- Haskell-Cafe - Haskell.org

HDBC's future and request for help
by John Goerzen 28 Feb '11

28 Feb '11

Hi folks, HDBC has been out there for quite some time now. I wrote it initially to meet some specific needs, and from that perspective, it has been done for awhile. It is clear, however, that there are some needs it doesn't meet. Most of them relate to specific backend driver items. I'd like to start some discussion in the community about what the future of HDBC and its backend drivers ought to look like. Some models might be: 1. I continue as maintainer for HDBC and HDBC-{postgresql,odbc,sqlite3} and act as patch manager/gatekeeper for patches that are discussed on some public mailing list. 2. Interested parties adopt the backend drivers while I continue to act as maintainer/patch manager/gatekeeper for HDBC itself. 3. Interested parties adopt all of HDBC and maintain it I am not expressing a particular preference for any of these options; just putting them forth. Here are some of the current issues I am aware of: 1. I have no Windows development platform. I can't test the releases on Windows. Many Windows users don't have the skill to diagnose problems. These problems do eventually get fixed when a Windows user with that skill comes along -- and I appreciate their efforts very much! -- but it takes longer than it ought to. 2. The ODBC documentation is monumentally terrible, and the API is perhaps only majestically terrible, and it is not 100% clear to me all the time that I have it right. A seasoned ODBC person would be ideal here. 3. Issues exist with transferring binary data and BLOBs to/from at least PostgreSQL databases and perhaps others. There appear to be bugs in the backend for this, but BLOB support in the API may need to be developed as well. 4. Although the API supports optimizations for inserting many rows at once and precompiled queries, most backends to not yet take advantage of these optimization. 5. I have received dueling patches for whether foreign imports should be marked "safe" or "unsafe" on various backends. There seems to be disagreement in the community about this one. 6. Many interactions with database backends take place using a String when a more native type could be used for efficiency. This project may be rather complex given varying types of column data in a database -- what it expects for bound parameters and what it returns. The API support is there for it though. 7. Various other more minor items. Thoughts? -- John

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Re: [Haskell-cafe] ANN: theoremquest-0.0.0
by Tom Hawkins 28 Feb '11

28 Feb '11

> I find this fairly interesting. Once you're finished grappling with the > logical core, I wouldn't mind helping out with a web interface, time > permitting. I suspect attracting mass appeal, or getting users at all, is > helped massively by having a web interface. Thanks for your interest. Yes, a web interface would be ideal. Currently the library has hooks to translate the transactions to JSON to ease construction of any Javascript client -- thought these translations still need to be implemented. Note the 'RspInJSON' request: http://hackage.haskell.org/packages/archive/theoremquest/0.0.0/doc/html/The…

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trac.haskell.org problem
by Niklas Broberg 28 Feb '11

28 Feb '11

Ah, I suppose that's to do with the trac server itself, that's beyond me. Moving to café: Does anyone know what's up with trac.haskell.org not sending out verification emails? Cheers, /Niklas ________________________________________ From: Jurriaan Hage [jur(a)cs.uu.nl] Sent: Monday, February 28, 2011 8:41 AM To: Niklas Broberg Subject: Re: Again a question about haskell-src-exts. On Feb 28, 2011, at 8:39 AM, Niklas Broberg wrote: > I suppose I do - to the extend I can. What's the issue? :-) The system does not send out verification e-mails, so I can not really register completely and submit new issues. I have tried it with two different e-mail accounts, and somebody else has tried too. They also do not show up in spam folders. Jur

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ANN: Hakyll 3
by Jasper Van der Jeugt 28 Feb '11

28 Feb '11

Hello all, I've just uploaded the 3.0.0.0 version of Hakyll [1] to Hackage [2]. This is a complete rewrite, and completely backward-incompatible with previous versions. Sorry for that. On the other hand, I believe almost all aspects of the library have improved tremendously. Hakyll now uses a declarative EDSL in Haskell to specify compilation rules, and an arrow-based EDSL to describe the compilation process. If you want to have a quick look, I recommend have a glance at the example in the tutorial [3]. More documentation needs to be written, but I wanted to make sure I can incorporate your feedback into that process. [1]: http://jaspervdj.be/hakyll/ [2]: http://hackage.haskell.org/package/hakyll-3.0.0.2 [3]: http://jaspervdj.be/hakyll/tutorial.html Cheers, Jasper

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http://www.cs.cornell.edu/icfp/task.htm
by Hauschild, Klaus (EXT) 28 Feb '11

28 Feb '11

Hi, Currently I'm trying to lern Haskell by doing. After doing some examples I plan to solve an ICFP task (see subject). In short: build an interpreter for a stack based language thata describes a 3D scene. Ray trace this scene in an image. My current source state can be found here: http://code.google.com/p/hgmltracer/source/browse/ My first goal is to develop the interpreter of the GML language. My source contains a data GmlToken with various constructors for the different operators, sequences, int, real, string an so on. Some code for the evaluation is also there und working. But the parser converting a string (contained in the program file provided as command line argument) is hard stuff and I'm stuck. Can any one help me with ideas or concepts for this parser? Thanks

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Re: [Haskell-cafe] [SOLVED] why is ghci trying to load hsc file ??
by briand＠aracnet.com 28 Feb '11

28 Feb '11

On Mon, 28 Feb 2011 02:58:58 +0000 Felipe Almeida Lessa <felipe.lessa(a)gmail.com> wrote: > On Sun, Feb 27, 2011 at 6:48 AM, by way of briand(a)aracnet.com > <briand(a)aracnet.com> wrote: > > the binding-DSL examples do NOT use the above PRAGMA anywhere in the > > code. > > Probably they have > > Extensions: ForeignFunctionInterface > > on their .cabal file. > as I do, hence the reason for my confusion :-( Something's not working right and I can't begin to guess why not. I had to put the actual pragma in my .hsc file to make things work, and now it works. Brian

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Render fonts in HopenGL
by er9999 27 Feb '11

27 Feb '11

Hello, Can anyone give me an example for fonts rendering? I tried to use FTGL library, but it is too complicated on Windows(compile). Is it possible to do it in a simple way? I need more than the given example in red book(Font.hs), something like outline font. By the way, I am new in OpenGL and Haskell, so the real exmaple should be the best. Thanks!

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Type problem
by michael rice 26 Feb '11

26 Feb '11

Why? Shouldn't this work for any type a? Michael ========== f :: [a] -> [a] f l = do x <- l return x ========== *Main> :r [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main> f "abcde" "abcde" *Main> f [1,2,3,4,5] [1,2,3,4,5] *Main> "abcde" >>= f <interactive>:1:12: Couldn't match expected type `Char' against inferred type `m b' In the second argument of `(>>=)', namely `f' In the expression: "abcde" >>= f In the definition of `it': it = "abcde" >>= f *Main>

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Simulating open datatypes with type families?
by Patrick Thomson 26 Feb '11

26 Feb '11

I'm currently working on a project that involves a C parser (using Parsec3) that needs to be dynamically extensible - i.e. end-users should be able to add new statement types, expression types, operators, and so on and so forth. Since Haskell ADT's are closed, I thought I would be able to simulate this by using type families, e.g.: {-# LANGUAGE TypeFamilies, DeriveDataTypeable #-} -- the statement type type CStatement = BreakStmt | CaseStmt CExpression CStatement -- rest of the statement types omitted here for brevity -- CStatement derives (Show, Eq, Typeable, Data) statement :: Parser CStatement -- how exactly this is parsed is not relevant, so its definition is also omitted data family StatementLike a data instance StatementLike CStatement = MkStmt CStatement -- okay, this is all well and good. let's create a new ADT, representing the Ruby/Perl-esque 'until' statement and add a StatementLike instance for it. data Until = UntilStmt CExpression CStatement deriving (Eq, Show, Typeable, Data) data instance StatementLike Until = MkUntil Until untilStatement :: Parser Until -- definition omitted -- Here's where it breaks down. I want to be able to have a function of type Parser (StatementLike a) so that I can pattern-match over the a in future code, but this function doesn't typecheck: GHC says that the function can't be that general - it expects either a Parser (StatementLike CStatement) or a Parser (StatementLike Until). newStatement :: Parser (CStatement a) newStatement = (MkUntil <$> untilStatement) <|> (MkStmt <$> statement) Am I expecting the wrong thing of type families? Are typeclasses the better way to do this? Is there any other way to simulate an open, extensible ADT, or am I totally barking up the wrong tree? Any assistance would be much appreciated. Please let me know if I need to provide more information. Thanks in advance, -- Patrick Thomson

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Auto elimination of MVars using a monad or monad transformer.
by Chris Dew 26 Feb '11

26 Feb '11

Hello, just like everyone else, I have a question about monads. I've read the tutorials, written one monad myself (not in this email), but I still consider myself a Haskell beginner. * Does GHC eliminate unneeded MVars during compilation? I'm expecting that it doesn't, as that would mean optimising away ForkIOs, which would be quite a thing to do. I've included example code below. * Is there a monad which allows their automatic elimination of MVars (or their creation only when necessary)? This would be similar to how the IO monad allows you to do purely functional things with a do block, using let. I've had a go at a lifting function, which wraps a pure function into an IO action which forever reads from one MVar and writes to another. What I'm looking for is some form of Monadic context in which many pure functions, MVar fillers and MVar consumers could be linked together, where only the necessary MVars remain (or were created) at compilation time. * Would this be a monad, or a monad transformer? * Can you specialise a monad transformer on a single base (in this case IO) so that you can use forkIO in the bind or return? Thanks, Chris. module Main ( main ) where import Control.Concurrent (forkIO, MVar, newEmptyMVar, putMVar, takeMVar, ThreadId, threadDelay) import Control.Monad (forever) stepA :: MVar String -> IO () stepA boxa = forever $ do line <- getLine putMVar boxa line stepB :: MVar String -> IO () stepB boxb = forever $ do line <- takeMVar boxb putStrLn line -- This simply wraps a string in brackets. bracket :: String -> String bracket x = "(" ++ x ++ ")" -- This lifts a function into an action which forever performs the function -- between the two MVars given. lft :: (a -> b) -> MVar a -> MVar b -> IO () lft f c d = forever $ do x <- takeMVar c putMVar d (f x) -- Just like C's main. main :: IO () main = do box <- newEmptyMVar box2 <- newEmptyMVar forkIO $ stepA box forkIO $ lft bracket box box2 forkIO $ stepB box2 threadDelay 10000000 -- Sleep for at least 10 seconds before exiting.

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