- Haskell-Cafe - Haskell.org

Compiling and installing glib and gtk+ on Snow Leopard
by Mark Spezzano 23 Oct '10

23 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 22 Oct '10

22 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 22 Oct '10

22 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 22 Oct '10

22 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 22 Oct '10

22 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 22 Oct '10

22 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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ICFP 2011: Call for Workshop Proposals
by Wouter Swierstra 22 Oct '10

22 Oct '10

CALL FOR WORKSHOP AND CO-LOCATED EVENT PROPOSALS ICFP 2011 16th ACM SIGPLAN International Conference on Functional Programming September 19 - 21, 2011 Tokyo, Japan http://www.icfpconference.org/icfp2011 The 16th ACM SIGPLAN International Conference on Functional Programming will be held in Tokyo, Japan on September 19-21, 2011. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops (and other co-located events, such as tutorials) to be affiliated with ICFP 2011 and sponsored by SIGPLAN. These events should be more informal and focused than ICFP itself, include sessions that enable interaction among the attendees, and be fairly low-cost. The preference is for one-day events, but other schedules can also be considered. NEW THIS YEAR: The workshops are scheduled to occur on September 18 (the day before ICFP) and September 22-24 (the three days after ICFP). ---------------------------------------------------------------------- Submission details Deadline for submission: November 19, 2010 Notification of acceptance: December 17, 2010 Prospective organizers of workshops or other co-located events are invited to submit a completed workshop proposal form in plain text format to the ICFP 2011 workshop co-chairs (Gabriele Keller and Derek Dreyer), via email to icfp11-workshops at mpi-sws.org by November 19, 2010. (For proposals of co-located events other than workshops, please fill in the workshop proposal form and just leave blank any sections that do not apply.) Please note that this is a firm deadline. Organizers will be notified if their event proposal is accepted by December 17, 2010, and if successful, depending on the event, they will be asked to produce a final report after the event has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2011/icfp11-workshops-form.txt Further information about SIGPLAN sponsorship is available at: http://acm.org/sigplan/sigplan_workshop_proposal.htm ---------------------------------------------------------------------- Selection committee The proposals will be evaluated by a committee comprising the following members of the ICFP 2010 organizing committee, together with the members of the SIGPLAN executive committee. Workshop Co-Chair: Gabriele Keller (University of New South Wales) Workshop Co-Chair: Derek Dreyer (MPI-SWS) General Co-Chair: Manuel Chakravarty (University of New South Wales) General Co-Chair: Zhenjiang Hu (National Institute of Informatics) Program Chair: Olivier Danvy (Aarhus University) ---------------------------------------------------------------------- Further information Any queries should be addressed to the workshop co-chairs (Gabriele Keller and Derek Dreyer), via email to icfp11-workshops at mpi-sws.org.

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Eta-expansion and existentials (or: types destroy my laziness)
by oleg＠okmij.org 22 Oct '10

22 Oct '10

Max Bolingbroke wrote: > Let's start with a simple example of an existential data type: > > data Stream a = forall s. Stream s (s -> Maybe (a, s)) > > ones :: Stream Int > > ones = cons 1 ones > > Unfortunately, 'ones' is just _|_! The reason is that cons is strict > in its second argument. The problem I have is that there is no way to > define cons which is > simultaneously: > > 1. Lazy in the tail of the list > 2. Type safe > 3. Non-recursive Really? Here are two 'cons' that seem to satisfy all the criteria > {-# LANGUAGE ExistentialQuantification #-} > > data Stream a = forall s. Stream s (s -> Maybe (a, s)) > > nil :: Stream a > nil = Stream () (const Nothing) > > -- One version > -- cons :: a -> Stream a -> Stream a > -- cons a str = Stream Nothing (maybe (Just (a, Just str)) run) > -- where run (Stream s step) = > -- step s >>= (\ (a,s) -> return (a, Just (Stream s step))) > > -- the second version > cons :: a -> Stream a -> Stream a > cons a str = Stream (Just (a,str)) step > where step Nothing = Nothing > step (Just (a, (Stream s step'))) = Just (a, > case step' s of > Nothing -> Nothing > Just (a',s') -> Just (a',(Stream s' step'))) > > > instance Show a => Show (Stream a) where > showsPrec _ (Stream s step) k = '[' : go s > where go s = maybe (']' : k) > (\(a, s) -> shows a . showString ", " $ go s) (step s) > > taken :: Int -> Stream a -> Stream a > taken n (Stream s step) = > Stream (n, s) (\(n, s) -> > if n <= 0 then Nothing else maybe Nothing > (\(a, s) -> Just (a, (n - 1, s))) (step s)) > > ones :: Stream Int > ones = cons 1 ones > > test2 = taken 5 $ ones > -- [1, 1, 1, 1, 1, ] Finally, if one doesn't like existentials, one can try universals: http://okmij.org/ftp/Algorithms.html#zip-folds http://okmij.org/ftp/Haskell/zip-folds.lhs The code implements the whole list library, including zip and zipWith. None of the list operations use value recursion. We still can use value recursion to define infinite streams, which are processed lazily. In fact, the sample stream2 of the example is the infinite stream.

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