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Haskell Weekly News: Issue 184
by Daniel Santa Cruz 02 Jun '11

02 Jun '11

Welcome to issue 184 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of May 22 to 28, 2011. Announcements The newsletter has not been posting new library announcements, but Ivan Lazar's announcement of his new wl-pprint-text library had to be an exception. Way to be creative Ivan! http://goo.gl/C6tu0 Simon Marlow wrote in to announce CamHac: (Haskell Hackathon in Cambridge, UK) during August 12 to 14, 2011. Registration is free, but they are limited to 50 people, so sign up soon! http://goo.gl/BySLB Quotes of the Week * Henning Thielemann: [On a slow solution to Euler's totien function] It's declarative and may help to verify more efficient implementations. * KC: Librarians have been struggling for years with classifying topics; I don't imagine classifying coding libraries as any easier. * max ulidtko: [on the *group hug* thread] Wow. I just subscribed to the list just an hour ago or so, and already receiving hugs! * Jacek Generowicz: What would be the point of asking non-ignorant questions, unless it is a rhethorical one? * Ketil Malde: Being brilliant doesn't mean the absence of mental bad hair days, but merely that they happen more rarely than for the rest of us. :-) * AndrewMorton: I was attacked by dselect as a small child and have since avoided Debian. * Cale: The hardest part of writing any binding is doing something to make up for the fact that libraries written for imperative languages are typically badly designed to begin with. * <lambdabot> *Exception: *Exception: *Exception: <Cale> E X C E P T I O N * Jafet: In strict languages, you implement Haskell, then use fix as usual. Top Reddit Stories * Multicore Garbage Collection with Local Heaps :: PDF Domain: community.haskell.org, Score: 45, Comments: 4 On Reddit: http://goo.gl/OcOqI Original: http://goo.gl/Zdkxv * Node.js creator first tried with haskell, but says he wasn't smart enough to hack GHC Domain: bostinnovation.com, Score: 41, Comments: 39 On Reddit: http://goo.gl/cDQWT Original: http://goo.gl/p8VpL * ANNOUNCEMENT: HaNS, the Haskell Network Stack. HaNS is a lightweight, pure Haskell network stack that can be used for Haskell networking in the context of the HaLVM, or with a Linux tap device. Domain: haskell.org, Score: 40, Comments: 12 On Reddit: http://goo.gl/hBlRB Original: http://goo.gl/H62UL * Haskell + FFI + Java + SWT: crazy, maybe, but working! Domain: jpmoresmau.blogspot.com, Score: 36, Comments: 17 On Reddit: http://goo.gl/9l45y Original: http://goo.gl/B6jO7 * Isomorphism lenses Domain: twanvl.nl, Score: 32, Comments: 10 On Reddit: http://goo.gl/rlBfs Original: http://goo.gl/BHrBh * Tail recursion makes your loops cleaner Domain: blog.ezyang.com, Score: 29, Comments: 14 On Reddit: http://goo.gl/7VD9g Original: http://goo.gl/YMqok * An insufficiently lazy map: the dangers of spine-strict data structures Domain: blog.ezyang.com, Score: 25, Comments: 0 On Reddit: http://goo.gl/DTBwg Original: http://goo.gl/dMEoI * From a 13M executable to an 84k one: use -dynamic with GHC for linking excellence Domain: stackoverflow.com, Score: 23, Comments: 13 On Reddit: http://goo.gl/tsN5H Original: http://goo.gl/poXbW * This is how a new package should be announced: Pretty-printing package for lazy Text values Domain: haskell.org, Score: 21, Comments: 3 On Reddit: http://goo.gl/iduqP Original: http://goo.gl/qyhrj * Happstack + JMacro + HSX Domain: happstack.blogspot.com, Score: 15, Comments: 2 On Reddit: http://goo.gl/33J1S Original: http://goo.gl/9mk8h Top StackOverflow Questions * Reading GHC Core votes: 27, answers: 4 Read on SO: http://goo.gl/wb6In * Small Haskell program compiled with GHC into huge binary votes: 26, answers: 2 Read on SO: http://goo.gl/poXbW * Force pre-computation of a constant votes: 8, answers: 1 Read on SO: http://goo.gl/Ri16N * Optimization of Function Calls in Haskell votes: 6, answers: 2 Read on SO: http://goo.gl/gPshb * How to make a CAF not a CAF in Haskell? votes: 6, answers: 6 Read on SO: http://goo.gl/KYTo7 * Concise if-then-else notation in do-blocks in Haskell votes: 6, answers: 4 Read on SO: http://goo.gl/dRlGY * Haskell (haskeline) word completion votes: 6, answers: 1 Read on SO: http://goo.gl/3TbFo * Why doesn't Haskell have symbols (a la ruby) / atoms (a la erlang)? votes: 6, answers: 5 Read on SO: http://goo.gl/4KxP8 * Why does Haskell appear to default to reading Int when reading Num? votes: 6, answers: 2 Read on SO: http://goo.gl/alCkY About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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[iteratee] how to do nothing .. properly
by Sergey Mironov 01 Jun '11

01 Jun '11

Hi. Would anybody explain a situation with iter6 and iter7 below? Strange thing - first one consumes no intput, while second consumes it all, while all the difference is peek which should do no processing (just copy next item in stream and return to user). What I am trying to do - is to write an iteratee consuing no input, but returning a constant I give to it. I thought (return a) should do it, but it seems I was wrong as return actually consumes all unparsed stream. iter6 experience tells me that (peek>>return a) is what I need, but it's completely confusing and not what I expected. Thanks, Sergey import Data.Iteratee as I import Data.Iteratee.IO import Control.Monad import Control.Exception import Data.ByteString import Data.Char import Data.String -- countBytes :: (..., Num b) => Iteratee s m a -> Iteratee s m (a, b) countBytes i = enumWith i I.length iter6 = do h <- countBytes $ (peek >> return 0) s <- I.stream2list return (h,s) iter7 = do h <- countBytes $ (return 0) s <- I.stream2list return (h,s) print6 = enumPure1Chunk [1..10] (iter6) >>= run >>= print print7 = enumPure1Chunk [1..10] (iter7) >>= run >>= print Here is example ghci session *Main> print6 ((0,0),[1,2,3,4,5,6,7,8,9,10]) -- read 0 items, returns 0 *Main> print7 ((0,10),[]) -- read 10 items (???) returns 0 *Main>

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Haskell School of Expression (graphics)
by michael rice 01 Jun '11

01 Jun '11

I installed the Haskell Platform (7.0.2) earlier, and I just downloaded package soegtk with cabal, apparently unsuccessfully. Any suggestions? Michael [michael@sabal ~]$ cabal updateConfig file path source is default config file.Config file /home/michael/.cabal/config not found.Writing default configuration to /home/michael/.cabal/configDownloading the latest package list from hackage.haskell.org[michael@sabal ~]$ cabal list SOE* soegtk Synopsis: GUI functions as used in the book "The Haskell School of Expression". Default available version: 0.11.1 Installed versions: [ Not installed ] Homepage: http://www.haskell.org/gtk2hs/ License: LGPL-2.1 [michael@sabal ~]$ cabal install soegtkResolving dependencies...Downloading cairo-0.11.1... /tmp/cairo-0.11.12906/cairo-0.11.1/Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [-Wcpp][1 of 2] Compiling Gtk2HsSetup ( /tmp/cairo-0.11.12906/cairo-0.11.1/Gtk2HsSetup.hs, /tmp/cairo-0.11.12906/cairo-0.11.1/dist/setup/Gtk2HsSetup.o ) /tmp/cairo-0.11.12906/cairo-0.11.1/Gtk2HsSetup.hs:190:70: Couldn't match expected type `[PackageDB]' with actual type `PackageDB' Expected type: PackageDBStack Actual type: PackageDB In the sixth argument of `registerPackage', namely `packageDb' In the expression: registerPackage verbosity installedPkgInfo pkg lbi inplace packageDbDownloading glib-0.11.2... /tmp/glib-0.11.22906/glib-0.11.2/Gtk2HsSetup.hs:25:2: warning: #warning Setup.hs is guessing the version of Cabal. If compilation of Setup.hs fails use -DCABAL_VERSION_MINOR=x for Cabal version 1.x.0 when building (prefixed by --ghc-option= when using the 'cabal' command) [-Wcpp][1 of 2] Compiling Gtk2HsSetup ( /tmp/glib-0.11.22906/glib-0.11.2/Gtk2HsSetup.hs, /tmp/glib-0.11.22906/glib-0.11.2/dist/setup/Gtk2HsSetup.o ) /tmp/glib-0.11.22906/glib-0.11.2/Gtk2HsSetup.hs:190:70: Couldn't match expected type `[PackageDB]' with actual type `PackageDB' Expected type: PackageDBStack Actual type: PackageDB In the sixth argument of `registerPackage', namely `packageDb' In the expression: registerPackage verbosity installedPkgInfo pkg lbi inplace packageDbcabal: Error: some packages failed to install:cairo-0.11.1 failed during the configure step. The exception was:ExitFailure 1gio-0.11.1 depends on glib-0.11.2 which failed to install.glib-0.11.2 failed during the configure step. The exception was:ExitFailure 1gtk-0.11.2 depends on glib-0.11.2 which failed to install.pango-0.11.2 depends on glib-0.11.2 which failed to install.soegtk-0.11.1 depends on glib-0.11.2 which failed to install.[michael@sabal ~]$

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Re: [Haskell-cafe] How on Earth Do You Reason about Space?
by John Lato 01 Jun '11

01 Jun '11

From: "Edward Z. Yang" <ezyang(a)MIT.EDU> > > Hello Aleksandar, > > It is possible that the iteratees library is space leaking; I recall some > recent discussion to this effect. Your example seems simple enough that > you might recompile with a version of iteratees that has -auto-all enabled. > Unfortunately, it's not really a safe bet to assume your libraries are > leak free, and if you've pinpointed it down to a single line, and there > doesn't seem a way to squash the leak, I'd bet it's the library's fault. > > Edward > I can't reproduce the space leak here. I tried Aleksander's original code, my iteratee version, the Ngrams version posted by Johan Tibell, and a lazy bytestring version. my iteratee version (only f' has changed from Aleksander's code): f' :: Monad m => I.Iteratee S.ByteString m Wordcounts f' = I.joinI $ (enumLinesBS I.><> I.filter (not . S.null)) $ I.foldl' (\t s -> T.insertWith (+) s 1 t) T.empty my lazy bytestring version > import Data.Iteratee.Char > import Data.List (foldl')import Data.Char (toLower) > > import Data.Ord (comparing) > import Data.List (sortBy) > import System.Environment (getArgs) > import qualified Data.ByteString.Lazy.Char8 as L > import qualified Data.HashMap.Strict as T > > f'2 = foldl' (\t s -> T.insertWith (+) s 1 t) T.empty . filter (not . L.null) . L.lines > > main2 :: IO () > main2 = getArgs >>= L.readFile .head >>= print . T.keys . f'2 None of these leak space for me (all compiled with ghc-7.0.3 -O2). Performance was pretty comparable for every version, although Aleksander's original did seem to have a very small edge. As someone already pointed out, keep in mind that this will use a lot of memory anyway, unless there's a lot of repetition of words. I'd be happy to help track down a space leak in iteratee, but for now I'm not seeing one. Best, John Lato

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maybe a GHC bug but not sure
by Edward Amsden 01 Jun '11

01 Jun '11

When building the Haskell Objective C bindings tool from <http://code.google.com/p/hoc> Using GHC 7.0.3 for Haskell Platform 2011.2.0.1, OS X 64 bit HOC compiles HOC_cbits.o before trying to build it into the rest of the program (I don't say link because this came up while trying to compile: > [18 of 31] Compiling HOC.StdArgumentTypes ( HOC/HOC/StdArgumentTypes.hs, dist/build/HOC/StdArgumentTypes.o ) > [loading packages, like you'd see from ghci...] > Loading object (static) dist/build/HOC_cbits.o ... ghc: dist/build/HOC_cbits.o: Bad magic. Expected: feedfacf, got: feedface. GHC says "the impossible happened" and tells me to report it as a GHC bug, but based on the odd behavior I think the HOC build script might be doing something odd. I just don't have any idea where to start looking. For instance, why the heck is it looking at the magic bits for an object file while compiling a source file? Any insight appreciated. -- Edward Amsden Student Computer Science Rochester Institute of Technology www.edwardamsden.com

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Re: [Haskell-cafe] How on Earth Do You Reason about Space?
by John Lato 01 Jun '11

01 Jun '11

> > From: Brandon Moore <brandon_m_moore(a)yahoo.com> > > > I was worried data sharing might mean your keys > retain entire 64K chunks of the input. However, it > seems enumLines depends on the StringLike ByteString > instance, which just converts to and from String. > That can't be efficient, but I suppose it avoids excessive sharing. That's true for 'enumLines', however the OP is using 'enumLinesBS', which operates on bytestrings directly. Data sharing certainly could be an issue here. I tried performing Data.ByteString.copy before inserting the key into the map, but that used more memory. I don't have an explanation for this; it's not what I would expect. The other parameter which affects sharing is the chunk size. I got a much better memory profile when using a chunksize of 1024 instead of 65536. Oddly enough, when using the large chunksize I saw lower memory usage from Data.Map, but with the small chunksize Data.HashMap has a significant advantage. John Lato

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ANN: dtd-types 0.2.0.0
by Yitzchak Gale 01 Jun '11

01 Jun '11

The dtd-types package provides types for processing XML DTDs in Haskell. These types are intended to be compatible with and extend the set of types provided by John Millikin's xml-types package. The consensus seems to be to leave this as a separate package and not to merge it with xml-types. This version fixes a number of issues with the preliminary version. Among those issues were: omission of processing instructions, pointed out by John Millikin, and Haddock problems pointed out by Michael Snoyman. Thanks! Hackage: http://hackage.haskell.org/package/dtd-types/ Darcs: http://code.haskell.org/dtd/dtd-types Enjoy, Yitz

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Re: [Haskell-cafe] ANNOUNCE: CamHac: Haskell Hackathon in Cambridge, UK, 12-14 August 2011
by Simon Marlow 01 Jun '11

01 Jun '11

On 26/05/2011 15:35, Simon Marlow wrote: > CamHac is happening - come and spend a long weekend in Cambridge hacking > Haskell code in great surroundings with fantastic company! > > Full details on the wiki page: > > http://www.haskell.org/haskellwiki/CamHac > > When: Friday-Sunday 12-14 August 2011 > Where: the Horobin Room of Homerton Conference Centre [1] > > Registration is free! See the wiki page above for details. We're limited > to 50 people, and registration is on a first-come first-served basis, so > sign up soon. We've reached 50 registrations already! Luckily the venue has a larger room available, so we've changed our booking and we can now fit 72 haskellers in. We're now in the Leah-Manning room: http://www.homertonconference.com/Leah-Manning.html Cheers, Simon

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Enabling GADTs breaks Rank2Types code compilation - Why?
by dm-list-haskell-cafe＠scs.stanford.edu 01 Jun '11

01 Jun '11

I'm using GHC 7.0.2 and running into a compiler error that I cannot understand. Can anyone shed light on the issue for me? The code does not make use of GADTs and compiles just fine without them. But when I add a {-# LANGUAGE GADTs #-} pragma, it fails to compile. Here is the code: ==== {-# LANGUAGE Rank2Types #-} {-# LANGUAGE GADTs #-} mkUnit :: (forall t. t -> m t) -> m () mkUnit f = f () data Const b a = Const { unConst :: b } sillyId :: r -> r sillyId r = unConst (mkUnit mkConst_r) -- This line can't compile with GADTs where mkConst_r t = mkConst r t mkConst :: b -> t -> Const b t mkConst b _ = Const b ==== And the error I get is: Couldn't match type `t0' with `t' because type variable `t' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: t -> Const r t The following variables have types that mention t0 mkConst_r :: t0 -> Const r t0 (bound at /u/dm/hack/hs/gadt.hs:11:11) In the first argument of `mkUnit', namely `mkConst_r' In the first argument of `unConst', namely `(mkUnit mkConst_r)' In the expression: unConst (mkUnit mkConst_r) I've found several workarounds for the issue, but I'd like to understand what the error message means and why it is caused by GADTs. Thanks in advance for any help. David

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Lazy Evaluation in Monads
by Scott Lawrence 01 Jun '11

01 Jun '11

I was under the impression that operations performed in monads (in this case, the IO monad) were lazy. (Certainly, every time I make the opposite assumption, my code fails :P .) Which doesn't explain why the following code fails to terminate: iRecurse :: (Num a) => IO a iRecurse = do recurse <- iRecurse return 1 main = (putStrLn . show) =<< iRecurse Any pointers to a good explanation of when the IO monad is lazy? === The long story === I wrote a function unfold with type signature (([a] -> a) -> [a]), for generating a list in which each element can be calculated from all of the previous elements. unfold :: ([a] -> a) -> [a] unfold f = unfold1 f [] unfold1 :: ([a] -> a) -> [a] -> [a] unfold1 f l = f l : unfold1 f (f l : l) Now I'm attempting to do the same thing, except where f returns a monad. (My use case is when f randomly selects the next element, i.e. text generation from markov chains.) So I want unfoldM1 :: (Monad m) => ([a] -> m a) -> [a] -> m [a] My instinct, then, would be to do something like: unfoldM1 f l = do next <- f l rest <- unfoldM1 f (next : l) return (next : rest) But that, like iRecurse above, doesn't work.

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