April 2011 - Haskell-Cafe

plugins and cabal build
by Michael Snoyman 27 Apr '11

27 Apr '11

Hi all, I'm experimenting with using the plugins package for yesod devel server. The basic approach is to use cabal for building the object files, and then load them with plugins. I can get everything to work when I compile with "ghc --make", but I believe name mangling is getting in the way with the "cabal build" route. Can someone give some guidance on how to properly mangle the function names so that plugins can load them? Thanks, Michael

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Creating polygons in qtHaskell
by Øystein Kolsrud 26 Apr '11

26 Apr '11

Hi! I'm using qtHaskell and I'm trying to create a QGraphicsPolygonItem object. From what I see I need to create a QPolygonF item that I can supply to the constructor, but to populate the QPolygonF I need to use methods of the QVector class that QPolygonF inherits and I can't figure out how to do that. It seems that QVector is not part of the qtHaskell library. What I would like to do is something like this (where mypoints is a list of QPointF): makePolygon = do polygon <- qPolygonF () mapM (pushBack polygon) mypoints qGraphicsPolygonItem_nf polygon However, pushBack doesn't seem to be available. Does anyone know a way to do this? -- Mvh Øystein Kolsrud

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ANNOUNCE: iteratee-compress 0.2.0.0
by Maciej Piechotka 26 Apr '11

26 Apr '11

Iteratee-compress provides compressing and decompressing enumerators including flushing (using John Lato's implementation). Currently only gzip and bzip is provided but LZMA is planned. Changes from previous version: - Add BZip support Next goals: - LZMA support - Generic interface for flushing To think about: - Should inner iteratee be able to request flushing Regards

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Re: [Haskell-cafe] Question about the Monad instance for Iteratee (from the enumerator package)
by John Lato 26 Apr '11

26 Apr '11

Joining slightly late... > From: John Millikin <jmillikin(a)gmail.com> > > John Lato's "iteratee" package is based on IterateeMCPS.hs[1]. I used > IterateeM.hs for "enumerator", because when I benchmarked them the non-CPS > version was something like 10% faster on most operations. > Based on tests I did before iteratee-0.4, the fastest implementation was Oleg's alternate design which is in the comments of IterateeM.hs. I don't think any current packages use that however, with good reason. I'd be interested to see the results of a shootout between iteratee and enumerator. I would expect them to be basically equivalent most of the time, with maybe two or three operations with a small (but consistent) difference one way or the other. John L.

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Haskell User Group starter kit
by Eric Y. Kow 26 Apr '11

26 Apr '11

Hi all, We may be coming to a time where we start to see Haskell User Groups or Functional Programming clubs popping up all the over the place. Maybe it's a good time to put our heads together and figure out what we can do to nurture these groups, to keep them going strong rather than fizzling out. One idea might be create some sort of Haskell User Group starter kit so that individual groups can hit the ground running with some concrete ideas. http://www.haskell.org/haskellwiki/Haskell_User_Group My hope is that it would provide just enough material for a group to reach critical mass. So just enough stuff to account for the different kinds of activities a HUG may want to do (eg. Coding Dojos vs Talks vs Reading Group), but not so much that HUGs feel intimidated or stifled. Have you started a HUG recently? Or are you thinking of starting one? Maybe you could contribute just one idea to the kit? More welcome, of course :-) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> For a faster response, try +44 (0)1273 64 2905 or xmpp:kowey@jabber.fr (Jabber or Google Talk only)

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Re: [Haskell-cafe] How to update the RNG per call (State monad) when generating QuickCheck arbitraries?
by Bryan O'Sullivan 26 Apr '11

26 Apr '11

On Tue, Apr 26, 2011 at 9:16 AM, Daniel Kahlenberg <d.kahlenberg(a)gmail.com>wrote: > > hold on I'd like to have the genArray call generating distinctive > results in one IO execution The problem you're seeing is due to the fact that you're not taking the final RNG state from the first execution of your code and passing it as the initial state to the second. Since you're initialising each one with the same RNG state, you're getting the same results in each case.

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typing hmatrix expressions
by Carter Schonwald 26 Apr '11

26 Apr '11

hello alberto, i've had some funny issues with using the cmap function, and i'd like understand what i'm doing wrong namely it wont correctly instantiate for vectors or matrices and I don't understand why. basically my question is: what is the correct type for the hmatrix expression mapMat f =liftMatrix $! cmap f I'd like to just give it a type like Storable a => (a->a)-> Matrix a -> Matrix a but i get an error message like Electrical.lhs:89:29: No instance for (Container Vector e0) arising from a use of `cmap' Possible fix: add an instance declaration for (Container Vector e0) In the second argument of `($!)', namely `cmap f' In the expression: liftMatrix $! cmap f In an equation for `mapMat': mapMat f = liftMatrix $! cmap f which i feel shouldn't be happening. is the type of cmap in some way too polymorphic, or am I just not turning on the right ghc type extensions for this to work, or is this something that would have been well typed prior to ghc 7? Or am i just not correctly understanding this type error and i'm making a simple mistake? thanks! -Carter

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converting prefixes of CString <-> String
by Eric Stansifer 26 Apr '11

26 Apr '11

I have been reading Foreign.C.String but it does not seem to provide the functionality I was looking for. Let 'c2h' convert CStrings to Haskell Strings, and 'h2c' convert Haskell Strings to CStrings. (If I understand correctly, c2h . h2c === id, but h2c . c2h is not the identity on all inputs; or perhaps c2h is not defined for all CStrings. Probably this is all locale dependent.) I have an infinite Haskell String transferred byte-wise over a network; I would like to convert some prefix of the bytes received into a prefix of the String I started with. However, if I understand correctly, if "s" is a Haskell String it is not necessarily true that "c2h (take n (h2c s))" is a prefix of s for all n. So I have two questions: Given a CString of the form "cs = take n (h2c s)", how do I know whether "c2h cs" is a prefix of s or not? Is there a way to recognize whether a CString is "valid" as opposed to truncated in the middle of a code point, or is this impossible? Better yet, given a CString "cs = take n (h2c s)", is there a way to find the maximal prefix cs' of cs such that c2h cs' is a prefix of s? If s == s1 ++ s2, is it necessarily true that s == (c2h (h2c s1)) ++ (c2h (h2c s2))? If so, then I can perform my conversion a bit at a time, otherwise I'd need to start from the beginning of the cstring each time I receive additional data. In practice, I think my solution will come down to restricting my program to only using the lower 128 characters, but I'd like to know how to handle this problem in full generality. Thanks, Eric

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Why does Cabal not choose the latest package?
by Emil Axelsson 26 Apr '11

26 Apr '11

Hello! I've had some of the usual problems with packages depending on multiple versions of another package. It seems the root of the hole problem was that I once attempted to run cabal install cabal-install This brought in a number of older packages (Cabal-1.8.0.6, containers-0.3.0.0, directory-1.0.1.2, filepath-1.1.0.4, process-1.0.1.5) to my local package DB, which led to inconsistencies when trying to install new packages. However, if I run cabal install cabal-install-0.10.2 things work just fine. So the problem is that it wants to install an older version of cabal-install. A session demonstrating the problem is found below. Why doesn't cabal always install the latest packages? Thanks! / Emil [emax@laptop:~]> ghc --version The Glorious Glasgow Haskell Compilation System, version 7.0.2 [emax@laptop:~]> cabal --version cabal-install version 0.10.2 using version 1.10.1.0 of the Cabal library [emax@laptop:~]> cabal install cabal-install Resolving dependencies... Downloading containers-0.3.0.0... ... [emax@laptop:~]> cabal install cabal-install-0.10.2 Resolving dependencies... Configuring cabal-install-0.10.2... Preprocessing executables for cabal-install-0.10.2... Building cabal-install-0.10.2... ...

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-- Extension for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell
by caseyh＠istar.ca 26 Apr '11

26 Apr '11

-- Extension for "Pearls of Functional Algorithm Design" by Richard Bird, -- 2010, page 25 #Haskell -- This version assumes 3 disjoint ordered sets represented as lists. -- So either: x<y XOR x>y -- Since it uses lists it is no faster than the divide and conquer approach. -- I might try to convert this version to sorted arrays for -- O(log|X|+log|Y|+log|Z|) performance -- If I can figure out how to do it without suffering from "indexitis". smallest3'' :: Ord a => Int -> ([a], [a], [a]) -> a smallest3'' k ([],[],ts) = ts !! k smallest3'' k (zs,[],[]) = zs !! k smallest3'' k ([],ws,[]) = ws !! k smallest3'' k ([],ws,ts) = smallest'' k (ws,ts) smallest3'' k (zs,[],ts) = smallest'' k (zs,ts) smallest3'' k (zs,ws,[]) = smallest'' k (zs,ws) smallest3'' k (zs,ws,ts) = ~~~~case (a<b, b<c, a<c) of ~~~~~~(True, True, True) -> smallest3h'' k ((zs,p,ys),(ws,q),(ts,o,rs)) -- a<b<c ~~~~~~(True, False, True) -> smallest3h'' k ((zs,p,ys),(ts,o),(ws,q,us)) -- a<c<b ~~~~~~(False, True, True) -> smallest3h'' k ((ws,q,vs),(zs,p),(ts,o,rs)) -- b<a<c ~~~~~~(False, True, False) -> smallest3h'' k ((ws,q,vs),(ts,o),(zs,p,xs)) -- b<c<a ~~~~~~(True, False, False) -> smallest3h'' k ((ts,o,ss),(zs,p),(ws,q,us)) -- c<a<b ~~~~~~(False, False, False) -> smallest3h'' k ((ts,o,ss),(ws,q),(zs,p,xs)) -- c<b<a ~~~~where ~~~~~~p = (length zs) `div` 2 ~~~~~~q = (length ws) `div` 2 ~~~~~~o = (length ts) `div` 2 ~~~~~~(xs, a : ys) = splitAt p zs ~~~~~~(us, b : vs) = splitAt q ws ~~~~~~(rs, c : ss) = splitAt o ts ~~~~~~smallest3h'' k ((zs,p,ys),(ws,q),(ts,o,rs)) = ~~~~~~~~case (k<=p+q+o) of ~~~~~~~~~~(True) -> smallest3'' k (zs,ws,rs) ~~~~~~~~~~(False) -> smallest3'' (k-p-1) (ys,ws,ts) smallest'' :: Ord a => Int -> ([a], [a]) -> a smallest'' k ([],ws) = ws !! k smallest'' k (zs,[]) = zs !! k smallest'' k (zs,ws) = ~~~~case (a<b, k<=p+q) of ~~~~~~(True, True) -> smallest'' k (zs,us) ~~~~~~(True, False) -> smallest''(k-p-1) (ys,ws) ~~~~~~(False, True) -> smallest'' k (xs,ws) ~~~~~~(False, False)-> smallest''(k-q-1) (zs,vs) ~~~~where ~~~~~~p = (length zs) `div` 2 ~~~~~~q = (length ws) `div` 2 ~~~~~~(xs, a : ys) = splitAt p zs ~~~~~~(us, b : vs) = splitAt q ws

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