- Haskell-Cafe - Haskell.org

Interpreter output in color
by dons＠cse.unsw.edu.au 15 Dec '06

15 Dec '06

Forward to haskell-cafe(a)haskell.org for further discussion. ----- Forwarded message from Walter Moreira <gwallter(a)gmail.com> ----- Date: Thu, 14 Dec 2006 06:46:44 +0000 From: Walter Moreira <gwallter(a)gmail.com> To: haskell(a)haskell.org Subject: [Haskell] interpreter ouput in color Hi all. I was wondering if some people miss the colored output of some applications, such like the IPython enhanced shell. I've been googling for similar options for Haskell but I found nothing. I feel that the type error messages, for example, would be a lot more readable (or at least quicker to read). Is there a way to write plugins for ghci to get this?, or do you people have some suggestion about what would be the easiest component to hack for applying colors? Thank you. Cheers, Walter ------------------------------------------------------------------------ Walter, I think this is a great idea. Currently, there is an existing tool, HsColour: http://www.cs.york.ac.uk/fp/darcs/hscolour/ which does a great job of marking up source files to ansi, html and css output. I think with only a little bit of effort, it could be integrated into ghci (or a ghci wrapper written), that runs each expression in ghci through HsColour. Here's a mockup (the result of dumping ghci's output through HsColour): http://www.cse.unsw.edu.au/~dons/tmp/colour-ghci.png Is someone interested in writing a wrapper over ghci that pipes input and output through HsColour? We might have to modify HsColour slightly to take input from stdin. I've put up an initial wiki page for this here: http://haskell.org/haskellwiki/GHCi_in_colour If you've thoughts on how to implement this- -- or want to just go ahead and do it, please dive in! -- Don

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Optimization again.
by szefirov＠ot.ru 15 Dec '06

15 Dec '06

From GHC documentation: "Once profiling has thrown the spotlight on the guilty time-consumer(s), it may be better to re-think your program than to try all the tweaks listed below." So, how should I rethink my program? Which way to take?

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haskell.org memory upgrade
by Paul Hudak 15 Dec '06

15 Dec '06

Dear Haskellers -- Haskell.org will go down today at 1500 EST for about 10 minutes for a memory upgrade. Sorry for the inconvenience, -Paul Hudak

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typeclass constraint question
by Jeff Polakow 14 Dec '06

14 Dec '06

Hello, I'm trying to understand what happens when a typeclass constraint with an uninstantiated type variable gets duplicated. Does the type checker treat the separate copies of the constraint as two constraints (with the same type variable) which are checked separately? Or does the type checker only consider one constraint which is checked (simultaneously) in different contexts? Also does it matter if the constraints are on datatype, or even gadt, constructors rather than functions? thanks, Jeff --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Re: Designing an object model in Haskell (RESOLVED)
by John Ky 14 Dec '06

14 Dec '06

Hi all, I'm now using existential types. I avoided learning about them because the name sounded so highly technical and obscure it did not occur to me they could be related to OO. Thanks, -John On 12/7/06, John Ky <newhoggy(a)gmail.com> wrote: > > Hi, > > I've got an object model that I have a difficult time conceptualising how > it might look like in Haskell: > > class Element { } > > class Inline : Element { } > > class ParentInline : Inline { > List<Inline> children; > } > > class Bold : ParentInline { } > class Underline : ParentInline { } > > class Link : ParentInline { > String link; > } > > class Text : Inline { > String text; > } > > class Block : Element { } > > class Paragraph : Block { > List<Inline> paragraph; > } > > class Heading : Block { > List<Inline> heading; > } > > class Document : Element { > List<Block> blocks; > } > > How best to represent this OO data model in Haskell? > > Thanks > > -John > >

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Re: Request for a Simple Pretty Printing library
by Doaitse Swierstra 14 Dec '06

14 Dec '06

I am sorry, I might have confused you. It appears that we have just a function for doing what you want; in UU.Pretty.Basic there is a function "invisible". From the manual page I quote: invisible ppd Makes the formatted element invisible (all its attributes are forgotten in order to always succeed, even if there is no space left!) In exceptional cases we want to escape to the page width limit, for example when you want to generate additional tags for markup languages such as LaTeX or HTML: > res_word rw = invisible (pp "{\\bf ") >|< rw >|< invisible (pp "}") and try render (res_word "let") 3. Doaitse On Dec 14, 2006, at 9:57 AM, Christian Maeder wrote: > Doaitse Swierstra schrieb: >> The Prettyprint library you can download from: >> >> http://www.cs.uu.nl/wiki/HUT/Download > > I've looked into your module UU.Pretty.Basic starting with: > > -- $Header: /data/cvs-rep/uust/lib/pretty/UU/Pretty/Basic.hs,v 1.2 > 2003/02/26 11:18:27 uust Exp $ > > And I did not find the type Doc that you describe below. > > Christian > >> and is described on: >> >> http://www.cs.uu.nl/wiki/HUT/PrettyPrintingCombinators >> >> contains amongst others the following code: >> >> data Doc = Empty >> | Char Char -- invariant: char is not >> '\n' >> | Text !Int String -- invariant: text doesn't >> contain '\n' >> | Line !Bool -- True <=> when undone by >> group, do not insert a space >> | Cat Doc Doc >> | Nest !Int Doc >> | Union Doc Doc -- invariant: first lines of >> first doc longer than the first lines of the second doc >> | Column (Int -> Doc) >> | Nesting (Int -> Doc) >> >> text "" = Empty >> text s = Text (length s) s >> >> so if you define now: >> >> wrap l t r = Text 0 l <|> text t <|> Text 0 r >> >> you should be set and done, >> >> Doaitse Swierstra >

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Re: [Haskell] interpreter ouput in color
by Neil Mitchell 14 Dec '06

14 Dec '06

Hi Walter, > Hi all. I was wondering if some people miss the colored output > of some applications, such like the IPython enhanced shell. I've > been googling for similar options for Haskell but I found nothing. WinHugs already colours (to some degree) and hyperlinks error messages. http://www-users.cs.york.ac.uk/~ndm/projects/winhugs.php (screenshot on that page) GuiHaskell does it a lot more, with full syntax colouring of error messages and hyperlinks. Unfortunately it can't be used reliably at the moment due to threading issues with Gtk+Gtk2Hs+GHC+Windows. http://www-users.cs.york.ac.uk/~ndm/projects/guihaskell.php GuiHaskell merely wraps GHCi (and Hugs and other stuff), so allows syntax colouring to be implemented without diving into the GHC source code. Thanks Neil PS. The Haskell-cafe mailing list is better for this sort of question, haskell tends to be more used for announcements, so i've sent the follow up there.

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type variable question
by Nicolas Frisby 14 Dec '06

14 Dec '06

I'm developing a typesafe record library (akin to HList but starting with key/val pairs and enforcing uniqueness and sorted order of keys). I'm having a GHC problem I've had with other projects and seen comments regarding it in other people's code (HList for instance: "GHC doesn't like it's own type" to paraphrase). I have a function |update| > --update :: forall sel val rec val' rec' . > -- SetField sel val rec val' rec' => sel -> (val -> val') -> rec -> rec' > update sel f rec = rec' > where --val :: val > val = value sel rec > --val' :: val' > val' = f val > --rec' :: rec' > rec' = set sel val' rec The commented out signature is the signature that GHC infers for the function. When I uncomment that signature, it will no longer type check. Why does this happen? Even with the forall and the explicit signatures in the where clause, it chokes. I don't plan on sharing the code for |SetField| because it's that wild looking type class hackery, and the problem isn't specific to this example--I remember having it happen in far simpler cases which I can't seem to find right now... Is there a canonical example example that exhibits this behavior? Or a ticket for it already? I'd like to understand what's happening. Thanks for your time, Nick

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Request for a Simple Pretty Printing library
by Neil Mitchell 14 Dec '06

14 Dec '06

Hi, I've been using the HughesPJ pretty printing library, but I need a new combinator: wrap :: String -> Doc -> String -> Doc wrap prepend doc append = ... The semantics of this would be that the text is prepended and appended to the doc when rendered, but does not alter the pretty printing at all. The idea of this is to support printing with HTML rendering: wrap "<span class='keyword'>" (text "case") "</span>" The HughesPJ library is 1000 lines long, so will be quite a lot of work to change this to get it working properly. Is there a simpler pretty printing library, with source code available somewhere? Ideally using the same interface. Of course, if anyone knows how to make this combinator using the existing library, or using another existing library, I'd appreciate it. Thanks Neil

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Re: [Haskell-cafe] Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Andrew Wagner 14 Dec '06

14 Dec '06

I think there are some great ideas here, and it would be a fantastic project to do as a community, via a wikibook. I, for one, have been studying haskell for several months, and am just starting to see a little bit of light when it comes to monads. I think it would be beneficial to work through a non-trivial construction of a new monad, and the larger examples given would be good opportunities to do that. Andrew

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c2hs examples?
by Magnus Therning 14 Dec '06

14 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 14 Dec '06

14 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 14 Dec '06

14 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 14 Dec '06

14 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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How to combine Error and IO monads?
by Cat Dancer 13 Dec '06

13 Dec '06

I've read Jeff Newbern's tutorial on monad transformers (http://www.nomaware.com/monads/html/index.html) but I don't grok it yet and I can't tell how to get started with this particular requirement, or even if I need monad transformers for this. I have a program that performs a series of IO operations, each which can result in an error or a value. If a step returns a value I usually want to pass that value on to the next step, if I get an error I want to do some error handling but usually want to skip the remaining steps. Thus I have a lot of functions with return types like IO (Either String x), where x might be (), Integer, or some other useful value type, and a lot of case statements like a :: Integer -> IO (Either String String) (a :: Either String Integer) <- some_io_action_returning_integer_or_error case a of -- got to get from (Either String Integer) to (Either String String) Left message -> return $ Left message -- continue on Right i -> do_more i If I was using just Either I know how to get rid of the case statements: import Control.Monad.Error p :: Integer -> Either String Integer q :: Integer -> Either String String r :: Either String String r = do a <- p 3 b <- q a return b which of course is much nicer. But how do I combine the Error and IO monads so that r could have the type IO (Either String String)?

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Why so slow?
by Lyle Kopnicky 13 Dec '06

13 Dec '06

The code below is using way more RAM than it should. It seems to only take so long when I build the 'programs' list - the actual reading/parsing is fast. For a 5MB input file, it's using 50MB of RAM! Any idea how to combat this? Thanks, Lyle {-# OPTIONS_GHC -fglasgow-exts #-} -- linear_importer.hs import Control.Monad (unless) import Data.ByteString.Char8 (ByteString) import qualified Data.ByteString.Char8 as BS import Text.Regex as RE import Data.Time.Calendar import Data.Time.LocalTime import System.Environment -- Consider adding strictness as necessary for performance. -- STB time channel data ChannelChange = ChannelChange Int LocalTime Int deriving Show -- channel program_no start end network_no data ScheduleProgram = ScheduleProgram Int Int LocalTime LocalTime (Maybe Int) deriving Show main = do fileNames <- getArgs ensure (length fileNames == 2) usageMessage let [eventFileName,programFileName] = fileNames putStrLn ("Reading program schedule file from '" ++ programFileName ++ "'...") text <- BS.readFile programFileName programs <- sequence $ map parseScheduleProgram (BS.lines text) print (take 20 programs) return () usageMessage = "Usage: linear_importer <channel change file> <schedule file>" parseScheduleProgram :: ByteString -> IO ScheduleProgram parseScheduleProgram s = do let fields = BS.split '|' s ensure (length fields == 7) ("Wrong number of fields in schedule program: " ++ BS.unpack s) let [_,channelNoText,programNoText,_,startTimeText,endTimeText,networkNoText] = fields let channelNo = read $ BS.unpack channelNoText programNo = read $ BS.unpack programNoText startTime <- parseProgramTime startTimeText endTime <- parseProgramTime endTimeText let networkNo = if BS.null networkNoText then Nothing else Just (read (BS.unpack networkNoText)) return $ ScheduleProgram channelNo programNo startTime endTime networkNo parseProgramTime :: ByteString -> IO LocalTime parseProgramTime s = do let parts = BS.split 'T' s ensure (length parts == 2) ("Expected exactly one T in eventChannelChange time: " ++ BS.unpack s) let [datePart,timePart] = parts ensure (BS.length datePart == 8) ("Expected 8 digits in date part of eventChannelChange time: " ++ BS.unpack s) let (yearPart, monthDayPart) = BS.splitAt 4 datePart (monthPart, dayPart) = BS.splitAt 2 monthDayPart year = read $ BS.unpack yearPart month = read $ BS.unpack monthPart day = read $ BS.unpack dayPart let date = fromGregorian year month day ensure (BS.length timePart == 6) ("Expected 6 digits in time part of eventChannelChange time: " ++ BS.unpack s) let (hoursPart,minutesSecondsPart) = BS.splitAt 2 timePart (minutesPart,secondsPart) = BS.splitAt 2 minutesSecondsPart hours = read $ BS.unpack hoursPart minutes = read $ BS.unpack minutesPart seconds = read $ BS.unpack secondsPart let time = TimeOfDay hours minutes (fromInteger seconds) return (LocalTime date time) ensure :: Bool -> String -> IO () ensure x s = unless x $ ioError (userError s)

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Ninety Nine Haskell Problems
by Andrew Wagner 13 Dec '06

13 Dec '06

Hello, fellow Haskellers! jcreigh, from #haskell, has found a great set of exercises to work through. Check out the new wiki pages indexed at http://haskell.org/haskellwiki/99_Haskell_exercises . These have all been done in lisp, but of COURSE they can be done at least as well in Haskell, if not better. This is a great way for relatively new haskell programmers to contribute something, since most of the problems are relatively simple. So have at them! The template is already there, there's existing lisp code, and examples; all you have to do is write the canonical haskell version. Please follow the instructions for 'reserving' a problem so we're not all duplicating each others' work. We should be able to knock these problems out in a matter of hours! Have fun! Andrew

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Re: [Haskell] Waiting on multiple Chan-nels
by dons＠cse.unsw.edu.au 13 Dec '06

13 Dec '06

sebastian.setzer.ext: > Hi, > How do you wait on multiple channels, but read only from one of them > (the first that gets an entry)? Is there a library-function I missed > which already does this? > > What do you think of this solution? Feels to heavy :) An alternative solution that strikes me, is to use a single chan, with a sum type to tag the elements to be received by each thread. Then lazily generate a list of filtered values for each thread. Then the reading threads need do no actual IO to read. In particular, doing the chan reading IO under a lazy stream greatly simplifies the reader threads. For example: -- -- ghc chantest.hs -o chantest -- Usage: Enter "1", "2" or "0" to exit. -- import System.IO import Control.Concurrent import Control.Concurrent.Chan import Text.Printf -- -- Rather tha wait on multiple channels, it strikes me that you could -- have a single channel, and tag each thread's input. then lazily -- stream the chans contents through a filter, passing the *pure* list -- of filtered elements to a consuming thread -- type Pipe = Chan (Either String String) main :: IO () main = do chan <- newChan :: IO Pipe s <- getChanContents chan -- lazy list of chan elements c1Thread <- forkIO $ reader "c1" (catLeft s) -- read only Lefts c2Thread <- forkIO $ reader "c2" (catRight s) -- read only Rights writer chan where catLeft ls = [x | Left x <- ls] catRight ls = [x | Right x <- ls] writer :: Pipe -> IO () writer chan = loop where loop = getChar >>= command command '0' = print "done" command '1' = writeChan chan (Left "main: 1") >> loop command '2' = writeChan chan (Right "main: 2") >> loop command '\n' = loop -- ignore command c = printf "Illegal: %c\n" c >> loop reader :: String -> [String] -> IO () reader name xs = mapM_ (printf "%s %s\n" name) xs {- $ ghc x.hs -threaded $ ./a.out 1 c1 main: 1 2 c2 main: 2 2 c2 main: 2 1 c1 main: 1 2 c2 main: 2 3 Illegal: 3 1 c1 main: 1 2 c2 main: 2 0 "done" -} -- Don P.s. redirected to haskell-cafe(a)haskell.org, which is more appropriate for design discussion.

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Name that function =)
by Louis J Scoras 13 Dec '06

13 Dec '06

Hi all; I'm new to the list, so I just wanted to say hi and -- of course -- ask a quick question: I have some IO actions that I want to map over a list of pairs -- these are just directory names and their down-cased versions. It wasn't difficult to actually get the right behavior by just doing mapM twice. -- putDirs just outputs something like "mv fst snd" mapM_ putDirs pairs mapM_ (uncurry renameFile) pairs This bothered me though, because I suspected that this could be done in one pass. Naively I proceeded to this. mapM_ (putDirs >> (uncurry renameFile)) pairs Which didn't work. I thought about it a little more before realizing that putDirs wouldn't get any parameters this way: I needed some way to distribute the pair over both operations. Here's the higher-order function I needed: foo h f g i = h (f i) (g i) which could then be curried and we get: mapM_ (foo (>>) putDirs $ uncurry renameFile) pairs Works great. So my question: is there a established name for foo? What about foo partially applied to (>>)? This was a fun exercise, but I'd like to use the standard implementations if they exist. Thanks! -- Lou.

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Refactoring recklessly
by Greg Fitzgerald 13 Dec '06

13 Dec '06

I'd like to be able to reorganize my code and then verify that I didn't change any functionality. That is, the old and new code have precisely the same meaning. Also, I'd like to be able to change a function and verify that efficiency was the only thing affected. Are either of these possible in Haskell or any language? Thanks, Greg

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Haskell and .NET
by Justin Bailey 13 Dec '06

13 Dec '06

I am learning Haskell, and I really want to be able to use .NET libraries from it. I've found the Hugs98.NET binary available from Galois, but it seems very, very stale. Is there anything more recent available? Or did I miss something? My motivation for this email is a recent series of posts[1] I saw on creating games using F# and XNA Express. If F# can do it, Haskell should be able to also. Any pointers are much appreciated! Justin [1] Part 1 - http://grammerjack.spaces.live.com/blog/cns!F2629C772A178A7C!156.entry Part 2 - http://grammerjack.spaces.live.com/blog/cns!F2629C772A178A7C!157.entry Part 3 - http://grammerjack.spaces.live.com/blog/cns!F2629C772A178A7C!158.entry

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Re: [Haskell-cafe] Name that function =)
by Louis J Scoras 13 Dec '06

13 Dec '06

On 12/12/06, Conal Elliott <conal(a)conal.net> wrote: > Try foo = liftM2, and check out the recent haskell thread "Cannot understand > liftM2". That's actually what I reached for first. The problem is that the arguments aren't monads: *Main> :t (uncurry renameFile) (uncurry renameFile) :: (FilePath, FilePath) -> IO () *Main> :t putDirs putDirs :: (String, String) -> IO () and liftM2 works on monadic arguments: liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r I need to somehow get the mapped elements to be applied to each of the arguments before they can be combined. Substituting liftM2 for (foo (>>)) yields: No instance for (Monad ((->) (String, String))) arising from use of `liftM2' at DCFiles.hs:30:9-14 Probable fix: add an instance declaration for (Monad ((->) (String, String))) The foo function does the job, so I assume that I'm trying to solve a different problem than liftM. I wouldn't be surprised if I was wrong, however. -- Lou.

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Translating Haskell to VHDL. What approach to follow?
by Alfonso Acosta 13 Dec '06

13 Dec '06

Hi all, As I wrote in the previous HLADPSA announcement, during the following moths I'm going to write a translator from Haskell to VHDL in order to accomplish my masters thesis goal. The main requirement is managing to translate from a ForSyDe (http://www.imit.kth.se/info/FOFU/ForSyDe/ ) specification which is Haskell after all, to VHDL. In order to get part of the work for free I decided to design the translator as a compiler backend. The best option so far seems to be Yhc's Core API (http://haskell.org/haskellwiki/Yhc/API/Core ), which unfortunately lacks type information. I discarded GHC due to the current "bit-rotted" state of External Core (http://www.haskell.org/pipermail/haskell-cafe/2006-October/018786.html ) and higher complexity of its core representation (SystemFC). I might have overlooked other approaches and/or intermediate representations such as GHC's C-- and STG (is it still used?). I'd be pleased to hear any suggestions. Thanks in advance, Alfonso Acosta

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Pretty printing HTML
by Neil Mitchell 12 Dec '06

12 Dec '06

Hi, I want to pretty print some text as HTML, using the standard HughesPJ combinators. The problem is that I want the "length" of a text string to be its textual context, not its character length. For example, a keyword will display as "case", but internally will be the string "<span class='keyword'>case</span>" Is there any way to acheive this? I have come up with something, but its not nice at all. It basically involves parsing the generated text and inserting the extra HTML back in. Thanks Neil

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New to Haskell - Question about hugsIORun from old Prelude
by Rahul Naik 12 Dec '06

12 Dec '06

Hello, I am trying to rewrite some older (2001) haskell : myReadFile :: String -> String myReadFile f = case hugsIORun (readFile f) of Right s -> s Left _ -> "" Can someone provide me with a up to date version of the above bit of code without using code from the old Prelude? Thanks, R _________________________________________________________________ Visit MSN Holiday Challenge for your chance to win up to $50,000 in Holiday cash from MSN today! http://www.msnholidaychallenge.com/index.aspx?ocid=tagline&locale=en-us

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Haskell Weekly News: December 12, 2006
by dons＠cse.unsw.edu.au 12 Dec '06

12 Dec '06

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/ Issue 53 - December 12, 2006 --------------------------------------------------------------------------- Welcome to issue 53 of HWN, a weekly newsletter covering developments in the Haskell community. Lots of new, practical Haskell libraries released this week, including support for ogg sound file parsing, a new user interface library, ftp clients and servers, database bindings as well as config files and logging. Announcements Visual Haskell 0.2. Krasimir Angelov [1]announced the final version of [2]Visual Haskell 0.2 is available! This is the first version that is: available for both VStudio 2003 and VStudio 2005; distributed with a stable GHC version (6.6). Additionally the plugin itself is much more stable than its first 0.0 version. 1. http://article.gmane.org/gmane.comp.lang.haskell.general/14608 2. http://www.haskell.org/visualhaskell Phooey: functional user interfaces for Haskell. Conal Elliott [3]announced Phooey, [4]a functional UI library for Haskell. GUIs are usually programmed in an 'unnatural' style, in that implementation dependencies are inverted, relative to logical dependencies. This reversal results directly from the imperative orientation of most GUI libraries. While outputs depend on inputs from a user and semantic point of view, the imperative approach imposes an implementation dependence of inputs on outputs. Phooey ('Phunctional ooser ynterfaces') retains the functional style, in which outputs are expressed in terms of inputs. In addition, Phooey supports dynamic input bounds, flexible layout, and mutually-referential widgets. It is [5]available via darcs. 3. http://article.gmane.org/gmane.comp.lang.haskell.general/14635 4. http://darcs.haskell.org/packages/phooey/doc 5. http://darcs.haskell.org/packages/phooey HOgg 0.2.0. Conrad Parker [6]announced HOgg 0.2.0. The [7]HOgg package provides a commandline tool for manipulating Ogg files, and a corresponding Haskell library. This is the initial public release. The focus is on correctness of Ogg parsing and production. The capabilities of the hogg commandline tool are roughly on par with those of the [8]oggz* tools, although hogg does not yet provide an equivalent to oggz-validate. HOgg supports chained and multiplexed Ogg bitstreams conformant with [9]RFC3533. HOgg can parse headers for CMML, FLAC, OggPCM, Speex, Theora and Vorbis media codecs, and can read and write Ogg Skeleton bitstreams. 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17102 7. http://snapper.kfish.org/~conrad/software/hogg/ 8. http://www.annodex.net/software/liboggz/index.html 9. http://www.ietf.org/rfc/rfc3533.txt 10. http://article.gmane.org/gmane.comp.lang.haskell.general/14610 ftphs. John Goerzen [10]announced ftphs, [11]an FTP client and server library for Haskell. Its features include: easy to use operation; full support of text and binary transfers; optional lazy interaction; server can serve up a real or a virtual filesystem tree; Standards compliant. ftphs was previously a part of the MissingH library. The code in this release is unchanged from its state in MissingH, other than the changes necessary to make it a standalone package. 11. http://software.complete.org/ftphs AnyDBM 1.0.0. John Goerzen [12]announced AnyDBM, a generic DBM-type interface. [13]AnyDBM provides a generic infrastructure for supporting storage of hash-like items with String-to-String mappings. It can be used for in-memory or on-disk storage. Two simple backend drivers are included with this package: one that is RAM-only, and one that is persistent and disk-backed. The hdbc-anydbm package provides another driver, which lets you use simple tables in any SQL database to provide a DBM-like interface. MissingPy also provides a Python driver which lets you use any Python anydbm driver under Haskell AnyDBM. 12. http://article.gmane.org/gmane.comp.lang.haskell.general/14613 13. http://software.complete.org/anydbm ConfigFile 1.0.0. John Goerzen [14]announced ConfigFile, a parser and writer for handling sectioned config files in Haskell. The [15]ConfigFile module works with configuration files in a standard format that is easy for the user to edit, easy for the programmer to work with, yet remains powerful and flexible. It is inspired by, and compatible with, Python's ConfigParser module. It uses files that resemble Windows .INI-style files, but with numerous improvements. 14. http://article.gmane.org/gmane.comp.lang.haskell.general/14612 15. http://software.complete.org/configfile hslogger. John Goerzen [16]announced hslogger, a logging framework for Haskell. [17]hslogger's features include: each log message has a priority and a source associated with it; multiple log writers can be on the system; configurable global actions based on priority and source; extensible log writers (handlers); default handlers that write to the console, file handles, or syslog; easy to use operation. hslogger is the first package to be split off from the MissingH source. 16. http://article.gmane.org/gmane.comp.lang.haskell.general/14611 17. http://software.complete.org/hslogger MissingH 0.18.0. John Goerzen [18]announced MissingH 0.18.0. [19]MissingH is a large collection of assorted utility libraries for Haskell. Its primary audience is Haskell programmers that want to get practical things done fast. This release implements the first part of the MissingH transition. The major changes are: most code from other sources that had been merged into MissingH has been split off; stable, mostly standalone components of MissingH have been split off: hslogger, ftphs, ConfigFile, AnyDBM, and all modules renamed to more closely follow current Haskell naming practices. 18. http://article.gmane.org/gmane.comp.lang.haskell.general/14614 19. http://software.complete.org/missingh ZMachine 0.0. Samuel Bronson [20]announced a (partial) Z-machine interpreter in Haskell. It uses gtk2hs for the interface, though other interfaces could be implemented without actually changing the interpreter proper. The Z-machine is a virtual machine that was developed by Joel Berez and Marc Blank in 1979 and used by Infocom for its text adventure games. 20. http://article.gmane.org/gmane.comp.lang.haskell.glasgow.user/11397 Halp: Haskell Logic Prover. Dan Mead [21]announced the release of Halp, the simple logic prover. Right now it only supports simple first order logic but predicates are on the way. Theres also a simple gui written in Java 5 which is in the [22]darcs repository. 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17104/ 22. http://taz.cs.wcupa.edu/~dmead/code/halp/ DList 0.1. Don Stewart [23]announced dlist 0.1, a library for difference lists in Haskell. Difference lists are a Haskell idiom for implementing O(1) append on lists, using functions to represent the lists. 23. http://article.gmane.org/gmane.comp.lang.haskell.general/14623 Haskell' This section covers the [24]Haskell' standardisation process. * [25]Infix multiparameter type classes * [26]Type sections * [27]Are pattern guards obsolete? 24. http://hackage.haskell.org/trac/haskell-prime 25. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1948/focus=1948 26. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1955/focus=1955 27. http://thread.gmane.org/gmane.comp.lang.haskell.prime/1956/focus=1956 Discussion Ninety Nine Haskell Problems. Andrew Wagner [28]mentioned that 99 Lisp exercises have been [29]ported to Haskell. Solving these exercises is a great way for relatively new haskell programmers to contribute something, since most of the problems are relatively simple. So have at them! All you need to do is write a canonical Haskell solution, and add it to the wiki. Please follow the instructions for 'reserving' a problem so we're not all duplicating each others' work. We should be able to knock these problems out in a matter of hours! 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17324 29. http://haskell.org/haskellwiki/99_Haskell_exercises MissingH status. John Goerzen [30]announced that MissingH now has [31]a new Trac-based homepage, complete with wiki, Darcs repository information, source browser, bug tracker, and the various pieces of MissingH have been split off already into separate packages. The full list of changes is [32]available. 30. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17156 31. http://software.complete.org/missingh 32. http://software.complete.org/missingh/wiki/TransitionPlanning Reversing a string of words: C# v Perl v Ruby v Haskell. Andrew Savige [33]invoked the Haskell cafe magicians with a comparison of string processing support across languages. 33. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17197/focus=17197 Unlifted type variables in GHC. Brian Alliet [34]asked about the intricacies of supporting type variables with an unlifted kind. 34. http://thread.gmane.org/gmane.comp.lang.haskell.glasgow.user/11397/focus=11… Wanted: libraries. Don Stewart [35]asked for information about missing or substandard libraries to be documented on the wiki. 35. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17222/focus=17222 Haskell for Dummies. Kirsten Chevalier [36]launched an interesting discussion to produce a new 'Practical Haskell' or 'Haskell for Dummies' textbook. 36. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17240/focus=17240 Stratified monads. Mark Carroll [37]asked about experience with Espinosa's stratified monads in Haskell. 37. http://thread.gmane.org/gmane.comp.lang.haskell.cafe/17266/focus=17266 Conference roundup hal: Haskell in Leipzig. [38]More than 25 Haskellers from all over Germany gathered in Leipzig for the first HaL, Haskell in Leipzig, meeting. The range of different backgrounds was broad and spanned both the academic and the business worlds, encompassing seasoned Haskell programmers as well as beginners. There were five talks and two discussion sessions, and a functional breakfast. One of the discussion sessions had IDE support (in particular, in Eclipse) as its topic, the other focused on the possibilities for promoting Haskell in the software industry. The talk topics ranged from the use of Haskell in web technology and in climate research, to specifics of programming (MonadFix, coding guidelines). A very Haskell-like language (CAL Quark) that offers access to Java libraries and can be compiled to JVM bytecode was also introduced. A second 'Haskell in Leipzig' meeting is planned for early spring 2007. [39]Summaries of the talks are available. 38. http://iba-cg.de/haskell.html 39. http://iba-cg.de/haskell.html The Haskell Hackathon. The Hac organisers [40]announced that extended registration for Hac 07, the Haskell libraries and infrastructure hackathon is open, via the [41]Hac website. 40. http://article.gmane.org/gmane.comp.lang.haskell.cafe/17132 41. http://haskell.org/haskellwiki/Hac_20007 Blog noise [42]Haskell news from the blogosphere. * [43]A Tree Grows Up in Haskell: Building a Dictionary Type * [44]Functional programming is a silver bullet * [45]Silver Bullets Incoming! * [46]The Division Bell Tolls for Me, Part One * [47]The Division Bell Tolls for Me, Part Two * [48]The Division Bell Tolls for Me, Part Three * [49]The Division Bell Tolls for Me, Part Four (Conclusion) * [50]The Divisive Aftermath * [51]Non-standard analysis, automatic differentiation, Haskell, and other stories * [52]Adventures in Haskell: parsing the game world * [53]Generalised Haskell * [54]Haskell tool tips * [55]Reconsidering my approach to learning Haskell * [56]A Sermon on Programming Languages * [57]Polyglot Programming * [58]Fibonacci series one-liner in Haskell * [59]An infinite list in Java * [60]The beauty of Haskell * [61]On Unit Testing and Type Safety * [62]Rewriting PLEAC Haskell? * [63]A newbie in Haskell land : The (->) monad * [64]Quick interpreters with the Reader monad * [65]bhc: Basic Haskell Compiler * [66]The lambda revolution: how you can help 42. http://planet.haskell.org/ 43. http://scienceblogs.com/goodmath/2006/12/a_tree_grows_up_in_haskell_bui_1.p… 44. http://cogito.blogthing.com/2006/12/06/no-silver-bullet-and-functional-prog… 45. http://cogito.blogthing.com/2006/12/08/silverbulletsincoming/ 46. http://praisecurseandrecurse.blogspot.com/2006/12/division-bell-tolls-for-m… 47. http://praisecurseandrecurse.blogspot.com/2006/12/division-bell-tolls-for-m… 48. http://praisecurseandrecurse.blogspot.com/2006/12/division-bell-tolls-for-m… 49. http://praisecurseandrecurse.blogspot.com/2006/12/division-bell-tolls-for-m… 50. http://praisecurseandrecurse.blogspot.com/2006/12/divisive-aftermath.html 51. http://vandreev.wordpress.com/2006/12/04/non-standard-analysis-and-automati… 52. http://jpmoresmau.blogspot.com/2006/12/adventures-in-haskell-parsing-game.h… 53. http://neilmitchell.blogspot.com/2006/12/generalised-haskell.html 54. http://monad.carnalreason.org/wordpress/?p=21 55. http://monad.carnalreason.org/wordpress/?p=20 56. http://www.dailyspeculations.com/ 57. http://memeagora.blogspot.com/2006/12/polyglot-programming.html 58. http://gimbo.org.uk/archives/2006/12/#001887 59. http://jpmoresmau.blogspot.com/2006/12/in-haskell-you-can-define-infinite.h… 60. http://clemens.endorphin.org/weblog/archives/2006-06.shtml#e2006-06-23T09_1… 61. http://blogs.teamb.com/craigstuntz/archive/2006/12/08/UnitTestingAndTypeSaf… 62. http://koweycode.blogspot.com/2006/12/rewriting-pleac-haskell.html 63. http://www.alpheccar.org/en/posts/show/61 64. http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/11#interpreters-with-reader-m… 65. http://neilmitchell.blogspot.com/2006/12/bhc-basic-haskell-compiler.html 66. http://www.cse.unsw.edu.au/~dons/blog/2006/12/11#release-a-library-today Quotes of the Week * Jim Apple: The Haskell list probably has the widest 'knowledge bandwidth' of any mailing list I've ever seen, from total beginner questions to highly abstruse stuff which probably represents the cutting edge of PhD research. All are answered with detail and good humour. * [Seen on the Haskell mailing list]: Wow! I found your help terrific! Thank you! Can I give you some money? * Paul Johnson: Is functional programming an actual bona-fide silver bullet as defined by Brooks? I believe it is. Functional programming is based on a sound theory of scalable software engineering, and the empirical evidence clearly and consistently supports the theory. * Clemens Fruhwirth: After all the elegance I have to conclude that Haskell definitely deserves to be looked at. * sorear: Haskell is amazing. I'm still working on my infinite-types unifier, and in a day I've added sums, products, and lambda abstractions, refactored the code until there was a net *decrease* in LOC, and helped man #haskell all at the same time. This'd take me a week in C, no doubt. * Claus Reinke: The aim of update programming is to transform a store whose contents describe a problem into a store whose contents describe a solution. The aim of functional programming is to transform a program which describes a problem into a program which describes a solution. * Daveman: What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? * Stinger: Sometimes Haskell seems like an arms race between weird mathematical constructs and my brain * astrolabe: Below a certain level of competence, coding is a destructive act. Like playing the violin. * dons: Gimme some lambda warez! * wy: [wy] lennart: Wow. You won the IOCCC three times! [augustss] I'm bad at C programming ;) * shapr: I think the Haskell approach works fine. That is, stick with research and do the best you can. Then after fifteen years of gestation, take over the world. * edwinb: Where does 'Oleg cornered me in a pub and explained delimited continuations to me' fit in? * emu: Design patterns are what you choose for wallpaper and carpets * kyevan: My skull is already mostly duct tape. And I'm only on page 25 of YAHT! * shapr: There's an important niche market for OSes that don't crash. Code Watch Tue Dec 5 23:18:45 PST 2006. Simon Peyton Jones. [67]Improve dependency analysis makes more dictionaries inlinable. 67. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18123 Sat Dec 9 09:42:03 PST 2006. Ian Lynagh. [68]Add PowerPC to the list of SMP arches 68. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18173 Mon Dec 11 14:25:34 PST 2006. Wolfgang Thaller. [69]Support MO_WriteBarrier in PowerPC NCG (lwsync instruction). 69. http://article.gmane.org/gmane.comp.lang.haskell.cvs.ghc/18217 About the Haskell Weekly News Each week, new editions are posted to [70]the Haskell mailing list as well as to [71]the Haskell Sequence and [72]Planet Haskell. [73]RSS is also available, and headlines appear on [74]haskell.org. Headlines are available as [75]PDF. The Haskell Weekly News is also [76]available in Spanish translation. To help create new editions of this newsletter, please see the [77]contributing information. Send stories to dons at cse.unsw.edu.au. The darcs repository is available at darcs get [78]http://www.cse.unsw.edu.au/~dons/code/hwn 70. http://www.haskell.org/mailman/listinfo/haskell 71. http://sequence.complete.org/ 72. http://planet.haskell.org/ 73. http://sequence.complete.org/node/feed 74. http://haskell.org/ 75. http://www.cse.unsw.edu.au/~dons/code/hwn/archives/20061212.pdf 76. http://haskell.org/haskellwiki/HWN/es 77. http://haskell.org/haskellwiki/HWN 78. http://www.cse.unsw.edu.au/~dons/code/hwn

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Stratified monads
by mark＠ixod.org 12 Dec '06

12 Dec '06

I was interested to read David Espinosa's "Stratified Monads" paper at http://www-swiss.ai.mit.edu/~dae/papers/sm.ps.Z I'm not sure I actually understand them properly yet, but I'm already curious about if anybody's played with them in Haskell, or how useful it would be to do so. Any comments? -- Mark

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[newbie]any nice code for newbie to read?
by 云杨 12 Dec '06

12 Dec '06

hello,all, I am new to haskell,and have read some tutorial, but I would like to read some "real" code from "real" haskell project, I believe this will help me study and use haskell quickly. would anyone please give me some suggestion about opensource project that a new haskell user should study? thanks you.

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Re: [Haskell-cafe] Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Eric Y. Kow 12 Dec '06

12 Dec '06

Since people keep saying the word wikibook, I will make the obligatory mention of http://en.wikibooks.org/wiki/Haskell It might not yet have the style/voice/sense of direction that we're discussing here, but maybe people might be interested in shaping that book into something really interesting? -- Eric Kow http://www.loria.fr/~kow PGP Key ID: 08AC04F9 Merci de corriger mon français.

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Re: Re: [Haskell-cafe] Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Andrew Wagner 12 Dec '06

12 Dec '06

Well, perhaps if nothing else, we could use a wikibook to collaboratively work on the structure of such a book, and then from that you could publish a "real" book. I don't really know the legal issues, though. I am thinking of several books though which have been written and released both as full paper books, and as free digital books. Could we do something similar?

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Re: Re: [Haskell-cafe] Writing "Haskell For Dummies Or At Least For People Who Feel Like Dummies When They See The Word 'Monad'"
by Andrew Wagner 12 Dec '06

12 Dec '06

Well, I'm not opposed at all to a written final form. I guess I just don't see that and using a wikibook to assist in our collaboration as mutually exclusive. Anyway, I'd love to help in any such project. By the way, I seem to be messing up the threads. What is considered the "correct" way to reply to a particular thread? I've been copying and pasting the subject line and writing to haskell-cafe(a)haskell.org.

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Num is such a fat and greedy class
by Dan Weston 11 Dec '06

11 Dec '06

The following observations are not new, insightful, or gracious, but I was lusting after the innocent +,-,* operators for my own evil ends and was mildly curious why... Num is such a fat and greedy class. If you want to marry Cinderella, you have to take her ugly stepsisters too. 1) Groups may only want to define addition. Why can't they use + (instead of <+>, >++++?&**+>, or other such perversion)? 2) Affine spaces have a (-) but no (+). Worse, the signature might be (-) :: Point -> Point -> Vector, which doesn't unify with (a -> a -> a). Wouldn't the following be more useful/general? class Subtraction a b | a -> b where (-) :: a -> a -> b Or would this require needless type annotation for the common subset of (a -> a -> a) instances? 3) Quaternions have no signum, unit quaternions have (*), (/) but no (+) or (-), abs would have a different signatures (Quaternion -> Double) which doesn't unify with (a -> a), and fields cannot be scaled with (*) as in (*) :: (Field f) => Double -> f -> f Would it not make sense to put each of these operators (division too) into their own individual superclasses that Num inherits? My (obviously naive) philosophy about type classes is that operations should be bundled only when they are mutually recursive (i.e. there is more than one useful minimal definition). If there is just one minimal set of operations, they can be in their own parent class too. Then again, I should get over my lust and stick with my own operators <+++++++++>, <---------------------->, and <*******>. Not too pretty, but they have a wonderful personality all their own! Dan

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IFF reader and writer
by Henning Thielemann 11 Dec '06

11 Dec '06

On AmigaOS there is a library called iffparse.library, which is used for processing the Interchange File Format, which is a binary container format developed by Electronic Arts for any kind of data. http://en.wikipedia.org/wiki/Interchange_File_Format The best known instances of this format are certainly the AIFF sampled sound format and WAV (which is RIFF, that is little endian IFF). Short question: Is there some Haskell library for parsing and constructing files of this format?

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Re: Haskell-Cafe Digest, Vol 40, Issue 27
by Kirsten Chevalier 11 Dec '06

11 Dec '06

On 12/11/06, haskell-cafe-request(a)haskell.org <haskell-cafe-request(a)haskell.org> wrote: > Message: 2 > Date: Mon, 11 Dec 2006 08:18:44 +0100 > From: Andy Georges <andy.georges(a)elis.ugent.be> > Subject: Re: [Haskell-cafe] Aim Of Haskell > To: Haskell-cafe <haskell-cafe(a)haskell.org> > Message-ID: <CD73F4B7-2B38-4958-9803-D1AE7CF5FC0B(a)elis.ugent.be> > Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed > > Hi, > > > one particular thing that we still lack is something like book > > "Haskell in > > real world" > > We need a 'Dive into Haskell' book. > "Head First Haskell"? Or perhaps it should be "unsafeHead First Haskell". Cheers, Kirsten -- Kirsten Chevalier* chevalier(a)alum.wellesley.edu *Often in error, never in doubt "The miracle of life is that, despite the best grip we can get on reality, it continuously manages to surprise us. The beauty of science is that, notwithstanding all our tacit assumptions, these surprises can get through." --Evelyn Fox Keller

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laziness
by Ranjan Bagchi 11 Dec '06

11 Dec '06

Hi -- I'm using ghc 6.6, and I had some questions about the extend of laziness and foldr. I've noticed the following: >x = foldr (:) [] [1..] >(take 10) x yields [1..10] Which is great.. however, what I'd like to fold the list over a tuple: >foo x (l,payload) = ((x:l), payload) >(x,_) = foldr foo ([], Nothing) [1..] >(take 10) x Doesn't terminate, until the stack overflows So I'm clearly expecting ghc to be more psychic than it's currently capable. But what I'd like to do is pass an infinite list through a processing function which maintains some state (foo could easily be a pseudo random number generator), and take the first bit of it. Any help? Thanks, Ranjan Bagchi

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One question...
by Waldemar Biernacki 11 Dec '06

11 Dec '06

Hello to everyone! Two days ago I have found Haskell in Internet. It sounds very nice. I have read some articles, few examples, ... yes it sounds nice. Now my problem is connected with the "non-update" object feature. I can't write "variable" instead object because - from the meaning of the word ( variable ) - it has the possibility to CHANGE its value. Yes that my problem :-( Let's assume that I've got to write the application that should works as follows: 1. At the beginning the user should write her/his name (for instance as the application parameter). 2. Let's assume that the application has many various functions defined inside, and - after printing each of the outputs - the name has to be printout. How should I think "properly" in Haskell to get described action? cheers Waldemar

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Re: [Haskell-cafe] One question...
by Waldemar Biernacki 11 Dec '06

11 Dec '06

apflemus, Thank for your answer: > Counter question: can you make quicksort shorter than three lines > > qsort :: Ord a => [a] -> [a] > qsort [] = [] > qsort (x:xs) = [y | y <- xs, y <= x] ++ [x] ++ [y | y <- xs, x < y] The classical Perl qsort has 5 lines - more but still short :-) (look how similar they are, I think it is because they're recursive and have sub-list selectors functions ) : ###################### #!/usr/bin/perl -w my @a = ('a',1,'c','z','h','oik','e','t','77','b'); my @b = &qqsort ( @a ); while ( @b ) { print shift @b,"\n" } sub qqsort { @_ || return (); my $p = shift; return (qqsort(grep $_ lt $p, @_), $p, qqsort(grep $_ ge $p, @_)); } ###################### however, I've questioned about client - server applications - (remote SQL database server (postgres for instance) + users GUI + remote resources (file, document etc ) - that is quite different world to compare with the "algorithm" one. Are there some people who perform such applications? What is your experienced in them? cheers Waldemar

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Designing an object model in Haskell
by John Ky 10 Dec '06

10 Dec '06

Hi, I've got an object model that I have a difficult time conceptualising how it might look like in Haskell: class Element { } class Inline : Element { } class ParentInline : Inline { List<Inline> children; } class Bold : ParentInline { } class Underline : ParentInline { } class Link : ParentInline { String link; } class Text : Inline { String text; } class Block : Element { } class Paragraph : Block { List<Inline> paragraph; } class Heading : Block { List<Inline> heading; } class Document : Element { List<Block> blocks; } How best to represent this OO data model in Haskell? Thanks -John

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SMP support in GHC 6.6 on PPC Macs
by Ivan Tomac 10 Dec '06

10 Dec '06

I've noticed that code compiled with -threaded doesn't like +RTS - N<thread count> as an argument. This seems to be the case with both the binary distribution of GHC 6.6 at www.haskell.org/ghc as well as the version I built from sources. I got rid of all the old versions of GHC on my system and ghc -- version reports 6.6. So I'm guessing this is a bug? Ivan

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interval arithmetic for integers?
by Nicolas Frisby 09 Dec '06

09 Dec '06

I'm looking to not reinvent the wheel. Is there an existing package that supports interval arithmetic on integers (or more)? A possible complication is that I'm hoping to include open intervals such as (GreaterEqThan 3). If there's not a package to go with, any pointers on the appopriate rules for this stuff? I started coding up multiplication and decided it was harder than I first thought... Thanks for your time, Nick

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throwTo & block statements considered harmful
by Chris Kuklewicz 09 Dec '06

09 Dec '06

Title: "throwTo & block statements considered harmful" This is a short essay to prove that the current GHC concurrency implementation has a critical flaw. And this is on the wiki at: http://haskell.org/haskellwiki/GHC/Concurrency#throwTo_.26_block_statements… The key problem is, at least in the presence of block/unblock, that Exceptions are never reliably delivered. And this is not a theoretical case, but can cause a hang in something as innocuous as "program A" given below. Simon Marlow wrote: >> I think people are misunderstanding the nature of a safepoint. The >> safepoint is a point at which you are prepared to have exceptions >> delivered. This does not mean that they *will* be delivered, just that they >> can. If you need to *wait* for an asynchronous exception, then you >> shouldn't be using them at all. > > Right. If a thread mostly runs inside 'block' with the occasional safe > point, then your exceptions are not really asynchronous, they're synchronous. > > > In this case, I'd say a better solution is to have an explicit event queue, > and instead of the safe point take an event from the queue. The action on > receiving an event can be to raise an exception, if necessary. > > Cheers, Simon The implementation of asynchronous signals, as described by the paper "Asynchronous exceptions in Haskell Simon Marlow, Simon Peyton Jones, Andy Moran and John Reppy, PLDI'01." is fatally inconsistent with the implementation in GHC 6.4 and GHC 6.6 today. The implemented semantics have strictly weaker guarantees and render programs using asynchronous expressions impossible to write correctly. The semantics in the paper were carefully designed to solve the problem laid out in the first sentence of the abstract: "Asynchronous exceptions, such as timeouts, are important for robust, modular programs, but are extremely difficult to program with -- so much so that most programming languages either heavily restrict them or ban them altogether." And I believe the paper succeeded. The paper shows how to replace other languages pervasive and intrusive error catching and handling code with much cleaner, clearer, and often more correct code. The implementation in GHC has changed the behavior of throwTo from asynchronous (not-interruptible) to synchronous (interruptible?) as discussed in section 8 of the paper. This change, in and of itself, is not the fatal problem; as described in the paper a (forkIO (throwTo ...)) recovers the asynchronous behavior. The fatal change between the paper and GHC comes from not following section 7.2 as published. Section "7.2 Implementation of throwTo" has two bullet point, and the second bullet point is (retyped, so typos are my own fault): "As soon as a thread exits the scope of a 'block', and at regular intervals during execution inside 'unblock', it should check its queue of pending exceptions. If the queue is non-empty, the first exception from the queue should be raised." A test of GHC 6.6 shows that this is not the case. Test program A: > loop = block (print "alive") >> loop main = do tid <- forkIO loop threadDelay > 1 killThread tid Program A, compiled with (-threaded) on a single CPU machine never halts. It will print "alive" forever while the the main thread is blocked on "killThread". This is wh As an aside, removing the threadDelay causes killThread to destroy the child before it can enter the block, thus showing the need to add "forkBlockedIO" or "forkInheritIO" to the library. This can be worked around using an MVar. Changing the definition of loop produces Test program B: > loop = block (print "alive") >> yield >> loop main = do tid <- forkIO loop > threadDelay 1 killThread tid This prints "alive" twice before the killThread succeeds. The paper demands that when the loop in Program A exits the scope of "block (print a)" that it check a queue of pending exceptions, see that it is non-empty, and raise the exception thrown by killThread. This can also be seen in "Figure 5. Transition Rules for Asynchronous Exceptions", where killThread should use throwTo to create an in-flight exception and exiting the scope of block in the presence of this in-flight exception should raise the exception. The implementation in GHC sleeps the main thread at the killThread command, and it is awoken when the block is exited and to succeed in delivering the exception it must execute while the child is still in the unblocked state. But the child re-enters a blocked state too quickly in Program A, so killThread never succeeds. The change in Program B has the child "yield" when unblocked and this gives the main thread a change to succeed. This trick using yield to make a safepopint was suggested by Simon Marlow: > The window in 'unblock (return ())' is tiny, I'm not really surprised if > nothing ever gets through it. You might have more luck with 'unblock yield'. It has been said on this mailing list thread that needing "yield" to program concurrently is a bug in the user's code and should be replaced by other mechanisms. In a complex program with many threads even the yield trick may not be enough to program reliably. Using blockY io = do x <- block io yield return x unblockY io = unblock (yield >> io) instead of 'block' and 'unblock' is at least a simple but unreliable workaround. This would not be a fatal flaw if there were a simple reliable workaround. The best workaround is the following, which is anything but simple, and amount to re-implementing the mechanisms in the paper: * When forking a thread: ** Create an (Chan Exception) (or (TChan Exception)) and pass it to the child thread. Call this the queue. ** Create a ((T)MVar queue) to indicate whether the thread is still alive or not and wrap the whole child action in a 'finally' to ensure this is set to the dead state when the child exits. (Alive iff the queue is in the MVar) ** To be sure the child thread is in the handler, use another MVar or re-use the "alive/dead" MVar to ensure the child is running before returning the ThreadId from the fork operation. * Replace all uses throwTo/killThread by ** Test the alive/dead state of the thread, proceeding only on living threads ** Putting the Exception in the queue ** Call throwTo/killThread with the Exception * The child thread must: ** implement safepoints by checking the queue ** At each 'block' to 'unblock' transition the child ought to add a safepoint This machinery closely recovers the queue of exceptions and the semantics described in the paper. When a thread has died the queue is removed from the alive/dead MVar and can be garbage collected, along with any pending exceptions for the dead thread. To ensure the above semantics are not violated by the caller, the now fork operation should not return an exposed ThreadId, but should return some proxy type or operation to ensure only the proper throwto/killThread are peformed. Next I will demonstrate how the paper's achievement of simple and clear error handling code is lost. The child thread is where the workaround becomes a nightmare. It is impossible to test the current 'block' versus 'unblock' state. So the only time the child is certain that a 'block' to 'unblock' transition has occurred is when it is explicit: > unblock ( .... block ( ... ) <*> .... ) The <*> is obviously such a transition and ought to have a safepoint added. But is only because the block had the explicit unblock around it. If there were a function such as > atomicUpdateFoo = block (...) <?> then there is no way to know if the <?> should be a safepoint. Therefore atomicUpdateFoo needs documentation so that the caller knows to add a safepoint in the case it is called from the unblocked case. In a more complex example: > atomicUpdateFooBar = block (...) <1?> block (...) <2?> the caller cannot add <1?> itself, and must therefore pass in code 'safe :: IO ()' to all such functions: > atomicUpdateFooBar safe = block (...) >> trans >> block (...) >> trans where 'trans' is "return ()" or "checkChan queue" depending on whether the the caller knows it is in a 'block' or 'unblock' state. If the parent never needed to use 'throwTo'/'killThread' then the child never needs 'block'/'unblock' and the nightmare is avoided. But if the child uses any operation which can block such as 'accept' or 'getChar' or 'takeMVar' then the parent cannot deliver the signal without waking up the child with a 'throwTo' which quickly leads to either an overwhelming use of 'catch' in the child (like other languages) or use of 'block' and the fragile workaround I just described. As an aside, if there was a "isBlockedThreadState" operation, then it would be possible to create a better client workaround: checkChan queue x = do isBlocked <- isBlockedThreadState -- perhaps needing =<< myThreadId case isBlocked of True -> return x False -> raiseQueue queue x raiseQueue queue x = do hasException <- isEmptyChan queue case hasException of False -> return x True -> readChan >>= raise checkedBlock queue io = do x <- block io checkChan queue x checkedUnblock queue io = checkChan queue >> io If that existed then the client should use checkedBlock and checkedUnblock instead of block and unblock. The net effect of this workaround is to implement the queue described in the original paper and wrap the forkIO/throwTo/block/unblock operations with new ones that implement the semantics described in the section 7.2 of the paper. We can try to implement isBlockedThread state by maintaining a stack of block/unblock operations: checkedBlock stack queue io = do x <- bracket_ (modifyIORef stack (True:)) (modifyIORef stack tail) (io) checkChan queue x checkedUnblock stack queue io = do checkChan queue () x <- bracket_ (modifyIORef stack (False:)) (modifyIORef stack tail) (io) isBlockedThread stack = liftM head (readIORef stack) Where "stack :: IORef [Bool]" is created and passed to the child by the modified fork operation. The 'stack' above grows without bound during recursion and is not tail recursive, so the stack management should be changed to the algorithm in the paper... in which case one really has recreated the paper's implementation inside Haskell. The only alternatives are to * write code that may hang forever at a throwTo/killThread * never use throwTo or killThread making block/unblock irrelevant * never use 'block' (and 'unblock') * only write children with explicit "(unblock ... block (... block () ...) >>= raiseQueue queue >>= ...." where the different between checked and unchecked block is obvious and explicit. * Track and propagate the blocked state as in atomicUpdateFooBar. This problem with throwTo/block was not obvious until I stated writing small programs to test the implementation in GHC and thus discovered that the paper's description had mislead me. The key problem is, at least in the presence of block/unblock, that Exceptions are never reliably delivered. And this is not a theoretical case, but can cause a hang in something as innocuous as "program A" given above. -- Chris Kuklewicz

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modelling problem
by Kurt Schelfthout 09 Dec '06

09 Dec '06

Hi Haskell'ers, I'm trying, more as a first excercise in Haskell than anything else, to code a simulation framework in Haskell. A simulation is a bunch of simulation state consisting of constituents (e.g. physical entities like a ball or properties like temperature), on which agents (e.g. simulated humans) in the system can do activities. Activities have a certain duration, and define how the simulation progresses. The simulator framework should just define the basics to maintain a normal representation of time and of cause and effect, i.e. make sure that the state evolves through the application of the activities in the right order. In a later step I will add "laws", which transform activities when they overlap or conflict (e.g. two drive activities leading to collision). I've run into the following problem. I defined a class that allows to define activities on constituents: class Activity a c where start :: c -> a -> Time --start of the activity (this isn't actually dependent on c, I guess) end :: c -> a -> Time --end of the activity delta :: a -> Time -> c -> c --how the constituent is changed at the given time Two parameter type class because some activities are only applicable to certain constituents: instance Activity Paint Ball where instance Activity Move Ball where instance Activity Paint Wall where but you can't move a wall for example, so no instance for Move Wall. My question is: How can I now represent the state of the simulation (i.e. all activites on all constituents). E.g. a list of activities won't do since the list is heterogeneous (i.e. [Paint Ball White, Move Ball (2,0)]) I know about existentials, but I'm at a loss at how to implement the "wrapper" datatype that is exemplified on http://www.haskell.org/hawiki/ExistentialTypes since my class has two parameters, in fact I'm at a loss at how to use existentials here completely. Then, how could I go back from the "general" lists (or whatever datatype) of [a]'s and [c]'s, to a list of [([a],c)] of all activities a that are applicable to a certain constituent c? I can't seem to be able to use the Typeable class for example, since this can not "cast" to typeclasses, only to concrete types (I think...). What I initially liked about this idea is that it can be encoded in the typesystem which activities apply to which consituents, but now it seems like I will have to encode it in the simulation framework more directly (i.e. giving each consituent a String name to encode its "type"). More straightforward ways of modelling this problem (avoiding multiple type class parameters and existentials :) )are also welcome. thanks for any pointers, Kurt

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known, I know: class contexts and mutual recursion
by Conor McBride 08 Dec '06

08 Dec '06

Hi folks I just tripped over the "Contexts differ in length" error message. I know it's not a new problem, but I thought I'd enquire as to its status. For those of you who haven't seen it, here's an example, contrived but compact. > data Thing > = Val Int > | Grok Thing (Maybe Int -> Int) > eval :: Monad m => Thing -> m Int > eval (Val i) = return i > eval (Grok t f) = return (f (eval t)) My eval function compiles ok. See? The recursive call to eval targets the Maybe monad, so I get *Mmm> eval (Grok (Val 5) (maybe 0 (1 +))) :: Maybe Int Just 6 However, when I try to decompose eval as a pair of mutually recursive functions, namely > foo :: Monad m => Thing -> m Int > foo (Val i) = return i > foo (Grok t f) = return (goo t f) > goo :: Thing -> (Maybe Int -> Int) -> Int > goo t f = f (foo t) I get Mmm.lhs:15:1: Contexts differ in length When matching the contexts of the signatures for foo :: forall (m :: * -> *). (Monad m) => Thing -> m Int goo :: Thing -> (Maybe Int -> Int) -> Int The signature contexts in a mutually recursive group should all be identical Poking about on the web, I got the impression that this was a known infelicity in ghc 6.4 (which I'm using), due to be ironed out. However, an early-adopting colleague with 6.6 alleges that foo-goo is still poisonous. I'm wondering what the story is. I mean, is there some nasty problem lurking here which prevents the lifting of this peculiar restriction? I'm not in a panic about this. I have a workaround for the problem as I encountered it in practice. Mind you, it's the sort of thing that's likely to happen more often, the more you localise the effects you tend to use. In the above, goo doesn't throw exceptions; rather, because goo has a handler, it can offer a /local/ exception-throwing capability to foo. Curious Conor

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Re: [Haskell] ANNOUNCE: Visual Haskell 0.2 final
by Bulat Ziganshin 08 Dec '06

08 Dec '06

Hello Krasimir, Friday, December 8, 2006, 11:12:26 AM, you wrote: > The final version of Visual Haskell 0.2 is ready: > - distributed with a stable GHC version (6.6) how about bundlng it with up-to-date GHC 6.6 build (which've fixes a lot of problems) or, better, allow to use it with user-installed ghc 6.6.* installation? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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