October 2010 - Haskell-Cafe

Re: [Haskell-cafe] concurrency vs. I/O in GHC
by Brandon Moore 24 Oct '10

24 Oct '10

On Oct 23, 2010, at 2:57 PM, Claude Heiland-Allen <claudiusmaximus(a)goto10.org> wrote: On 23/10/10 17:42, Gregory Crosswhite wrote: On 10/23/10 7:54 AM, John Lato wrote: On Fri, Oct 22, 2010 at 6:16 PM, Bulat Ziganshin <bulat.ziganshin(a)gmail.com <mailto:bulat.ziganshin@gmail.com>> wrote: Hello John, Monday, October 18, 2010, 8:15:42 PM, you wrote: > If anyone is listening, I would very much like for there to be a > mechanism by which external functions can be called "unsafe"-ly, but > without blocking all other Haskell threads. I have code that does this: +RTS -N2 This doesn't work, which was why the OP asked in the first place. When a thread calls an unsafe foreign function, it blocks everything until that function returns. Is that true? The last time we discussed this in Haskell Cafe the conclusion I drew from the discussion was that unsafe foreign functions block the current thread but not any other thread. The conclusion I drew was that "unsafe" foreign functions block the current "capability" (OS thread) and any "threads" (Haskell forkIO etc) currently scheduled on that capability, but other capabilities and threads continue executing as normal. I haven't tested it recently, but I think that is mostly correct except that an unsafe call will also prevent major collections, which will eventually tie up the rest of the capabilities as well.

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Redy to release gtk2hs-0.12.0!
by Andy Stewart 24 Oct '10

24 Oct '10

Dear gtk2hs user: We have ready to release next version : gtk2hs-0.12.0 Please pull gtk2hs darcs to test, if no bug report, we will release. You can use below commands (Debian) to install gtk2hs darcs: darcs get --lazy http://code.haskell.org/gtk2hs/ cd ./gtk2hs sudo chmod +x ./bootstrap.sh ./bootstrap.sh Then recompile your gtk2hs program to test. Of course, any successful feedback are welcome! Thanks, -- Gtk2hs Team

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Compiling and installing glib and gtk+ on Snow Leopard
by Mark Spezzano 24 Oct '10

24 Oct '10

Hi, I'm trying to install the Leksah IDE and I've run into the problem that it requires glib and gtk+. Whenever I type 'configure' for glib-2.26.0 everything goes fine. But when I type 'make' it goes through necessary actions and then suddenly stops halfway through stating the following (annoying) error message: error: #error GNU libiconv not in use but included iconv.h is from libiconv What, exactly is happening here? I've compiled libiconv and put it under /usr/bin (so iconv is there). Yet it still complains...I don't get it. I've spend the best part of a day mucking around with this to no avail. Also: I used to have Leopard installed on my machine and then I upgraded to Snow Leopard--does this mean that I would need to rebuild from scratch most of the packages on the entire system? Could this change from 32-bit to 64-bit be causing an issue? Thanks, Mark

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[ANNOUNCE] text-xml-qq - XML quasiquoter for Text.XML.Light
by Oscar Finnsson 23 Oct '10

23 Oct '10

Hi, I've just released the first version of text-xml-qq, It's a Template Haskell quasiquoter that converts XML code into Text.XML.Light.Element compile time. Feature requests, bug fixes etc are welcomed. The package only got one backend (xml-package) at the moment but it should be trivial to add more backends (e.g. for a bytestring-based xml package). You can embed Haskell variables in the xml code. Inside xml-elements you use the syntax {foo} for embedded variables. If you wish to embed an element or text you use the syntax <<foo>>. See example or the test-file http://github.com/finnsson/text-xml-qq/blob/master/src/Text/XML/TestQQ.hs at the project site http://github.com/finnsson/text-xml-qq. -- Oscar === Example === Given the variables url = "google.se" elem = "gmail" attrNs = "something" attrName = "Pelle" attrValue = "Arne" elemCont = CRef "testing" cont1 = Elem $ blank_element { elName = QName "hej" Nothing Nothing } cont2 = CRef "other test" the code [$xmlQQ| <{url}:{elem} {attrNs}:{attrName}={attrValue} attr="cool"> <elem ns1:elem1="1" ns2:elem2="2"><<elemCont>></elem> <elem /> <el /> <<cont1>> <<cont2>> </{url}:{elem}> |] will generate the data structure element { elName = QName elem Nothing (Just url), elAttribs = [Attr (QName attrName Nothing (Just attrNs)) attrValue, Attr (qname "attr") "cool"], elContent = [ (Elem $ blank_element { elName = QName "elem" Nothing Nothing, elAttribs = [Attr (QName "elem1" Nothing (Just "ns1")) "1", Attr (QName "elem2" Nothing (Just "ns2")) "2"], elContent = [elemCont] }), (Elem $ blank_element { elName =QName "elem" Nothing Nothing}), (Elem $ blank_element { elName = QName "el" Nothing Nothing}), cont1, cont2] } === End example ===

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The Haskell theme
by Christopher Done 23 Oct '10

23 Oct '10

To kick off discussion about Haskell's general theme, as discussed recently, here's some random ideas. Going with the original colours of the nominated Haskell logo, and kinda what's been done here: http://new-www.haskell.org/haskellwiki/Haskell http://img840.imageshack.us/img840/3577/ideasv.png To download the Inkscape SVG grab it here: http://chrisdone.com/designs/haskell-theme-1.svg I don't know what I was thinking here but it seemed like fun: http://img412.imageshack.us/img412/1827/rect5935.png I get bored really quickly, I was alright for 15 minutes and then I lost the will to point and click. Anyway, I was thinking rather than trying to come up with a brilliant all-encompassing design, we could agree on conventions: * colours * headings * spacing * links * the particular incarnation of the logo etc. I'm pretty happy to go with the colour theme of http://new-www.haskell.org/haskellwiki/Haskell Haskellers.com also makes me think we need an umbrella theme that encompasses all Haskell sites. Maybe we should do a theme poll like the logo poll. I don't know. Regardless, I think we do need to decide on something and stick with it. Should this discussion be taken to the web devel mailing list? Is that appropriate?

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readProcess exception
by Leskó Dániel 23 Oct '10

23 Oct '10

Hi everyone! I run into a problem with readProcessWithExitCode (from System.Process module). Basically what i want is to start an exe file, giving it some input on stdin and receiving the results on stdout. But id the stdin and out is bigger that 5000-7000 char (nondeterministically), i get commitAndReleaseBuffer exception and sometimes hFlush exception. I'm getting these errors with GHCi 6.12.3, if it matters. Have you guys any idea what causes the problem, to how to avoid it? Thanks Daniel Here is a dummy example, which produces the exception: Haskell code: import System.Process test = do result <- sequence $ Prelude.map (\x -> runOnce x) [1..10000] return () where runOnce :: Int -> IO [Int] runOnce i = do putStrLn (show i) a <- caller i b <- caller i return (a++b) caller a = do (exit_code,stdout) <- readProcessWithExitCode "a.exe" [show a] (unlines (map show [1..850])) return (read stdout) C code (for executable) #include <stdio.h> #include <complex.h> #include <stdlib.h> int main(int argc, char *argv[]) { printf("[%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d]",100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100, 100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100,100); }

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Re: Scrap your rolls/unrolls
by Max Bolingbroke 23 Oct '10

23 Oct '10

Forgot to reply to list On 22 October 2010 12:14, Dan Doel <dan.doel(a)gmail.com> wrote: > Another solution, though, is SHE. With it, you can write: > > data ListF a r = NilF | ConsF a r > newtype List a = Roll (ListF a (List a)) > > pattern Nil = Roll NilF > pattern Cons x xs = Roll (ConsF x xs) > > And not worry about Rolls anymore. Ah yes, pattern synonyms. This solution is somewhat unsatisfying because you will also need some smart constructors: """ nil = Roll NilF cons x xs = Roll (ConsF x xs) """ Now the names of the smart constructors for building the data type are not the same as the things you pattern match on, which is a slightly annoying asymmetry. It's certainly less complicated than the TypeFamilies based solution though! Cheers, Max

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HSCurses - Hello World? (can't find much documentation)
by Anonymous Void 23 Oct '10

23 Oct '10

Hi everyone, Haven't posted here since I was a kid, when this language first warped my poor imperative brain. My main work environment is a three-split terminal session using `Yakuake' as a dropdown-terminal (one for vim, one for interactive interpreter, one for MySQL). I don't mind using the MySQL cli *that* much, but I find typing my queries all the time much takes up precious time when I'm developing things, and it annoys me. Since there is currently no TUI for mysql administration (besides links/phpadmin? o.0). I decided to make one, choosing Haskell because, well, it's awesome. I've made curses apps before, but never in Haskell (only wx, cgi, cli, etc). Couldn't find anything besides an api reference; Which was impossible to find before typing haddock specifically, even after an exhausting search on google, krugle, archives, etc... It's livable (I've learned more complex things with less). But since I've never seen how the HSCurses functions really integrate together, or any tutorials/examples (besides hsFishEx), I fear toying with it is going to be annoying until I figure it out on my own, especially since I'm already doing a project for someone... So, I was wondering if anyone would be kind enough to point me to (or write me) a basic example of how HSCurses works, maybe w/ some very simple example of interactivity (e.g. getch processing, screen refresh, ..) support. Also, maybe warn me of some of the common pitfalls and differences in hscurses vs ncurses in other languages. Having found the haddock, and having done curses apps in other languages, that should be enough for me to get a handle on it. I'm just dreading the "read the haddock, assume it works this way, find out you do it this way, repeat" loop of learning w/o a starting point. Any help would be greatly appreciated, and will save me some (anticipated) hassle. TYIA, -Pete

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A rant against the blurb on the Haskell front page
by Ben Franksen 23 Oct '10

23 Oct '10

This is a critique of the current 'Haskell Blurb', the first paragraph on www.haskell.org. This blurb should, IMO, give a concise description of what Haskell, the programming language, is, what makes it different from other languages, and why I should be interested in it. What it does, instead, is to make me scratch my head and ask myself: what marketing idiot has written this inclonclusive mumble-jumble of buzz-words? Let me explain. "Haskell is an advanced purely functional programming language." Good start, if only the "advanced" were replaced with something more characteristic, like "lazy", or "statically typed". Which, BTW, both do not appear in the whole blurb, even though they are *the* characteristics of Haskell, lazyness being even something that sets it apart from most other languages. I hear the marketeers crying "but the average visitor has no idea what lazyness means". So what? Give them a link to the wiki with an explanation. So, a better introductory sentence would be -> "Haskell is a lazily evaluated, purely functional programming language with a very flexible and powerful static type system." Next sentence: "An open source product of more than twenty years of cutting edge research, it allows rapid development of robust, concise, correct software." This really gets me every time I read it. How can anyone write such a nonsense? Haskell is not an "open source product"! It is no product at all. That most (maybe all) implementations are opens source is certainly an interesting fact, but IMO not something that should appear at the top of the page right under the header "The Haskell Programming Language". The second and third sentences deliberately conflate language and implementation(s). This is a well known falacy and I am ashamed that it appears on the front page of my favourite programming language. The blurb talks about "robust, concise, correct software", but misses itself most of these goals: it is imprecise, incorrect, and not robust (because implementations vary), and therefore not a good advertisement, though quite possibly "rapidly developed". The blurb promises "rapid development of robust, concise, correct software" lest one think this were something akin to Perl which certainly allows rapid development, yet typically neither robust nor correct, especially if done rapidly. So, how does Haskell differ from that? Well, I'd say this is where lazyness and a static yet flexible type system come into play. But no, I forgot, we don't want to explain anything or even be logical, dear reader, we want to pound slogans into your head! That cutting edge research is done for Haskell as well as for its implementations is of course good to know, but just stating it is not nearly enough: such a statement must be corroberated with evidence, otherwise it is just idle marketing. (Not that there wouldn't be evidence amass, it's just that none is given.) On we go: "With strong support for integration with other languages, built-in concurrency and parallelism, debuggers, profilers, rich libraries and an active community, Haskell makes it easier to produce flexible, maintainable high-quality software." Let us take that apart: (1) Fact: Haskell has a good and very easy to use FFI. To the C language. I have never heard of integration with any other langauge being directly supported. (2) Fact: Built-in concurrency and parallelism is not exactly part of the langauage, although purity makes it possible to support them in a more precise and less error-prone way; which is exploited by ghc's concurrency and parallelism extensions. (3) Fact: Traditional debuggers are practically useless in Haskell, due to lazy evaluation. But, oh, we forgot to mention the small fact that Haskell is lazy. Too bad. Profiling is supported by ghc only (AFAIK), but is supposed to be very useful (never seriously used it, so I can't judge.) (4) Fact: there are libraries. Some of them are good, others are not so good. Most leave a lot to be desired, but I would be hard pressed to come up with something better myself. What really distinguishes Haskell libraries from what is found in other languages is the level of abstraction. I know no other language where stuff like Monad, (Applicative) Functor, Monoid, Category etc. is *at the heart* of all the libraries. But, oh, I forgot, we don't want to scare people off, so better not talk about this in public. (5) Fact: Haskell has an active community. No question. (6) Fact: Haskell makes certain things easier (than other languages). Other things that are easy in other langauges are hard in Haskell. Or at least very non-obvious. Whether Haskell programs are more flexible, when compared with dynamically typed OO languages like Python, I seriously doubt. Maintainable high-quality software is the real selling point, IMO; or rather *reliably correct* software. However, I cannot see how this dirtectly follows from the previous points, which is what the sentence is saying. What has an FFI do to with high quality? What has the supposed availability of debuggers (where? which?) to do with it? Enough. I think I have made my point. Cheers Ben

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tried to use the example given in the source of network.browser
by Michael Litchard 23 Oct '10

23 Oct '10

This is what is in HTTPbis/Network/Browser.hs > do > rsp <- Network.Browser.browse $ do > setAllowRedirects True -- handle HTTP redirects > request $ getRequest "http://google.com/" > fmap (take 100) (getResponseBody rsp) And how I changed it slightly to test it out > import Network.HTTP > import Network.Browser > main = > do > rsp <- Network.Browser.browse $ do > setAllowRedirects True -- handle HTTP redirects > request $ getRequest "http://google.com/" > fmap (take 100) (getResponseBody rsp) but I got this errortest.lhs:10:39: Couldn't match expected type `Network.Stream.Result (Response [a])' against inferred type `(Network.URI.URI, Response String)' In the first argument of `getResponseBody', namely `rsp' In the second argument of `fmap', namely `(getResponseBody rsp)' In the expression: fmap (take 100) (getResponseBody rsp) how did I get this error? I'm perplexed as this came right from the source.

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