- Haskell-Cafe - Haskell.org

Bug in writeArray?
by Grzegorz Chrupała 24 Sep '09

24 Sep '09

Hi all, This seems like a bug in the implementation of writeArray: when passed an out-of-range index it silently writes to an incorrect index in the array. -- import Data.Array.IO import Data.Array.Unboxed main = do let (l,u) = ((0,10),(20,20)) marr <- newArray (l,u) 0 :: IO (IOUArray (Int,Int) Int) let badi = (10,9) print (inRange (l,u) badi) writeArray marr badi 1 arr <- freeze marr :: IO (UArray (Int,Int) Int) print . filter ((/=0) . snd) . assocs $ arr -- Grzegorz

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Status of GHC as a Cross Compiler
by John Van Enk 24 Sep '09

24 Sep '09

Hi, This may be more appropriate for a different list, but I'm having a hard time figuring out whether or not we're getting a cross compiler in 6.12 or not. Can some one point me to the correct place in Traq to find this information? /jve

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gtk2hs and runghc
by Günther Schmidt 24 Sep '09

24 Sep '09

Hi, I'm trying to test some gtk2hs gui code without compiling it first, just by using runghc. Gtk2hs then complains about running in a multithreaded ghc, ie. one with several "real" OS threads. Is it possible to start runghc single-threaded? Günther

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Ctrl-C handling in Haskell with curl on Linux
by Vasyl Pasternak 24 Sep '09

24 Sep '09

Hi, Yesterday I tried to implement simple tool to download pages, and wanted catch Ctrl-C (and other 'killing' messages) from haskell to handle state saving. Without curl (when I perform some long operation) haskell throws UserInterrupt exception immediately, but if I put long operation, which downloads page from the WEB (from the far-far-away server :) ) than I noticed following issues: - to break my program I have to press Ctrl-C twice - haskell doesn't throw an exception - when I rewrite this code to use signals, haskell, after I press Ctrl-C several times exits with error "too many pending signals" I put the test code in the end of the letter. Shortly the longTask doesn't handle Ctrl-C and longTask' handles it. I couldn't find any solutions to this problem, I am afraid that this problem could occur in other non-native haskell modules (bindings to C libraries) Many thanks in advance, Vasyl pasternak ------------------------------------------ Test code: module Main where import Prelude hiding (catch) import Network.Curl import Control.Exception import Control.Monad import System.IO errorHandler defVal e = do putStrLn $ "Error: " ++ (show (e :: ErrorCall)) return defVal link = "far-far-away-site.com.net" getSite curl l = do r <- do_curl_ curl l method_GET :: IO (CurlResponse) if respCurlCode r /= CurlOK then error "get page failed" else return $ respBody r -- this long task doesn't throw user interrupts longTask = do putStrLn "Long task started" curl <- initialize setopts curl [CurlCookieJar "cookies"] handle (errorHandler ()) $ mapM_ (\_ -> getSite curl link >> return ()) [0..100] return () -- this trows longTask' = do putStrLn "long task started" let fib n = foldr (*) 1 [1..n] h <- openFile "/dev/null" WriteMode -- never ends mapM_ (hPutStr h . show . fib) [1..] return () onAbort e = do let x = show (e :: AsyncException) putStrLn $ "Aborted: " ++ x return () main :: IO () main = do handle onAbort longTask putStrLn "Exiting"

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ANNOUNCE: graphviz-2999.5.1.1
by Ivan Lazar Miljenovic 24 Sep '09

24 Sep '09

I'm pleased to announce version 2999.5.1.1 [1] of the graphviz library, which provides bindings to the GraphViz [2] suite of tools for drawing graphs. [1] http://hackage.haskell.org/package/graphviz-2999.5.1.1 [2] http://www.graphviz.org/ This is yet another bugfix release, fixing the problem spotted by Kathleen Fisher where Dot keywords need to be explicitly quoted if used as labels, etc. Once again, this is done automagically with no change to the API. -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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[uvector] derive UA instance for newtype
by Khudyakov Alexey 24 Sep '09

24 Sep '09

Hello I want to derive UA instance for newtypes. Say I have following newtype: > newtype Foo a = Foo a Attempts to derive UA automatically fails because it have associated types. It seems that it's not possible to define from outside. However it's possible to derive instance in the uvector's internals. > deriving instance UAE a => UAE (Foo a) > deriving instance UPrim a => UPrim (Foo a) > > instance UPrim a => UA (Foo a) where > newtype UArr (Foo a) = UAFoo (BUArr (Foo a)) > newtype MUArr (Foo a) s = MUFoo (MBUArr s (Foo a)) > > lengthU = primLengthU > indexU = primIndexU > sliceU = primSliceU > lengthMU = primLengthMU > newMU = primNewMU > readMU = primReadMU > writeMU = primWriteMU > copyMU = primCopyMU > unsafeFreezeMU = primUnsafeFreezeMU > > memcpyMU = primMemcpyMU > memcpyOffMU = primMemcpyOffMU > memmoveOffMU = primMemmoveOffMU Am I moving in right direction and are there other/better solutions to problem? -- Khudyakov Alexey

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best practices for monad transformers and exceptions?
by Evan Laforge 24 Sep '09

24 Sep '09

I don't have an especially deep understanding of monad transformers... basically, I asked around (thanks #haskell!) and struggled and with helpful advice came up with something that seemed to work for me, only now I'm having trouble. What I'm trying to do is hardly revolutionary or even advanced, so I'd like to come up with some kind of standard best practices way to do this. Basically, the question is, "how do you combine multiple sets of monad transformers, each of which may contain their own exception mechanism, and throw and catch exceptions for each one?" This seems like a perfectly normal thing to want to do, so someone must have figured this out. For more details: I have several subsystems, each of which runs in its own monad. For instance, the Ui level has a monad that provides state, logging of updates, and an exception (LoggerT is simply a type alias for a WriterT with DList): module Ui.State where type StateStack m = State.StateT State (Logger.LoggerT Update.Update (Error.ErrorT StateError m)) newtype StateT m a = StateT (StateStack m a) deriving (Functor, Monad, Trans.MonadIO, Error.MonadError StateError) I wished to wrap up this Ui.StateT as its own transformer that can be reused inside other transformers.... because another subsystem has its own set of needs, including its own logging, state, and exception, in addition to Ui.StateT: module Cmd.Cmd where type CmdStack m = Ui.State.StateT (Monad.State.StateT State (Error.ErrorT Abort (Log.LogT m))) newtype CmdT m a = CmdT (CmdStack m a) deriving (Functor, Monad, Trans.MonadIO) I want to provide unlifted access to Ui.State operations, so I have: class (Monad m, Functor m) => UiStateMonad m where get :: m State put :: State -> m () modify :: (State -> State) -> m () update :: Update.Update -> m () throw :: String -> m a Along with instances for Ui.StateT and Cmd.CmdT where Ui.StateT does the appropriate lifting and CmdT just "delegates": 'put st = CmdT (State.put st)' etc. So now I can throw a StateError within Ui.State by calling State.throw. I can throw the error within Cmd with: abort = (CmdT . lift . lift . lift) (Error.throwError Abort) So this all works fine and each "level" of transformers can provide access to its throw by exporting a 'throw' with the appropriate lifts already applied. But then I want to catch an exception, and there's a problem because catchException takes a monadic argument in addition to having a monadic result. I can easily lift the result into the proper layer, but I can't so easily "lower" the result of the monadic argument back down to the StateT or CmdT or whatever. I can use fancy newtype deriving to put Ui.StateT into MonadError and get catchError for free, but if I try to do the same with CmdT then I get: Couldn't match expected type `Abort' against inferred type `State.StateError' When using functional dependencies to combine Error.MonadError State.StateError (State.StateT m), arising from the dependency `m -> e' So evidentally they are not co-existing. In any case, I would need different names for the throw and catch in the different monads. So all I want to do is to be able to catch an Abort. The closest I can think of is something vaguely like: catch_abort :: (Monad m) => CmdT m a -> CmdT m (Maybe a) catch_abort cmd = do ustate <- State.get cstate <- get_state (cmd_result, logs) <- run_cmd ustate cstate mapM_ Log.record logs case cmd_result of Left Abort -> return Nothing Right ui_result -> case ui_result of Left ui_err -> State.throw ui_err Right (val, ustate, updates) -> do State.put ustate mapM_ State.update updates return (Just val) All of this manual "unlifting" is quite ugly and clearly doesn't scale... and it's quite easy to introduce a subtle bug by forgetting to merge one of the many results that come out. So... what should I have done? What do other people who have a project with >1 kind of exception do? Is there an easier way to write 'catch_abort'? Should I break modularity by making a global exception type and putting them all in StateError? Then (I think) I could use catchError directly since I'd only have one occurrance of ErrorT in any transformer stack.

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Re: [Haskell-cafe] accessible layout proposal?
by Jimmy Hartzell 24 Sep '09

24 Sep '09

> I am a bit puzzled here. > This seems to mean something like > "If you take readable code using an operator you can > make it less readable, and when you do that you create > another problem as well, and an even less readable hack > can fix that." > > "I know an old lady who swallowed a fly..." This proposal notwithstanding, I find 'concat [a, b, c]' much more readable than (a) ++ (b) ++ (c), especially since the parentheses in the latter might be necessary (or become necessary unexpectedly upon editing), whereas in the former order of operations is never a concern. Furthermore, why repeat the (++) over and over again, when really you are making a *list* of things to add, and should be using a list?

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help with cabal; trying to escape from configuration hell
by S. Doaitse Swierstra 23 Sep '09

23 Sep '09

I am trying to run happstack on my Mac, but unfortunately I am getting error messages as described in: http://code.google.com/p/happstack/issues/detail?id=88 The cure seems to be to downgrade to network-2.2.0.1, but unfortunately my installed cabal depends on network-2.2.1.4. I tried to re-install happstack using: cabal install happstack --reinstall --constraint="network==2.2.0.2" but unfortunately the ghc happily reports to link against network-2.2.1.4: ... Loading package parsec-2.1.0.1 ... linking ... done. Loading package hsemail-1.3 ... linking ... done. Loading package network-2.2.1.4 ... linking ... done. Loading package SMTPClient-1.0.1 ... linking ... done. Loading package time-1.1.4 ... linking ... done. ... Can someone rescue me? Doaitse

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FFMpeg, SDL and Haskell
by Vasyl Pasternak 23 Sep '09

23 Sep '09

Hi all, Last few days I was playing with FFI, FFMpeg and Haskell. Currently I am trying to make this tutorial http://www.dranger.com/ffmpeg/ on Haskell. Now I have done tutorial 01 and tutorial 02 (show video stream in SDL window). The third tutorial is about audio, and I found that audio doesn't supported in SDL bindings completely. So I'd like to fill this gap, but I don't know where to get latest version and to whom I have to send SDL patches. Anyway, does anybody interested in FFMpeg bindings ? Should I put it on Hackage ? The preliminary version I've put on google code: http://hs-ffmpeg.googlecode.com/files/hs-ffmpeg-0.2.0.tar.gz, but this is "work on my PC" version. Vasyl

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ANNOUNCE: Darcs Hacking Sprint - 14-15 November Vienna
by Eric Kow 23 Sep '09

23 Sep '09

Dear Haskellers, The Darcs Team will soon be hosting its third hacking sprint. Details ------- When : 14-15 November 2009 Where: University of Technology, Vienna, Austria Who : Anybody who wants to hack on Darcs (or Camp, Focal, SO6, etc) Beginners especially welcome! Why : Darcs aims to have bi-annual hacking sprints so that we can get together on a regular basis, hold design discussions, hack up a storm and have a lot fun. What : We plan to put some finishing touches on Darcs-2.4. Darcs 2.4 is a pretty exciting release because we expect it to offer nice performance enhancements from Petr's Google Summer of Code Project, and also a nice new 'hunk splitting' feature. We also intend to set aside at least one Darcs hacker for mentoring beginners, so if you're new to Haskell or to Darcs hacking, here's a good chance to plunge in and start working on a real world project. How: Add yourself to http://wiki.darcs.net/Sprints/2009-11 or email me to let me know you're interested! Thanks ------ Thanks to our local team David Markvica and Benedikt Huber for generously offering to host this event! Thanks also to donors from the last fundraising drive. We'll be using that money to help Darcs hackers get to Vienna. Supporting Darcs ---------------- Darcs is a very happy member of the Software Freedom Conservancy. Among other things, the Software Freedom Conservancy makes it easy for open source projects to raise funds and hold assets (as part of a legal entity). For more information on supporting Darcs, see http://darcs.net/donations.html See you there! -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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accessible layout proposal?
by Jimmy Hartzell 23 Sep '09

23 Sep '09

I am in love with this proposal: http://www.haskell.org/haskellwiki/Accessible_layout_proposal However, after some Google searching and contacting its original author, I have still not found any implementation or project to implement it. Are there any I'm missing? And, if not, who would be willing to help undertake a project to patch ghc to support it? Alternatively, you can try to talk me out of liking the proposal so much, but that is much less likely to work. Jimmy Hartzell

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Re: [Haskell-cafe] Problem with Text.XHtml.Strict.Formlets.file on Mac OSX
by Chris Eidhof 23 Sep '09

23 Sep '09

OK, the trick is to set the attribute enctype="multipart/form-data" on the form whenever your form contains a file upload item. Using the third parameter you can detect if this is the case. This is typical of using file-upload fields in web programming and independent of the formlets. It's also covered in the Happstack tutorial [1] and in the HTML form spec [2]. Good luck, -chris [1] http://tutorial.happstack.com/tutorial/file-uploads [2] http://www.w3.org/TR/html401/interact/forms.html#h-17.13.4 On 23 sep 2009, at 15:35, Colin Adams wrote: > I'm not sure, but the other components of the form work (before I > added the file bit, that is), so they will all be using the same > method. > > I'm using a generic form-handling function named withForm that I got > from one of the examples - this code appears to ignore the third > component of the tuple. > > Note that I don't really understand all of this yet. > > 2009/9/23 Chris Eidhof <chris(a)eidhof.nl>: >> Hey Colin, >> >> The code looks OK to me. Are you sure you are setting the right >> method for >> your form? It's the third component of the tuple returned by the >> runFormState. >> >> Thanks, >> >> -chris >> >> On 22 sep 2009, at 08:56, Colin Adams wrote: >> >>> I'm writing a form that involves picking a file to upload, and so >>> uses >>> Text.XHtml.Strict.Formlets.file. The form displays OK, but when I >>> click the Browse button, and select a file from the dialog (no >>> matter >>> what the file type), and then click on the Submit button, I get an >>> error: >>> >>> "fval[2] is not a file" >>> >>> I haven't added any validation yet. >>> >>> The generated html looks ok to me: >>> >>> <input type="file" name="fval[2]" id="fval[2]"> >>> >>> This is on my laptop running Mac OSX. I can't test it on Linux >>> until I >>> get home at the weekend, so I don't know if I've made a silly >>> error in >>> my coding, but it is very simple: >>> >>> imageInputForm = F.plug (\xhtml -> X.p << (X.label << "Image file:") >>> +++ xhtml) F.file >>> >>> Are there any known problems with the file formlet? Might it be OSX >>> specific? >>> -- >>> Colin Adams >>> Preston, >>> Lancashire, >>> ENGLAND >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > > > > -- > Colin Adams > Preston, > Lancashire, > ENGLAND

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Problem with Text.XHtml.Strict.Formlets.file on Mac OSX
by Colin Adams 23 Sep '09

23 Sep '09

I'm writing a form that involves picking a file to upload, and so uses Text.XHtml.Strict.Formlets.file. The form displays OK, but when I click the Browse button, and select a file from the dialog (no matter what the file type), and then click on the Submit button, I get an error: "fval[2] is not a file" I haven't added any validation yet. The generated html looks ok to me: <input type="file" name="fval[2]" id="fval[2]"> This is on my laptop running Mac OSX. I can't test it on Linux until I get home at the weekend, so I don't know if I've made a silly error in my coding, but it is very simple: imageInputForm = F.plug (\xhtml -> X.p << (X.label << "Image file:") +++ xhtml) F.file Are there any known problems with the file formlet? Might it be OSX specific? -- Colin Adams Preston, Lancashire, ENGLAND

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Re: Multiple-selection formlet?
by Chris Eidhof 23 Sep '09

23 Sep '09

Hey Colin, Currently, I don't think this is supported. It would help a lot if you could send me a dumbed-down version of your code, I'll file an issue for it on github. Thanks, -chris On 23 sep 2009, at 14:35, Colin Adams wrote: > Just in case you missed it on the cafe. > > ---------- Forwarded message ---------- > From: Colin Adams <colinpauladams(a)googlemail.com> > Date: 2009/9/21 > Subject: Multiple-selection formlet? > To: Haskell Cafe <haskell-cafe(a)haskell.org> > > > I can't work out how to get multiple selections delivered from a > formlet. I can add the multiple attribute to get the formlet to allow > the user to make multiple selections, but I only get a single answer > back. This seems inevitable from the type of rawSelect (or select) - > since the list of values to select from is of type [(a, h)] and the > result type is a. > > Am I missing something? > > -- > Colin Adams > Preston, > Lancashire, > ENGLAND > > > > -- > Colin Adams > Preston, > Lancashire, > ENGLAND

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Got it working, now trying to abstract it
by Günther Schmidt 23 Sep '09

23 Sep '09

Hi all, I'm finished with my implementation, an app that analyses financial data for budgeting. But there are no abstractions in my code whatsoever, everything is coded exactly to the point. So now that I'm done I'd like to start it over, this time using far more abstractions than I have, I was thinking along the lines of creating a DSL for it first. The aspects I have to consider are two-fold: ETL-ish - I need to assemble the "records" from a set of csv files BI-ish - I need then to group, filter and aggregate the data. I've come across "How to write a financial contract" S. Peyton Jones in the "fun of programming" and it gives me some ideas on how to go about the BI-ish part of the application. Does anyone know of a DSL example which could fit the bill? Günther

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How to install GD library on Mac OSX?
by Colin Adams 23 Sep '09

23 Sep '09

It needs some missing C libraries - gd, png, jpeg, fontconfig and freetype. Does anyone know what to do to install these on OSX? -- Colin Adams Preston, Lancashire, ENGLAND

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Re: [Haskell-cafe] accessible layout proposal?
by Jimmy Hartzell 23 Sep '09

23 Sep '09

> On Sep 23, 2009, at 3:04 PM, Evan Laforge wrote: > Let's not forget what the proposal actually _is_. > It is that "#(" "#[" "#$" and "#" should become magic > tokens such that > > #( {a;b;c} is read as (a,b,c) > #[ {a;b;c} is read as [a,b,c] > f #$ {a;b;c} is read as f (a) (b) (c) > # {a;b;c} is read as {a,b,c} > > [http://www.haskell.org/haskellwiki/Accessible_layout_proposal] > > Are you not troubled by the inconsistencies here? I am indeed. I concede that the proposal in its present form is completely unacceptable. How about this (which I believe to be backwards compatible): * For data-type declarations, simple indentation may be used: data Tree a Leaf a Branch (Tree a) (Tree a) deriving Show -- this bit may be overkill Read * For records, simple indentation is extended: data Tree a = Branch left::a right::a Leaf a * Export lists are similarly without extra symbols: module Foo Zap Con1 Con2 Con3 mkZap count where * [~] and (~) introduce newline-separated lists and tuples, respectively: wrapWithParens str = concat [~] "(" str ")" Ideally, I'd want something even more light-weight, but I can't think of anything backwards-compatible that satisfies the bracket-matching requirement (which I also care about, as I don't particularly want to hack my editor for this layout proposal either). * For the magic uncurried-function operator, I don't want to have the monstrosity of $$(~). A simple ~ should do here: main = between_ ~ enter a exit a body a This is also allowable in data declarations: data Tree a = Branch ~ Tree a Tree a Leaf a I'd also be interested in knowing if any of these seem interesting separately. I would personally use this new layout for virtually all datatypes, records, and long (where long = (> 2) . length) lists and tuples. I would probably use the '~' notation heavily in writing parsec parsers (the following based loosely off of code from a compiler for a toy programming language): classDeclaration = LiftM4 ClassDeclaration ~ scopeDesignator >>~ keyword "class" typeNameDeclaration option rootClass $ parens qualifiedConstantName -- look, no parens list fieldVariable >>~ operator ";" I like how this version allows me to plainly list the components of a ClassDeclaration. Certainly, if I was actually designing a programming language for writing parsers in, I wouldn't require a bunch of parentheses for productions like this. The fact that, with this proposal, Haskell would no longer require them I take as a good sign. (>>~) is defined as per an article I can't seem to find right now: a >>~ b = do res <- a b return res Furthermore, a function 'if_ a b c = if a then b else c' would be a lot more natural to use. I prefer programming languages where an if-function is as natural to use as the programming language's native 'if', especially those that have a function to implement the functionality of 'if', as those languages are usually the most flexible in their control structures (in Haskell, the only awkwardness with 'if_' at present is syntactic): if_ $ a < 3 ~ errorResult "insufficient allocations" Right $ f b c I feel that with the existence of ($), do notation, the significant whitespace for both do-notation and case statements, Haskell already leans in the direction of allowing for minor awkwardness and inconsistency to eliminate parentheses and to make the structure of the program easily visibly accessible. Jimmy Hartzell

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Re: [Haskell-cafe] accessible layout proposal?
by Jimmy Hartzell 23 Sep '09

23 Sep '09

> Am Mittwoch 23 September 2009 04:06:11 schrieb Jimmy Hartzell: >> Daniel Fischer wrote: >> > Or, what I do: >> > >> > concat >> > [ "(" >> > , str >> > , ")" >> > ] You're right: my objections to this seem mostly to be matters of taste -- as I think about it, I find fewer and fewer practical reasons for disliking it. I still would prefer it if the proposal were implemented (and I would modify it so brackets weren't mismatched), and I'm still interested in implementing it, if only so I (and others) can do programming projects both ways and see if it actually makes a big difference.

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Re: [Haskell-cafe] accessible layout proposal?
by Jimmy Hartzell 23 Sep '09

23 Sep '09

> > On Sep 22, 2009, at 8:01 PM, Jimmy Hartzell wrote: > >> I am in love with this proposal: >> http://www.haskell.org/haskellwiki/Accessible_layout_proposal > > I hadn't read it before. Now that I have, I really do not like > it. "Syntactic sugar causes cancer of the semicolon" as Alan > Perlis once said, and to my taste this proposal definitely > counts as cancer of the semicolon. In effect, its purpose > is to overload vertical white space. > > Any time that you have something where you think you need > this, it's likely that a better solution is to break what > you are doing into smaller pieces. Well, look at code like this: wrapParens str = concat [ "(", str, ")" ] (And yes, I realize you can do something like this with 'printf "(%s)" str'.) First off, there is a serious issue with the commas. You should at least be allowed to have a comma after the last element, a la Python. Otherwise, the last one is randomly special in this list, and in a format like this, I regularly edit code accidentally leaving off commas, yielding stuff like: concat [ "(", str, ")" -- (oops, no comma!) lineEnd -- forgot I needed this ] which (of course) results in a very confusing type error. Meanwhile, you have to format your code very awkwardly, as the closing bracket can't be in the left-most column, and, all in all, you have lots and lots of commas cluttering up your otherwise clean-looking layout. You get humans reading the code based off of the physical layout, while the computer is interpreting it based on the presence of commas, which the human mind will naturally filter in favor of the layout. And as to the "simplifying your code" idea: however "pure" your code is, you will regularly have to embed lists of things in it (ADTs, export lists, data from the domain). And unless you're claiming it is *almost always bad style to have code looking like (from the proposal): main = do a <- getChar bracket_ (enter a) (exit a) (do putChar a putStrLn "hello") then the same argument holds for $# (which I expect is the most controversial part of the proposal): the humans read the layout, the computers read the parentheses, and there are many opportunities for error. I mean, probably in this case the code could stand to be changed to: with a = bracket_ (enter a) (exit a) main = do a <- getChar with a $ do putChar a putStrLn "hello" But I would *still* rather have: with a = bracket_ $# enter a exit a Layout is easier to read than parentheses. In summary, I have to spend a good portion of my time coding Haskell dealing with the fact that I have a lot of {'s, ['s, and ,'s to keep track of, and they rarely fit on one line (records, ADTs, lists). I have to spend a significant amount of my coding time finagling the layout to look sensible, and I don't think anyone would claim that I just shouldn't use records or ADTs.

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Re: [Haskell-cafe] accessible layout proposal?
by Richard O'Keefe 23 Sep '09

23 Sep '09

On Sep 23, 2009, at 3:40 PM, James Hartzell wrote: >> I asked for the trailing comma in Erlang for _social_ reasons, >> not because I believed it would fix all problems of this type. > > What do you mean, for social reasons? The proposal is http://www.erlang.org/eeps/eep-0021.html The abstract says "Allow an extra comma at the end of a list or tuple. Darren New proposed this change; Richard O'Keefe, who doesn't like it very much, wrote it up as an EEP." Later on, we find the text "I don't actually feel any need for this proposal; I believe that the answer is better tool support. However, many people are wedded to their tools, even more than their programming languages. Darren New is not the only one to have asked for it, and with about 1 SLOC in 110 of the Erlang/OTP sources reflecting a list or tuple where this feature could have been used, it's very much a low cost high public appreciation feature." It was a feature that people *wanted* because they were used to in other languages and it could be added at comparatively low cost (there's only one Erlang implementation worth bothering about) without breaking existing tools. Adding it would please many existing and potential Erlang programmers without XXXXing off too many others (you don't _have_ to write the extra comma, and it's not that obtrusive when you read other people's code), and it would remove some complaints and criticisms from the mailing list, so, social reasons. I don't believe it solves any problem that can't be better solved any other way, but at least optional trailing commas don't *hurt* anything. This might actually be different in Haskell, which allows things like (,,). You can't add a trailing comma here without breaking things. The #( #[ #$ # proposal, on the other hand, WOULD break things. "#" is currently an 'ascSymbol', so the character sequences #( and #[ may occur in legal Haskell code. infix # x # y = (x,y) foo x y = x #( y ) bar x z = x #[ z ] > As for "all problems of this type", I agree in theory, but often > enough, > for an operator @@, you can use foldl1 (@@) [...] and reduce it to the > trailing comma problem (notable exceptions include (.) and ($) and > other > operators where the type is different for the intermediate return > values > -- and this, I take it, is why '$#' is in the proposal). I am a bit puzzled here. This seems to mean something like "If you take readable code using an operator you can make it less readable, and when you do that you create another problem as well, and an even less readable hack can fix that." "I know an old lady who swallowed a fly..." > I don't see how this is all that revolutionary, I don't think anyone claims that it is. The claims are that it is - not needed - not pleasant to look at - not good for existing syntax-(semi-)aware tools - not backwards compatible with existing code > I am not at all committed to the specifics. Would it be better if > there > were keywords like 'list#', 'tuple#', or maybe '(#)', '[#]' etc? But of course! > > I guess where I'm coming from is: I don't like my expressions to be > full > of parentheses or hard-to-track operator precedences, and find it > harder > to read, to write, and to maintain, even for toy examples like the > ones > we've been discussing. There's no accounting for taste. I find invisible operators much harder to deal with than parentheses. So we agree to differ. Changing the proposal to something that doesn't spoil bracket matching and doesn't break previously legal Haskell' code (note the prime; I'm not fighting old battles again) would make a huge difference. I still wouldn't _like_ it, but wouldn't have such strong reasons to _oppose_ it. For what it's worth, I've used an experimental language where - immediately after ( [ { or an infix or prefix operator, newlines were treated like any other white space - in other contexts, newlines inside ( [ were treated as commas - in other contexts, newlines were treated as semicolons - trailing commas and semicolons were allowed In Haskell terms, this would mean that ( 1 2 3 ) meant (1,2,3) and [ 1 2 3 ] meant [1,2,3] and 'do' and 'case' worked as now. By adding f $$ (x,y) = f x y f $$ (x,y,z) = f x y z f $$ (x,y,z,a) = f x y z a ... you could also have f $( 1 2 3 ) meaning f 1 2 3. I am aware that this is not backwards compatible with existing code, and I am not proposing this for adoption.

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Re: [Haskell-cafe] accessible layout proposal?
by Richard O'Keefe 23 Sep '09

23 Sep '09

On Sep 23, 2009, at 12:24 PM, James Hartzell wrote: >> Well, look at code like this: > > wrapParens str = concat [ > "(", > str, > ")" > ] I just did. I'd write it as wrap_parens s = "(" ++ s ++ ")" although I wouldn't use that name, because it's not the parentheses that are wrapped, it's s. > First off, there is a serious issue with the commas. > You should at least > be allowed to have a comma after the last element, a la Python. Oh, I agree about that. I've written an Erlang Extension Proposal for Erlang about that. > concat [ > "(", > str, > ")" -- (oops, no comma!) > lineEnd -- forgot I needed this > ] "(" ++ str ++ ")" ++ line_end I'm actually quite serious here. Using infix ++ we have the SAME problem about missing ++ as we do about mssing , After all, someone might have started with ( "(" ++ str ++ ")" ) and ended up with ( "(" ++ str ++ ")" -- (oops, no ++!) lineEnd -- forgot I needed this ) I asked for the trailing comma in Erlang for _social_ reasons, not because I believed it would fix all problems of this type. > And as to the "simplifying your code" idea: however "pure" your code > is, > you will regularly have to embed lists of things in it (ADTs, export > lists, data from the domain). And unless you're claiming it is *almost > always bad style to have code looking like (from the proposal): > > main = do > a <- getChar > bracket_ > (enter a) > (exit a) > (do > putChar a > putStrLn "hello") The thing about toy examples is that they are toy size. On an example this size, we don't NEED fancy new stuff. Actually, I'd write main = getChar >>= \a -> bracket_ (enter a) (exit a) (putChar a >> putStrLn "hello") so even _with_ this proposal, I'd still need exactly the same parentheses. One thing I might do is this: enter_exit_bracket a body = bracket_ (enter a) (exit a) body and then main = getChar >>= \a -> enter_exit_bracket a (putChar a >> putStrLn "hello") because it looks very much as if enter and exit exist only to be passed (with suitable parameters) to bracket_. > > > But I would *still* rather have: > with a = bracket_ $# > enter a > exit a > > Layout is easier to read than parentheses. It all depends on size (small things being easy pretty much no matter what you do) and tools (tools where you click just inside parentheses and the bracketed thing lights up, like the traditional Smalltalk interface make parentheses very very easy to read). > > In summary, I have to spend a good portion of my time coding Haskell > dealing with the fact that I have a lot of {'s, ['s, and ,'s to keep > track > of, and they rarely fit on one line (records, ADTs, lists). I have to > spend a significant amount of my coding time finagling the layout to > look > sensible, and I don't think anyone would claim that I just shouldn't > use > records or ADTs. For what it's worth, the editor I normally use has an "add new element to list" command. Where I'm coming from is this: I do not find meaningless jumbles of special characters combined with overloaded white space readable. It would be so much better if we could discuss a _real_ example.

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Re: [Haskell-cafe] accessible layout proposal?
by Jimmy Hartzell 23 Sep '09

23 Sep '09

Richard O'Keefe wrote: > After all, someone might have started with > ( > "(" ++ > str ++ > ")" > ) > and ended up with > ( > "(" ++ > str ++ > ")" -- (oops, no ++!) > lineEnd -- forgot I needed this > ) > > I asked for the trailing comma in Erlang for _social_ reasons, > not because I believed it would fix all problems of this type. What do you mean, for social reasons? And are there any projects/feature requests to add the trailing comma to ghc? As for "all problems of this type", I agree in theory, but often enough, for an operator @@, you can use foldl1 (@@) [...] and reduce it to the trailing comma problem (notable exceptions include (.) and ($) and other operators where the type is different for the intermediate return values -- and this, I take it, is why '$#' is in the proposal). If I need to use 'a @@ b @@ c' for any operator @@, I rewrite it with a fold (if possible), because I find it more readable to not repeat the operator over and over again. I find lists more semantically appealing in this situation: you really are listing things. > Actually, I'd write > > main = > getChar >>= \a -> > bracket_ (enter a) (exit a) (putChar a >> putStrLn "hello") > > so even _with_ this proposal, I'd still need exactly the same > parentheses. Doing monads without do-notation gets me into precedence trouble pretty quickly -- I find myself adding lots and lots of parentheses, until it ends up looking like Lisp (which I've always found hard on the eyes, keeping track of how many layers you have -- indentation is better for this, in my opinion). The fact that $ can't be used for the last expression of the bracket_ is an example of the precedence trouble I get into. I agree that this proposal makes a lot less sense if you don't also use do-notation. > Where I'm coming from is this: > I do not find meaningless jumbles of special characters > combined with overloaded white space readable. I find the combination of characters very meaningful and intuitive, especially if '(#' is chosen over '#(': use an # if you want to put a list on separate lines (which you'd lay out like that anyway: why not give the compiler some extra knowledge), and don't use it if it fits on the same line. I don't see how this is all that revolutionary, especially as we already have whitespace-significant and non-whitespace-significant versions of 'case', 'do', etc. I don't even see it as syntactic sugar, but more as an alternative syntax -- with the exception of '$#' it parses pretty much directly to the same tree, does it not? It's just changing what's significant. I am not at all committed to the specifics. Would it be better if there were keywords like 'list#', 'tuple#', or maybe '(#)', '[#]' etc? I would suggest using colon like Python, except for it's taken by the (:) operator. I guess where I'm coming from is: I don't like my expressions to be full of parentheses or hard-to-track operator precedences, and find it harder to read, to write, and to maintain, even for toy examples like the ones we've been discussing.

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Strange problem with type classes (ghc 6.8.2 looks allright and ghc 6.10.1 doesn't).
by Serguey Zefirov 22 Sep '09

22 Sep '09

(followup to my previous letter with the same subject) I found the way to break 6.8.2 type checker. It's as easy as to uncomment Div case alternative in valueIndex. Weird. I found a solution, though. Instead of (valueIndex (x_38,(x_36,x_37))) in that Div alternative I should create expression (valueIndex x_38*valuesCount x_36*valuesCount x_37 + valueIndex x_36 * valuesCount x_37+valueIndex x_37) which is equivalent to above one. Slightly more work. And I still cannot figure why valueIndex (a,(b,c)) works in one type and doesn;t in another.

1 0

Strange problem with type classes (ghc 6.8.2 looks allright and ghc 6.10.1 doesn't).
by Serguey Zefirov 22 Sep '09

22 Sep '09

I try to create yet another hardware description language embedded in Haskell and faced a (perceived) bug in ghc 6.10.1 type checker. In ghc 6.8.2 everything works fine. I need a type class that can express relationship between type and its' "size in wires" (the number of bits needed to represent any value of a type). The type class can be easily generalized to most haskell data types and it's easy to write instance derivation using Template Haskell. I used multiparameter type classes because support for type families in Template Haskell was incomplete in 6.10.1 and prior versions. The source code below. Do not forget to include -fcontext-stack=100 when trying to load the code into ghci. The problem lines are between "THE PROBLEM!!" comments. Please, tell me, is it geniune bug or I misunderstand something about Haskell type checker? How do you solve problems like that? and, btw, you can take a look on how we can describe something MIPS-alike in Haskell here: http://thesz.mskhug.ru/svn/hhdl/MIPS.hs it is not finished by any means, but it gives a feel on how it will look. I head to obtain (pretty) efficient simulation functions on infinite streams (almost done, transformations are very simple) and synthesable VHDL/Verilog description all from the same code that looks like ordinary Haskell code. ----------------------------------------------------------------------------------------------------------------------------- -- "A problem with functional dependencies. -- use -fcontext-stack=100 ghc switch. {-# LANGUAGE GADTs, FunctionalDependencies, MultiParamTypeClasses, TypeSynonymInstances, FlexibleInstances, FlexibleContexts, UndecidableInstances #-} module Problem where import Data.Bits import Data.Word ------------------------------------------------------------------------------- -- Type level arithmetic. data Zero = Zero data Succ n = Succ n type ZERO = Zero type ONE = Succ ZERO type TWO = Succ ONE type THREE = Succ TWO type FOUR = Succ THREE type FIVE = Succ FOUR type SIX = Succ FIVE type SEVEN = Succ SIX type EIGHT = Succ SEVEN type NINE = Succ EIGHT type SIZE10 = Succ NINE type SIZE11 = Succ SIZE10 type SIZE12 = Succ SIZE11 type SIZE13 = Succ SIZE12 type SIZE15 = Succ (Succ SIZE13) type SIZE16 = Succ SIZE15 type SIZE20 = Succ (Succ (Succ (Succ (Succ SIZE15)))) type SIZE25 = Succ (Succ (Succ (Succ (Succ SIZE20)))) type SIZE26 = Succ SIZE25 type SIZE30 = Succ (Succ (Succ (Succ (Succ SIZE25)))) type SIZE32 = Succ (Succ SIZE30) class Nat n where fromNat :: n -> Int instance Nat Zero where fromNat = const 0 instance Nat n => Nat (Succ n) where fromNat ~(Succ n) = 1+fromNat n class (Nat a, Nat b, Nat c) => CPlus a b c | a b -> c, a c -> b --instance Nat a => CPlus a Zero a instance Nat a => CPlus Zero a a instance (CPlus a b c) => CPlus (Succ a) b (Succ c) --instance (CPlus a b c) => CPlus (Succ a) b (Succ c) --instance (CPlus a b c) => CPlus a (Succ b) (Succ c) class (Nat a, Nat b, Nat c) => CMax a b c | a b -> c instance CMax Zero Zero Zero instance Nat a => CMax (Succ a) Zero (Succ a) instance Nat a => CMax Zero (Succ a) (Succ a) instance CMax a b c => CMax (Succ a) (Succ b) (Succ c) class (Nat a, Nat b) => CDbl a b | a -> b instance CPlus a a b => CDbl a b class (Nat a, Nat b) => CPow2 a b | a -> b instance CPow2 Zero (Succ Zero) instance (CPow2 a b, CDbl b b2) => CPow2 (Succ a) b2 ------------------------------------------------------------------------------- -- Sized integer. data IntSz n where IntSz :: Nat sz => Integer -> IntSz sz intSzSizeType :: IntSz n -> n; intSzSizeType _ = undefined intSzSize :: Nat n => IntSz n -> Int; intSzSize n = fromNat $ intSzSizeType n instance Nat sz => Eq (IntSz sz) where (IntSz a) == (IntSz b) = a == b instance Show (IntSz n) where show n@(IntSz x) = show x++"{"++show (intSzSize n)++"}" instance Nat sz => Num (IntSz sz) where fromInteger x = IntSz x (IntSz a) + (IntSz b) = IntSz $ a+b (IntSz a) - (IntSz b) = IntSz $ a-b (IntSz a) * (IntSz b) = IntSz $ a*b abs = error "No abs for IntSz." signum = error "No signum for IntSz." ------------------------------------------------------------------------------- -- ToWires class, the main source of problems. class Nat aSz => ToWires a aSz | a -> aSz where wireSizeType :: a -> aSz wireSizeType _ = undefined -- size of a bus to hold a value in bits. wireSize :: a -> Int wireSize x = fromNat $ wireSizeType x -- 2^wireSize, currently unused. wireMul :: a -> Integer wireMul x = Data.Bits.shiftL 1 (wireSize x) -- selector size for decode (unused). wireSelSize :: a -> Int wireSelSize = const 0 -- Integer should occupy no more than wireSize bits, the remaining bits should be 0 toWires :: a -> Integer -- fromWires should work with integers that have non-zero bits -- above wireSize index. fromWires :: Integer -> a -- enumeration. -- valuesCount of (Maybe a) returns 1+valuesCount (x::a) -- valuesCount of Either a b returns valuesCount (x::a) + valuesCount (yLLb) -- valuesCount of (a,b) returns valuesCoutn (x::a) * valuesCount (y::b) valuesCount :: a -> Integer -- valueIndex. -- get a value and return an index inside enumeration. valueIndex :: a -> Integer ------------------------------------------------------------------------------- -- Instances. instance (ToWires x a,ToWires y b, CPlus a b c) => ToWires (x,y) c where toWires (a,b) = Data.Bits.shiftL (toWires a) (wireSize b) .|. toWires b fromWires x = (a,b) where b = fromWires x a = fromWires $ Data.Bits.shiftR x (wireSize b) valuesCount (a,b) = valuesCount a*valuesCount b valueIndex (a,b) = valueIndex a*valuesCount b + valueIndex b instance (ToWires x1 a,ToWires x2 b,ToWires x3 c, CPlus b c bc, CPlus a bc abc) => ToWires (x1,x2,x3) abc where toWires (a,b,c) = toWires (a,(b,c)) fromWires abc = (a,b,c) where (a,(b,c)) = fromWires abc valuesCount (a,b,c) = valuesCount (a,(b,c)) valueIndex (a,b,c) = valueIndex (a,(b,c)) instance (ToWires x1 a,ToWires x2 b,ToWires x3 c,ToWires x4 d,ToWires x5 e, CPlus d e de, CPlus c de cde, CPlus b cde bcde, CPlus a bcde abcde) => ToWires (x1,x2,x3,x4,x5) abcde where toWires (a,b,c,d,e) = toWires (a,(b,(c,(d,e)))) fromWires abcde = (a,b,c,d,e) where (a,(b,(c,(d,e)))) = fromWires abcde valuesCount (a,b,c,d,e) = valuesCount (a,(b,(c,(d,e)))) valueIndex (a,b,c,d,e) = valueIndex (a,(b,(c,(d,e)))) instance CPow2 FIVE size32 => ToWires Int size32 where toWires n = fromIntegral n .&. 0xffffffff fromWires x = fromIntegral x valuesCount = const (Data.Bits.shiftL (1::Integer) 32) valueIndex = fromIntegral instance CPow2 FIVE size32 => ToWires Word32 size32 where toWires n = fromIntegral n .&. 0xffffffff fromWires x = fromIntegral x valuesCount = const (Data.Bits.shiftL (1::Integer) 32) valueIndex = fromIntegral instance Nat size => ToWires (IntSz size) size where toWires x@(IntSz n) = n .&. (Data.Bits.shiftL 1 (wireSize x) - 1) fromWires x = fromIntegral x valuesCount x = Data.Bits.shiftL (1::Integer) (wireSize x) valueIndex x = toWires x ------------------------------------------------------------------------------- -- Neccessary types for a problem. -- A hack around bugs (??) in ghc 6.10.1. data RI = RI (IntSz FIVE) deriving (Eq,Show) data Imm5 = Imm5 (IntSz FIVE) deriving (Eq,Show) data Imm16 = Imm16 (IntSz SIZE16) deriving (Eq,Show) data Imm26 = Imm26 (IntSz SIZE26) deriving (Eq,Show) instance ToWires RI FIVE where toWires (RI x) = toWires x fromWires x = RI $ fromWires x valuesCount = const 32 valueIndex (RI x) = toWires x instance ToWires Imm5 FIVE where toWires (Imm5 x) = toWires x fromWires x = Imm5 $ fromWires x valuesCount = const 32 valueIndex (Imm5 x) = toWires x instance ToWires Imm16 SIZE16 where toWires (Imm16 x) = toWires x fromWires x = Imm16 $ fromWires x valuesCount (Imm16 x) = valuesCount x valueIndex (Imm16 x) = valueIndex x instance ToWires Imm26 SIZE26 where toWires (Imm26 x) = toWires x fromWires x = Imm26 $ fromWires x valuesCount (Imm26 x) = valuesCount x valueIndex (Imm26 x) = toWires x instance ToWires a sz => ToWires (Maybe a) (Succ sz) where toWires Nothing = 0 toWires (Just x) = shiftL (toWires x) 1 .|. 1 fromWires = undefined valuesCount x = 1+valuesCount y where ~(Just y) = x valueIndex Nothing = 0 valueIndex (Just x) = 1+valueIndex x ------------------------------------------------------------------------------- -- And a problem itself. data Signed = Signed | Unsigned deriving (Eq,Show) data MIPSCmd regv = Nop | Trap -- we mark destination register with RI. It never gets fetched. -- we mark source registers as regv, because they can change their size -- at FETCH stage (became Word32). | AddU regv regv RI | And regv regv RI | Div Signed regv regv deriving (Eq,Show) instance ToWires Signed (Succ Zero) where toWires (Signed) = Data.Bits.shiftL 0 1 .|. 0 toWires (Unsigned) = Data.Bits.shiftL 0 1 .|. 1 fromWires x = undefined valuesCount x = 1 + (1 + 0) where ~(Signed) = x ~(Unsigned) = x valueIndex x = case x of Signed -> 0 + 0 Unsigned -> (0 + 0) + 0 where ~(Signed) = x ~(Unsigned) = x instance (ToWires regv_0 v_1, ToWires RI v_2, CPlus v_1 v_1 vCPlus_3, CPlus vCPlus_3 v_2 vCPlus_4, CPlus v_1 v_1 vCPlus_5, CPlus vCPlus_5 v_2 vCPlus_6, ToWires Signed v_7, CPlus v_7 v_1 vCPlus_8, CPlus vCPlus_8 v_1 vCPlus_9, CMax vCPlus_4 vCPlus_6 vCMax_10, CMax vCMax_10 vCPlus_9 vCMax_11) => ToWires (MIPSCmd regv_0) (Succ (Succ (Succ vCMax_11))) where toWires (Nop) = Data.Bits.shiftL 0 3 .|. 0 toWires (Trap) = Data.Bits.shiftL 0 3 .|. 1 toWires (AddU a_12 a_13 a_14) = Data.Bits.shiftL (Data.Bits.shiftL (Data.Bits.shiftL (Data.Bits.shiftL 0 (wireSize a_12) .|. toWires a_12) (wireSize a_13) .|. toWires a_13) (wireSize a_14) .|. toWires a_14) 3 .|. 2 toWires (And a_15 a_16 a_17) = Data.Bits.shiftL (Data.Bits.shiftL (Data.Bits.shiftL (Data.Bits.shiftL 0 (wireSize a_15) .|. toWires a_15) (wireSize a_16) .|. toWires a_16) (wireSize a_17) .|. toWires a_17) 3 .|. 3 toWires (Div a_18 a_19 a_20) = Data.Bits.shiftL (Data.Bits.shiftL (Data.Bits.shiftL (Data.Bits.shiftL 0 (wireSize a_18) .|. toWires a_18) (wireSize a_19) .|. toWires a_19) (wireSize a_20) .|. toWires a_20) 3 .|. 4 fromWires x = undefined valuesCount x = 1 + (1 + ((valuesCount x_21 * (valuesCount x_22 * (valuesCount x_23 * 1))) + ((valuesCount x_24 * (valuesCount x_25 * (valuesCount x_26 * 1))) + ((valuesCount x_27 * (valuesCount x_28 * (valuesCount x_29 * 1))) + 0)))) where ~(Nop) = x ~(Trap) = x ~(AddU x_21 x_22 x_23) = x ~(And x_24 x_25 x_26) = x ~(Div x_27 x_28 x_29) = x -- Please don't mind that code is wrong, I can't get past typechecker here. valueIndex x = case x of Nop -> 0 + 0 Trap -> (0 + 0) + 0 -- THE PROBLEM!!! AddU ax_30 ax_31 ax_32 -> ((0 + 0) + 0) + valueIndex (ax_32,(ax_30, ax_31)) -- /THE PROBLEM!!! {- And x_33 x_34 x_35 -> (((0 + 0) + 0) + valuesCount (x_32, (x_30, x_31))) + valueIndex (x_35, (x_33, x_34)) Div x_36 x_37 x_38 -> ((((0 + 0) + 0) + valuesCount (x_32, (x_30, x_31))) + valuesCount (x_35, (x_33, x_34))) + valueIndex (x_38, (x_36, x_37)) -} where ~(Nop) = x ~(Trap) = x ~(AddU x_30 x_31 x_32) = x ~(And x_33 x_34 x_35) = x ~(Div x_36 x_37 x_38) = x -----------------------------------------------------------------------------------------------------------------------------

1 0

What is
by DavidA 22 Sep '09

22 Sep '09

Hi, When I try to build my HaskellForMaths library (http://hackage.haskell.org/package/HaskellForMaths) using GHC6.10.4 on Mac OS X (Leopard), I get several "ld warning: atom sorting error" (see below). The same code built without problems under GHC6.10.3 on Windows. The code in question is using phantom types to parameterise type constructors. Should I be worried? (It's only a warning - I believe the build is fine.) ld warning: atom sorting error for _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Grevlex_closure_tbl and _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Lex_closure_tbl in dist/build/Math/Algebra/Commutative/Monomial.o ld warning: atom sorting error for _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Glex_closure_tbl and _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Grevlex_closure_tbl in dist/build/Math/Algebra/Commutative/Monomial.o ld warning: atom sorting error for _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Elim_closure_tbl and _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Glex_closure_tbl in dist/build/Math/Algebra/Commutative/Monomial.o ld warning: atom sorting error for _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Grevlex_closure_tbl and _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Lex_closure_tbl in dist/build/Math/Algebra/Commutative/Monomial.o ld warning: atom sorting error for _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Glex_closure_tbl and _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Grevlex_closure_tbl in dist/build/Math/Algebra/Commutative/Monomial.o ld warning: atom sorting error for _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Elim_closure_tbl and _HaskellForMathszm0zi1zi8_MathziAlgebrazi CommutativeziMonomial_Glex_closure_tbl in dist/build/Math/Algebra/Commutative/Monomial.o

2 1

Cabal packages -> cabbages
by Jason Dusek 22 Sep '09

22 Sep '09

Some day, we're going to need a short, catchy name for Cabal packages. Let's call them cabbages. -- Jason Dusek

9 15

Building com-1.2.3
by Andrew Coppin 22 Sep '09

22 Sep '09

C:\com-1.2.3>runhaskell Setup configure Configuring com-1.2.3... Setup: Missing dependencies on foreign libraries: * Missing header file: include/WideStringSrc.h * Missing C libraries: kernel32, user32, ole32, oleaut32, advapi32 This problem can usually be solved by installing the system packages that provide these libraries (you may need the "-dev" versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. Any hints? (It is conspicuous that the "hissing header file" is actually present at the exact path indicated... WTH?)

3 6

Problem with type families
by Victor Nazarov 21 Sep '09

21 Sep '09

I've tried to reimplement code presented in the following blog post: http://cdsmith.wordpress.com/2009/09/20/side-computations-via-type-classes/ But I've got stuck with the following error: Triangulable.hs:41:8: `addRows' is not a (visible) method of class `Triangulable' Triangulable.hs:43:8: `addCols' is not a (visible) method of class `Triangulable' I think the minimal example to reproduce this error is the following: ... class Triangulable m where type Elem m (@@) :: m -> (Int, Int) -> Elem size :: m -> Int swapRows :: Int -> Int -> m -> m swapCols :: Int -> Int -> m -> m addRow :: (Num Elem) => Elem -> Int -> Int -> m -> m addCol :: (Num Elem) => Elem -> Int -> Int -> m -> m newtype ListMatrix a = ListMatrix [[a]] instance Num a => Triangulable (ListMatrix a) where type Elem (ListMatrix a) = a (ListMatrix xs) @@ (i, j) = xs !! i !! j size (ListMatrix xs) = size xs swapRows m n (ListMatrix xs) = ListMatrix $ swap m n xs swapCols m n (ListMatrix xs) = ListMatrix $ map (swap m n) xs addRows q m n (ListMatrix xs) = ListMatrix $ modifyNth n (zipWith comb (xs !! m)) where comb a b = q * a + b addCols q m n (LisMatrix xs) = ListMatrix $ map (\row -> modifyNth n (comb (row !! m)) row) where comb a b = q * a + b ... How can I fix this? -- Victor Nazarov

2 2

readline does not work on gentoo amd64 (ghc-6.10.4)
by Sergei Trofimovich 21 Sep '09

21 Sep '09

Hi, dear list! libraries(a)haskell.org treats me as a spammer, so I'm writing here. Today I've stuck with following problems: 1. After recent readline upgrade (5 -> 6) in my distro I got unbuildable lambdabot. After investigating a little I've noticed I'm having broken readline: //===-------------------- $cabal install readline ... $ ghci -package readline GHCi, version 6.10.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package syb ... linking ... done. Loading package base-3.0.3.1 ... linking ... done. Loading package filepath-1.1.0.2 ... linking ... done. Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.2 ... linking ... done. Loading package unix-2.3.2.0 ... linking ... done. Loading package directory-1.0.0.3 ... linking ... done. Loading package process-1.0.1.1 ... linking ... done. Loading package readline-1.0.1.0 ... <command line>: can't load .so/.DLL for: readline (/usr/lib64/libreadline.so: invalid ELF header) //===-------------------- It's true, there is no real ELF library, but none of packages complained before. //===-------------------- $ cat /usr/lib64/libreadline.so /* GNU ld script Since Gentoo has critical dynamic libraries in /lib, and the static versions in /usr/lib, we need to have a "fake" dynamic lib in /usr/lib, otherwise we run into linking problems. This "fake" dynamic lib is a linker script that redirects the linker to the real lib. And yes, this works in the cross- compiling scenario as the sysroot-ed linker will prepend the real path. See bug http://bugs.gentoo.org/4411 for more info. */ OUTPUT_FORMAT ( elf64-x86-64 ) GROUP ( /lib64/libreadline.so.6 ) //===-------------------- Looks like readline package can't resolve real ELF library path to readline in readline.buildinfo (right after cabal configure): //===-------------------- $ cat readline.buildinfo buildable: True cc-options: ld-options: extra-libraries: readline ncurses frameworks: //===-------------------- And second minor nit: readline tarball is unusable after issuing 'cabal clean': //===-------------------- $ cabal clean && cabal configure cleaning... Resolving dependencies... Configuring readline-1.0.1.0... configure: error: cannot find sources (include/HsReadline.h) in . or .. //===-------------------- Practically these issues can be solved by moving lambdabot to haskeline tracks. Thanks! -- Sergei

1 0

Suggested additions to System.FilePath.Posix/Windows
by Marcus D. Gabriel 21 Sep '09

21 Sep '09

Hello Neil I used System.FilePath.Posix quite extensively recently, and I thank you for the package filepath. There were however two words that I needed which I could not construct from those in System.FilePath.Posix. They are maybe of interest to you and others. I submit these two words to you for consideration for inclusion in System.FilePath.Posix. Please change the names as you see fit. I do not know if they make sense for System.FilePath.Windows. If the do not make sense, then please feel free to drop them so as to preserve the interface. As requested, I Cc'ed the haskell-cafe, but I am not at the moment following these threads, so if anyone else responds, please Cc me if you wish. Thanks again and cheers, - Marcus P.S. Here they are. Although I use ksh(1) as an example, this is a feature of POSIX shells. > -- | 'reduceFilePath' returns a pathname that is reduced to canonical > -- form equivalent to that of ksh(1), that is, symbolic link names are > -- treated literally when finding the directory name. See @cd -L@ of > -- ksh(1). Specifically, extraneous separators @(\"/\")@, dot > -- @(\".\")@, and double-dot @(\"..\")@ directories are removed. > > reduceFilePath :: FilePath -> FilePath > reduceFilePath = joinPath . filePathComponents This is in turn built on filePathComponents that does all the work. > filePathComponents :: FilePath -> [FilePath] > filePathComponents "" = [] > filePathComponents (c:cs) = > reverse $ snd $ foldl accumulate > (if c == pathSeparator then ([],["/"]) else > ([c],[])) > (cs++[pathSeparator]) > where > accumulate :: (String,[String]) -> Char -> (String,[String]) > accumulate (cs, css) c = > if c == pathSeparator > then ([],(if null cs then id else cons cs) css) > else (cs++[c],css) > cons :: String -> [String] -> [String] > cons cs css > | cs == "." = css > | cs /= ".." || null css = cs : css > | otherwise = > let hd = head css > tl = tail css > in if hd == [pathSeparator] > then css > else if hd == ".." > then cs : css > else if null tl > then ["."] > else tl // -- Marcus D. Gabriel, Ph.D. Saint Louis, FRANCE http://www.marcus.gabriel.name mailto:marcus@gabriel.name Tel: +33.3.89.69.05.06 Portable: +33.6.34.56.07.75

5 6

line intersection code
by staafmeister 21 Sep '09

21 Sep '09

Hi, I wrote a O(n log n) line segment intersection code. Couple of questions 1) I did not find such a library on hackage (at least google gave no results:)), and I notice a lot of people submit packages. Would there be any interest to submit this code (would give incentive to make the code cleaner) 2) In the algo I needed a balanced binary tree, but with a compare function that changes with each iteration. Is there a balanced binary tree available that give more freedom in specifying the compare function. Data.Set does not have this option (I think). Using unsafePerformIO to change the compare function seemed a little bit ugly. -- View this message in context: http://www.nabble.com/line-intersection-code-tp25530313p25530313.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell Weekly News: Issue 131 - September 19, 2009
by Joe Fredette 21 Sep '09

21 Sep '09

Hopefully the line endings come out okay this week, I did a test before sending it to the list, please let me know if you notice anything awry. Just put a [HWN] in the subject line so my filter's will catch it. </metaeditorial> --------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090918 Issue 131 - September 18, 2009 --------------------------------------------------------------------------- Welcome to issue 131 of HWN, a newsletter covering developments in the [1]Haskell community. Last week, I received an email from Mark Wotton about his project [2]Hubris. I totally forgot to put it in the HWN last week, too busy trying to figure out all the tools. So, I thought I'd make it up and give him some special editorial status this week. Hubris is a bridge between Ruby and Haskell, allowing you to call Haskell from Ruby. It's very cool, I highly suggest playing with it. Also, I've been posting a bit about the new HWN tools (dubbed "HWN2") on my [3]blog, there is also a repo up at [4]patch-tag which will have all the code. If there is some interest in helping me, I'll try to come up with a TODO list/Trac. Announcements hssqlppp, sql parser and type checker, pre-alpha. Jake Wheat [5]announced his parser/type checker for SQL. It currently parses a subset of PostGreSQL and PL/pgSQL, and can type check some statements. LambdaINet-0.1.0, Graphical Interaction Net Evaluator for Optimal Evaluation. Paul L [6]announced a LambdaINet 0.1.0, available on [7]Hackage. LambdaINet implements an interaction net based optimal evaluator. With an interactive graphical interface allowing the user to view and directly manipulate the interaction net. arbtt-0.1. Joachim Breitner [8]announced the Automatic Rule-Based Time Tracking tool on hackage. he has an introduction available [9]here. A statistics library. Bryan O'Sullivan [10]announced the imaginatively named [11]statistics library. Which supports common discrete and continuous probability distributions, Kernel density estimation, Auto-correlation analysis, Functions over sample data, Quantile estimation, and Re-sampling techniques. CFP: JSC Special Issue on Automated Verification and Specification of Web Systems. [12]A Special Issue of the Journal of symbolic computation was announced. This issue is related to the topics of the Automated Specification and Verification of Web Systems Workshop (WWV'09). Read the announcement for more details. Haskeline 0.6.2. Judah Jacobson [13]announced the release of Haskeline 0.6.2, available [14]here. Improvements over the last version include, new emacs and vi bindings, a new preference to remove repeated history entries, recognition of page-up and page-down keys, and more. PEPM'10 - Last CFP (Submission: 6 Oct 09, Notification: 29 Oct 09). Janis Voigtlaender [15]announced the Last Call for Papers for PEPM'10, see the announcement for more details. Videos of HIW 2009. Malcolm Wallace [16]announced videos of all the presentations/discussions at the recent Haskell Implementers Workshop 2009, in Edinburgh, are now [17]online. The program of talks is available [18]here. Unification in a Commutative Monoid (cmu 1.1) and a new release of Abelian group unification and matching (agum 2.2). John D. Ramsdell [19]announced cmu 1.1, which provides unification in a commutative monoid, also know as ACU-unification. The core computation finds the minimal non-zero solutions to homogeneous linear Diophantine equations. The linear equation solver has been place in a separate module so it can be used for other applications. He also announced agum 2.2, which provides unification and matching in an Abelian group, also know as AG-unification and matching. graphviz-2999.5.1.0. Ivan Lazar Miljenovic [20]announced a bug-fix release of the GraphViz package, no major API changes occurred. levmar-0.2, bindings-levmar-0.1.1. Bas van Dijk and Roel van Dijk announced [21]new [22]versions of the levmar and bindings-levmar packages. New features include automatic calculation of the Jacobian via Conal Elliot's automatic differentiation from his vector-space library. CmdArgs - easy command line argument processing. Neil Mitchell [23]announced CmdArgs 0.1. CmdArgs is a library for parsing command-line arguments. It offers several improvements over GetOpts, namely that the Command Line Argument Processors are shorter and CmdArgs can support multiple-mode command lines such as those found in darcs, cabal, hpc, etc. OpenGL 2.4.0.1. Sven Panne [24]announced a new version of the OpenGL package, this version fixes a bug that didn't make it into the previous release. OpenGLRaw 1.1.0.0. Sven Panne [25]announced a new version of the OpenGLRaw package has been uploaded to Hackage. Discussion Thank you guys. Cristiano Paris took some time to thank us all from -Cafe for helping him learn Haskell. You're welcome, Cristiano! Unicode lexing in GHC and GHCi. Sean McLaughlin [26]asked about why certain unicode characters parsed in GHCi without error, but not in compiled code. Help with FFI. Jose Prous [27]asked for some help with the foreign function interface. Ambiguous type variable with subclass instance. Andy Gimblett [28]asked about a particular ambiguous type error Haskell -> .NET. Peter Verswyvelen [29]asked about the possibilities for a .NET version of Haskell. A thought about liberating Haskell's syntax. George Pollard [30]suggested a new way to do templates, so that brace-like syntax could be added without having to seriously hack GHC. Blog noise [31]Haskell news from the [32]blogosphere. Blog posts from people new to the Haskell community are marked with >>>, be sure to welcome them! * Neil Brown: [33]Concurrent Pearl: The Sort Pump. * Roman Cheplyaka: [34]CCC #6: HWN. A comic revealing the _real_ reason Brent left the HWN to me. * Don Stewart (dons): [35]Data.Binary: performance improvements for Haskell binary parsing. * Thomas M. DuBuisson: [36]Kernel Modules in Haskell. * Neil Brown: [37]Boids Simulation: Part 4. * Edward Kmett: [38]Iteratees, Parsec, and Monoids, Oh My!. * Edward Kmett: [39]Remodeling Precision. * Alex McLean: [40]Hackpact documentation (week 3). The continuation of Alex's series on Hackpact * Luke Palmer: [41]IO-free splittable supply. * Thomas M. DuBuisson: [42]Kernel Modules in Haskell. * Bryan O'Sullivan: [43]A video demo of my Haskell benchmarking framework. * Don Stewart (dons): [44]Haskell for Everyone: Hackage and the Haskell Platform : Haskell Implementers Workshop 2009. * David Sankel: [45]Applied Functional Programming: Part 1. * Neil Brown: [46]Boids Simulation: Part 4. * Darcs: [47]darcs weekly news #40. * Chris Smith: [48]On Inverses of Haskell Functions. * David Amos: [49]Finite geometries, part 1: AG(n,Fq). A multipart series from David, part of his Haskell For Maths project. Quotes of the Week * quicksilver: no, you mispelt >> as ; * dons: Cale's my alter-ego. I talk about applications and benchmarking, he talks about theory and math. We've been doing this for years :) * gwern: #haskell: because none of us are as offtopic as all of us * some-crazy-hwn-editor: A monster! HAH! It will not be a monster, but a god! ALL SHALL BOW BEFORE MY SPAWN AND DESPAIR! ALL HAIL THE PROGRAMMER CHILD! ALL HAIL THE HYPNOTOAD! * AlanJPerlis: Purely applicative languages are poorly applicable. About the Haskell Weekly News New editions are posted to [50]the Haskell mailing list as well as to [51]the Haskell Sequence and [52]Planet Haskell. [53]RSS is also available, and headlines appear on [54]haskell.org. To help create new editions of this newsletter, please see the information on [55]how to contribute. Send stories to jfredett . at . gmail . dot . com. The darcs repository is available at darcs get [56]http://patch-tag.com/r/HWN2/home . References 1. http://haskell.org/ 2. http://github.com/mwotton/Hubris 3. http://lowlymath.net/?p=45 4. http://patch-tag.com/r/HWN2/snapshot/current/content/pretty 5. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63413 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63390 7. http://hackage.haskell.org/package/LambdaINet 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63369 9. https://www.joachim-breitner.de/blog/archives/336-The-Automatic-Rule-Based-… 10. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63362 11. http://hackage.haskell.org/package/statistics 12. http://article.gmane.org/gmane.comp.lang.haskell.general/17502 13. http://article.gmane.org/gmane.comp.lang.haskell.general/17497 14. http://hackage.haskell.org/package/haskeline 15. http://article.gmane.org/gmane.comp.lang.haskell.general/17496 16. http://article.gmane.org/gmane.comp.lang.haskell.general/17494 17. http://www.vimeo.com/album/126462 18. http://haskell.org/haskellwiki/HaskellImplementorsWorkshop 19. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63572 20. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63542 21. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63499 22. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63498 23. http://article.gmane.org/gmane.comp.lang.haskell.general/17493 24. http://article.gmane.org/gmane.comp.lang.haskell.general/17492 25. http://article.gmane.org/gmane.comp.lang.haskell.general/17490 26. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63580 27. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63574 28. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63545 29. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63512 30. http://article.gmane.org/gmane.comp.lang.haskell.cafe/63492 31. http://planet.haskell.org/ 32. http://haskell.org/haskellwiki/Blog_articles 33. http://chplib.wordpress.com/2009/09/18/the-sort-pump/ 34. http://ro-che.blogspot.com/2009/09/ccc-6-hwn.html 35. http://donsbot.wordpress.com/2009/09/16/data-binary-performance-improvments… 36. http://tommd.wordpress.com/2009/09/13/kernel-modules-in-haskell/ 37. http://chplib.wordpress.com/2009/09/16/boids-simulation-part-4/ 38. http://comonad.com/reader/2009/iteratees-take-2/ 39. http://comonad.com/reader/2009/remodeling-precision/ 40. http://yaxu.org/hackpact-documentation-week-3/ 41. http://lukepalmer.wordpress.com/2009/09/14/io-free-splittable-supply/ 42. http://tommd.wordpress.com/2009/09/13/kernel-modules-in-haskell/ 43. http://www.serpentine.com/blog/2009/09/14/a-video-demo-of-my-haskell-benchm… 44. http://donsbot.wordpress.com/2009/09/14/haskell-for-everyone-hackage-and-th… 45. http://netsuperbrain.com/blog/posts/applied-functional-programming-part-1/ 46. http://chplib.wordpress.com/2009/09/14/boids-simulation-part-4/ 47. http://blog.darcs.net/2009/09/darcs-weekly-news-40.html 48. http://cdsmith.wordpress.com/2009/09/14/on-inverses-of-haskell-functions/ 49. http://haskellformaths.blogspot.com/2009/09/finite-geometries-part-1-agnfq.… 50. http://www.haskell.org/mailman/listinfo/haskell 51. http://sequence.complete.org/ 52. http://planet.haskell.org/ 53. http://sequence.complete.org/node/feed 54. http://haskell.org/ 55. http://haskell.org/haskellwiki/HWN 56. http://patch-tag.com/r/HWN2/home

3 3

Multiple-selection formlet?
by Colin Adams 21 Sep '09

21 Sep '09

I can't work out how to get multiple selections delivered from a formlet. I can add the multiple attribute to get the formlet to allow the user to make multiple selections, but I only get a single answer back. This seems inevitable from the type of rawSelect (or select) - since the list of values to select from is of type [(a, h)] and the result type is a. Am I missing something? -- Colin Adams Preston, Lancashire, ENGLAND

1 0

GHC will not let me install
by James Michael Stephens 21 Sep '09

21 Sep '09

I am trying to install Xmonad on my Mac. I download GHC installer for mac the .dmg from Haskell.org and when I install it gets stuck here [img]http://www.jmstephens.99k.org/picture.png[/img] As you can see the install button is grey and will not let me click it.

4 3

Type Families
by Alexander Solla 21 Sep '09

21 Sep '09

(I'm using a fixed width font, so if you don't see nice formatting, you need to use a fixed width font. This is literate Haskell, but I copied and pasted the code. YMMV) Hi Everybody! (Hi Dr. Nick) I've been looking for a good way to use some richer notions of polymorphism than Haskell98 allows. First, I tried to define a monad "of" the things I want to quantify over (so each "type of view" or "type of evaluable thing" would be a constructor in a monadic type.) But that didn't seem to offer the kind of extensibility I wanted, since I basically want to join several types together. I tried FunDeps next, and that worked okay, but it was a bit difficult to keep track of what was "meant" to do what. I guess I could have plowed through the work, but it wasn't any fun. I finally heard about TypeFamilies, and they seem to give me the kind of extensibility I want, while keeping the theoretical foundations relatively clean. But I am not so sure I understand them. Let us consider the code: > type AbstractValue = Int > class Evaluate asset where > data Value asset :: * > value :: (Value asset) -> AbstractValue That's easy enough. "Value asset" is an indexed type. That is reflected in the instance declaration: > instance Evaluate Abstract where > data Value Abstract = AbstractValue Abstract > value (AbstractValue int) = int Okay, easy enough. But what happens when we want to "add" Evaluate instances? > data Add a b = Add a b > instance ( Evaluate a > , Evaluate b > ) => Evaluate (Add a b) where > data Value (Sum a b) = SumValue (Sum a b) Even this much is straightforward. We require a and b to be Evaluate'able before we can find the sum of a and b as "values". Now I want to write my definition for the "value" function. But... how is that supposed to work? My first guess is value (SumValue (Sum a b)) = (value a) + (value b) But I more-or-less expected that to fail. I realize I need some more typing information. What am I supposed to fill in? My next guess was > value (SumValue (Sum a b)) = (value $ Value a) + (value $ Value b) But Value doesn't exist as a type constructor. So now that I am starting to "get" what's going on, I wonder why I don't get what's going on. Since I need to use a type constructor for the (Value a) and (Value b) "things", it kind of defeats the point. (I hesitate to say "value", since I have been using "value" to mean the result/blah of an Evaluate instance) Speaking of which, I am still not sure what the difference between associate data type families and associated type constructor families are. The former use the data keyword in class declarations, and the latter use type keywords. What can I do with one and not the other? I would really appreciate some guidance. Thanks! Alex

3 2

first the platform, the cabal
by Peter J. Veger 20 Sep '09

20 Sep '09

I just installed the Haskell Platform 2009.2.0.2 on Windows. There is no cabal.exe, only Cabal-1.6.0.3 How to continue: build cabal? Then what to download from where and what to execute? greetings, Peter J. Veger, Best Netherlands

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[ANN] histogram-fill, library for creating hitograms
by Khudyakov Alexey 20 Sep '09

20 Sep '09

Hello. I'm glad to announce library for filling histograms. Its purpose to provide generic and convenient API and be fast. Features list: * Allows to fill many histograms at once. I used it to fill about hundred. * It provide support for arbitrary binning algorithm. Currently there are integer bins, equal sized floating point bins and generic 2D bins. One however could implement more exotic variants. Bins with variable width for example. * Immutable histograms. It's relatively recent addition so it could have few rough edges. * Histogram serialization to/from human readable text. So data could be used with other tools. -- Khudyakov Alexey

2 2

Re: [Haskell-cafe] How to generate random string?
by Snouser 20 Sep '09

20 Sep '09

Snouser wrote: > > I need to generate a random string from 1 to 30. > > This is the parts I've done so far. > > unikString xs | let x = unsafePerformIO (randomRIO (1,30)) elem x xs = x : > unikString xs > | otherwise = unikString xs > > How do I proceed? > > I need the string/list to look like this: > > [1,9,3,6,2] et.c with only unik numbers. > > Thanks! > I wasnt added to the mailinglist, but now I'm. -- View this message in context: http://www.nabble.com/How-to-generate-random-string--tp25512293p25512298.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

2 1

Is it safe to use unsafePerformIO here?
by Cristiano Paris 20 Sep '09

20 Sep '09

Hi Cafè, I've the following problem: I have a (possibly very long) list of files on disk. Each file contains some metadata at the beginning and continues with a (possibly very large) chunk of data. Now, the program I'm writing can be run in two modes: either read a specific file from the disk and show the whole chunk of data on screen, or read all the files' metadata, sort the file list based on the metadata, and display a summary of those without reading the chunk of data from each file. I've factored out the file access machinery in a single module so as to use it indifferently under the two scenarios. At first, I wrote a piece of code which, in spirit, works like the following reduced case: ------ module Main where import System.IO import Control.Applicative import Data.List import Data.Ord import Debug.Trace data Bit = Bit { index :: Integer, body :: String } readBit fn = withFile fn ReadMode $ \h -> Bit <$> (hGetLine h >>= return . read) <*> readBody where readBody = withFile fn ReadMode $ \h -> do b <- hGetContents h seq b $ trace ("Read body from: " ++ fn) $ return b main = do bl <- mapM readBit ["file1.txt","file2.txt"] mapM_ (putStrLn . show . index) $ sortBy (comparing index) bl ---- which is very expressive as it's written in applicative style. Each file is like the following: ---- file1.txt ---- 1 foo ---- I've created a separate IO action for reading the body in the hope that it wouldn't get executed when the file list is sorted. But, to my surprise, it didn't work as the trace message gets written for each file before the summary is displayed. Thinking about this, I came to the conclusion that the IO Monad is enforcing proper IO ordering so that the IO action for file1's body must be executed right before IO action for file2's index one. If this is true, the only solution that came into my mind was to wrap the IO action for reading the body in an unsafePerformIO call. I actually ran the program with this modification and it works properly. So, as using unsafePerformIO is a Great Evil (TM), I'm wondering if there's a different way to do this which doesn't rely on retyping body as an IO action returning a String, which would break my pure code manipulating the files. My opinion is that using unsafePerformIO here is like ensuring the compiler that there're no observable side effects in running the IO action for reading the body and that no other side effects would impact this IO action. Thank you for any thoughts. Cristiano

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TFM09: Call for Participation (FMWeek, Eindhoven, November 2009)
by J.N. Oliveira 20 Sep '09

20 Sep '09

TFM2009 2nd Int. FME Conference on Teaching Formal Methods Friday, November 6th 2009, co-located with FM2009 : 16th Int. Symposium on Formal Methods Eindhoven, the Netherlands, November 2 - November 6, 2009 CALL FOR PARTICIPATION (URL: http://www.di.uminho.pt/tfm09) 1. About the conference ----------------------- Ten years after the First World Formal Methods Congress (FM'99) in Toulouse, formal methods communities from all over the world will once again have an opportunity to come together. As part of the First Formal Methods Week event surrounding the FM2009 conference in Eindhoven, Formal Methods Europe will be organizing TFM2009, the Second International Conference on Teaching Formal Methods. The conference will serve as a forum to explore the successes and failures of Formal Methods (FM) education, and to promote cooperative projects to further education and training in FMs. TFM2009 will include a panel discussion on the idea of building a 'Guide to the Formal Methods Body of Knowledge' (FMBoK), inspired by similar efforts for software engineering (SWEBoK) and for project management (PMBoK); such a resource would provide guidance to teachers, managers, and developers on what should be expected from a comprehensive, balanced programme of education in FMs. 2. Invited speaker ------------------ Jeff Kramer (Imperial College London, UK) 3. Accepted papers ------------------ * Teaching Concurrency: Theory in Practice (Luca Aceto, Anna Ingolfsdottir, Kim Guldstrand Larsen, Jiri Srba) * Integrated and Tool-Supported Teaching of Testing, Debugging, and Verification (Wolfgang Ahrendt, Richard Bubel, Reiner Haehnle) * What Top-Level Software Engineers Tackle after Learning Formal Methods - Experiences from the Top SE Project (Fuyuki Ishikawa, Kenji Taguchi, Nobukazu Yoshioka, Shinichi Honiden) * Teaching program specification and verification using JML and ESC/Java2 (Erik Poll) * Chief Chefs of Z to Alloy: Using A Kitchen Example to Teach Alloy with Z (Sureyya Tarkan, Vibha Sazawal) * Teaching Formal Methods based on Rewriting Logic and Maude (Peter Olveczky) * Which Mathematics for the Information Society? (Joao Ferreira, Alexandra Mendes, Roland Backhouse, Luis Barbosa) * How to explain mistakes (Stefan Hallerstede, Michael Leuschel) * On Teaching Formal Methods: Behavior Models and Code Analysis (Jan Kofron, Ondrej Sery, Pavel Parizek) * Teaching Formal Methods for the Unconquered Territory (Nestor Catano, Camilo Rueda) 4. Sponsors -------------- * Formal Methods Europe Association (FME) * Software Improvement Group (SIG) , Amsterdam, Netherlands 5. Programme Committee ---------------------- Izzat Alsmadi (North Dakota State University, USA) Dines Bjorner (IIMM Institute, Denmark) Eerke Boiten (University of Kent, UK) Raymond Boute (Universiteit Gent, Belgium) Andrew Butterfield (Trinity College, Dublin) Jim Davies (University of Oxford, UK) David Duce (Oxford Brookes University, UK) John Fitzgerald (University of Newcastle upon Tyne, UK) Jeremy Gibbons (University of Oxford, UK) Randolph Johnson (National Security Agency, USA) Michael Mac an Airchinnigh (Trinity College, Dublin) Dino Mandrioli (Politecnico di Milano, Italy) Jose Oliveira (Universidade do Minho, Portugal) Kees Pronk (Technische Universiteit Delft, NL) Bernhard Schaetz (Tecnical University of Munique, Germany) Wolfgang Schreiner (Johannes Kepler University Linz, Austria) Simao Melo de Sousa (Universidade da Beira Interior, Portugal) Kenji Taguchi (National Institute of Informatics, Japan) Jeannette Wing (Carnegie-Mellon University, USA)

1 0

Ambiguous type variable with subclass instance (also: is there a better way to do this?)
by Andy Gimblett 19 Sep '09

19 Sep '09

Hi all. This email is in literate Haskell; you should be able to load it into ghci and verify what I'm saying (nb: it won't compile without alteration: see below). I'm trying to do something which may anyway be stupid / not the best approach to what I'm trying to achieve; however, it's not working and I can't see why not. So I'm asking for help on two fronts: 1) Why is this failing? 2) Maybe more usefully, how should I actually be doing this? It seems an ugly approach; a voice in my head is saying "scrap your boilerplate", but I've no idea yet if that's actually applicable here; should I look at it? On with the show... I need these for "subclass" stuff later on... > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE OverlappingInstances #-} > {-# LANGUAGE UndecidableInstances #-} > module Ambig where I wish to define a number of algebraic data types with the ability to turn Int values into instances of those types. So I define a typeclass saying this is possible. I use Maybe so I can encode the existence of out-of-range Int values, which will vary from target type to target type. > class Target a where > convert :: Int -> Maybe a E.g. here's a type Foo which only wants values between 1 and 10: > data Foo = Foo Int deriving (Show) > instance Target Foo where > convert n | n `elem` [1..10] = Just $ Foo n > | otherwise = Nothing (That's a simple example; some are rather more complex. How to do this isn't what I'm asking about, really.) So we have, for example: *Ambig> (convert 1) :: Maybe Foo Just (Foo 1) *Ambig> (convert 11) :: Maybe Foo Nothing Now, some of those algebraic data type types happen to be enumerations; in this case, my idea is to list the constructors, with the rule that each constructor's position in the list is the Int which gets converted into that constructor. > class Enumerated a where > constructors :: [a] E.g. here's a type Bar with three constructors: > data Bar = X | Y | Z deriving (Show) > instance Enumerated Bar where > constructors = [X, Y, Z] (This is certainly ugly. Any suggestions?) Now we get to the crux. If a type is an instance of Enumerated, it should also be a Target, because we should be able to convert from Int just by list lookup. But we include a bounds check, naturally... > instance (Enumerated a) => Target a where > convert n | n `elem` [0..len-1] = Just $ constructors !! n > | otherwise = Nothing > where len = length constructors So I would _hope_ that then, e.g., we'd have: *Ambig> (convert 0) :: Maybe Bar Just X *Ambig> (convert 1) :: Maybe Bar Just Y *Ambig> (convert 3) :: Maybe Bar Nothing Sadly, this function doesn't compile, dying with an "Ambiguous type variable" error: Ambig.lhs:75:29: Ambiguous type variable `a' in the constraint: `Enumerated a' arising from a use of `constructors' at Ambig.lhs:74:29-40 Probable fix: add a type signature that fixes these type variable(s) If we replace "length constructors" with "3" (say), it compiles (but is useless). Adding a type signature doesn't help: it's "misplaced" in that context. If I break it out of the instance declaration so I can add one, I still get the same problem: > convert' :: (Enumerated a, Target a) => Int -> Maybe a > convert' n | n `elem` [0..len-1] = Just $ constructors !! n > | otherwise = Nothing > where len = length constructors I guess I see roughly what's going on; the question is "which constructors instance is meant?", right? In the "Just" part it's OK, because it can be inferred from the function's return type (right?). But in the guard we don't have that help, so it could be any Enumerated instance? Any advice appreciated! Particularly if this is just a dumb approach. For context, this is related to deserialisation of binary data (they'll actually be Word8's, not Int's) into a variety of data structures. Hmmm, maybe I should just be using Data.Binary... Many thanks, -Andy

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Why the stack overflow?
by staafmeister 19 Sep '09

19 Sep '09

Hi haskell-cafe, Why does rlist 100000 [] gives stack overflow in ghci? rlist 0 l = return l rlist n l = do {x <- randomRIO (1,maxBound::Int); let nl = x:l in nl `seq` rlist (n-1) nl} I first uses replicateM then foldM and finally an explicit function. But they give all stack overflow I don't know why 100000 is not absurd and it is tail recursive. Or is it not, due to the monad structure? greetings Gerben -- View this message in context: http://www.nabble.com/Why-the-stack-overflow--tp25520431p25520431.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Win32 API
by Andrew Coppin 19 Sep '09

19 Sep '09

Hi guys. Last time I looked at the Win32 bindings, it covered a few basic file I/O things (e.g., special access modes, file permissions, etc.), registry editing, and that was about it. Yesterday I took another look, and was pleasently surprised to find that GDI is now covered. Suffice it to say, yesterday I wrote my very first ever program using the Win32 API directly. (Who'd have thought Haskell would be the place to do that?) This was complicated by a small glitch: Graphics.Win32.Window exposes SendMessage() but does not expose PostMessage(). Kind of an important difference there. Fortunately, it's not actually especially hard to fix this deficiency. (Basically few the source code for the module, copy and paste the line for SendMessage(), and edit it to say PostMessage(). The type signature just happens to be identical.) Is there a reason why this is missing to start with? What other functions are missing? (I didn't see PostQuitMessage() anywhere...) Still, 85 lines of code to make a working, native-looking Windows program isn't too shabby, really... (Not that any sane person writes nontrivial programs directly like this of course.)

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Compiling Yi's Dependencies on Windows
by Jeff Wheeler 19 Sep '09

19 Sep '09

Hey all, Yi has a lot of dependencies that currently make it a pain to install on Windows, so I'm wondering which of those can be streamlined for easier install on Windows. So far, I haven't succeeded at getting them to work on my own machine, and I've already spent a lot of time on it. The first major impediment is Gtk2Hs. Duncan has mostly resolved this issue with a minimal build that works on 6.10.4 [1], but I think that this should at least be mentioned on the Gtk2Hs site if it is impossible for now to get cabal-install to work by itself. Then comes bindings for things like curl (this may only be needed for Darcs, I don't remember), zlib, and regex (POSIX). If I remember correctly, zlib, or some other library, is able to ship with its *.c/*.h files and compile them as necessary on Windows. Can this be done for regex-posix and curl, too? On my system now, I've got both GnuWin32 and MSYS (MinGW) installed in attempts to get regex headers that work with regex-posix, but, at the moment, I've run into a wall, in which I get this error when loading or linking to regex-posix (it compiles fine itself): ghc.exe: C:\...\cabal\regex-posix-0.94.1\ghc-6.10.4\HSregex-posix-0.94.1.o: unknown symbol `_regerror' Loading package regex-posix-0.94.1 ... linking ... ghc.exe: unable to load package `regex-posix-0.94.1' I doubt most of these changes are significant (like linking on the gtk2hs site to the 6.10.4 build), but changing them could make many apps easier to install on Windows, not just Yi. Are there any other dependencies that have trouble on Windows which I haven't run into yet? If they're library dependencies, can the headers be included in the Hackage tarball? Jeff Wheeler

1 0

big array allocation too slow?
by Paul L 19 Sep '09

19 Sep '09

I'm trying to use newArray to allocate something that has 100M unboxed doubles. It takes quite a few seconds to do so on GHC 6.10.2. But doing the same thing (and initialize all to 0) in C returns immediately. Setting RTS heap size doesn't help. Does anybody happen to know why? -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale

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code-build-test cycle
by Michael Mossey 18 Sep '09

18 Sep '09

I'm working on a GUI application in qtHaskell, and I have a bit of a bind. Using ghci, it launches quickly but runs slowly. On the other hand, compiling (mainly linking) takes a while---several minutes. The truth is that I can compile it much faster if I selectively import the needed modules, so figure the actual compilation/link time is more like 15 to 30 seconds. (This is Windows on a very old laptop.) I'm used to working in Python, so I'm used to a nearly instant code-build-test cycle, and GUI applications in PyQt run briskly, faster than ghci/qtHaskell. Now I'm wondering if Hugs is a faster interpreter. So during development I don't want to give up the quick cycle you get with an interpreter, but the application may be much too slow to use in any meaningful way without compilation. Any advice welcome. Maybe there is a way to speed up the interpretation. -Mike

4 3

memoization
by staafmeister 18 Sep '09

18 Sep '09

Hi, I participating in de google code jam this year and I want to try to use haskell. The following simple http://code.google.com/codejam/contest/dashboard?c=90101#s=p2 problem would have the beautiful haskell solution. import Data.MemoTrie import Data.Char import Data.Word import Text.Printf newtype ModP = ModP Integer deriving Eq p=10000 instance Show ModP where show (ModP x) = printf "%04d" x instance Num ModP where ModP x + ModP y = ModP ((x + y) `mod` p) fromInteger x = ModP (x `mod` p) ModP x * ModP y = ModP ((x * y) `mod` p) abs = undefined signum = undefined solve _ [] = 1::ModP solve [] _ = 0::ModP solve (hs:ts) t@(ht:tt) | hs==ht = solve ts tt + solve ts t | otherwise = solve ts t go (run, line) = "Case #"++show run++": "++show (solve line "welcome to code jam") main = interact $ unlines . map go . zip [1..] . tail . lines Which is unfortunately exponential. Now in earlier thread I argued for a compiler directive in the lines of {-# Memoize function -#}, but this is not possible (it seems to be trivial to implement though). Now I used memotrie which runs hopelessly out of memory. I looked at some other haskell solutions, which were all ugly and more clumsy compared to simple and concise C code. So it seems to me that haskell is very nice and beautiful until your are solving real algorithmic problems when you want to go back to some imperative language. How would experienced haskellers solve this problem? Thanks -- View this message in context: http://www.nabble.com/memoization-tp25306687p25306687.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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∀ lexing in ghc and ghci
by Sean McLaughlin 18 Sep '09

18 Sep '09

Hi, I'm getting different behavior in ghci and ghc with the identifier ∀. In ghc I need to wrap it with parens, as in > (∀) :: Var -> Base -> Formula -> Formula > (∀) = All In ghci, I get an error this way Formula.lhs:112:2: Invalid type signature In ghci I can do > ∀ :: Var -> Base -> Formula -> Formula > ∀ = All fine. But then ghc complains. What's going on here? Thanks! Sean

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Thank you guys
by Cristiano Paris 18 Sep '09

18 Sep '09

I wish to thank Cafè's people for their great support in understanding Haskell. Thank you all! Cristiano

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