- Haskell-Cafe - Haskell.org

Do we need Monad fail (or MonadFail)?
by John Smith 23 Dec '10

23 Dec '10

Monads seem to use fail in one of three ways: -Regular monads leave it at the default definition of error -MonadPlus sometimes redefines it to mzero -IO redefines it to failIO Are there any other definitions of fail? If not, does the special case of IO really need a class-level definition, or could there be another way of dealing with failed pattern matches?

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Why is Haskell flagging this?
by michael rice 23 Dec '10

23 Dec '10

I don't understand this error message. Haskell appears not to understand that 1 is a Num. Prelude> :t 1 1 :: (Num t) => t Prelude> :t [1,2,3,4,5] [1,2,3,4,5] :: (Num t) => [t] Prelude> Michael =================== f :: [Int] -> IO [Int] f lst = do return lst main = do let lst = f [1,2,3,4,5] fmap (+1) lst =============================== Prelude> :l test [1 of 1] Compiling Main ( test.hs, interpreted ) test.hs:5:17: No instance for (Num [Int]) arising from the literal `1' at test.hs:5:17 Possible fix: add an instance declaration for (Num [Int]) In the second argument of `(+)', namely `1' In the first argument of `fmap', namely `(+ 1)' In the expression: fmap (+ 1) lst Failed, modules loaded: none. Prelude>

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Help!! Cannot get RandT and liftIO to work together...
by JP Moresmau 23 Dec '10

23 Dec '10

Hello all, sorry I must have taken my stupid pills this morning, I cannot get the following code to compile, what am I missing? data MyState=MyState Integer newState:: (RandomGen g) => RandT g IO MyState newState = do time<-liftIO getCPUTime rand<-getRandomR (1,6) return $ MyState (time+rand) newStateIO :: IO MyState newStateIO = do r<-getStdGen evalRandT newState r -> Could not deduce (MonadIO (RandT g IO)) from the context (RandomGen g) arising from a use of `liftIO' at ...:4:15-31 Possible fix: add (MonadIO (RandT g IO)) to the context of the type signature for `newState' or add an instance declaration for (MonadIO (RandT g IO)) Pirates.hs /Pirates/src line 167 Problem But the MonadRandom docs say: Instances: MonadIO<http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/C…> m => MonadIO<http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/C…> (RandT<http://hackage.haskell.org/packages/archive/MonadRandom/0.1.6/doc/html/Cont…> g m) And the MonadIO docs say: Instances: MonadIO<http://hackage.haskell.org/packages/archive/transformers/0.2.2.0/doc/html/C…> IO<http://hackage.haskell.org/packages/archive/base/4.3.1.0/doc/html/System-IO…> So it looks to me it should work! I've also tried to replace IO by MonadIO m=> in the signature, or remove all signatures. No joy. Thanks for any help. -- JP Moresmau http://jpmoresmau.blogspot.com/

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Re: [Haskell-cafe] Functor => Applicative => Monad
by Simon Marlow 23 Dec '10

23 Dec '10

On 22/12/10 19:17, John Smith wrote: > On 22/12/2010 19:03, Simon Marlow wrote: >> On 14/12/2010 08:35, Isaac Dupree wrote: >>> On 12/14/10 03:13, John Smith wrote: >>>> I would like to formally propose that Monad become a subclass of >>>> Applicative, with a call for consensus by 1 February. The change is >>>> described on the wiki at >>>> http://haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal, >>> >>> That page isn't written as a proposal yet, it's written as a bunch of >>> ideas. I would be happy to see something along the lines of Bas van >>> Dijk's work >>> http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/14740 . >> >> This is a proposal with far-reaching consequences, and with several >> alternative designs. I'm not sure I understand all >> the tradeoffs. Some parts of the proposal are orthogonal to the rest >> (e.g. changing fmap to map), and should probably be >> considered separately. >> >> Could someone please write a detailed proposal, enumerating all the >> pros and cons, and the rationale for this design >> compared to other designs? >> >> Cheers, >> Simon > > The wiki page was admittedly too broad of scope when I first wrote it, > and it's getting broader with time. > > The ticket at http://hackage.haskell.org/trac/ghc/ticket/4834 has > patches for only one change, making Monad a subclass of Applicative. (I > would update the description to make this clear, but I can't edit the > ticket description, so this has been relegated to a comment.) Dealing with one issue at a time is great. But even with this single change, we need to weight up the advantages and disadvantages - it's difficult to make an assessment without trawling through long email threads, and yet I don't want to let the proposal pass without comment. So it would help a lot if someone could take the time to explain the design space and the tradeoffs. I know for example I have lots of code that defines a Monad instance and doesn't need an Applicative instance, so this change will force me to jump through hoops that I don't need to. Cheers, Simon

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non_lazy_ptr ld error?
by Evan Laforge 23 Dec '10

23 Dec '10

Lately I've been seeing this error occasionally when recompiling: Undefined symbols: "_UiziTrackC_d26DB", referenced from: _UiziTrackC_d26DB$non_lazy_ptr in TrackC.o ld: symbol(s) not found If I delete the .o file for the mentioned module and recompile, it links fine. This makes me think something in the dependency analysis is mixed up and not recompiling something that should be recompiled. But I'm using --make and letting ghc figure out the deps. Does this error look familiar to anyone? This is ghc 6.12.3 on OS X 10.5.8. On a probably unrelated note, a few times lately I've had a startup segfault on a program which was previously fine. The stack trace on the crashing thread is: Thread 2 Crashed: 0 libSystem.B.dylib 0x954f9fa0 strcmp + 80 1 elaforge.seq.seq 0x0143f7dd lookupHashTable + 131 2 elaforge.seq.seq 0x01444243 ghciInsertStrHashTable + 24 3 elaforge.seq.seq 0x01445a87 ocGetNames_MachO + 1803 4 elaforge.seq.seq 0x014447bf loadObj + 500 5 elaforge.seq.seq 0x007064a4 sYy_info + 12 This is a little worrying because I've never seen this before and I don't know why it's started. It seems to happen consistently until I make clean and recompile. My app uses hint and I suspect this is part of the hint initialization and has to do with inconsistent .o files or something... anyone seen this either?

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Yet Another Haskell Tutorial missing (maybe lost on server move?)
by Henning Thielemann 23 Dec '10

23 Dec '10

The page http://www.haskell.org/haskellwiki/Yet_Another_Haskell_Tutorial says that there is a darcs repository at http://darcs.haskell.org/yaht/ but it is not. Also in the canonical replacement location http://code.haskell.org/yaht/ I cannot find it. The Wikiboook https://secure.wikimedia.org/wikibooks/en/wiki/Haskell/YAHT mentions the same URL and additionally a PDF at http://darcs.haskell.org/yaht/yaht.pdf which is missing, too. Can someone please tell me, where it is, update HaskellWiki and Wikibook and add a forward from darcs.haskell.org to the new location?

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New simplified primes; no VIP necessary.
by Will Ness 23 Dec '10

23 Dec '10

Hi, For those who remember the discussion about this about a year ago, it turns out there was a simpler version after all lurking somewhere in there (or is it _out_?). I've just posted it to the haskellwiki's Prime Numbers page: primes = 2: primes' where primes' = 3: 5: [7,9..] `minus` tfold [ [p*p,p*p+2*p..] | p <- primes' ] tfold ((x:xs):t) = x : xs `union` tfold (pairs t) pairs ((x:xs):ys:t) = (x: union xs ys) : pairs t The full code with double-feed, wheel and better folding structure is also there. Speed and complexity weren't changed by this. But it is rather _short_. :) Cheers,

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ANN: cabal-dev 0.7.3.1 -- now with ghci!
by Rogan Creswick 23 Dec '10

23 Dec '10

Cabal-dev is now capable of launching ghci with the project's package database and local modules (if the package under development exposes a library). For example: # First, invoke cabal-dev install the package to populate the # package database: $ cabal-dev install .... <snip> .... $ cabal-dev ghci GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> The ghci shell should have access to all the libraries your application/library is using, as well as any modules that your library exposes. Note that this is not quite as natural as your traditional ghci shell, namely: Source modifications are not visible without exiting, re-issuing `cabal-dev install` *and* `cabal-dev ghci`. This will eventually get better, but that's where things are right now. The reason for this is that `cabal-dev ghci` just issues ghci with the cabal-dev package database (and excluding the user package db, to best reflect what cabal-dev does when it causes compilation). Cabal-dev is available on hackage and github: - http://hackage.haskell.org/package/cabal-dev - https://github.com/creswick/cabal-dev There is a short write-up with a (slightly) longer example here: - http://blog.ciscavate.org/2010/12/cabal-dev-sandboxing-your-haskell-develop… --Rogan

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Behaviour of System.Directory.getModificationTime
by Arnaud Bailly 23 Dec '10

23 Dec '10

Hello, I am writing a program which scans a bunch of directories to detect changes (added, modified, deleted files). To detect modification, I would like to use getModification, doing something like the following: -- |Stores last modified timestamp of the file -- Yes, I come from Java... type HashMap = M.Map FilePath ClockTime data (Show a, Eq a) => Edit a = Mod a | Add a | Del a deriving (Eq,Show) checkChanges :: [String] -> HashMap -> IO ([Edit FilePath], HashMap) checkChanges [fs] m = do addedFilesList <- lsRecursive fs timestamps <- mapM getModificationTime addedFilesList let allts = zip addedFilesList timestamps let ret = (findDeletedFiles allts.updateScannedFiles allts) ([],m) return ret -- | Returns an updated map and a list of modified/added/deleted files updateScannedFiles :: [(FilePath,ClockTime)] -> ([Edit FilePath], HashMap) -> ([Edit FilePath], HashMap) updateScannedFiles [] r = r updateScannedFiles ((path,ts):files) (updates,m) = case M.lookup path m of Nothing -> updateScannedFiles files ((Add path:updates), M.insert path ts m) Just ts' -> if ts' < ts then updateScannedFiles files ((Mod path:updates), M.adjust (const ts) path m) else updateScannedFiles files (updates, m) -- omitting findDeletedFiles which is obvious and works I got the following test case that fails: "recursively marks changed files as modified" `for` do complexFileSetup -- create a small FS tree with 3 files root <- tempDir state <- checkChanges [root] M.empty writeFile (root </> "subdir" </> "cFile.txt") "this is another toast" checkChanges [root] (snd state) >>= (\r -> assertEqual "there should be 1 changed file in 3 files" 1 ((length.modified.fst) r)), In words: the getModificationTime of the overwritten file appears identical before and after the write. What am I doing wrong ? BTW, I checked the content of the file which has been correctly updated. Thanks for your help Arnaud

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$ do?
by Jonathan Geddes 23 Dec '10

23 Dec '10

Quick question: Why do I need the $ in the following bits of code? > main = withSocketsDo $ do > --do something with sockets > foo = fromMaybe 0 $ do > --do something in the maybe monad I don't see (after admittedly only a minute or so thinking about it) where any grammar ambiguities would be if 'do' had an implicit $ in front of it: > foo = fromMaybe 0 do > --do something in maybe Though now that I've written it down, that is hard for me to visually parse at a glance. I'm still curious though. --Jonathan

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Re: [Haskell-cafe] Proof in Haskell
by Patrick Browne 23 Dec '10

23 Dec '10

On 22/12/2010 14:48, Artyom Shalkhakov wrote: > ..Do you want to prove a property of > a function formally, using some kind of formal logic? I am aware that functional languages do not do proofs at term level, but the motivation for my question is to get a precise reason why this is so. The replies from the café have clearly articulated the reasons. Thanks to all, 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

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Darcs failure
by Andrew Coppin 23 Dec '10

23 Dec '10

I'm running a VM with Ubuntu 10.10 (Maverick Meerkat). I installed Darcs 2.4.4 using apt-get, but it keeps CONSTANTLY failing with the error message darcs: bug at src/URL.hs:246 compiled Sep 12 2010 20:24:56 Another possible bug in URL.waitNextUrl: curl_multi_perform() - no running handles See http://wiki.darcs.net/BugTracker/Reporting for help on bug reporting. Does anybody have any clue how I can get the hell rid of this stupid message and actually get some stuff done? (At this point, Darcs has hung, and I have to use the kill command with just the right signal number to make Darcs exit. Ctrl+C just tells me "Couldn't handle interrupt since darcs was in a sensitive job." So every single time this error happens, I have to get a process list, find Darcs, issue the kill command, and then reissue the original Darcs command and hope it actually completes this time...)

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Identity type
by Pierre-Etienne Meunier 23 Dec '10

23 Dec '10

Hi, Is there something like an identity type, transparent to the type-checker, in haskell ? For instance, I'm defining an interval arithmetic, with polynomials, matrices, and all that... defined with intervals. The types are : Polynomial Interval (instead of Polynomial Double for instance) Matrix Interval etc. Now I'd like to be able to define some typeclass "intervalize" that would convert a IEEE-754 version of my types into the interval ones : class Intervalize a where intervalize::a Double-> a Interval Wouldn't it be very natural wanting to use this class on Doubles directly : intervalize' :: Double->Interval intervalize'=intervalize Does this exist in haskell ? in GHC ? Cheers, Pierre

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Missing Parsec library in latest stable GHC
by Aaron Gray 22 Dec '10

22 Dec '10

Missing Parsec library :- scheme.o(.text+0x4fa):fake: undefined reference to `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziCombinator_skipMany1_closure' scheme.o(.text+0x501):fake: undefined reference to `parseczm2zi1zi0zi0_TextziPaserCombinatorsziParsecziChar_space_closure' scheme.o(.text+0x5c2):fake: undefined reference to `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziChar_oneOf_closure' scheme.o(.text+0x63a):fake: undefined reference to `mtlzm1zi1zi0zi0_ControlziMonadziTrans_zdf1_closure' GHC from :- http://www.haskell.org/ghc/dist/stable/dist/ ghc-7.0.1.20101221-i386-windows.exe<http://www.haskell.org/ghc/dist/stable/dist/ghc-7.0.1.20101221-i386-windows…> and earlier. Aaron

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Re: [Haskell-cafe] Missing Parsec library in latest stable GHC
by Antoine Latter 22 Dec '10

22 Dec '10

ghc doesn't, by default, go searching for packages to link in to the resulatant executable.. If you try 'ghc --make scheme.hs' your example will work better. You can also specify what to link manually, but --make works pretty well most of the time. In ghc version 7 '--make' is the default, but until that's more widely distributed you'll want to get used to using the switch. Take care, Antoine On Wed, Dec 22, 2010 at 10:36 AM, Aaron Gray <aaronngray.lists(a)gmail.com> wrote: > On 22 December 2010 16:27, Antoine Latter <aslatter(a)gmail.com> wrote: >> >> What commands did you enter to produce this error? >> > > ghc scheme.hs > Aaron > >> >> Ahanks, >> Antoine >> >> On Wed, Dec 22, 2010 at 9:54 AM, Aaron Gray <aaronngray.lists(a)gmail.com> >> wrote: >> > Missing Parsec library :- >> > scheme.o(.text+0x4fa):fake: undefined reference to >> > >> > `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziCombinator_skipMany1_closure' >> > scheme.o(.text+0x501):fake: undefined reference to >> > `parseczm2zi1zi0zi0_TextziPaserCombinatorsziParsecziChar_space_closure' >> > scheme.o(.text+0x5c2):fake: undefined reference to >> > `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziChar_oneOf_closure' >> > scheme.o(.text+0x63a):fake: undefined reference to >> > `mtlzm1zi1zi0zi0_ControlziMonadziTrans_zdf1_closure' >> > GHC from :- >> > http://www.haskell.org/ghc/dist/stable/dist/ >> > ghc-7.0.1.20101221-i386-windows.exe >> > and earlier. >> > Aaron >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> > > >

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Re: [Haskell-cafe] Functor => Applicative => Monad
by Isaac Dupree 22 Dec '10

22 Dec '10

On 12/14/10 03:13, John Smith wrote: > I would like to formally propose that Monad become a subclass of > Applicative, with a call for consensus by 1 February. The change is > described on the wiki at > http://haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal, That page isn't written as a proposal yet, it's written as a bunch of ideas. I would be happy to see something along the lines of Bas van Dijk's work http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/14740 . -Isaac

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Rendering of <hask> in new wiki (MSIE6)
by Dimitry Golubovsky 22 Dec '10

22 Dec '10

Hi, In MSIE6, <hask> tags are rendered like this (from the Monad_Transformers page): transformers: provides the classes MonadTrans and MonadIO , as well as concrete monad transformers such as StateT ... etc. The Wiki source: [http://hackage.haskell.org/package/transformers transformers]: provides the classes <hask>MonadTrans</hask> and <hask>MonadIO</hask>, as well as concrete monad transformers such as <hask>StateT</hask>. HTML (a small piece of it): provides the classes <div class="inline-code"><div dir="ltr" style="text-align: left;"><div class="source-haskell" style="font-family: monospace;">MonadTrans</div></div></div> Words MonadTrans, MonadIO, StateT etc are enclosed in <hask> tags. They show up in monospace, each starting a new line. Is this only MSIE6, or this is how it is supposed to render? Thanks. PS I am not attaching a screenshot to the mailing list; if anyone needs to see it I'll send via personal e-mail. -- Dimitry Golubovsky Anywhere on the Web

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Latest Haskell Platform for Windows
by Aaron Gray 22 Dec '10

22 Dec '10

Could someone please point me at a copy of the latest Haskell platform or a working GHC please. Many thanks in advance, Aaron

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Windows Haskell Platform download link is broken
by Aaron Gray 22 Dec '10

22 Dec '10

Windows Haskell Platform download link goes nowhere :- http://hackage.haskell.org/platform/windows.html The "Download Haskell for Windows" is broken :- http://lambda.galois.com/hp-tmp/2010.2.0.0/HaskellPlatform-2010.2.0.0-setup… Aaron

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Problem with class Control.Monad.Error noMsg usage
by Aaron Gray 22 Dec '10

22 Dec '10

I have been trying to build the Scheme in 24 Hours on WikiBooks :- http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours http://jonathan.tang.name/files/scheme_in_48/code/listing10.hs But I am getting an error :- scheme.hs:289:6: `noMsg' is not a (visible) method of class `Error' scheme.hs:290:6: `strMsg' is not a (visible) method of class `Error' with the following :- instance Error LispError where noMsg = Default "An error has occurred" strMsg = Default if I try importing them :- import Control.Monad.Error (noMsg, strMsg) I get the following :- scheme.hs:4:29: Module `Control.Monad.Error' does not export `noMsg' scheme.hs:4:36: Module `Control.Monad.Error' does not export `strMsg' Many thanks in advance, Aaron

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Re: [Haskell-cafe] Proof in Haskell
by Daniel Fischer 21 Dec '10

21 Dec '10

I wrote: > On Tuesday 21 December 2010 19:34:11, Felipe Almeida Lessa wrote: >> >> Theorem mirror_mirror : forall A (x : Tree A), mirror (mirror x) = x. >> induction x; simpl; auto. >> rewrite IHx1; rewrite IHx2; trivial. >> Qed. > Since mirror (mirror x) = x doesn't hold in Haskell, I take it that Coq > doesn't allow infinite structures? Oops, mirroring infinite binary trees should work. Ignore above.

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type class design
by Uwe Schmidt 21 Dec '10

21 Dec '10

Dear Haskellers, I've a question about type class design. When developing the set of functions for a class, there are often two or more functions, let's say f and g, where the semantics of g can be expressed by f. When writing down the code, there are two choices for g. First g is included in the type class, second g is defined outside and the signature has a context referencing the class. 1. case class Foo a where f :: ... a ... g :: ... a ... g = expr[f] 2. case class Foo a where f :: ... a ... g :: Foo a => ... a ... g = expr[f] What are the reasons for choosing 1., what are the arguments for 2.? My arguments for 1.: a) Efficiency: g may be overwritten in an instance declaration by a more efficient piece of code. b) Readability: All functions, which are logically connected are grouped together in a single syntactic construct and can be found in one place (similar to an interface in Java). My argument for 2.: c) Correctness: The semantics for g are fix (relative to f), All laws for g hold everywhere, assuming f is implemented correctly. I've sometimes the problem to balance the reasons for 1. and 2 and I doubt, that this list of criteria is complete. In the standard Haskell classes we can find both cases, even within a single class. Eq with (==) as f and (/=) as g belongs to the 1. case, so does Monad and (>>=) as f and (>>) as g. But (>=>) and the Monad class fall into the 2. case. It seem, there is no uniform or consistent strategy in the core Haskell classes. What are your best practice rules for this problem? Expecting your advices, Uwe

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ANN: The attosplit package splits lazy bytestrings lazily
by Yitzchak Gale 21 Dec '10

21 Dec '10

This is a belated announcement of a package I uploaded to hackage last week. When processing a large input stream, one of the most important techniques is to split the input into smaller pieces and process each piece separately. If that large input stream happens to be a lazy bytestring, the attosplit package provides a simple but powerful way to use that technique. The attosplit package exports a single function: split :: Parser Strict.ByteString -> Lazy.ByteString -> [Lazy.ByteString] It splits a lazy bytestring into a lazy list of lazy bytestrings, at boundaries defined by an attoparsec parser. Whenever the parser matches, the result of the match is prepended to the new lazy bytestring which begins at that point. For example, suppose lazyH is a lazy bytestring read from a gigantic HTML file, and you want to process each section beginning with an <H1> tag separately. Each section could itself still be quite large, so you want to process each section lazily as well. Then split (string "<H1>") lazyH is a lazy list of lazy bytestrings. The first lazy bytestring gives you all of the bytes up to the first <H1> tag. The rest of the lazy bytestrings give each section beginning with <H1> separately. This allows you to read the entire file lazily, while processing each individual section lazily on its own. Of course, you are not limited to split only at boundaries defined by a string. You have the full power of attoparsec at your fingertips. http://hackage.haskell.org/package/attosplit Enjoy! -Yitz

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Re: [Haskell-cafe] Do we need Monad fail (or MonadFail)?
by Jonathan Geddes 21 Dec '10

21 Dec '10

I'd be really interested in learning the rationale behind those changes. I'm sure it wasn't done for capricious or arbitrary reasons, but I can't help but see it as a step back. --Jonathan Geddes (sent from android mobile) On Dec 21, 2010 8:47 AM, "Lauri Alanko" <la(a)iki.fi> wrote: On Tue, Dec 21, 2010 at 08:31:08AM -0700, Jonathan Geddes wrote: > I'd love for the compiler to give... You will be interested to know that everything you ask for already was in Haskell ages ago: http://www.cs.auckland.ac.nz/references/haskell/haskell-report-1.4-html/exp… They decided to get rid of it in Haskell 98, for reasons that someone else can probably explain. Lauri _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org ...

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Installing snap-server with gnutls on OS X with Homebrew
by Benedict Eastaugh 21 Dec '10

21 Dec '10

Hi -café, I installed Snap 0.3 last night on my Mac, and had some problems getting it to build with SSL support. This is just a quick note of the extra flags I had to pass in when installing it in case someone else has a setup similar to mine. These instructions presume that you are using the Homebrew package manager on OS X, and that Homebrew is installing packages into /usr/local. You first need to install the gnutls library. brew install gnutls Then install snap-server. cabal update cabal install snap-server -fgnutls --extra-lib-dirs=/usr/local/lib/ --extra-include-dirs=/usr/local/include/ Note that while gnutls' dependencies libgcrypt and libgpg-error also need to be in the path, they are dependencies of gnutls and will both be installed when you install gnutls with Homebrew. Cheers, Benedict.

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internal Haddock or GHC error: de-package: openBinaryFile:
by Johannes Waldmann 21 Dec '10

21 Dec '10

Hi. I try to use haddock (2.8.1) but it breaks with haddock: internal Haddock or GHC error: de-package: openBinaryFile: does not exist (No such file or directory) Why would it want to read a file with the name "de-package"? (Is "de" related to the locale? But the error also happens with LC_ALL=C) If I do "touch de-package" in the current directory, then I get haddock: internal Haddock or GHC error: Data.Binary.getWord8: end of file I am running this in a directory where I did ghc --make before, and then haddock -d $(find . -name "*.hi" | sed -e 's/\.hi/.hs/') I can't try this with haddock-2.9 since I am using ghc-6.12.3 (?) Thanks, J.W.

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Haskell Parse Tree
by Jane Ren 21 Dec '10

21 Dec '10

Hi, Does anyone know how to get the parse tree of a piece of Haskell code? Any recommended documentation? Thanks

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ANNOUNCE: new version of uu-parsinglib:2.5.6
by S. Doaitse Swierstra 21 Dec '10

21 Dec '10

An old problem popped up in the uu-parsinglib. When combining two parsers with <|>, it is checked to see which alternative accepts the shortest input; this is done in order to prevent infinite insertions, which may occur as a result of choosing a recursive alternative when inserting of some recursive non-terminal is needed. In such a case we want to choose a non-recursive alternative. This approach however does not combine well with the permuting parsers: here all the alternatives have the same length and we do not want to compute all those lengths, since this defeats the whole purpose of building these permutations lazily. So I added a non-length-checking version of <|> to ExtApplicative with is used in pPerms. If you are using the permuting parsers you are strongly advised to upgrade. Have a nice Christmas and a happy new year, Doaitse

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Data.Judy and StablePtr
by Diego Souza 21 Dec '10

21 Dec '10

Hello Haskellers, I'm playing a bit with Data.Judy. However, I noticed that using StablePtr incurs in some performance problems, I don't have any numbers but as a example use Int and ByteString as values and you can notice the difference without any benchmarking tool with the example that comes in the documentation. The question that I have is: is there any way to make it faster for types other than those that fits in word size? I understand that the way the binding to the C code has been done, stableptr is required. Am I wrong assuming this? I would be happy to submit a patch but I failed to find a way to improve the performance. The second question is what is a common practice to avoid memory leaks with StablePtr? I have zero experience programming with the Haskell FFI interface, so probably the most basic stuff would help me a lot in this case. Thanks in advance, -- ~dsouza yahoo!im: paravinicius gpg key fingerprint: 71B8 CE21 3A6E F894 5B1B 9ECE F88E 067F E891 651E gpg pub key: http://bitforest.org/~dsouza/pub/gpg-pubkey.txt authorized_keys: http://bitforest.org/~dsouza/pub/authorized_keys.txt

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unable to load package `regex-posix-0.94.2' (On Vista x64)
by __kaveh__ 21 Dec '10

21 Dec '10

I am using Vista x64. I have Haskell platform installed (Latest varsion -available for Windows, 2010.2.0.0, GHC 6.12.3). When I am trying to run picnic.hs from "How To Organize a Picnic on a Computer" tutorial. I get this: C:\...>runhaskell picnic.hs picnic.hs:46:18: Warning: accepting non-standard pattern guards (use - XPatternGuards to suppr ess this message) Just [x, y] <- matchRegex (mkRegex "([0-9.]+), ([0-9.]+)") s picnic.hs:181:21: Warning: accepting non-standard pattern guards (use - XPatternGuards to suppr ess this message) Just (_, e) <- find ((== p) . fst) a picnic.hs: C:\Program Files (x86)\Haskell Platform\2010.2.0.0\lib \extralibs\rege x-posix-0.94.2\ghc-6.12.3\HSregex-posix-0.94.2.o: unknown symbol `_regerror' picnic.hs: picnic.hs: unable to load package `regex-posix-0.94.2' I have tried to update using (cmd is running as administrator): C:\Windows\system32>cabal update Downloading the latest package list from hackage.haskell.org And then installing package using cabal: C:\Windows\system32>cabal install regex-posix [--reinstall] --global Resolving dependencies... Configuring regex-posix-0.94.4... Preprocessing library regex-posix-0.94.4... Building regex-posix-0.94.4... [1 of 6] Compiling Text.Regex.Posix.Wrap ( dist\build\Text\Regex\Posix \Wrap.hs, dist\build\Text\Regex\Posix\Wrap.o ) [2 of 6] Compiling Text.Regex.Posix.String ( Text\Regex\Posix \String.hs, dist\bu ild\Text\Regex\Posix\String.o ) [3 of 6] Compiling Text.Regex.Posix.Sequence ( Text\Regex\Posix \Sequence.hs, dis t\build\Text\Regex\Posix\Sequence.o ) [4 of 6] Compiling Text.Regex.Posix.ByteString ( Text\Regex\Posix \ByteString.hs, dist\build\Text\Regex\Posix\ByteString.o ) [5 of 6] Compiling Text.Regex.Posix.ByteString.Lazy ( Text\Regex\Posix \ByteStrin g\Lazy.hs, dist\build\Text\Regex\Posix\ByteString\Lazy.o ) [6 of 6] Compiling Text.Regex.Posix ( Text\Regex\Posix.hs, dist\build \Text\Regex \Posix.o ) Registering regex-posix-0.94.4... Installing library in C:\Program Files (x86)\Haskell\regex-posix-0.94.4\ghc-6.12.3 Registering regex-posix-0.94.4... And why the cabal package installs at "C:\Program Files (x86)\Haskell"? Shouldn't it be "C:\Program Files (x86)\Haskell Platform\2010.2.0.0"? Thanks

3 6

identity function
by A Smith 21 Dec '10

21 Dec '10

I've been reviewing the library, and have come unstuck with the *id*function. What's its purpose and can someone give me an example of its practical use. -- Andrew

3 2

Where are all the Taiwanese Haskellers?
by Pasqualino "Titto" Assini 21 Dec '10

21 Dec '10

Hello, I am going to Taiwan with my family for about five weeks (I am Italian, my wife is Taiwanese and we have two kids) and I wondered if there was any Taiwanese Haskeller that would be interested in meeting for a chat about Haskell, Life and Everything else, maybe in front of a nice cup of 高山茶. We arrive in Taiwan on the 4th of December, spend a few days nearby Taipei and then down to Pu-Li and Kaoshong, where we will spend about 4 weeks and then probably back to Taipei for a few more days around the 10 of January. I am afraid that my Chinese is almost non-existent (yes I know, that's so stupid of me) so the conversation will have to be in English (Italian or French would be fine as well :-)). All the very best, titto -- Dr. Pasqualino "Titto" Assini http://quid2.org/

2 1

non-hackage cabal repos? unpacking non-repo package tarballs?
by Claus Reinke 21 Dec '10

21 Dec '10

I often find myself writing example code that I'd like to distribute via cabal, but without further burdening hackage with not generally useful packages. 1. The simplest approach would be if cabal could expose its internal 'unpackPackage' as a command, so that author: cabal sdist user: cabal unpackPackage Example.tar.gz would work (the point being that cabal handles .tar.gz, which not all users have installed separately; distributing the .tar.gz would be handled via the usual channels; and after unpacking, cabal can build the source package). Note that 'cabal unpack' does not seem helpful here, as it touches the package index instead of just looking at the tarball. Could this be made possible, please? Or is it already? 2. Failing that, I remembered that cabal used to be designed without a fixed package repo address, and a little digging found options --remote-repo and --local-repo, as well as the directory layout for package repositories: http://hackage.haskell.org/trac/hackage/wiki/HackageDB So this scenario seems possible, in principle: author: expose a package repo http://myrepo user: cabal --remote-repo=http://myrepo fetch Example Is it possible to add a (temporary) repo location like this, especially for 'cabal fetch'/'cabal unpack'? I've managed to get 'cabal --local-repo=myrepo list Example' to work (needs uncompressed 00-index.tar), but not the remote variant, and fetching from the local repo doesn't work, either. Are there any usage examples of these features? Claus

3 2

ANN: cabal-dev - sandboxed haskell development
by Rogan Creswick 21 Dec '10

21 Dec '10

Galois is happy to announce cabal-dev: a tool for managing development builds of Haskell projects within sandboxes. Performing consistent builds is critical in software development, but the current system in GHC/Haskell of per-user and per-system GHC package databases interferes with this need for consistency. It is difficult to precisely identify the dependencies of a given project, and changes necessary to enable one project to build may render another project inoperable. If each project had a separate package database, each project could be built in a sandbox. Cabal-dev manages cabal-install repositories and sandboxed ghc package databases to prevent interactions between disparate projects or user package databases. Cabal-dev is similar to capri, which was coincidentally developed concurrently, but the two projects take slightly different approaches to sandboxing and exhibit slightly different behaviors depending on the state of the global package database and the versions of the tools installed. For most packages, just use cabal-dev instead of cabal, and you will get a sandboxed build that will not install anything (even automatically installed dependencies) into the user or global ghc package databases. If your build depends on patched or unreleased libraries, you can add them to your sandboxed build environment so they can be installed by cabal-dev or cabal by running: > cabal-dev add-source /path/to/source/code Where /path/to/source/code is either a path to a source directory containing a .cabal file, or an sdist tarball. Cabal-dev has been in use at Galois for roughly six months now, but it should currently be treated as alpha software: we've exercised a few execution paths heavily, but it still has a number of rough edges (the rough edges we know about are documented in the TODO file: https://github.com/creswick/cabal-dev/blob/master/TODO.md) Cabal-dev is hosted on hackage and git-hub: - http://hackage.haskell.org/package/cabal-dev - https://github.com/creswick/cabal-dev A brief tutorial can be found in the README.md: - https://github.com/creswick/cabal-dev/blob/master/README.md We're looking forward to your feedback! --Rogan Creswick

1 0

haskell platform windows download url
by Albert Y. C. Lai 21 Dec '10

21 Dec '10

On http://hackage.haskell.org/platform/windows.html , the link for "download haskell for windows" is: http://lambda.galois.com/hp-tmp/2010.2.0.0/HaskellPlatform-2010.2.0.0-setup… Is this normal?

3 3

REPL loop
by Aaron Gray 21 Dec '10

21 Dec '10

Is it possible to implement a REPL (Read-eval-print loop) in Haskell ? Many thanks in advance, Aaron

3 2

Haskell on Debian - gcc compilation problem with cabal installed libs like hmatrix, etc.
by gutti 20 Dec '10

20 Dec '10

Hi, I'm very new to Haskell and this Forum, just doing my first steps ... -- Try to use the hmatrix package for vector and matrix calculations. The haskell compilation works (no problem in GHCi mode), the gcc however compilation fails with messages like: EFA.o: In function `r1bo_info': (.text+0x48): undefined reference to `hmatrixzm0zi10zi0zi0_NumericziVector_zdfNumVector1_closure' EFA.o: In function `r1bs_info': (.text+0x10a): undefined reference to `hmatrixzm0zi10zi0zi0_NumericziVector_zdfFractionalVector_closure' Obviously gcc doesn't know where the according libraries are installed. Same problem with other packages. I installed the haskell base system over debian (debian squeeze), gcc version 4.4.5. - Hmatrix and other packages are installed and build over cabel into local folder ~/.cabal/lib Do I need to set a system path, or change cabal options or .. ? Thanks in advance, Phil -- View this message in context: http://haskell.1045720.n5.nabble.com/Haskell-on-Debian-gcc-compilation-prob… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

3 5

Snap 0.3 Released!
by Gregory Collins 20 Dec '10

20 Dec '10

Hi all, After many months of hard work, the Snap team is proud to announce the release of the next major version of the Snap Framework: Snap 0.3. New Features ------------ - Snap now has SSL support! To enable SSL support, install the gnutls library and pass the "gnutls" in when you build snap-server: $ cabal install snap-server -fgnutls - The snap project starter executable has been pulled out of snap-core and put into its own package, called "snap". The snap package also contains some library code which the generated projects make use of, and the generated projects have a simplified and much cleaner layout. - Snap now comes with support for working in a "development mode" which runs your web handlers in an interpreter using the hint library. This makes working on Snap applications quicker and more convenient, since you no longer need to perform a kill/recompile/reload cycle to see your code changes reflected in your running application. - One of the new things in the snap package is an experimental "extensions" mechanism which contains some interfaces/helper code for writing reusable web components. - A new MonadSnap typeclass has been defined: class (Monad m, MonadIO m, MonadCatchIO m, MonadPlus m, Functor m, Applicative m, Alternative m) => MonadSnap m where liftSnap :: Snap a -> m a Most of the basic functions from Snap.Types have been redefined to accept any monad implementing MonadSnap, and MonadSnap instances have been defined for most of the standard monad transformers; this should make it much more convenient to use monad transformers with Snap without having to use "lift" all over the place. - Added getCookie :: MonadSnap m => ByteString -> m (Maybe Cookie), which allows you to fetch cookies from the request by name. - Snap now depends on the enumerator package for iteratee support instead of iteratee. - The snap-server now has a convenient configuration mechanism which will parse arguments from the command line for you. Bugfixes/Improvements --------------------- - Changed type signature of finishWith from Response -> Snap () to Response -> Snap a. - When using fileServe, file paths are now properly URL-decoded, meaning you can now download a file with spaces in its name. - Parameters grabbed by variable capture using route are also now URL-decoded. - Under the hood: snap server backend mechanism has been improved/factored out, making it possible to listen to two or more ports using different backends at once (e.g. SSL and non-SSL). Thanks, G -- Gregory Collins <greg(a)gregorycollins.net>

1 0

handling multiple versions of a data structure
by Dmitry V'yal 20 Dec '10

20 Dec '10

Greetings, while developing my neural net simulator I stumbled upon a problem. I have a data type NeuralNet and use Show and Read instances for saving and loading configurations. As time passed, I changed the data type, so the program can no longer load files saved in previous versions. I want fix it. My current idea looks as follows. I'm going to create a bunch of types NN1, NN2, NN3..NNn for different versions and write converters c12 :: N1 -> N2, c23 :: N2 -> N3 and so on. But how to organize the whole process of parsing String into NNn so it's easy to change formats? Something based on using a list of parsers [read, c43 . read, c43 . c23 . read, c43, c23 . c12 . read, c43 . c32 . c21 . read] looks rather verbose and grows quadratically with N. I'm sure there must be a more elegant way. Any ideas? Dmitry

6 11

cabal dependency on code repositories.
by Greg Weber 20 Dec '10

20 Dec '10

Michael Snoyman and I were discussing the need for beta releases of Yesod and he encourage me to post this to the cafe. Beta releases could be built into the hackage system. However, this can be viewed as a more general problem of distributing multiple versions of code (stable vs. experimental, forks, etc). This is a problem that has been largely solved by version control, but in this instance just needs some integration with an installer system. In the ruby world there is a tool called bundler that is now the standard installer- basically bundler combined with rubygems it is like cabal combined with hackage. In addition to referencing a published library version, with bundler one can reference a git repository, and even reference a particular branch or revision. Essentially with beta releases you are trying to release multiple branches of your repository. This is also very useful for dealing with multiple forks of a package. If I have problems with a package I can look at the github repo and look for forks with bug fixes, and then point the Gemfile (cabal file) to that repo. The repo must have a gemspec (cabal file). Essentially this removes the step of the code writer of needing to edit the cabal file or publish to hackage. This is good because many code writers do not want to publish their minor variations of packages on hackage or do not want to publish new packages that they feel are of low quality. This is also good from the perspective of the code user- I normally don't want to see minor variations on hackage. If I am interested in minor variations I will go to the github repo and look at the different branches and forks where it will be clear to me what the difference is- it would be difficult to accurately portray this information on hackage. Productivity in modern programming is largely about code re-use, and I only see this as becoming more important. Type safety and Cabal do a great job facilitating code re-use. I think we can make cabal even more useful here. Has this ability been discussed before for cabal? t would be great if it could at least be worked on for the next Google Summer of Code. Sorry if I offended anyone by exclusively mentioning git, but that is the only version control system used by Rubyists :)

2 2

Is there a tutorial interpreter to teach haskell?
by Larry Evans 20 Dec '10

20 Dec '10

By tutorial interpreter, I means something like an expert system having a list of rules and than a problem which is solved by using those list of rules. The tutorial means the trace of the "problem state" before and after each rule is applied along with which parts of the rule are matched with which part of the "problem state". W.R.T. haskell, the "problem state" would be a haskell expression and each "rule application" would be simply reducing the haskell expression (using some rule, which would be cited during the reduction) to a simpler form until the final answer was achieved. Obviously the rule applications to be trace should be user selectable to avoid way too much output, but that seems similar to setting breakpoints in selected functions; hence, I guess it wouldn't be hard to do. The attached file illustrates what I'm after. I laboriously composed that attached to enable me to understand what the haskell code was doing. It would help other novices if such a trace could be automated. I'm currently trying to understand: sequence (c:cs) = return (:) `ap` c `ap` sequence cs from p. 2 of: http://www.soi.city.ac.uk/~ross/papers/Applicative.pdf and again I'm having a lot of difficulty what the code is doing. So far I've got: --{--eshell-- Prelude Monad> :t ap ap :: (Monad m) => m (a -> b) -> m a -> m b Prelude Monad> :t return (:) return (:) :: (Monad m) => m (a -> [a] -> [a]) Prelude Monad> (:) 'a' "bc" "abc" Prelude Monad> :t return (:) `ap` "a" return (:) `ap` "a" :: [[Char] -> [Char]] --}--eshell-- so now I must "manually" figure out what the a and b in the ap declaration correspond to in the return(:) type: m ( a -> b ) __ _ _ 1: [] Char -> [Char]->[Char] 2: [] Char->[Char] -> [Char] IOW, is it choice 1: in the above table for choice 2:? I'd guess, since application associates to the left, that it's choice 1:. But you see, I'm not sure, and I have to work too hard to figure it all out. I *may* get it eventually, but it's a *lot* of work. I tutorial interpreter would make this so much easier! -regards, Larry

3 3

enumsets in ArchLinux but not in Hackage
by Henning Thielemann 20 Dec '10

20 Dec '10

I found the package 'enumsets' in ArchLinux: http://aur.archlinux.org/packages.php?ID=18116 This points to Hackage, but there is no such package. It sounds like something I could need: A simple Haskell 98 wrapper around Word32, Word16, Word8, that are interpreted as bitfields, providing set operations with elements of an Enum type. I frequently like to have this when interfacing to C. I know of Edison.EnumSet, but my emphasis is on simplicity (i.e. Haskell 98).

2 4

IO, sequence, lazyness, takeWhile
by Jacek Generowicz 20 Dec '10

20 Dec '10

-- Is it possible to rewrite code written in this style untilQuit = do text <- getLine report text if text == "quit" then return () else untilQuit -- in a style using higher order functions for abstract iteration? For -- example, something along these lines: untilQuit' = (fmap (takeWhile (/= "quit"))) (sequence $ map (>>= report) (repeat getLine)) -- The latter version shows the report, but it doesn't stop at the -- appropriate place, so I'm guessing that I'm being bitten by my -- ignorance about the interaction of actions and lazyness. -- For completeness, here's a definition of report report text = do putStrLn $ "You wrote " ++ text return text

6 6

Re: [Haskell-cafe] Bifold: a simultaneous foldr and foldl
by Henning Thielemann 19 Dec '10

19 Dec '10

Noah Easterly wrote: > Somebody suggested I post this here if I wanted feedback. > > So I was thinking about the ReverseState monad I saw mentioned on r/haskell a > couple days ago, and playing around with the concept of information flowing > two directions when I came up with this function: > > bifold :: (l -> a -> r -> (r,l)) -> (l,r) -> [a] -> (r,l) > bifold _ (l,r) [] = (r,l) > bifold f (l,r) (a:as) = (ra,las) > where (ras,las) = bifold f (la,r) as > (ra,la) = f l a ras > > (I'm sure someone else has come up with this before, so I'll just say I > discovered it, not invented it). > > Basically, it's a simultaneous left and right fold, passing one value from > the start of the list toward the end, and one from the end toward the start. I also needed a bidirectional fold in the past. See foldl'r in http://code.haskell.org/~thielema/utility/src/Data/List/HT/Private.hs You can express it using foldr alone, since 'foldl' can be written as 'foldr': http://www.haskell.org/haskellwiki/Foldl_as_foldr You may add your example to the Wiki.

2 1

how to write a loop in haskell way
by ender 19 Dec '10

19 Dec '10

Hi all: I'v been learning haskell for several months, and now I'm trying to write some "real word" program in haskell, like finding files under one directory or something My problem is that, I dont know the way of writing a loop in haskell. I searched google and found some code that translate c loop into haskell like this one: > > > I am new to haskell and would look to write a function equivalent > > > to the following loop in C > > > > > > int value = 500000; > > > int part_stack[4]; > > > int *part_ptr = part_stack; > > > for (; value; value /= 10000) > > > *part_ptr++ = value % 10000; > > > > part_stack :: [Int] > > part_stack = [0,50] > > > > Note that I've performed a memoization optimization--this > > makes the code both smaller, faster and easier to read! :P > > Ignore David, he's pulling your leg. Here's the proper translation: > > do > alloca $ \value -> do > poke value (500000::Int) > allocaArray 4 $ \part_stack -> do > alloca $ \part_ptr -> do > poke part_ptr part_stack > let loop = do > val <- peek value > if val == 0 then return () else do > p <- peek part_ptr > poke p (val `rem` 10000) > poke part_ptr (p `plusPtr` 1) > poke value (val `quot` 10000) > loop > loop and I really think that's not a "haskell way", it's just translate c code into haskell code byte by byte My question is: how to translate above c code into haskell in "haskell way" Thanks and BR Ender

5 8

Re: [Haskell-cafe] [Haskell] macports, cabal, cabal-install difficulty
by Antoine Latter 19 Dec '10

19 Dec '10

I don't know about the ld warning, but cabal-install can never know which version of cabal-install is installed, because it doesn't track executables - it only tracks and knows about libraries. Right after the warning is the line: > Installing executable(s) in /Users/matthew/.cabal/bin So it looks like it might have worked :-) You can either add ~/.cabal/bin to your path, or move/soft-link the cabal-install executable to your preferred path. Take care, Antoine On Sat, Dec 18, 2010 at 7:52 AM, Matthew Fairtlough <research(a)fairtlough.net> wrote: > Hello, I'm new to this list and to hackage/cabal/cabal-install and haskell > on the mac generally. I don't dare post on cabal-devel because I'm a > newbie. > > I came to this question because I haven't been able to install the unix > package properly (some packages I want depend on it) but I've posted > separately in haskell-cafe on that. > I've port version 1.9.2 and haskell-platform @2009.2.0.2 (devel, haskell) > > My question: Is cabal currently out-of-the-box compatible with macports? I > seem to have problems with cabal-install, in particular the ld warning and > the fact that cabal doesn't seem to know what version of cabal-install is > installed. > > bash-3.2# cabal info cabal > * Cabal (library) > Synopsis: A framework for packaging Haskell software > Latest version available: 1.10.0.0 > Latest version installed: 1.10.0.0 > > bash-3.2# cabal install cabal-install-0.8.2 > Resolving dependencies... > Configuring cabal-install-0.8.2... > Preprocessing executables for cabal-install-0.8.2... > Building cabal-install-0.8.2... > ....<lots of build information deleted>... > [40 of 40] Compiling Main ( Main.hs, > dist/build/cabal/cabal-tmp/Main.o ) > Linking dist/build/cabal/cabal ... > ld: warning: -read_only_relocs cannot be used with x86_64 > Installing executable(s) in /Users/matthew/.cabal/bin > > bash-3.2# cabal info cabal-install > * cabal-install (program) > Synopsis: The command-line interface for Cabal and Hackage. > Latest version available: 0.8.2 > Latest version installed: [ Unknown ] > > ....and lots more:... > > Homepage: http://www.haskell.org/cabal/ > Bug reports: http://hackage.haskell.org/trac/hackage/ > Description: The \'cabal\' command-line program simplifies the process > of > managing Haskell software by automating the fetching, > configuration, compilation and installation of Haskell > libraries and programs. > Category: Distribution > License: BSD3 > Author: Lemmih <lemmih(a)gmail.com> > Paolo Martini <paolo(a)nemail.it> > Bjorn Bringert <bjorn(a)bringert.net> > Isaac Potoczny-Jones <ijones(a)syntaxpolice.org> > Duncan Coutts <duncan(a)haskell.org> > Maintainer: cabal-devel(a)haskell.org > Source repo: http://darcs.haskell.org/cabal-install/ > Executables: cabal > Flags: old-base, bytestring-in-base > Dependencies: unix >=1.0 && <2.5, Win32 ==2.*, bytestring >=0.9, > base <2.0 || >=3.0, base >=2.0 && <2.2, old-time >=1 && > <1.1, > array >=0.1 && <0.4, containers >=0.1 && <0.4, > random >=1 && <1.1, pretty >=1 && <1.1, > directory >=1 && <1.1, process >=1 && <1.1, base >=3, > base <3, time ==1.1.*, zlib >=0.4 && <0.6, > HTTP >=4000.0.2 && <4001, network >=1 && <3, filepath >>=1.0, > Cabal ==1.8.*, base >=2 && <5 > Cached: Yes > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

1 0

DNS Problems at haskell.org
by John Peterson 19 Dec '10

19 Dec '10

Just to clarify - I was the contact for Haskell.org but the email address was no longer being forwarded. I had assumed I had been removed as the contact but unfortunately only the technical contact had been changed. All was finally resolved with the help of Yale, where they resurrected the old email address, and Galois, who fixed things up once they had access to the account at Network Solutions. Everything is now in capable hands - sorry about the problems. John

1 0

Re: [Haskell-cafe] Why is Haskell flagging this?
by michael rice 19 Dec '10

19 Dec '10

Here's (attached) a screen shot of the post I received from you. There's your name after the "From:" at the top. Also, this post is a reply, so your message follows. Was there a mix up? Michael --- On Sat, 12/18/10, Thomas Davie <tom.davie(a)gmail.com> wrote: From: Thomas Davie <tom.davie(a)gmail.com> Subject: Re: [Haskell-cafe] Why is Haskell flagging this? To: chrisdone(a)googlemail.com Cc: "michael rice" <nowgate(a)yahoo.com>, haskell-cafe(a)haskell.org Date: Saturday, December 18, 2010, 2:38 PM On 17 Dec 2010, at 21:44, Christopher Done wrote: On 17 December 2010 18:04, michael rice <nowgate(a)yahoo.com> wrote: =================== f :: [Int] -> IO [Int] f lst = do return lst main = do let lst = f [1,2,3,4,5] fmap (+1) lst The problem is that you are applying fmap to a type IO a. fmap (+1) (return [1,2,3]) But to achieve the behaviour you expect, you need another fmap: fmap (fmap (+1)) (return [1,2,3]) Which can be more neatly written with Conal's semantic editor cominators as (fmap . fmap) (+1) (return [1,2,3]) Of course, I question why the list is put in the IO monad at all here... surely this would be much better return $ fmap (+1) [1,2,3] Finally, that has the wrong type for main... perhaps you meant to print it out? main :: IO ()main = print $ fmap (+1) [1,2,3] Bob

1 0

(re)newcomer
by Matthew Fairtlough 19 Dec '10

19 Dec '10

Hello Haskell-cafers, I used to teach Haskell (and Clean!) at University level but haven't touched Haskell in years and certainly never used it with a Mac. Now I work in publishing and want to experiment with Haskell's web services and see if I can help set up an open-source system for handling ONIX xml data. So I tried to install happstack-server (among several other things) and this depends on unix but cabal can't install unix. Error and diagnostics below as best as I could see to report them. Any tips/pointers much appreciated; I'm not sure if this is a bug or where to report it if it is one... thanks for any help. Matthew. bash-3.2# cabal install unix Resolving dependencies... cabal: cannot configure unix-2.4.1.0. It requires base >=4.2 && <4.4 For the dependency on base >=4.2 && <4.4 there are these packages: base-4.2.0.0, base-4.2.0.1, base-4.2.0.2 and base-4.3.0.0. However none of them are available. base-4.2.0.0 was excluded because of the top level dependency base -any base-4.2.0.1 was excluded because of the top level dependency base -any base-4.2.0.2 was excluded because of the top level dependency base -any base-4.3.0.0 was excluded because of the top level dependency base -any bash-3.2# uname -a Darwin Matthew-Fairtloughs-MacBook-Pro.local 10.5.0 Darwin Kernel Version 10.5.0: Fri Nov 5 23:20:39 PDT 2010; root:xnu-1504.9.17~1/RELEASE_I386 i386 bash-3.2# port version Version: 1.9.2 bash-3.2# port info haskell-platform haskell-platform @2009.2.0.2 (devel, haskell) Description: This is the the Haskell Platform: a single, standard Haskell distribution for every system. The Haskell Platform is a blessed library and tool suite for Haskell distilled from Hackage. Homepage: http://hackage.haskell.org/platform/ Runtime Dependencies: ghc, hs-platform-cgi, hs-platform-fgl, hs-platform-editline, hs-platform-GLUT, hs-platform-haskell-src, hs-platform-html, hs-platform-HUnit, hs-platform-mtl, hs-platform-network, hs-platform-OpenGL, hs-platform-parallel, hs-platform-parsec, hs-platform-QuickCheck, hs-platform-regex-base, hs-platform-regex-compat, hs-platform-regex-posix, hs-platform-stm, hs-platform-time, hs-platform-xhtml, hs-platform-zlib, hs-platform-HTTP, hs-platform-alex, hs-platform-happy, hs-platform-cabal Platforms: darwin License: unknown Maintainers: gwright(a)macports.org bash-3.2# cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library bash-3.2# cabal info base * base (library) Synopsis: Basic libraries Latest version available: 4.3.0.0 Latest version installed: 4.1.0.0 Homepage: [ Not specified ] Bug reports: http://hackage.haskell.org/trac/ghc/newticket?component=libraries/base Description: This package contains the Prelude and its support libraries, and a large collection of useful libraries ranging from data structures to parsing combinators and debugging utilities. License: BSD3 Maintainer: libraries(a)haskell.org Source repo: http://darcs.haskell.org/packages/base/ Flags: integer-simple Dependencies: rts -any, ghc-prim -any, integer-simple -any, integer-gmp -any Documentation: /opt/local/share/ghc-6.10.4/doc/ghc/libraries/base Cached: No Modules: <long list elided>

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Haskell, Step by Step, Tutorial, Developing a Whole Application
by __kaveh__ 18 Dec '10

18 Dec '10

Is there a (or more; the more, the better) tutorial for Haskell, developing a whole application (of any kind: web, windows, console)? I mean something like NerdDinner or MVC Music Store for ASP.NET MVC; Or those whole applications in "in Action" books. Thanks Edit 1: Thanks to all for your replies. I just wanted to insist on "step by step developing a whole application" aspect of what I want; not those tutorials that teach Haskell (they are not bad - They are not this). I believe if one could understand Haskell well and even understand it's underlying mathematical foundations, that (absolutely) does not mean "he can develop an application in Haskell". The problem with Haskell is not failing to understand it (I can use LINQ and even developed a DSL based on LINQ - a LINQ provider - and see how it makes composition tidy) but failing to understand the context (system thinking). I can use same concepts in C# because I "know" the context and it is easy to "feed in" new tools/concepts. In Haskell, one can understand every bit of it (well almost) but there is no context there to fit in! So (I might not be wrong - but who knows!) there is an absolute need for this kind of "tutorial" (I wish there was a better name for this). Edit 2: I believe there is a way to learn and teach Haskell in a neat and straight way (That does not mean I think Haskell is a pragmatic programming language - don't get me wrong; I appreciate it; but start- up barrier with Haskell it too high; It has still a long way ahead to be considered as a common purpose programming language and may never does. But Haskell is definitely father of future programming languages - and also not so future ones!) Note: I have asked this question on stackoverflow (http://goo.gl/ bCFf7) and I will try to sync useful information as much as possible.

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