { public: int do_x(int x,int y) { ...} }; Any help would be greatly appreciated. Thanks Simon courtenage(a)gmail.com

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Criteria for determining if a recursive function can be implemented in constant memory
by Steffen Schuldenzucker 06 Jul '10

06 Jul '10

Dear Cafe, since a little discussion with my tutor I am wondering how the following problem can be solved and if it is decidable: Given the definition of a recursive function f in, say, haskell, determine if f can be implemented in O(1) memory. First I thought the solution would be "check if f is tail-recursive". However, consider the following definition: > -- for the sake of uniformity > if' c t e = if c then t else e > > f :: (Num a) => a -> a -> a > f x y = if' (x <= 0) y $ > if' (x >= 2) (f (x-2) (y+1)) (f (x-1) y) (I don't really care what this function computes as long as it terminates, which is obvious) Although ghc will probably not perform this optimization, f can be realized in O(1) mem: // trying to duplicate f's structure as closely as possible double f( double x, double y ) { START: if ( x <= 0 ) return y; else { if ( x >= 2 ) { x = x - 2; y = y + 1; goto START; } else { x = x - 1; y = y; goto START; } } } It is crucial that (the second) if' does not use both of its last two arguments, but only one. If we replace the second if' by, say > g :: (Num a) => c -> a -> a -> a > g c t e = if' c (t + e) (t - e) , then we have to compute *both* (f (x-2) (y+1)) and (f (x-1) y), and x and y have to be kept in memory during the call to (f (x-2) (y+1)), therefore f cannot be implemented in constant memory. (read: I haven't found a way which does not radically alter f's structure). So, does someone know how to solve this or can prove that it can't be solved? Best regards, Steffen

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Re: [Haskell-beginners] upgrade Hackage show to QuickCheck 2 for lambdabot
by Antoine Latter 06 Jul '10

06 Jul '10

Including the café. On Jul 2, 2010 8:49 AM, "Mark Wright" <markwright(a)internode.on.net> wrote: Hi, I'm trying to upgrade Hackage show to QuickCheck 2, after applying the diffs below (which may not be correct, since I am a beginner), I am left which this error message: runghc ./Setup.hs build Preprocessing library show-0.3.4... Building show-0.3.4... [4 of 4] Compiling ShowQ ( ShowQ.hs, dist/build/ShowQ.o ) ShowQ.hs:104:20: Not in scope: `generate' Compilation exited abnormally with code 1 at Fri Jul 2 23:07:17 The error occurs in this method: tests :: Gen Result -> StdGen -> Int -> Int -> [[String]] -> IO String tests gen rnd0 ntest nfail stamps | ntest == 500 = done "OK, passed" ntest stamps | nfail == 1000 = done "Arguments exhausted after" ntest stamps | otherwise = case ok result of Nothing -> tests gen rnd1 ntest (nfail+1) stamps Just True -> tests gen rnd1 (ntest+1) nfail (stamp result:stamps) Just False -> return $ "Falsifiable, after " ++ show ntest ++ " tests:\n" ++ reason result where result = generate (((+ 3) . (`div` 2)) ntest) rnd2 gen (rnd1,rnd2) = split rnd0 The QuickCheck 1 generate method is near the bottom this page: http://hackage.haskell.org/packages/archive/QuickCheck/1.2.0.0/doc/html/Tes… but I can not find generate in QuickCheck 2. I am wondering if you have any ideas on how to fix it? I'm trying to package lambdabot on Solaris. I have already packaged the Haskell Platform and about 90 packages, they are in: http://pkgbuild.sourceforge.net/spec-files-extra/ Thanks very much, Mark here are the diffs: goanna% diff -wc show-0.3.4-orig/ShowQ.hs show-0.3.4/ShowQ.hs *** show-0.3.4-orig/ShowQ.hs Wed Jan 20 11:24:11 2010 --- show-0.3.4/ShowQ.hs Fri Jul 2 23:07:13 2010 *************** *** 12,22 **** --- 12,25 ---- import qualified Test.SmallCheck (smallCheck, Testable) import Test.QuickCheck + import Test.QuickCheck.Arbitrary import Data.Char import Data.List import Data.Word import Data.Int import System.Random + import Control.Exception (evaluate) + import Test.QuickCheck.Property (ok, stamp) type T = [Int] type I = Int *************** *** 23,36 **** --- 26,45 ---- instance Arbitrary Char where arbitrary = choose (minBound, maxBound) + + instance CoArbitrary Char where coarbitrary c = variant (ord c `rem` 4) instance Arbitrary Word8 where arbitrary = choose (minBound, maxBound) + + instance CoArbitrary Word8 where coarbitrary c = variant (fromIntegral ((fromIntegral c) `rem` 4)) instance Arbitrary Ordering where arbitrary = elements [LT,EQ,GT] + + instance CoArbitrary Ordering where coarbitrary LT = variant 1 coarbitrary EQ = variant 2 coarbitrary GT = variant 0 *************** *** 37,42 **** --- 46,53 ---- instance Arbitrary Int64 where arbitrary = sized $ \n -> choose (-fromIntegral n,fromIntegral n) + + instance CoArbitrary Int64 where coarbitrary n = variant (fromIntegral (if n >= 0 then 2*n else 2*(-n) + 1)) instance (Integral a, Arbitrary a) => Arbitrary (Ratio a) where *************** *** 48,53 **** --- 59,65 ---- else (b % a) else (a % b) + instance (Integral a, CoArbitrary a) => CoArbitrary (Ratio a) where coarbitrary m = variant (fromIntegral $ if n >= 0 then 2*n else 2*(-n) + 1) where n = numerator m *************** *** 87,93 **** Just False -> return $ "Falsifiable, after " ++ show ntest ++ " tests:\n" ! ++ unlines (arguments result) where result = generate (((+ 3) . (`div` 2)) ntest) rnd2 gen (rnd1,rnd2) = split rnd0 --- 99,105 ---- Just False -> return $ "Falsifiable, after " ++ show ntest ++ " tests:\n" ! ++ reason result where result = generate (((+ 3) . (`div` 2)) ntest) rnd2 gen (rnd1,rnd2) = split rnd0 goanna% _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://www.haskell.org/mailman/listinfo/beginners

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Please check your dependencies on fgl
by Ivan Miljenovic 06 Jul '10

06 Jul '10

Thomas Bereknyei are currently re-writing fgl (just about completely from scratch) and we plan to make an initial release to get feedback on the API in the next few weeks. However, I'm sending this email out now to warn people that I highly doubt any code that was written for the current version of fgl (http://hackage.haskell.org/package/fgl-5.4.2.2) will work with the new version. As such, if your code uses fgl you should ensure that the specification of the package in your .cabal file has an explicit upper bound (or indeed just set "fgl == 5.4.2.2") to avoid any problems when we release 6.0.0.0 (we're trying to avoid the problems that plagued KDE-4 by not claiming in the slightest that the initial few versions are intended for end users). I have already directly emailed the maintainers of any and all packages I could find on Hackage (using Roel van Dijk's wonderful reverse-dependency mirror at http://bifunctor.homelinux.net/~roel/hackage/packages/hackage.html) but just in case I've missed one or there's an as-yet-unreleased package somewhere in the works, here is the official "heads up". -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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cereal vs. binary
by braver 06 Jul '10

06 Jul '10

I dump results of a computation as a Data.Trie of [(Int,Float)]. It contains about 5 million entries, with the lists of 35 or less pairs each. It takes 8 minutes to load with Data.Binary and lookup a single key. What can take so long? If I change from compressed to uncompressed (and then decode), it's the same time... It's not IO, CPU is loaded 100%. I'm now thinking of using cereal. Given I have Data.Binary in place, what needs to be changed to work with cereal? Is it binary- compatible? How can one construct a cereal instance for Data.Trie? -- Alexy

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More experiments with ATs
by Andrew Coppin 06 Jul '10

06 Jul '10

Currently we have class Functor c where fmap :: (x -> y) -> (c x -> c y) This relies on c having kind * -> *. For example, Bytestring cannot be an instance of Functor. A cleaner solution would be to have something like class Container c where type Element c :: * I then attempted to write class Container c => Functor c where fmap :: (Functor cx, Functor cy, Element cx ~ x, Element cy ~ y) => (x -> y) -> (cx -> cy) However, this fails horribly: The type signature fails to mention c. And even if it did, this type signature still isn't correct: It says that fmap can transform *any* functor into *any* other functor, which is wrong. It seems that we have gained the ability to talk about containers holding any type of value (even hard-coded types or types with class constraints), but lost the ability to refer to a particular "type of" functor without also specifying the element type. Does anybody know a straightforward way to fix this? Or is this just a dead end?

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emacs tags
by Michael Mossey 06 Jul '10

06 Jul '10

(I asked this question on the logical email list to ask, haskell-mode, but got no reply, so I'm hoping someone here might know the answer.) I tried to use tags with emacs. I ran :etags in ghci, then tried to use C-c M-. inside Emacs to find the definition of a symbol. It reported "no source information available". I did some basic checking---a TAGS file does exist in the same directory as the file I was visiting, and contains some kind of entry for the specific symbol (whether the right syntax I don't know). I'm using haskell-mode 2.4 and emacs 21.3.1 on Windows XP. Also ghci 6.12.1. Thanks, Mike

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Is my code too complicated?
by Ertugrul Soeylemez 06 Jul '10

06 Jul '10

Hello fellow Haskellers, I'd like to discuss an interesting topic. My experience is that there are two worlds in Haskell, which are quite separate: the pure, algorithmic world, where you use idiomatic pure Haskell, and the IO-driven world of state, change, threads and execution, where you use idiomatic concurrent Haskell. If you understand these two concepts well, then you are usually a good Haskell programmer. Interestingly this doesn't mean that you get applications done faster or done at all. Most languages today provide a certain "glue" to bring everything together. I think that term originates from "Why Functional Programming Matters" by John Hughes. Haskell provides a lot of low level glue like laziness, currying and other very helpful language features. But what is different in Haskell is that it doesn't seem to provide any high level glue like other languages do, especially when it comes to the IO world. There is a somewhat powerful module system, but nothing to bring modules and the objects they define together in a consistent way. In my opinion this is both a disadvantage and an advantage. It's bad because there is no standard way of gluing, nothing everybody learns and uses. On the other hand it's good, because you can make your own glue. This has proven very useful for me. My usual way is writing monad transformers and sticking them together, often together with concurrent programming. The problem with that approach is: This makes my code harder to understand for beginners. Usually they can tell /what/ my code is doing, because it's written in natural language as much as possible, but they couldn't reproduce it. And when they try to learn it, they give up fast, because you need quite some background for that. Also sometimes when I write Haskell, my friend sits beside me and watches. When he asks (as a PHP programmer with some C background), say, about my types, I can't give a brief explanation like I could in other languages. Yesterday I was writing a toy texture handler for OpenGL (for loading and selecting textures). He asked about my TextureT type. I couldn't explain it, because you couldn't even express such a thing in PHP or C. type TextureT = StateT Config -- Note that this is MonadLib. -- BaseM m IO corresponds to MonadIO m. selectTexture :: BaseM m IO => B.ByteString -> TextureT m () I fear that my code is already too difficult to understand for beginners, and it's getting worse. But then I ask myself: I've got a powerful language, so why shouldn't I use that power? After all I haven't learnt Haskell to write C code with it. And a new Haskell programmer doesn't read my code to learn Haskell. They first learn Haskell and /then/ read my code. Is this a real problem or am I just exaggerating? What do you think? Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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Getting started
by Mrwibbly 05 Jul '10

05 Jul '10

I'm having real trouble starting this project. Basically I have to create a record store that store information about artists and albums and also the number of sales that they have had. It also needs to generate a list of the top 5 sellers. So far I have: recordSale :: Sales -> String -> String -> Sales where recordSale sales anArtist aTrack returns a modified version of the sales. Any help getting started on this would be gratefully received. I don't want answers, I just want help getting started. Thank you -- View this message in context: http://old.nabble.com/Getting-started-tp29046956p29046956.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Fwd: [Haskell-cafe] Re: Is my code too complicated?
by Alberto G. Corona 05 Jul '10

05 Jul '10

Very often the type system assures safety to a point where some coding is done trough trial and error: -I think that this composition is right, let´s try it-... -oh, some missing type here, Let´s put it here and here-...-go on-... -The typechecker say t´s OK. I run it and, well it works. Later when I look at the code, I have to think twice to understand it. Sometimes I mix transactions, threading, IO and some other abstractions in the same line, things that I would fear much to work with in any other language. This playing thig is fine, because the typechecking cut a lot of dead branches in the entrophic tendency of humans to commit mistakes. For this matter I praise math as the hidden god that handles reality and haskell its prophet. but the resulting code is not a product of my intended design, but something made together with the compiler, and sometimes it needs more than a look to understand it, even to myself. 2010/7/5 Ertugrul Soeylemez <es(a)ertes.de> Felipe Lessa <felipe.lessa(a)gmail.com> wrote: > > > On Sat, Jul 3, 2010 at 9:25 AM, Ertugrul Soeylemez <es(a)ertes.de> wrote: > > > Haskell provides a lot of low level glue like laziness, currying and > > > other very helpful language features. But what is different in > > > Haskell is that it doesn't seem to provide any high level glue like > > > other languages do, especially when it comes to the IO world. There > > > is a somewhat powerful module system, but nothing to bring modules > > > and the objects they define together in a consistent way. > > > > When I first read this paragraph, I thought: "STM to the rescue!". > > STM is one of the best concurrent world glues, IMHO. > > I found that I get along with the basic concurrency constructs. STM may > be handy in a few applications, but in none that I write. > > > > If you want, you may use Haskell just as you as PHP or C: just put > > everything in IO. Your code will get uglier and the type system won't > > catch many bugs, but that's what we get when doing C or PHP, right? > > Not really. Even when programming in such a style, Haskell is much > safer than PHP with its braindead type system, and still somewhat safer > than C. > > > > > The problem with that approach is: This makes my code harder to > > > understand for beginners. Usually they can tell /what/ my code is > > > doing, because it's written in natural language as much as possible, > > > but they couldn't reproduce it. And when they try to learn it, they > > > give up fast, because you need quite some background for that. Also > > > sometimes when I write Haskell, my friend sits beside me and > > > watches. When he asks (as a PHP programmer with some C background), > > > say, about my types, I can't give a brief explanation like I could > > > in other languages. > > > > I agree that it gets harder to reason about the code. In fact, > > sometimes I stack monad transformers in the wrong order. However, as > > Ivan says, if the feature is useful for you, don't be afraid of using > > it. Beginners may have a hard time grasping the concepts for the > > first time, but that's only until they "get it". > > I find monad transformers easy to reason about, and in most cases the > stacking order doesn't make a difference at all. Just remember to > change the running function, too. The problem with them is that > beginners learn them very late. > > > > > Yesterday I was writing a toy texture handler for OpenGL (for > > > loading and selecting textures). He asked about my TextureT > > > type. I couldn't explain it, because you couldn't even express such > > > a thing in PHP or C. > > > > > > type TextureT = StateT Config > > > > > > -- Note that this is MonadLib. > > > -- BaseM m IO corresponds to MonadIO m. > > > selectTexture :: BaseM m IO => B.ByteString -> TextureT m () > > > > "It is the type of functions that may access and modify a state of > > type Config." > > Then you need to explain "type of functions" and this explicitly > implicit "state" and why you have to do it that way in Haskell. > > > Greets, > Ertugrul > > > -- > nightmare = unsafePerformIO (getWrongWife >>= sex) > http://ertes.de/ > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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How easy is it to hire Haskell programmers
by Paul Johnson 05 Jul '10

05 Jul '10

I'm starting to see job adverts mentioning Haskell as a "nice to have", and even in some cases as a technology to work with. However right now I'm looking at it from the other side. Suppose someone wants to hire a Haskell developer or three. How easy is this? I'd appreciate replies from people who have actually done this. * How many applications did you get? * How many of those applicants knew what a monad is, or how to write FizzBuzz in Haskell? Thanks, Paul.

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Re: [Haskell-cafe] music-related problem
by Michael Mossey 05 Jul '10

05 Jul '10

erik flister wrote: > I am dealing with ties because I am converting a MusicXML document > into a more natural form for my purposes. The initial form of the > document will have tied notes (as it comes that way from MusicXML), > and I want to convert that into a form that makes it possible to > ignore ties and see notes as having a single duration. > > > but can't you just say that the first note of a tie has the indicated > start time and a duration which is the sum of the tied notes? i don't > know musicXML, but i'm not seeing why that would be hard... That's what I want to do. I'm asking about a good way to write the algorithm that traverses the notes and reconstructs the document with the correct duration in each note. Actually, I don't want to lose information about the original form of the document, so I have separate fields for the duration of the graphical single note, and the duration of the tied chain. Actually, it is better to speak of the end time than the duration, because what units do you put duration in? Beats? The time signature could be changing measure to measure. MusicXML "position"? The meaning of "one position" changes measure to measure and can be different in different parts. It can get confusing. So I use this concept of "location": data Loc = Loc Int Rational -- measure number and beat within the measure I.e. measure 1, beat 2 measure 7, beat 3 1/2 I use Rational so there is no worry about precision of Floats. Thanks, Mike

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Undecidable Instances [Was: Is my code too complicated?]
by oleg＠okmij.org 05 Jul '10

05 Jul '10

Ertugrul Soeylemez wrote: > Essentially UndecidableInstances turns the type system into a > Turing-complete programming language. One direct consequence is that > type checking may not terminate Actually, the type checking (specifically, instance resolution) always terminates, due to the recursion depth limit. When the type checker hit the limit, one can't generally say either it is because the program is ill-typed or the program is well-typed but needs a bigger limit. In practice, it is usually quite easy to tell the difference. The trace emitted by GHC helps. I'd like to emphasize however that UndecidableInstances is only a sufficient condition of a decidable type checking -- but by no means necessary. There are many good, type-decidable programs that are not accepted without UndecidableInstances. One of the simplest examples is shown in http://okmij.org/ftp/Haskell/types.html#undecidable-inst-defense The situation with UndecidableInstances seems similar to the primitive recursion criterion: all primitive recursion functions surely terminate; non-primitive recursion functions generally don't. That is not the reason to disregard the latter: There are many classes of non-primitive recursive functions that are total. To see that, one has to use more complex criteria. The added complexity is not worthwhile as far as GHC is concerned (The tradeoff is different in Agda, where the added complexity of more powerful termination checkers may be worthwhile).

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Mining Twitter data in Haskell and Clojure
by braver 05 Jul '10

05 Jul '10

I'm computing a communication graph from Twitter data and then scan it daily to allocate social capital to nodes behaving in a good karmic manner. The graph is culled from 100 million tweets and has about 3 million nodes. First I wrote the simulation of the 35 days of data in Clojure and then translated it into Haskell with great help from the glorious #haskell folks. I had to add -A5G -K5G to make it work. It does 10 days OK hovering at 57 GB of RAM; I include profiling of that in sc10days.prof. At first the Haskell executable goes faster than Clojure, not by an order of magnitude, but by 2-3 times per day simulated. (Clojure also fits well in its 32 GB JVM with compressed references.) However, Haskell gets stuck after a while, and for good. Clearly I'm not doing Haskell optimally here, and would appreciate optimization advice. Here's the code: http://github.com/alexy/husky The data and problem description is in http://github.com/alexy/husky/blob/master/Haskell-vs-Clojure-Twitter.md -- also referred from the main README.md. The main is in sc.hs, and the algorithm is in SocRun.hs. The original Clojure is in socrun.clj. This is a continuation of active Twitter research and the results will be published, and I'd really like to make Haskell work at this scale and beyond! The seq's sprinkled already did no good. I ran under ghc 6.10 with -O2 with or without - fvia-C, with no difference in stallling, and am working to bring 6.12 to bear now. Cheers, Alexy

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Re: [Haskell-cafe] music-related problem
by Michael Mossey 05 Jul '10

05 Jul '10

Henning Thielemann wrote: > > On Sun, 4 Jul 2010, Michael Mossey wrote: > >> Henning Thielemann wrote: >>> >>> On Sun, 4 Jul 2010, Michael Mossey wrote: >>> >>>> I can solve a simpler problem which is >>>> >>>> computeSoundedEnd :: Item -> [Item] -> Loc >>>> computeSoundedEnd firstNote notes = compSndEnd (pitch firstNote) notes >> >>> You will certainly not be able to make use of foldl or foldr, but you >>> may use a manual recursion instead. Just like >>> >>> computeAllEnds :: [Item] -> [Item] >> >> What makes it harder than this is that the original document is not a >> single list of Item's--they are broken into measures. > > That is, you want a function of type > > computeAllEnds :: [Measure] -> [Measure] > > ? Right, although it occurs to me that it might be superior to represent the document as [Item] and give Item a measure-number attribute. There are also other measure attributes, so in that case the doc would look like: data Doc = Doc [MeasureInfo] [Item]

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GHC AT inference bug?
by Kevin Quick 04 Jul '10

04 Jul '10

I started with the following: {-# LANGUAGE TypeFamilies #-} class DoC a where type A2 a op :: a -> A2 a data Con x = InCon (x (Con x)) type FCon x = x (Con x) foldDoC :: Functor f => (f a -> a) -> Con f -> a foldDoC f (InCon t) = f (fmap (foldDoC f) t) doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) doCon (InCon x) = op x fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) fCon = foldDoC op I then changed the rank of op, but forgot to update the foldDoC accordingly---see below. Attempting to compile this causes GHC to run forever using 100% cpu. The corrected definition of foldDoC works fine. Should the GHC (6.12.1) behavior in the face of my foolishness be reported as a bug or is this a legitimate infinite recursion of type deduction? {-# LANGUAGE TypeFamilies #-} class DoC a where type A2 a type A3 a op :: a -> A2 a -> A3 a data Con x = InCon (x (Con x)) type FCon x = x (Con x) -- should have been changed to this, which works -- foldDoC :: Functor f => (f a -> a) -> A2 (FCon f) -> Con f -> a -- foldDoC f i (InCon t) = f (fmap (foldDoC f i) t) -- this original version causes GHC to hang foldDoC :: Functor f => (f a -> a) -> Con f -> a foldDoC f (InCon t) = f (fmap (foldDoC f) t) doCon :: (DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) doCon (InCon x) = op x -- note that if this is commented out then there's no hang: presumably because GHC doesn't have to perform type deduction for foldDoC. fCon :: (Functor x, DoC (FCon x)) => Con x -> A2 (FCon x) -> A3 (FCon x) fCon = foldDoC op -- -KQ

1 0

Type classes
by Andrew Coppin 04 Jul '10

04 Jul '10

Yesterday I read the Typeclassopedia [again]. In the course of doing this, I discovered that several types have class instances that do something useful. (Indeed, some of these instances do things that I've written custom functions to do.) The trouble is, when you read the documentation, it might say something like data Foo instance Monoid Foo This tells me that Foo is an instance of Monoid. It utterly fails to tell me what the heck the Monoid instance actually *does*. Now, sometimes it's quite obvious. (E.g., just about the only binary operation on lists is (++).) But sometimes it's not less obvious, and sometimes it's so utterly mystifying that just about the only way to find the answer is to either read the source code or run example code and try to deduce some kind of pattern in the answers. In summary, I think we need to devise a way of better-documenting class instances. Sometimes important functionallity can only be accessed using a class, and if it isn't too obvious what a particular class instance does, you can miss that functionallity and end up implementing it again by hand. (Or worse, thinking it's just impossible.) And there's also little quirks like the way Eq usually means value-equality, but for mutable structures it's usually defined as reference-equality. And then of course there are non-standard user-defined classes and their instances...

5 13

One problem with Cabal-install and dependencies
by Michael Lazarev 04 Jul '10

04 Jul '10

Hi, everybody! I think there's either an error in cabal-install, or I don't have an idea on haskell packages. I tried to install one particular library that depends on containers-0.2.0.1 and got the following error: Data/IntMap.hs:182:7: Could not find module `Data.Data': It is a member of the hidden package `base'. Perhaps you need to add `base' to the build-depends in your .cabal file. Use -v to see a list of the files searched for. Then I tried to install containers-0.2.0.1 separately, andin the process of the struggle I found that issuing the following command in containers-0.2.0.1 directory: ~/.cabal/bin/cabal install --constraint="base >= 4" -v ... prints this: Resolving dependencies... selecting containers-0.2.0.1 (hackage) selecting array-0.3.0.1 (installed or hackage) and discarding array-0.1.0.0, 0.2.0.0 and 0.3.0.0 selecting base-3.0.3.2 (installed) and 4.2.0.1 (installed) and discarding syb-0.1.0.0, 0.1.0.1, 0.1.0.2, 0.1.0.3, 0.2 and 0.2.1 selecting ffi-1.0 (installed) selecting ghc-prim-0.2.0.0 (installed) selecting integer-gmp-0.2.0.1 (installed) selecting rts-1.0 (installed) selecting syb-0.1.0.2 (installed) In order, the following would be installed: containers-0.2.0.1 (new version) Configuring containers-0.2.0.1... Dependency array ==0.3.0.1: using array-0.3.0.1 Dependency base ==3.0.3.2: using base-3.0.3.2 And proceeds to building the library: /usr/local/bin/ghc --make -package-name containers-0.2.0.1 -hide-all-packages -fbuilding-cabal-package -i -idist/build -i. -idist/build/autogen -Idist/build/autogen -Idist/build -Iinclude -optP-include -optPdist/build/autogen/cabal_macros.h -odir dist/build -hidir dist/build -stubdir dist/build -package-id array-0.3.0.1-fb28b07fd38a6011f4f73844e0d8281f -package-id base-3.0.3.2-260693a92016991a03206b5a55f6a411 -O -XDeriveDataTypeable -XMagicHash -XRank2Types -XCPP Data.Graph Data.IntMap Data.IntSet Data.Map Data.Sequence Data.Set Data.Tree where, as you see, it really uses base-3.0.3.2, and thus fails. So, my qusetion is, whether I'm doing something wrong, or there are really two errors: 1) cabal-install makes a wrong choice for dependency version 2) cabal-install does not respect the constraint, and proceeds to building the library without any warning

3 4

The Haskell module landscape
by Christopher Done 04 Jul '10

04 Jul '10

I was pondering how naming Haskell packages with a proper hierarchy is quite important, and wondered what the general hierarchy of the Hackage packages was like. So I grabbed the tarball of pakage descriptions[1] from Hackage and ran a little messy Haskell script[2] and got this list: http://gist.github.com/raw/463423/f8458d83b1a7cc26cdbf812747188993e50cd8a2/… Here's a snippet: hsdns: ADNS hsdns: ADNS.Base hsdns: ADNS.Endian hsdns: ADNS.Resolver cv-combinators: AI.CV.ImageProcessors HOpenCV: AI.CV.OpenCV.CV HOpenCV: AI.CV.OpenCV.CxCore HOpenCV: AI.CV.OpenCV.HighGui It's interesting to see how there are quite a few packages that could be easily put into the proper namesace, e.g. ADNS below should be in the Network hierarchy. It's also cool to see clusters of separate packages providing functionality for the same module, e.g. check out Control.Applicative! special-functors: Control.Applicative base: Control.Applicative applicative-extras: Control.Applicative.Backwards applicative-extras: Control.Applicative.Compose applicative-extras: Control.Applicative.Error InfixApplicative: Control.Applicative.Infix category-extras: Control.Applicative.Parameterized action-permutations: Control.Applicative.Permutation applicative-extras: Control.Applicative.State yjtools: Control.Applicative.Tools unicode-symbols: Control.Applicative.Unicode base-unicode-symbols: Control.Applicative.Unicode Control.Concurrent also has a lot going on inside it. There are quite a lot of packages under Network, too. One idea: Hackage could have an additional page which displays this, fully linked up to the packages in question, with an MSDN-style complete collapsible/expandable module hierarchy menu on the left and documentation on the right. I'm not saying that's the right way to do it, but it certainly makes for fun browsing for Haskell's entire open source codebase. (It would also encourage library writers to put their libraries in a useful namespace.) [1]: http://hackage.haskell.org/packages/archive/00-index.tar.gz [2]: http://gist.github.com/463472

1 0

Haskell and Standard ML modules
by José Romildo Malaquias 04 Jul '10

04 Jul '10

Hello. I am writing a Haskell program based on some guidelines written for Standard ML (The Tiger compiler from the "Modern Compiler Implementation in ML" book). The author uses modules for abstracting things like specifities of the taraget machine. For intance, there is a general Frame signature that abstracts the frame representation which depends on the target machine. For each target machine there is a structure compliant to that signature. For instante MipsFrame, PentiumFrame and SparcFrames would be structures implementing the Frame signature. Many parts of the program just see the Frame signature. Is there any guidelines on how to achieve somethying similar to this in Haskell? Regards, Romildo

2 2

Rank2Types and pattern matching
by Yves Parès 04 Jul '10

04 Jul '10

Hello everybody, I'm trying to implement the type protection used by ST to prevent a monad from returning a certain type. There's my code: import Control.Monad.Identity newtype SomeMonad s a = SomeMonad { unSome :: Identity a } deriving (Monad) newtype SomeType s = SomeType Int runSomeMonad :: (forall s. SomeMonad s a) -> a runSomeMonad (SomeMonad x) = runIdentity x And when compiled, I got this error: phantom.hs:10:14: Couldn't match expected type `forall s. PwetMonad s a' against inferred type `PwetMonad s a1' In the pattern: PwetMonad x In the definition of `runPwetMonad': runPwetMonad (PwetMonad x) = runIdentity x But when I change line 10 to: runSomeMonad x = runIdentity . unSome $ x then it works, so obviously, the trouble is about pattern matching. What was I doing wrong?

5 8

understanding Peano
by Patrick Browne 04 Jul '10

04 Jul '10

Hi, I would like to understand the Peano module below. I do wish to implement or improve the code. I am unsure of the semantics of (suc = Suc) and then the subsequent use of (Suc n) Is this an example of type-level programming? module Peano where data Nat = Zero | Suc Nat deriving Show class Peano n where suc :: n -> n eq :: n -> n -> Bool plus :: n -> n -> n instance Peano Nat where suc = Suc eq Zero Zero = True eq (Suc m) (Suc n) = eq m n eq _ _ = False plus m Zero = m plus m (Suc n) = Suc (plus m n) -- Some evaluations let t1 = plus (Suc (Zero)) (Suc ( Suc (Zero))) let t2 = plus (Suc (Suc(Zero))) ( Suc (Zero)) is 1+2=2+1? eq t1 t2 -- true *Peano> :t plus (Suc (Suc(Zero))) ( Suc (Zero)) 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

4 3

What is the point of many and some functions in Control.Applicative (for Alternative?)
by Thomas Hartman 04 Jul '10

04 Jul '10

When I load up Control.Applicative in ghci and try, eg many [1,2] or many (Just 1) or some [1,2] or some (Just 1) this never returns. What are the practical uses of these combinators, or for using the Alternative class in general?

4 3

Read Instance code.
by Andy Stewart 03 Jul '10

03 Jul '10

Hi all, I have some incorrect "Read instance" make i got error "Prelude.read: no parse", and i don't know how to fix it. ------------------------------> code start <------------------------------ newtype SerializedWindow = SerializedWindow (Maybe DrawWindow) instance Show SerializedWindow where show _ = "SerializedWindow Nothing" instance Read SerializedWindow where readsPrec _ str = [(SerializedWindow Nothing, idStr) | (val :: String, idStr) <- reads str] ------------------------------> code end <------------------------------ Any help? Thanks! -- Andy

2 3

Spiking neural networks in Haskell
by Dmitry V'yal 03 Jul '10

03 Jul '10

Greetings anyone, Some time ago I with a friend of mine implemented a toy interactive simulator of neural network activity. It has a totally imperative design and uses Gtk2hs for user interface parts. The darcs repository is available here: http://vyal.ru/code/spiking_neuro Now we decided to develop it further to try out some nice biologically inspired ideas. At first I wanted to rewrite the simulator in C++, since that language seems to match the current design more closely. But recently I've read some FRP related papers and have decided to rethink the overall approach. I'd like to hear some comments from the community on the current code and advises on possible improvements and other designs and their pros and conses. The performance is not critical for me, I'm looking for maximal flexibility and clearness so the model can easily be modified and extended. Best wishes, Dmitry

1 0

Parsec combinator like Prolog's cut operator?
by Erik de Castro Lopo 03 Jul '10

03 Jul '10

Hi all, I'm reading John Hughes' paper "Generalizing Monads to Arrows" and found the statement regarding parser combinators: "... depend on the programmer using an additional combinator similar to Prolog's 'cut' operator do declare that a parser need never backtrack beyond a certain point." Does something like this already exist in Parsec? If not is there a way to write it? Having this would really help with a parsing problem I have. Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

6 7

a very pedestrian question about maillists
by Vasili I. Galchin 03 Jul '10

03 Jul '10

Hello, Suppose I have the following fragment of a posting: Message: 3 Date: Fri, 2 Jul 2010 12:32:43 -0500 From: aditya siram <aditya.siram(a)gmail.com> Subject: Re: [Haskell-cafe] How easy is it to hire Haskell programmers To: Andrew Coppin <andrewcoppin(a)btinternet.com> Cc: haskell-cafe(a)haskell.org Message-ID: <AANLkTil-p8qMNkPdqEwDSciqw2RjASzbRL2-sPyf079u(a)mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1 On another posting I tried counter-posting using the "Message-ID" but the counter-post bounced back to me as non-deliverable. How do I post on a "thread" like "How easy is it to hire Haskell programmers"? Thanks, Vasili

2 1

Status of status on freenode
by Walt Rorie-Baety 03 Jul '10

03 Jul '10

My work environment is what I'd call typical US corporate - IRC nodes are blocked, but I can use a web-based client to access it. A couple of days ago, someone had the poor taste to throw a pack of rabid spam bots into the #haskell channel, forcing the short-term solution of shutting down all web-based users. I noticed today that I can now access #haskell again, but have no voice. I'm wondering: is there any way to modify access to allow registered users to speak? I've tried using other web-based clients, but none of then connect to the freenode server. At home, I can (and do) use mIRC, so I'm not totally cut off - and now I can read, but not communicate back to, the #haskell channel. BTW, please direct me to another list, if this issue does not belong on it. Walt "BMeph" Rorie-Baety "A mountain that eats people? I want one!." - Richard, of LFGComic.com

3 2

ANNOUNCE: hs-cryptohash 0.4
by Vincent Hanquez 03 Jul '10

03 Jul '10

Hi Haskell-cafe, I'ld like to announce the existence of hs-cryptohash [1] which provides most common digests (sha1, sha2 family, md[245], ripemd160) in a incremental and one-pass api. It's also much faster than pure haskell implementation i've played with, since the underlaying algorithms are all fairly optimized in C. The API remains pure with the use of unsafePerformIO, which after reading everything i could about it and some testing, seems to be safe in this context. The main reason for this library is the lack of incremental api exposed by current digest libraries, and filling the void about some missing digest algorithms; Also the speed comes as a nice bonus. I'm looking forward to hear any comments, [1] http://github.com/vincenthz/hs-cryptohash -- Vincent Hanquez

9 20

MState: A consistent State monad for concurrent applications
by Nils Schweinsberg 03 Jul '10

03 Jul '10

Hi, I have been trying to use the State monad for concurrent applications and came up with a little library.[1] My "MState" uses an IORef to maintain the state between different threads. The library also offers a simple way to fork off new threads using its own "forkM" function. This function will asure that every thread in an "evalMState"/"execMState" call will be finished before the final result is returned. The library is also on github.com.[2] Please let me know what you think of it and whether or not I should put it on hackage. Cheers, Nils Schweinsberg [1] http://n-sch.de/hdocs/mstate/Control-Concurrent-MState.html [2] http://github.com/mcmaniac/mstate

5 11

Re: [Haskell-cafe] ANN: Haskell-mode 2.8.0
by Ivan Lazar Miljenovic 03 Jul '10

03 Jul '10

Deniz Dogan <deniz.a.m.dogan(a)gmail.com> writes: > Doing so would only mean that haskell-mode will most likely never be a > part of Emacs, meaning that anyone looking for programming Haskell in > Emacs would have to download an external library, be it through > package.el or not. And what's so bad about that? I suppose it might be possible to have a cut-down haskell-mode in Emacs for casual hacking, the way there's python-mode and pymacs. > To put it short, there are no pros in making the new Haskell indenter > a strict part of haskell-mode. There are if it brings in a lot of cool features. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

Re: [Haskell-cafe] ANN: Haskell-mode 2.8.0
by Svein Ove Aas 03 Jul '10

03 Jul '10

On Sat, Jul 3, 2010 at 1:07 AM, Deniz Dogan <deniz.a.m.dogan(a)gmail.com> wrote: > > Since there is no reason what so ever to integrate this external > Haskell indenter tightly into haskell-mode, I don't see any reason to > do so. Doing so would only mean that haskell-mode will most likely > never be a part of Emacs, meaning that anyone looking for programming > Haskell in Emacs would have to download an external library, be it > through package.el or not. > > To put it short, there are no pros in making the new Haskell indenter > a strict part of haskell-mode. > Sure there is. I expect there to be quite a lot of shared code. It's not solely an indenter; the same interface and capabilities will be used for a debugger, refactoring, and type signature info. Haskell-mode should still work without it, to about the same degree as today, but actually doing so would be a *major* degradation compared to the state of the art once I'm done with it. In any case, I expect the actual installation to be via cabal-install. -- Svein Ove Aas

1 0

Re: [Haskell-cafe] ANN: Haskell-mode 2.8.0
by Ivan Lazar Miljenovic 03 Jul '10

03 Jul '10

Deniz Dogan <deniz.a.m.dogan(a)gmail.com> writes: > Why should it not become part of Emacs? > > I would like it if this Haskell indenter was external to haskell-mode > itself instead of making haskell-mode external to Emacs. Why should it? Look at all the effort that the Haskell community went through to _unbundle_ libraries from GHC, and now Emacs is bundling more and more with it, which means it's harder to use a newer version of a package than the bundled one. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

Looking for GUI (WX) example code
by Günther Schmidt 03 Jul '10

03 Jul '10

Hi all, I'd be interested in studying GUI (wxhaskell) code. Does anyone have links to good gui code? Best regards Günther

2 1

GHCi and State
by corentin.dupont＠ext.mpsa.com 02 Jul '10

02 Jul '10

Another couple of reflexions (sorry for monopolizing): 1. Since i am making a Nomic game, players will have to submit rules. These rules will be written in a sub-set of haskell. Instead of writing my own reader/interpreter, i'd like to use GHC to compil them on the fly, and then add them to the current "legislation". What would you suggest me to do that? Any pointers? 2. For now, the game is more or less playable in GHCi. But my concern is: When you use GHCi, you are in the IO monad, right? How to had state to this monad? I would like that the player can compose his rule in GHCi, and when he is done, he can submit it in GHCi with something like: *Nomic> submitRule <myrule> And then the game takes the rule, possibly modify the current legislation, and give the hand back to GHCi. So the "current legislation" has to be a state of the GHCi's loop. Is this possible at all? submitRule would have a type more or less like that (GameState contains the legislation): submitRule :: Rule -> StateT GameState IO () Thanks for your attention! I know this is a bit confused! Best, Corentin

7 7

Re: [Haskell-cafe] ANN: Haskell-mode 2.8.0
by Ivan Lazar Miljenovic 02 Jul '10

02 Jul '10

Alex Ott <alexott(a)gmail.com> writes: > Re > > Svein Ove Aas at "Fri, 2 Jul 2010 15:36:06 +0200" wrote: > SOA> On Fri, Jul 2, 2010 at 3:23 PM, Deniz Dogan <deniz.a.m.dogan(a)gmail.com> wrote: > >> Why do you want to rewrite the indenter in Haskell? If haskell-mode > >> has any external dependencies such as that one, it will most likely > >> never make it into Emacs. > >> > SOA> In no particular order: > > SOA> - For any given line, there are typically several possible > SOA> indentations. We can cycle between these, but it's better to minimize > SOA> them. > > SOA> - I want the parser to be as accurate as possible. This essentially > SOA> means reimplementing haskell-src-exts, unless I *use* > SOA> haskell-src-exts. > > May be you can use semantic to make a parser? Semantic is already included > into emacs, and I hope, that it will make life easier. Plus we'll > automatically get lot of other functionality - name completion, navigation, > etc. My understanding is that semantic is more aimed at C-like languages (I seem to recall reading a discussion a while back about someone trying to use semantic to parse Python or something like that and failing), and as such wouldn't work too well for Haskell. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

Re: [Haskell-cafe] ANN: Haskell-mode 2.8.0
by Svein Ove Aas 02 Jul '10

02 Jul '10

On Fri, Jul 2, 2010 at 3:23 PM, Deniz Dogan <deniz.a.m.dogan(a)gmail.com> wrote: > Why do you want to rewrite the indenter in Haskell? If haskell-mode > has any external dependencies such as that one, it will most likely > never make it into Emacs. > In no particular order: - For any given line, there are typically several possible indentations. We can cycle between these, but it's better to minimize them. - I want the parser to be as accurate as possible. This essentially means reimplementing haskell-src-exts, unless I *use* haskell-src-exts. - In many cases, the legal indentations depend on the types of the expressions involved. This can be figured out, but requires part of the code to be running in GHC *anyway*. - Elisp is a lousy language compared to haskell, and I don't really care about getting haskell-mode into the emacs distribution. It makes more sense to bundle it with GHC, if it has to be bundled. - Most of haskell-mode will work without this helper program, including the three other indentation modes. -- Svein Ove Aas

1 0

Re: [Haskell-cafe] ANN: Haskell-mode 2.8.0
by Ivan Lazar Miljenovic 02 Jul '10

02 Jul '10

Deniz Dogan <deniz.a.m.dogan(a)gmail.com> writes: > 2010/7/2 Svein Ove Aas <sveina(a)gmail.com>: >> My current project is to rewrite the indenter in haskell, using >> haskell-src-exts and such. [snip] > > Why do you want to rewrite the indenter in Haskell? If haskell-mode > has any external dependencies such as that one, it will most likely > never make it into Emacs. Why should it become part of Emacs (especially when they finally seem to be starting to work on standardising packaging, etc. to make it easier to install third party modes)? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

1 0

Re: [Haskell-cafe] ANN: Haskell-mode 2.8.0
by Christopher Done 02 Jul '10

02 Jul '10

On 2 July 2010 14:45, Svein Ove Aas <sveina(a)gmail.com> wrote: > My current project is to rewrite the indenter in haskell, using > haskell-src-exts and such. Aligning imports will probably be trivial > once that is done, but the work of integrating it now would probably > also be wasted. That sounds like a good idea. I considered using haskell-src-exts myself but didn't bother at the time. > I might do it anyway, if it's sufficiently little work. Let's see. It should just be case of choosing what keybinding to use.

1 0

Re: ANN: Haskell-mode 2.8.0
by Johan Tibell 02 Jul '10

02 Jul '10

On Fri, Jul 2, 2010 at 2:26 PM, Svein Ove Aas <sveina(a)gmail.com> wrote: > I'm happy to (finally) announce haskell-mode 2.8.0, which can be found at > http://projects.haskell.org/haskellmode-emacs/ > > This is a feature release. I'll let the changelog speak for itself: > > > * Minimal indentation support for arrow syntax > > * Autolaunch haskell-mode for files starting with #!/usr/bin/runghc > and similar > > * Added minimal major mode for parsing GHC core files, courtesy of Johan > Tibell. > There is a corresponding Haskell menu entry. > > * Allow configuration of where-clause indentation; M-x customize-group > haskell-indentation. > > > You can expect a 2.8.1 or even 2.8.2 in the near future, as I attempt > to empty the bug list, but I thought I might as well get the new > features out there quickly. There's bound to be new bugs associated > with them. > Thank you for working on this. Getting the where-clause indentation issue fixed removed a small but frequent annoyance in my Haskell programming. Johan

1 0

ANN: Haskell-mode 2.8.0
by Svein Ove Aas 02 Jul '10

02 Jul '10

I'm happy to (finally) announce haskell-mode 2.8.0, which can be found at http://projects.haskell.org/haskellmode-emacs/ This is a feature release. I'll let the changelog speak for itself: * Minimal indentation support for arrow syntax * Autolaunch haskell-mode for files starting with #!/usr/bin/runghc and similar * Added minimal major mode for parsing GHC core files, courtesy of Johan Tibell. There is a corresponding Haskell menu entry. * Allow configuration of where-clause indentation; M-x customize-group haskell-indentation. You can expect a 2.8.1 or even 2.8.2 in the near future, as I attempt to empty the bug list, but I thought I might as well get the new features out there quickly. There's bound to be new bugs associated with them. -- Svein Ove Aas

2 1

State of the Hackage: Q1, Q2 2010
by Don Stewart 02 Jul '10

02 Jul '10

Downloads and popular packages on Hackage for Q1 and Q2 this year. http://donsbot.wordpress.com/2010/06/30/popular-haskell-packages-q2-2010-re… -- Don

4 5

Associated types
by Andrew Coppin 02 Jul '10

02 Jul '10

Consider the following: class Path p where ... class Path p => CompletePath p where ... class Path p => IncompletePath p where type CompletedPath p :: * Obviously, the idea is that CompletedPath Foo (where Foo is an IncompletePath) should yield some type which is a CompletePath. However, the source code does not actually state this, so GHC (rightly) complains that it was unable to deduce this constraint from the actual code written. Is it possible to state this constraint? If so, how? (And if not, the answer, presumably, is to fundamentally redesign the whole thing...)

6 8

chart "broken" under 6.12 according to criterion
by Tom Doris 02 Jul '10

02 Jul '10

According to the criterion.cabal file shipped with the latest (0.5.0.1) version of criterion, the Chart package is broken under GHC 6.12: flag Chart

4 5

Type-Level Programming
by Walt Rorie-Baety 02 Jul '10

02 Jul '10

I've noticed over the - okay, over the months - that some folks enjoy the puzzle-like qualities of programming in the type system (poor Oleg, he's become #haskell's answer to the "Chuck Norris" meme commonly encountered in MMORPGs). Anyway,... are there any languages out there whose term-level programming resembles Haskell type-level programming, and if so, would a deliberate effort to appeal to that resemblance be an advantage (leaving out for now the hair-pulling effort that such a change would entail)? Or, better yet, is there an Interest Group or committee (Working, or not), that is looking at a coherent architecture or design for a possible future version of Haskell (no offense to Tim Sheard's excelent Ωmega project)? Walt "BMeph" Rorie-Baety "A mountain that eats people? I want one!." - Richard, of LFGComic.com

15 37

ghc api printing of types
by Phyx 02 Jul '10

02 Jul '10

I was wondering how given a Type I can get a pretty printed type out of it. I'm currently using showSDocUnqual . pprType . snd . tidyOpenType emptyTidyEnv But this has the problem that predicates don't get printed, anyone know how GHCi does this? Thanks, Phyx

1 0

Inferring the most general type
by Job Vranish 01 Jul '10

01 Jul '10

Esteemed fellow haskellers, I recently ran into a very simple real life case where Haskell's rules for inferring the types for mutually recursive definitions resulted in a type that was less general than it could be. It took me a while to realize that the type error I was getting wasn't actually a problem with my code. I understand why Haskell does this (it infers the strongly connected mutually recursive definitions monomorphically), but I think it _could_ infer the more general type even with recursive definitions like this. Here is a simplified example that illustrates the problem: > import Data.Maybe > -- The fixed point datatype > data Y f = Y (f (Y f)) > -- silly dummy function > maybeToInt :: Maybe a -> Int > maybeToInt = length . maybeToList > -- f :: Y Maybe -> Int > f (Y x) = g maybeToInt x > g h x = h $ fmap f x This is the type it wants to infer for g g :: (Maybe Int -> Int) -> Maybe (Y Maybe) -> Int This is the type I think it should have, note you can't force the type with a typesig without -XRelaxedPolyRec g :: (Functor f) => (f Int -> b) -> f (Y Maybe) -> b If I use -XRelaxedPolyRec I can manually specify the more general type, but then I have to convince myself that there isn't a more general type that I'm missing. Are there other known algorithms that yield a more general type? and if so, what was the rational for Haskell keeping the current method? I worked out an alternative algorithm that would give a more general type (perhaps the most general type) but it has factorial complexity and probably wouldn't be good for strongly connected groups with 7 or more members. Even so, I would much rather have the inferred types always be the most general ones and be required to add type signatures for mutually recursive groups with 7 or more members (which probably need to be redesigned anyway) than be always required to manually figure out the more general signatures. What do you think? - Job

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