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c2hs examples?
by Magnus Therning 13 Dec '06

13 Dec '06

Does anyone know of any good source of examples if one wants to learn to use c2hs? The documentation that comes with c2hs is a little too sparse when it comes to examples to actually work out how things work. I don't mind looking at some reasonably sized project that uses c2hs (gtk2hs is a little too overwhelming). /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. "Zaphod grinned two manic grins, sauntered over to the bar and bought most of it." -- Zaphod in paradise.

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cpphs and #pragma
by Dimitry Golubovsky 13 Dec '06

13 Dec '06

Hi, Is this a bug or a feature that cpphs does not pass the #pragma cpp directive (they disappear in the output)? In version 0.6 of cpphs: * Recognise and ignore the #pragma cpp directive. Or what command line options could be used to preserve #pragma's in cpphs output? Thanks. -- Dimitry Golubovsky Anywhere on the Web

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Re: Haskell and .NET
by Monique Monteiro 13 Dec '06

13 Dec '06

Hi Justin, I've runned a research project about this topic (in fact, it was the subject of my MSc dissertation). Please see the Haskell.NET Project (http://www.cin.ufpe.br/~haskell/haskelldotnet) We have compiled a subset of the Haskell language to .NET, but we still don't have an available release (we don't support the full Haskell prelude yet). The compiler is an extension to GHC. In the future, we intend to provide full interoperability with .NET in this way. There are some interesting references in the website about our compilation strategies. If you need further information about the project's status, please contact Prof. André Santos (alms(a)cin.ufpe.br) and/or Guilherme Avelino (gaa(a)cin.ufpe.br) Best regards, -- _____________________________________________ Monique Monteiro, MSc http://www.cin.ufpe.br/~mlbm http://thespoke.net/blogs/moniquelouise/default.aspx monique(a)qualiti.com.br +55 81 34198137 Project Manager Recife Microsoft Innovation Center

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Re: [Haskell] ANNOUNCE: Phooey -- a Functional UI library for Haskell
by Brian Hulley 13 Dec '06

13 Dec '06

Brian Hulley wrote: > Conal Elliott wrote: > >> GUIs are usually programmed in an "unnatural" style, in that ... > > Also, suppose you have a gui consisting of an edit widget such that > when the user types something it gets lexed, parsed, and fontified as > a Haskell program, ie from the user's point of view the widget > maintains a syntax highlighted view of the code which is > "continuously" updated. Would your system do the lexing/parsing for > every key that was pressed or only once, when the widget needs to be > re-rendered (a typical gui for Windows would only propagate changes > and re-draw the gui (ie lex/parse/fontify) when there are no event > messages pending)? Thinking about this more, a better example would be a gui with 2 widgets: an edit widget to type in code and a list widget which displays a list of top level functions defined in that code. (The code could be Haskell or some simple toy language.) Then the question becomes: how to set things up such that the code in the edit widget is only parsed when the list widget (and/or edit widget) is asked to render it's contents rather than every time the user presses a key. I think perhaps the problem would be solved by the edit widget passing a lazy parse tree (ie an expression evaluating to the parse tree) to the list widget. But an interesting thing is if you compare this to imperative guis, in the functional gui each widget passes lazy expressions eagerly whereas in imperative guis each widget passes eagerly evaluated expressions lazily ie functional is "push messages" whereas imperative is "pull changes when I'm asked to pick or render something etc and I'm dirty". (Have I got this wrong?) I imagine that the overhead of eager passing in the functional gui is a small price to pay for the ease of using a functional gui, since the important thing is that in both guis, the hard work of parsing the code in the edit buffer would only be done on demand, unless I'm much mistaken. If you're still thinking of examples for your paper it would also be really great to see how you'd create a widget that displays the result of another process (eg a window that displays the output as ghc compiles a program) or some other example of how to use the IO monad inside a widget for those unfamiliar with combining arrows with monads. It would also be useful to know more about the relationship between the functional gui and WxWidgets to see what characteristics an imperative gui toolkit must have in order to be usable with it. (Like the extremely useful discussion of low level details in the Fudgets thesis.) Best regards, Brian. -- http://www.metamilk.com

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Good Haskell introduction for an Ocaml programmer?
by Brian Hurt 13 Dec '06

13 Dec '06

Greetings, all. I'm an experienced Ocaml programmer, looking to broaden my horizons yet further and pick up Haskell, and I'm wondering if there's a good introduction to Haskell for me. I have Simon Thompson's "Haskell: The Craft of Functional Programming", which isn't a bad book, but I'm something of a special case. I'm already familiar with and comfortable with a lot of concepts which are new to your average C++/Java programmer- things like symbolic computation and recursion as looping and applicative data structures. So churning through introductions to these concepts looking for the rare nugget of new information is, well, kinda boring. On the other hand there are a lot of Haskell concepts I'm not comfortable with, like monads. So I was wondering if there was a better introduction for me out there? I'm willing to pay for a book or read something online, whichever. Thanks, Brian

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Trivial database access in Haskell
by Paul Moore 13 Dec '06

13 Dec '06

I'm an Oracle DBA, and I have been looking at Haskell with interest for a long while now. But I don't feel particularly that I could use it in my "day job", mainly because I can't seem to find a nice, simple way of doing (Oracle) database access in Haskell. What I'm after is very simple - I want to be able to connect to a database and issue SQL queries and DML (updates, inserts, etc). There's nothing particularly "functional" in this - it's more of a basic building block (like file I/O) as far as I'm concerned, so I'm happy for database access to sit in the IO monad. I've looked at the various database access libraries for Haskell, but they all seem to be missing a couple of key pieces if I want to use them: 1. Simple documentation of how to install the library (starting from a vanilla GHC installation on Windows, plus Oracle software, to the point where I can use the library in my code). All I need is Oracle access, so other database client libraries can be ignored. I'd rather not use ODBC, but will if required. 2. Simple examples on how to actually run a query. For example, how do I code the follwing SQL*Plus code (should be pretty self-explanatory) in Haskell: connect scott/tiger@mydb select * from emp; (Connect to the database "mydb" as user "scott", password "tiger", and issue the query "select * from emp"). Ideally, I'd like the Haskell code to not be *too* much more verbose than this (although I can accept that Haskell, beng a general language, won't be able to be quite as concise as a special tool can manage). Without this basis, I don't really have a way to evaluate the various libraries available... If anyone can offer any assistance, I'd be very grateful. Paul.

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Type problem with simple takusen code (was: Trivial database access in Haskell)
by Paul Moore 13 Dec '06

13 Dec '06

(Wow, it looks like my message has generated quite a bit of traffic! Thanks, guys!) On 12/11/06, Taral <taralx(a)gmail.com> wrote: > The magic commands are: > > runhaskell Setup.lhs configure > runhaskell Setup.lhs build > runhaskell Setup.lhs install Excellent! Now I'm getting somewhere. I even found the sample code in the takusen documentation. I'm trying to modify the sample code to work with a local database I have, and I'm getting some type errors which mean nothing much to me. The code I have, stored in a file called db.hs and run via "runhaskell db.hs" is: import Control.Monad.Trans import Database.Oracle.Enumerator import Database.Enumerator query1Iteratee :: (Monad m) => String -> IterAct m [String] query1Iteratee a accum = result' (a:accum) main :: IO () main = do withSession (connect "USER" "PASSWORD" "DB") $ do -- simple query, returning reversed list of rows. r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ putStrLn $ show r The error I'm getting is db.hs:10:64: Couldn't match expected type `forall mark. DBM mark Session a' against inferred type `a1 b' In the second argument of `($)', namely `do r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ (putStrLn $ (show r))' In the expression: (withSession (connect "USER" "PASSWORD" "DB")) $ (do r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ (putStrLn $ (show r))) In the expression: do (withSession (connect "USER" "PASSWORD" "DB")) $ (do r <- doQuery (sql "select username from all_users") query1Iteratee [] liftIO $ (putStrLn $ (show r))) I'm not sure what might be wrong here - or even, for that matter, how to diagnose the problem. To complicate the matter, it's possible that I'm getting a database connection error which I've not got code in to report. (I've no easy way to test that the database connection from Haskell is working, or rather *this* is the easy way :-)) Can anyone offer any pointers? (I take the suggestions about asking on IRC, but I can't get at IRC from here due to firewall issues, so I'll have to stick to email). Paul.

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Large data structures
by Alex Queiroz 13 Dec '06

13 Dec '06

Hi all, I'm considering the use of Haskell to manipulate large data structures for Computer Graphics (large geometric datasets). I'm wondering what's the best way to do it. As "objects" (not in the OO sense) in Haskell are immutable, how can I add a vertex to a large mesh without using obscene amounts of memory? Making an analogy: > enlarge :: a -> [a] -> [a] > enlarge c cs = c : cs > > enlarge 15 [1..100000] Will this copy the whole list to make a new one with an element more? Cheers, -- -alex http://www.ventonegro.org/

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Micro-contributions to MissingH
by Yang 13 Dec '06

13 Dec '06

Hi, I'm at the moment writing a very simple function, spanM (a la mapM). I realize that this is a rather general-purpose function, so I'm wondering if I should submit this as a patch for MissingH. If so, where in the new hierarchy should this be placed? Thanks in advance. Yang

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Request for improvements
by Chris Kuklewicz 13 Dec '06

13 Dec '06

I have written a thin but useful layer on top of parts of Control.Concurrent and Control.Exception to aid in forking new threads. I think I have written it correctly and understand the guarantees (when running under GHC 6.4 or 6.6). It is useful when * you need to be able to check whether a thread has finished running * you need to be able to wait until a thread has finished running * when you want to run within the scope of 'block' * you need to run in another thread with ** an exception handler (catch) ** a finally clause ** bracket commands But it may need improving, especially if I have missed a subtle blocking or race condition. And suggestions on improved names are welcome. The code is on the wiki at http://haskell.org/haskellwiki/WrapConc -- Chris

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