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Type of a function?
by Jenny678 14 Jul '06

14 Jul '06

Hallo Can somebody tell me the type of the following function? func :: ????????????? func f x = (z,y) where (z,y) = f x Thanks -- View this message in context: http://www.nabble.com/Type-of-a-function--tf1945843.html#a5335607 Sent from the Haskell - Haskell-Cafe forum at Nabble.com.

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can't WashNGo (abstract tables)
by Ferenc Wagner 14 Jul '06

14 Jul '06

Hi, trying to put WashNGo-2.9 to a nontrivial prototyping job gave some very compelling results so far, but also got me stumped on occasions. I'd be grateful for some guidance on the following points, concerning abstract tables mainly. * selectionDisplay: looks like displayFun (fourth arg) mustn't fail for null arguments, as it's invoked with them after leaving the page it appears on. An error or a feature? Is it documented? * trying to get around the "Strings only" restriction of AT's I threw in some show-read pairs, but got the impression that the channel is not Char-clean: Base64.encode . UTF8.encode helped. What's the spec? * should a click on an AT radio button result in submission of the form? It does not, which is OK with me. But the following code fails in a rather strange way: > data Record = P String Person | R String Role | Unknown String deriving (Read,Show) > > unAtSafe :: String -> Record > unAtSafe = read . fst . UTF8.decode . Base64.decode > > edit sg = case record of > P dn p -> editPerson dn p > R dn r -> editRole dn r > Unknown s -> standardQuery "No dice" $ CGI.div $ -- provokes error in AbstractSelector.hs:115 > do p $ text s > goBack > where record = unAtSafe $ head $ unAR $ value sg > > editPerson :: String -> Person -> CGI () > editPerson dn p = > standardQuery "Edit person" $ CGI.div $ > do inputs <- personTable p > actionPar "Modify" (createPerson dn) inputs In this case Person and Role contain String fields only. The abstract table contains Records, and the above function 'edit' dispatches on the type of the selected Record. The first two cases work, the Unknown case trips on an 'error' in the WashNGo code as indicated. It's in the valueSelectionGroup function, if selectionValue is Nothing. How could I fix this? * Lastly, how can I go back some queries? goBack above is basically a 'submit0 (return ())' which continues the main 'forever' loop. But what if I need to go back in the middle of the current loop, as if the user pressed Back several times? -- Thanks, Feri.

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Building Data.Time on WinXP GHC6.4.1
by Alistair Bayley 14 Jul '06

14 Jul '06

Has anyone built Ashley's Data.Time package on Windows? I ask because timestuff.c fails to compile for me because it refers to fields (tm_zone, tm_gmtoff) in struct tm which don't seem to exist on Windows (at least, mingw). Alistair

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ANN: Takusen 0.5
by Alistair Bayley 14 Jul '06

14 Jul '06

Oleg and I are pleased to announce the release of a new version of Takusen (it's been a while; so long that we don't remember the last version number we used). The most significant code change is a new internal design (courtesy of Oleg) which gives better separation of concerns like statement preparation, binding, and result-set processing. Also of note is a conversion to darcs, and hosting at haskell.org (thanks to Simon Marlow): darcs get http://darcs.haskell.org/takusen ... which also has the nice property that the Haddock documentation is browseable online: http://darcs.haskell.org/takusen/doc/html/index.html A comprehensive description of API usage can be found in the documentation for module Database.Enumerator: http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html We hope that the use of darcs will encourage contributions... Other changes: - support for multiple result sets returned from functions (PostgreSQL only). Oracle support for this is in the pipeline, which will allow use of nested cursors, and processing RefCursors returned as output parameters from procedure calls. Future plans: - Cabalisation - use of Data.Time instead of System.Time - ODBC and MS Sql Server backends For those of you unfamiliar with Takusen, here is our HCAR blurb: Takusen is a library for accessing DBMS's. It is a low-level library like HSQL, in the sense that it is used to issue SQL statements. Takusen's "unique-selling-point" is a design for processing query results using a left-fold enumerator. For queries the user creates an iteratee function, which is fed rows one-at-a-time from the result-set. We also support processing query results using a cursor interface, if you require finer-grained control. Currently we fully support Oracle, Sqlite, and PostgreSQL. Alistair

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Why is Day and Month bounded?
by Johan Holmquist 13 Jul '06

13 Jul '06

I fail to see why the types Day and Month in System.Time are instances of Bounded. In my world there is no last day and the successor of Saturday is Sunday -- not an exception! :) I would like to believe there is a good reason for this boundedness, but I can't see it.

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Using of C constants in Haskell sources; Determining compilation environment (Unix vs Windows)
by Bulat Ziganshin 13 Jul '06

13 Jul '06

Hello Haskell, what is a best way to bring C constant (defined in header file) into the Haskell source? Haskell project is cabalized and should work with both Win and Unix while the constants are OS-specific. the best way i found at this moment is to use the following scheme: mmap.h: #if defined(mingw32_HOST_OS) || defined(__MINGW32__) || defined(_MSC_VER) .... #else INLINE int const_MAP_FILE( void ) { return MAP_FILE; } #endif mmap.hs: #if defined(mingw32_HOST_OS) || defined(__MINGW32__) || defined(_MSC_VER) .... #else foreign import ccall unsafe "mydefs.h const_MAP_FILE" mAP_FILE :: CInt #endif Second question is how to test whether program compiles under Unix or Windows environment. Posix/Win32 packages availability would be the best criterion but it can't be tested :( -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Hackathon hesitation
by Chad Scherrer 13 Jul '06

13 Jul '06

Hi, I'm interested in attending the Hackathon, but I don't have any previous experience working on compilers. I think it could be a great learning experience, but I certainly don't want to slow progress on any work by just hanging around asking questions. I'm a mathematician/statistician, and my programming experience is limited to high-level languages like Python, R, and Haskell. I can read C (sort of), but I've been too frustrated by the lack of high-level features to do anything useful with it. I've been using Haskell for about 2 years now (every chance I get), and I'm very interested in using it to allow high-performance (preferably parallel) code to be written at a very high level of abstraction. Given this, does it seem there would be much I could help with? Thanks, Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Re: [Haskell] Type-Level Naturals Like Prolog?
by Greg Buchholz 13 Jul '06

13 Jul '06

Jared Warren wrote: > Haskell's type checking language is a logical programming language. > The canonical logical language is Prolog. > Why can't Haskell (with extensions) do type-level Peano naturals in > the same fashion? The code would be something like: Also of possible interest, _Fun with Functional Dependencies_... http://www.cs.chalmers.se/~hallgren/Papers/wm01.html "This paper illustrates how Haskell's type class system can be used to express computations. Since computations on the type level are performed by the type checker, these computations are static (i.e., performed at compile-time), and, since the type system is decidable, they always terminate. Haskell thus provides a means to express static computations, and has a clear distinction between static and dynamic computations. "

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Comma in the front
by Joel Reymont 13 Jul '06

13 Jul '06

Are cool kids supposed to put the comma in front like this? , foo , bar , baz Is this for historical or other reasons because Emacs formats Haskell code well enough regardless. Thanks, Joel -- http://wagerlabs.com/

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Haskore dependency trouble
by Yitzchak Gale 13 Jul '06

13 Jul '06

I am having trouble installing Haskore due to a dependency issue. I am a pampered user of Debian. Thanks to wonderful work by the Cabal people and the Debian package maintainers, I have never had to use Cabal manually to install a package before. In order to learn how to do that, I practiced by installing the ever-important HNOP package. After a few tries, I found that the following sequence of commands works: ./Setup.hs configure --prefix=/usr ./Setup.hs build sudo ./Setup.hs install But similar commands for Haskore do not fair as well. The configure command fails with the following error: Setup.lhs: cannot satisfy dependency Hsc-any How can I satisfy that dependency? I am using ghc on Debian testing. I obtained Haskore from http://cvs.haskell.org/darcs/haskore Thanks! Incidentally, why is Cabal not smart enough to know that with ghc on linux the default prefix is /usr? -Yitz

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