- Haskell-Cafe - Haskell.org

Best Introduction To Monads For Newbies (& Especially Imparative Minds) I'v Ever Read!!!
by Kaveh Shahbazian 18 Oct '06

18 Oct '06

This is the best introduction to "Monads" for newbies and imparative mined that I'v ever read! For a long time I was searching for such an excellent text on monads. Ofcourse there are many good text there. But all of them are about mathematics and how clever the monads are. After all most of them are useless for a practical approach. Take a look at this http://sigfpe.blogspot.com/2006/08/you-could-have-invented-monads-and.html !! Have a good time with it!!

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6.4 -> 6.6, scoped type variables, prescribing poly-/mono-morphism
by Nicolas Frisby 18 Oct '06

18 Oct '06

I've been writing some code that relies heavily on the type system. Some of my polymorphic functions end up too polymorphic. I'm looking for some tips and references regarding prescribing poly-/mono-morphism. I know of three ways to do it: 1) rely on the monomorphism-restriction -- kind of scary in that its not explicitly obvious 2) introducing scoped-type variables of interest -- explicit and modular 3) introducing all type variables with a forall -- explicit but not modular The rest of this email is a discussion of a particular problem I'm having. Scoped-type variables worked in 6.4, but no longer in 6.6. I'm sure there's good reasons for this (I think I found a Peyton-Jones message regarding it in 6.4->6.5) I just didn't quite catch on to them. The problem is in the |bop| function: class SubType a v where -- the lack of fundeps is required by design inj :: a -> v lazyPrj :: v -> Maybe a bop :: forall x y z m c v . ( SubType x v, SubType y v, SubType z v , SubType (Fun m c v) v , Monad m, Comonad c ) => (x -> y -> z) -> Fun m c v bop op = Fun fn where fn = wrap fn' where fn' v0 = (inj . Fun) (wrap fn'') where fn'' v1 = inj (lazyPrj v0 `op` lazyPrj v1) wrap :: (v -> v) -> (c v -> m v) wrap = (return .) . (. counit) The explicit forall is needed to introduce the scoped type variables so that I can prescribe the type of |wrap|. After that, the monomorphism restriction on |wrap| takes care of the rest of my worries (that there is only one |m|, only one |c|, and only one |v|). If I leave the types to inference (i.e. drop the forall and the type ascription for |wrap|), then I end up with this type bop :: ( SubType x v2, SubType y v1, SubType z v1 , SubType (Fun m c v1) v2 , Monad m, Comonad c ) => (x -> y -> z) -> Fun m c v2 Note the multiple |v|s; they wreak havoc in the rest of my code. (The monomorphism restriction still takes care of the |m| and |c|.) I'd like to not have to introduce the forall, because with that comes introducing the entire context of the type. Thus I have to maintain any changes to the context, which I would prefer to leave to inference for modularity's sake. In 6.4 I was able to fix this without having to introduce the forall; I could introduce scoped type-variables in patterns to get the job done. That felt a bit like voodoo, especially when trying to get it right for 6.6--which I gave up on doing. I can post the code that works with 6.4 if anyone would like to see it (I just can't find it at the moment). I know there's some work on introducing partial signatures (fun pat | False = ...), but I don't see how to prescribe monomorphism for particular type variables in that way. Any thoughts on how to get the preferred signature using scoped-type variables in 6.6? I'd also welcome any references you might think are relevant. Thanks for your time, Nick

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Re: [Haskell-cafe] Newbie and working with IO Int and Int
by Víctor A. Rodríguez 18 Oct '06

18 Oct '06

Hi, > What's wrong with doing it this way? > > -- ** UNTESTED CODE ** > > verifyAdd :: Int -> Int -> Int -> Bool > verifyAdd a b sum | a + b == sum = True > otherwise = False > > testAddMundane :: Int -> Int -> Bool > testAddMundane a b = verifyAdd a b (a + b) > > -- all the IO-dependent stuff is below this line -- > > testAddRandom :: IO Bool > testAddRandom = do a <- randomIO > b <- randomIO > return verifyAdd a b (a + b) > First "thanks", but the problem is that I can't check if testAddRandom was sucessfull or not. If I replace (a+b) with (a+b-1) it still executes but couldn't check if it's True or False :-P -- Víctor A. Rodríguez (http://www.bit-man.com.ar) El bit Fantasma (Bit-Man) Perl Mongers Capital Federal (http://cafe.pm.org/) GNU/Linux User Group - FCEyN - UBA (http://glugcen.dc.uba.ar/)

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Newbie and working with IO Int and Int
by Víctor A. Rodríguez 18 Oct '06

18 Oct '06

Hi all, I'm really newbie to Haskell, and working on a program I'm trying to make some testing. I make some test on certain know values ( e.g. adding 10 to 15 must return 25) and some test on random values (eg. adding rnd1 to rnd2 must return rnd1+rnd2). The problem that makes me mad is the random number generation. I can obtain random numbers through module Random but all of them return IO Int values (all I need are Ints) instead of Int. I know that I can adjust my own functions to use IO Int instead of Int but the call to certain functions must contain Int parameters, because these ones can't be changed to accept IO Int (I read http://haskell.org/hawiki/ThatAnnoyingIoType and know that can convert from IO Int to Int :-P). How can I deal with this problem ?? Thanks in advance. -- Víctor A. Rodríguez (http://www.bit-man.com.ar) El bit Fantasma (Bit-Man) Perl Mongers Capital Federal (http://cafe.pm.org/) GNU/Linux User Group - FCEyN - UBA (http://glugcen.dc.uba.ar/)

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Error detection in GLR Happy grammar
by Iván Pérez Domínguez 17 Oct '06

17 Oct '06

Hi! I'm trying to detect parse errors in a happy GLR grammar, but I can't! I insert the special "token" 'error', and call the error function when an error is found. However, the program prints no error messages, and simply returns a ParseError at the end. Does any of you have an good example of a GLR parser with error detection and, if possible, error recovery as well?

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Extended functionality for record field accessors
by Henning Thielemann 16 Oct '06

16 Oct '06

Here is some proposal for Haskell 2 1/2 or so: {- | In Haskell 98 the name of a record field is automatically also the name of a function which gets the value of the according field. E.g. if we have @ data Pair a b = Pair {first :: a, second :: b} @ then @ first :: Pair a b -> a second :: Pair a b -> b @ However for setting or modifying a field value we need to use some syntactic sugar, which is often clumsy. @ modifyFirst :: (a -> a) -> (Pair a b -> Pair a b) modifyFirst f r@(Pair {first=a}) = r{first = f a} @ We propose to extend the meaning of the record field names to a function which allows setting, getting and modifying values easily. -} module RecordAccess where import Control.Monad.State (MonadState) import qualified Control.Monad.State as State {- | The access functions we propose, look very similar to those needed for List.mapAccumL (but parameter order is swapped) and State monad. They get the new value of the field and the record and return the old value of the field and the record with the updated field. -} type Accessor r a = a -> r -> (a, r) {- * Access helper functions, these are similar to State methods and should be in Prelude -} {- | Set the value of a field. -} set :: Accessor r a -> a -> r -> r set f x = snd . f x {- | Get the value of a field. -} get :: Accessor r a -> r -> a get f = fst . f undefined {- | Transform the value of a field by a function. -} modify :: Accessor r a -> (a -> a) -> (r -> r) modify f g rOld = let (a,rNew) = f (g a) rOld in rNew {- * Access helper functions in a State monad. -} setState :: MonadState r m => Accessor r a -> a -> m () setState f x = State.modify (set f x) getState :: MonadState r m => Accessor r a -> m a getState f = State.gets (get f) modifyState :: MonadState r m => Accessor r a -> (a -> a) -> m () modifyState f g = State.modify (modify f g) {- * Example accessors for the pair type -} {- | Access to the first value of a pair. -} first :: Accessor (a,b) a first xNew (xOld,y) = (xOld, (xNew,y)) {- | Access to the second value of a pair. -} second :: Accessor (a,b) b second yNew (x,yOld) = (yOld, (x,yNew)) {- * Example accessors for the pair type -} {- | Example of using 'set', 'get', 'modify'. -} example :: Int example = get second $ modify second succ $ set first 'a' $ ('b',7) exampleState :: State.State (Char,Int) Int exampleState = do setState first 'a' modifyState second succ getState second

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Debugging Newton's method for square roots
by Vraj Mohan 16 Oct '06

16 Oct '06

I am new to Haskell and need help in debugging my code. I wrote the following function for calculating square roots using Newton's method: my_sqrt :: Float -> Float my_sqrt x = improve 1 x where improve y x = if abs (y * y - x) < epsilon then y else improve ((y + (x/y))/ 2) x epsilon = 0.00001 This works for several examples that I tried out but goes into an infinite loop for my_sqrt 96. How do I go about debugging this code in GHC or Hugs? (The equivalent code is well-behaved on MIT Scheme)

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Re[4]: [Haskell-cafe] windows file locking
by Bulat Ziganshin 16 Oct '06

16 Oct '06

Hello Stefan, Wednesday, October 11, 2006, 4:53:19 PM, you wrote: > So far the windows API seems to work for me. Currently I'm still struggling to not > write garbage to the file but the shared access works now. Does anyone have an > example how to use it (e.g. the implementation of hPutStr on windows or something, > i did not find it in the ghc source code)? > I tried to use Streams before but without the windows API. sorry, i'm a bit to fool you. there is a way to apply Handle API to HANDLE. you should just 1) open file using win32 API and get HANDLE 2) convert this HANDLE to FD using _open_osfhandle from win32 api 3) open Handle over FD using fdToHandle function from GHC.Handle module alternatively, you can use Streams with the same mechanism: 1) open file using win32 API and get HANDLE 2) convert this HANDLE to FD using _open_osfhandle from win32 api // it's in C language: fd = _open_osfhandle (someHANDLE, _O_TEXT | _O_WRONLY); 3) use bufferBlockStream to make full-featured Stream from this FD: do stream <- bufferBlockStream fd it is from MS docs: SUMMARY There are multiple types of file handles that can be opened using the Win32 API and the C Run-time: Returned Type File Creation API API Set --------------------------------------------- HANDLE CreateFile() Win32 HFILE OpenFile()/_lcreat() Win32 int _creat()/_open() C Run-time FILE * fopen() C Run-time In general, these file I/O "families" are incompatible with each other. On some implementations of the Win32 application programming interfaces (APIs), the OpenFile()/_lcreat() family of file I/O APIs are implemented as "wrappers" around the CreateFile() family of file I/O APIs, meaning that OpenFile(), _lcreat(), and _lopen() end up calling CreateFile(), returning the handle returned by CreateFile(), and do not maintain any state information about the file themselves. However, this is an implementation detail only and is NOT a design feature. NOTE: You cannot count on this being true on other implementations of the Win32 APIs. Win32 file I/O APIs may be written using different methods on other platforms, so reliance on this implementation detail may cause your application to fail. The rule to follow is to use one family of file I/O APIs and stick with them--do not open a file with _lopen() and read from it with ReadFile(), for example. This kind of incorrect use of the file I/O APIs can easily be caught by the compiler, because the file types (HFILE and HANDLE respectively) are incompatible with each other and the compiler will warn you (at warning level /w3 or higher) when you have incorrectly passed one type of file handle to a file I/O API that is expecting another, such as passing an HFILE type to ReadFile(HANDLE, ...) in the above example. MORE INFORMATION Compatibility ------------- The OpenFile() family of file I/O functions is provided only for compatibility with earlier versions of Windows. New Win32-based applications should use the CreateFile() family of file I/O APIs, which provide added functionality that the earlier file I/O APIs do not provide. Each of the two families of C Run-time file I/O APIs are incompatible with any of the other file I/O families. It is incorrect to open a file handle with one of the C Run-time file I/O APIs and operate on that file handle with any other family of file I/O APIs, nor can a C Run-time file I/O family operate on file handles opened by any other file I/O family. _get_osfhandle() ---------------- For the C Run-time unbuffered I/O family of APIs [_open(), and so forth], it is possible to extract the operating system handle that is associated with that C run-time handle via the _get_osfhandle() C Run-time API. The operating system handle is the handle stored in a C Run-time internal structure associated with that C Run-time file handle. This operating system handle is the handle that is returned from an operating system call made by the C Run-time to open a file [CreateFile() in this case] when you call one of the C Run-time unbuffered I/O APIs [_open(), _creat(), _sopen(), and so forth]. The _get_osfhandle() C Run-time call is provided for informational purposes only. Problems may occur if you read or write to the file using the operating system handle returned from _get_osfhandle(); for these reasons we recommend that you do not use the returned handle to read or write to the file. _open_osfhandle() ----------------- It is also possible to construct a C Run-time unbuffered file I/O handle from an operating system handle [a CreateFile() handle] with the _open_osfhandle() C Run-time API. In this case, the C Run-time uses the existing operating system handle that you pass in rather than opening the file itself. It is possible to use the original operating system handle to read or write to the file, but it is very important that you use only the original handle or the returned C Run-time handle to access the file, but not both, because the C Run-time maintains state information that will not be updated if you use the operating system handle to read or write to the file. -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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searching haskell.org
by Tamas K Papp 15 Oct '06

15 Oct '06

Hi, I noticed that searching on Haskell.org (using the Search feature at the bottom) doesn't work as I expected. For example, searching for "memoise" produces no results. http://www.google.com/search?q=memoise+site%3Ahaskell.org produces 18 hits. Is this intentional? Thanks, Tamas

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Haskellers in London (UK)?
by Ben Moseley 15 Oct '06

15 Oct '06

Hi, Just thought I'd send out a quick mail to see if there are any other Haskellers based in London who might be interested in getting together occasionally. Anyway, if you're interested please reply to this. Cheers, --Ben

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[off-topic / administrative] List Reply-to
by Matthias Fischmann 15 Oct '06

15 Oct '06

In my last post, I hit the wrong button again, with the effect of some noise in innocent mailboxes (sorry!). Some lists have the Reply-To: set to the list address. I think you can even configure the From: to be haskell-cafe instead of the poster, making the poster merely identifiable by the Sender: field. Do you have strong opinions on this subject? I would opt for setting Reply-To:; I think most of the follow-ups here are of some public interest, or easily filterable if not. cheers, matthias

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function result caching
by Silviu Gheorghe 15 Oct '06

15 Oct '06

hello,I'm new to Haskell and I'm using it to do some simulations for a game, and I have a some functions that have as argument just one int (the current situation), but they do a lot of computations after that (future evolutions etc) I'd like to know if the results are "cached" by the compiler (there are only a few thousand values i call the functions on, but they are distributed on a fairly large interval (0-1000000), because of the codification. if they are not I'd like to know what is the best way to cache them manually, and where can I read more about this, and the optimizations the compiler does, because I've searched the web before and i found very little on this topic. thank you very much Silviu P.S. sorry for my English :)

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What's going on in our courses?
by Walt Potter 15 Oct '06

15 Oct '06

All, Last Spring my Functional Programming class implemented a Genetic Algorithm with Neural Networks that learned to play Nim. The students had a really good time--they also learned lots about Functional Programming with Haskell. Part of the final exam was a tournament. This Fall in AI we'll be doing GA's again and Genetic Programming. Is there a list or forum that talks about teaching our students about Haskell? Thanks, Walt

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Howto Haskell in PocketPC?
by Iván Pérez Domínguez 15 Oct '06

15 Oct '06

Hi. Here a simple question: Is there any haskell compiler/interpreter or similar for PocketPC?

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Fwd: [GHC] #939: Possible bug building wxhaskell
by Jason Dagit 14 Oct '06

14 Oct '06

I reported the following bug and received the response below. Any wxhaskell hackers up to the challenge? I'd really love to see the makefiles go away and see wxhaskell just use cabal but I don't know if that's possible given the wxhaskell build requirements. Any help is much appreciated. Thanks Jason ---------- Forwarded message ---------- From: GHC <trac(a)galois.com> Date: Oct 13, 2006 5:21 PM Subject: Re: [GHC] #939: Possible bug building wxhaskell To: dagit(a)codersbase.com #939: Possible bug building wxhaskell -------------------------------------+-------------------------------------- Reporter: dagit(a)codersbase.com | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 6.6 Severity: normal | Resolution: invalid Keywords: | Difficulty: Unknown Architecture: x86 | Os: Unknown -------------------------------------+-------------------------------------- Changes (by duncan): * resolution: => invalid * status: new => closed Comment: It's not a GHC bug. It's a problem with the way wxHaskell is built. Admitidedly this is due to a slight change in the way GHC manages packages but it's not a bug as such, wxHaskell just needs fixing. What needs to happen is that the packages must be registered locally before they get used in other packages. Also, instead of importing the modules from the other package directly from the directory with -i foo/, it must be via -package foo. So you'll need to get in contact with someone who knows enough about wxHaskell's build system to fix it. Try asking on the ghc-users or haskell-cafe mailing lists. ------------------------ The bug report: I appologize if this isn't a GHC bug but since it started happening after a GHC upgrade I suspect it's a GHC bug. After upgrading to ghc6.6 on windows (using the release binary) I tried to install the current wxhaskell release found here: http://wxhaskell.sourceforge.net/download.html (version wxhaskell-src-0.9.4-1.zip) I have used wxhaskell on this machine before with older version of ghc (including ghc6.5, specifically visual haskell) so my environment for building the wxhaskell source was already setup. When I first got the error below I thought maybe my version of wxhaskell was messed up so I redownloaded the above zip file and unzipped a fresh copy. I unziped the source then I typed: configure && make Things went along fairly smoothly for a while then suddenly I got this error message: ghc -c wx/src/Graphics/UI/WX/Types.hs -o out/wx/imports/Graphics/UI/WX/Types.o -iout/wx/imports -odir out/wx/imports -hidir out/wx/imports -fvia-C -package-name wx -iout/wx/imports wx/src/Graphics/UI/WX/Types.hs:94:0: Bad interface file: out/wx/imports/Graphics/UI/WXCore/Types.hi Something is amiss; requested module wx:Graphics.UI.WXCore.Types differs from name found in the interface file wxcore:Graphics.UI.WXCore.Types make: *** [out/wx/imports/Graphics/UI/WX/Types.o] Error 1

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Re: [Haskell-cafe] Gtk2Hs and Glade Tutorial
by Duncan Coutts 14 Oct '06

14 Oct '06

On Fri, 2006-10-13 at 15:18 +0200, Hans van Thiel wrote: > Hello Duncan, > > Thanks for your reply, and here's the pdf. Thanks. > One of the things to be improved, which I note myself, is something on > how to do it on the Windows platform. It's much the same of course, but yes that would be nice. > (This is assuming there is a Glade for Windows.) There is. It comes with the dev flavour of the Gtk+ installer. > Another thing is the terminology, for example about handles, actions and > functions. Glade uses 'signals' and 'callback functions' and you can > specify them visually. I believe (I'll have to check this) you can look > up the names in a file 'callbacks.c'. Of course you have to substitute > those for the Gtk2Hs equivalents, but it might make it easier to find > the appropriate ones in the Gtk2Hs API. We do have slightly different terminology in a few places and usually for good reason. For example the idea of a callback function makes a lot of sense in C where you really do have to have a named function that gets called in response to a signal. I Haskell we don't need a named function of course we can just say: onClicked button $ do ... ... > I'll check out the link you provided in the next few days, and I'm > looking forward to your comments. I'll also have to wait and see what > Eddy Ahmed, the author of the Glade tutorial, thinks about this idea. > It would be nice to reuse that, and to put the Haskell version on his > more general web site as well. Why re-invent the wheel.. I'll send you some more comments when I've had a full read through. Perhaps we should use the haskell.org wiki for this. Duncan

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Porting content from the old wiki
by dons＠cse.unsw.edu.au 13 Oct '06

13 Oct '06

The main thing holding up porting of content from the old wiki is licensing. In order to help this, could people who've written for the old wiki, and are happy to have that work moved to the new wiki and relicensed, add their names to the list here: http://haskell.org/haskellwiki/HaWiki_migration Or email me and I'll add the name. Then anyone can move your work over, which should really speed things up. Thanks, Don

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quick trivial question on records
by Imam Tashdid ul Alam 13 Oct '06

13 Oct '06

I was reading Daan's paper on scoped labels and it struck me... has anyone tried a variation of the language where records are by dafault strict? and unboxed when appropriate? extended question: doesn't it makes more sense? laziness and partial applications go very well together, and the ADT constructors are often partially applied... __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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A type in search of a name...
by Brian Hulley 13 Oct '06

13 Oct '06

Hi, You'll never believe it but I've been struggling last night and all of today to try and think up a name for the following type and I'm still nowhere near a solution: data ??? = VarId | VarSym | ConId | ConSym this is part of a type to describe Haskell lexemes: data Token = TName !??? !ByteString.T | ... Here are some of my attempts to fill in ??? and my reasons for rejecting them: 1) VarConIdSym - the problem is that it's too long and the first letter is 'v' which could be confused with the letter 'V' in "VarId" or "VarSym" 2) Name - the problem is that this suggests the string itself rather than the VarId/VarSym/ConId/ConSym'ness of the token 3) NameType - I'm trying to avoid using the words "Type" "Kind" etc because I'll probably want to use these later for other things and in any case NameType suggests the type of a lexical name ie Token itself 4) Space - this can be confused with something to represent whitespace or the namespaces introduced by modules 5) Just using data Token = TVarId !BS.T | TVarSym !BS.T | ... -- explodes into too many different cases when you add tokens for qualified names and all their variations (since I'm also representing incomplete tokens like "Foo." and "Foo.where" (as a prefix of "Foo.whereas" etc since it can't stand by itself because "where" is a reserved word)) 6) Using Bool as in data Token = TName !Bool !Bool !BS.T -- problem is that then I won't get any help from the typechecker if I accidentally confuse the order of Bools when matching etc. I could use the record syntax but then code can become very clunky to look at and it would still allow the Bools to get confused when they are passed about Any ideas? I must say that trying to think up names for things is absolutely the most difficult task in programming imho. The problem is that once fixed down, a name gets used all over the place and becomes totally entrenched in the code and affects how one thinks about the code thereby influencing its whole future development in a very deep and pervasive way (think suffixes/ prefixes/ relationships with other concepts etc). I would also be satisfied with just good names for the following types: data ???1 = Var | Con data ???2 = Id | Sym data ???3 = Op | Plain (I just combined ???1 and ???2 in ??? to try and save some space in the representation but ideally they would be separate types if only there were a way to tell the compiler to represent each by 1 bit in a word at the machine level like C's struct {VarCon vc : 1; IdSym is : 1;}) Any suggestions welcome, Thanks, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

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nightnightita
by George Brewster 13 Oct '06

13 Oct '06

hope you're feeing better too.

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Nim Game in Haskell - Someone plz?
by Luis Felipe 13 Oct '06

13 Oct '06

Hi, I need help to develop an implementation of nim game in Haskell. Could anyone send me a implementation of this game in haskell?? thanks -- Luis

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Having typing problem with "tan" function.
by Edward Ing 13 Oct '06

13 Oct '06

Hi, I am new to Haskell, so the following problem is probably easy for you to spot but difficult for me. Any pointers would be appreciated. In GHCI on a load I am getting the following message: SSnowflake.hs:41:45: No instance for (Floating Int) arising from use of `pi' at Snowflake.hs:41:45-46 Probable fix: add an instance declaration for (Floating Int) In the first argument of `(/)', namely `pi' In the first argument of `tan', namely `(pi / 3)' In the second argument of `(*)', namely `(tan (pi / 3))' Failed, modules loaded: none. When I put the expression "tan ( pi / 3)" at the GHCI interpreter prompt I get a value, but this is not accepted on a load the code. Here is the code: >>>> code triangle :: Window -> (Point,Point) -> Int -> IO () triangle window ((x1,y1), (x2,y2)) size = let height = fromIntegral(y2 - y1) * 3 / 2 halfWidth = height * (tan (pi / 3 )) in drawInWindow window (withColor (sizeColorMap size) (polygon [(x1,y1),( x1 - halfWidth, height ), (x1 + halfWidth, height)] ))

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Is Haskell a Keynesian language?
by Henning Thielemann 13 Oct '06

13 Oct '06

Here is another approach of questionable classification of languages. :-) A lazy functional program is demand driven, an imperative program is supply driven. That is, if I request some information by calling a function in GHCi or Hugs, the interpreter develops a plan a how to produce the information I need and then executes the necessary steps. In contrast to that, an imperative program executes what's next on the schedule, whether it is need or not. So is Haskell a Keynesian language and C++ a Say language?

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tail-recursing through an associative list
by Seth Gordon 13 Oct '06

13 Oct '06

In my first posting, I mentioned that I was going to try to translate some of our code to Haskell and see how it worked. Well, I don't have a stunning demonstration of the power of pure functional programming, but I do have an interesting problem. I chose to port a program that we use in our build system that takes a table of geographic data and groups the rows in the table according to [REDACTED]. The description of what the existing program does takes up only a few paragraphs, but the source code is eight pages of dense C++ that has obviously been optimized up the wazoo (and beyond the point where a mortal like myself can understand what's going on). On one of our servers, it can process 200,000 rows in about three minutes. My Haskell translation is three and a half pages of gorgeous lucid almost-entirely-functional code ... that in its first draft, took about three seconds to process 2,000 rows, eight minutes to process 20,000 rows, and overflowed a 1-MB stack when processing 200,000 rows. Oops. After banging together a version that took input from stdin instead of the database (it was easier to do that then to learn enough about Cabal to get HSQL recompiled with profiling), I profiled the code and observed that the most-called-upon function in the program (200 million entries for those 20,000 rows) was structured like this: type AList = [(Key, [MetaThingies])] myFunction :: AList -> Thingie -> AList myFunction [] x = [(key x, [meta x])] myFunction ((k, v):tail) x | matchKeys k (key x) = case tryJoining v x of Nothing -> (k, v) : (myFunction tail x) Just v' -> (k', v') : tail where v' = bestKey k (key x) | otherwise = (k, v) : (myFunction tail x) I'm wondering if the slowness of the program can be attributed to that case statement in the middle--perhaps unwrapping the Maybe type returned by tryJoining is preventing the function from being properly tail-recursive, or something. (I tried making the list construction strict by replacing "(k, v) : (myFunction tail x)" et al. with "(:) (k, v) $! (myFunction tail x)", and it actually slowed the program down, so either I'm not understanding how to improve things with strictness or laziness isn't the problem here.) Any advice from the gurus?

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Gtk2Hs and Glade Tutorial
by Hans van Thiel 13 Oct '06

13 Oct '06

Hello, There is a very nice tutorial on the Glade Interface Designer at http://www.writelinux.com/glade and I've been using it with 'An Introduction to Gtk2Hs, A Haskell GUI Library', by Kenneth Hoste at http://haskell.org/~shae/memory.pdf to learn Gtk2Hs and Glade. The 'Gtk2Hs Introduction', however, is not really aimed at the absolute beginner and so I've written down my experience as an 'appendix' to the glade tutorial, which really is for getting started. I've just asked the author, Eddy Ahmed, for permission to 'translate' his tutorial to Haskell Gtk2Hs, but meanwhile I'd like to submit my separate concept (attached) for comments. Thanks, Hans van Thiel

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do ghci
by Greg Fitzgerald 12 Oct '06

12 Oct '06

Just curious, why does ghci run in the context of a 'do'? This tripped me up when I first started learning Haskell. It's fine once you know what's going on, but why the restriction? Why can't I write the code below without 'let' and ':module'? > two = 1 + 1 > import Data.List > cols = transpose [[1,2,3], [4,5,6]] Thanks, Greg

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Quantifying Partial Type Annotations
by Philippa Cowderoy 12 Oct '06

12 Oct '06

I've done a bit more thinking about partial type annotations (as proposed on the Haskell' list), and I have a somewhat more concrete proposals for some of the extensions to them that perhaps also makes more sense of the original basic idea as well. I'm sending it to the Cafe this time as it's a bit early to consider this for standardisation. I'd like to propose a new quantifier for type variables, which for now I'll call unknown[1] - correspondingly I'll talk about "unknown-quantified variables" and probably "unknown variables" where it's not ambiguous. Unknown quantifiers will never be introduced by the typechecker without a corresponding annotation - only propagated inwards. Whereas universal type variables must not accumulate additional constraints during typechecking (and in a traditional Hindley-Milner implementation only become universally quantified during a generalisation step), unknown type variables can - indeed this is their raison d'etre. Furthermore, they are never propagated 'outwards' - either the variable is constrained sufficiently that it can be replaced with a monotype or, having otherwise finished typechecking the corresponding term, the unknown quantifier is replaced with a forall. For example: add' :: unknown x. x -> x -> x add' = (+) add'' :: unknown x. x -> x -> x add'' x y = (x::Int) + (y::Int) will typecheck, resulting in these types when the identifiers are used: add' :: forall x. Num x => x -> x -> x add'' :: Int -> Int -> Int It's probably also sensible to allow "wildcard" variables written _, generated fresh and implicitly unknown-quantified much as universal variables are now. Type synonyms seem to present an interesting question though - it seems to me most sensible to hang on to the quantification at top-level and generalise it only as we finish type-checking a module, rather than copying out the quantifier anew each time. Any comments? The amount of thinking I've done about interactions with rank-n variables is limited - I guess we'd need to prohibit unifying with type variables that're of smaller scope than the unknown variable? I'm don't think I can see any other worrying issues there. Unknown variables in method declarations seem... meaningless to me, they're not proper existentials and I don't think there's a sane meaning for them that isn't a kludge for associated types instead. I don't think they belong in instances for similar reasons. Incidentally, I think there's also a cute use case in .hs-boot files, where an unknown-quantified variable could be used to tie some knots in a manner similar to the way recursive bindings are checked now. If so, I have an interesting use for this. [1] Other names that have occurred to me are "solve" (as in "solve for x"), and "meta" (by analogy to metavariables in the typechecker - because by the time we've finished checking the annotated term we'll have removed the quantifier), but unknown seems by far the strongest to me. No doubt someone'll suggest a far better name shortly after I post this! -- flippa(a)flippac.org There is no magic bullet. There are, however, plenty of bullets that magically home in on feet when not used in exactly the right circumstances.

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Yet Another Haskell Tutorial Question
by Bill Mill 12 Oct '06

12 Oct '06

I've been working through the "Yet Anther Haskell Tutorial" at http://www.cs.utah.edu/~hal/docs/daume02yaht.pdf , and I've gotten up to Excercise 4.6. It says: === YAHT === Write a datatype Tuple which can hold one, two, three or four elements, depending on the constructor (that is, there should be four constructors, one for each number of arguments). Also provide functions tuple1 through tuple4 which take a tuple and return Just the value in that position, or Nothing if the number is invalid (i.e., you ask for the tuple4 on a tuple holding only two elements). === /YAHT === I've got a brute-force answer, but I'm sure that there's a better way, and I can't find it. The code I've currently got is at http://paste.lisp.org/display/27806 . Anybody want to point me in the right direction? Thanks Bill Mill bill.mill at gmail.com

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ghc 6.2.2 seems to ignore -package when there's a -prof option
by Seth Gordon 12 Oct '06

12 Oct '06

I have a program using HSQL that I'm trying to profile. When I do ghc program.hs -package hsql -o program it compiles fine, but when I do ghc -prof -auto-all program.hs -package hsql -o program I get error messages saying "failed to load interface for `Database.HSQL'" and so forth, the same way I do if I leave off the -package argument entirely. How can I work around this?

2 1

Trying to understand HList / hMapOut
by Matthias Fischmann 11 Oct '06

11 Oct '06

Hello, I am using a heterogenous list as in [1] all elements of which are of a given class C. I am awed by the beauty of the code (HList, not mine :-). Here is what I am trying doing: import HListPrelude data T = T Int class C a where foo :: a -> Int instance C T where foo (T i) = i class HList l => CList l instance CList HNil instance (C c, CList cs) => CList (HCons c cs) Since foo maps all class members to Int, hMapOut should be a straight-forward way to produce homogenous Int lists from heterogenous CLists: test :: (CList l) => l -> [Int] test = hMapOut foo That would be too easy, though... ghci (6.4) sez: /home/fis/tmp/Main.hs:16:7: Could not deduce (HMapOut (a -> Int) l Int) from the context (CList l) arising from use of `hMapOut' at /home/fis/tmp/Main.hs:16:7-13 Probable fix: add (HMapOut (a -> Int) l Int) to the type signature(s) for `test' or add an instance declaration for (HMapOut (a -> Int) l Int) In the definition of `test': test = hMapOut foo hugs (version 20050308) sez: ERROR "/home/fis/esim/HList/FakePrelude.hs":113 - Overlapping instances for class "Show" *** This instance : Show (HSucc a) *** Overlaps with : Show (HSucc HZero) *** Common instance : Show (HSucc HZero) (oops? should i upgrade, or is there a switch that i missed?) I am trying to understand the ghci error message, which looks like I might have missed a point. It seems like I should make it more obvious to ghc that CLists actually consist of values of type C. Is that a good guess? But how do I actually do it? I have unsuccessfully to extend the context to (HMapOut (a -> Int) l Int, CList l) or even to (C a, HMapOut (a -> Int) l Int, CList l) and I can't see any reason why the instance declarations of HMap in HListPrelude shouldn't cover my code. I am feeling a little stuck again... thanks, Matthias [1] http://homepages.cwi.nl/~ralf/HList/

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windows file locking
by Stefan Aeschbacher 11 Oct '06

11 Oct '06

Hi I need to open a file and keep it open for writing (a log file). Another process has to read from this file. On windows the second process (e.g. tail -f) can not open the file. How can I open a file without this locking? regards Stefan

4 4

Vertical tabs in source code and other obscure chars
by Brian Hulley 11 Oct '06

11 Oct '06

Hi, In the Haskell98 report at http://haskell.org/onlinereport/lexemes.html section 2.2 has the rule: whitechar -> newline | vertab | space | tab | uniWhite Does anyone know what a vertical tab is supposed to do? Is there any reason to allow them as whitespace? (Does anyone in the universe actually use them?) Thanks, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

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Re: Haskell-Cafe Digest, Vol 38, Issue 22
by Kirsten Chevalier 11 Oct '06

11 Oct '06

On 10/11/06, haskell-cafe-request(a)haskell.org <haskell-cafe-request(a)haskell.org> wrote: > Meanwhile, if you want to use External Core, but can't because it > doesn't work properly for you, don't be afraid to yell. (E.g. File a > Trac bug report.) I don't want to promise an immediate fix, but the > more people that use it the keener we are to get it done. > I did file a bug on this nine months ago: http://hackage.haskell.org/trac/ghc/ticket/670 but haven't had a chance to work on it. Cheers, Kirsten -- Kirsten Chevalier * chevalier(a)alum.wellesley.edu * Often in error, never in doubt "That's the thing about people who think they hate computers. What they really hate is lousy programmers."-- Larry Niven and Jerry Pournelle

1 0

GHC Core still supported?
by Jim Apple 11 Oct '06

11 Oct '06

In http://www.haskell.org/ghc/dist/current/docs/users_guide/ext-core.html , I see two notes that I can't verify: 1. I don't see any CORE pragma on http://www.haskell.org/ghc/dist/current/docs/users_guide/pragmas.html 2. Using GHC 6.5.20060920, I compile module Core where data Foo = Bar with -fext-core to get %module main:Core %data main:Core.Foo = {Bar}; I then compile the resulting hcr file with no flags to get <no location info>: 1: Parse error : %data main:Core.Foo = {Bar}; Jim

3 2

Multiple stages in Template Haskell
by Lyle Kopnicky 11 Oct '06

11 Oct '06

Hi folks, I noticed that in Template Haskell, you can only have one level of splicing or quasi-quoting. E.g., you can't write: $(zipN ($(sel 2 3) ("zip level",3,()))) ['a'..'Z'] [1..] (words "now is the time") Because you can't have a splice inside a splice. But wouldn't it be handy to use all these macros when defining other macros? This would be like stages in MetaML, which have no such limitation. But all the stages but the last one would be evaluated at compile time. The compiler would look for the deepest level of nested splices and evaluate it first. On the other hand, I can understand why you wouldn't include splices and quasi-quotes in the Language.Haskell.TH.Syntax - programmers could write functions that dynamically created arbitrarily (even infinitely) nested splices - so it would be undecidable where to start. I'm wondering if there's some reason this restriction is imposed. Is it particularly difficult to implement, or are there more theoretical problems? Does that mean I'm volunteering? ;) - Lyle

2 1

a monad for secret information
by Seth Gordon 11 Oct '06

11 Oct '06

I finally (think I) understand monads well enough to make one up: > module Secret (Secret, classify, declassify) > where > > data Secret a = Secret a > > classify :: a -> Secret a > classify x = Secret x > > declassify :: Secret a -> String -> Maybe a > declassify (Secret x) "xyzzy" = Just x > declassify (Secret x) _ = Nothing > > instance Monad Secret where > return = classify > (Secret x) >>= f = f x The nice thing about this is that (correct me if I'm wrong) clients of the module can't sneak information out of the monad without either using the right password or by using one of the unsafe*IO methods. (Or by running the program in a debugger. But you get the idea.) The not-so-nice thing is that the literal text of the password is baked into the data definition. I'd like to have a more general version of Secret that allows someone to pass the password in when constructing a secret, and preserves that password when "return" is used, but doesn't let the second argument of (>>=) see the password. Something like this:... > data Classification pw a = Classification pw a > declassify (Classification pw a) pw' = case pw' of > pw -> Just a > _ -> Nothing > > type Secret = Classification "xyzzy" ...but that doesn't parse. Is it even possible to have a type like this that still observes the monad rules? Is this the sort of thing that I need to understand arrows to pull off?

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Haskell performance (again)!
by Yang 11 Oct '06

11 Oct '06

This email actually turned out much longer than I expected, but I hope it sparks interesting (and hopefully, thorough!) discussion on points that weren't touched on by previous threads on this topic. What follows describes my journey thus far exploring what I see (as a newcomer to Haskell) as a major problem with Haskell - the ease with which users can write inefficient code, and have no idea that they did. Some of this has to do with laziness, some of it with the functional programming in general. My goal is to achieve (as a link further down says) *good* performance, not blazing performance. I.e., I want to avoid what should really be no-brainers such as little-omega(1) space utilization for simple folds. The first version of a simple program I wrote was reasonably "clean." (Throughout this email, "clean" is supposed to mean some combination of: clear, compact, modular, elegant, etc.) It's a polynomial adder, which takes in lists of (coefficient, degree) tuples, combining terms of the same degree and maintaining a sorted order of least-to-greatest degree: type Poly = [(Int,Int)] addPoly1 :: Poly -> Poly -> Poly addPoly1 p1@(p1h@(p1c,p1d):p1t) p2@(p2h@(p2c,p2d):p2t) | p1d == p2d = (p1c + p2c, p1d) : addPoly1 p1t p2t | p1d < p2d = p1h : addPoly1 p1t p2 | p1d > p2d = p2h : addPoly1 p1 p2t addPoly1 p1 [] = p1 addPoly1 [] p2 = p2 addPoly1 [] [] = [] But this doesn't use tail recursion/accumulation (which seems to me like a complete hack that is a mandatory price to pay for using FP), so I rewrote it: addPoly :: Poly -> Poly -> Poly addPoly p1 p2 = let addPoly' p1@(p1h@(p1c,p1d):p1t) p2@(p2h@(p2c,p2d):p2t) result | p1d == p2d = addPoly' p1t p2t ((p1c + p2c, p1d):result) | p1d > p2d = addPoly' p1 p2t (p2h:result) | p1d < p2d = addPoly' p1t p2 (p1h:result) addPoly' (p1:p1s) [] result = addPoly' p1s [] (p1:result) addPoly' [] (p2:p2s) result = addPoly' [] p2s (p2:result) addPoly' [] [] result = reverse result in addPoly' p1 p2 [] But laziness will cause this to occupy Theta(n)-space of cons-ing thunks. (See appendix for a simpler example.) My third iteration will become even uglier because I will have to incorporate strictness and things like $! or seq. And there's probably a whole host of other issues that I haven't even thought of or don't know about (boxing, space leaks, etc.). >From #haskell, I got a general piece of advice: Oct 07 22:37:20 <Cale> Actually, a lot of the time, code gets messy when people optimise because they take the wrong road to optimisation Oct 07 22:37:31 <Cale> There are two ways to optimise a piece of Haskell code Oct 07 22:37:43 <Cale> You can make it stricter, forcing evaluation to occur sooner Oct 07 22:38:01 <Cale> Or you can make it lazier, ensuring that things are not demanded until actually needed Oct 07 22:38:09 <Korollary> @wiki performance Oct 07 22:38:09 <lambdabot> http://www.haskell.org/haskellwiki/performance Oct 07 22:38:13 <Cale> the latter route actually tends to result in cleaner code Of course, to want to learn route 2 was a no-brainer. I read through that wiki page on laziness and various other resources, but was disappointed in what I found: http://www.haskell.org/haskellwiki/Performance/Laziness only discusses the aspect of laziness where you only evaluate what you need, which by now has been repeatedly beaten into my head and sounds obvious. Aren't there other advantages to laziness? Or at least, am I not fully appreciating the "obvious" work-avoiding part of laziness? For instance, the example seems to allude to sharing (between the even and odd functions), which I think ties in strongly with laziness. I was hoping for more in-depth insights on how to take advantage of laziness to write cleaner AND more efficient code. (A loose analogy exists in computer architecture - the smaller the chip, the less power it can consume/the faster it can clock.) http://en.wikibooks.org/wiki/Haskell/Laziness_revisited does slightly better but still just scratches the surface - it gives an example of a clean (compact and modular) yet efficient piece of code (isSubstringOf), then completely neglects explaining it (e.g., exactly why/how does it run more efficiently?). http://users.aber.ac.uk/afc/stricthaskell.html seems to suggest that strictness is the way to go. Please say it ain't so! http://www.algorithm.com.au/mt/haskell/haskells_performance.html says that between code optimized for performance in Haskell and in another language (like O'Caml), "which is more declarative, high-level, and most importantly, which one doesn't require an expert in the language to write" is an unfortunate (for Haskell fans) no-brainer: "The problem then is you have to know why something is slow, and there's where Haskell can get complex. Is there a space leak? (Do you even know what a space leak is?) Is your array strict instead of lazy? If it's strict, is it boxed instead of unboxed? (Do you even know what unboxing is?) Can you use unsafeRead instead of readArray? (Did you know that unsafeRead exists? I sure didn't!)" So, how do I take advantage of laziness to write clean *and* performant code? I've been seeking resources with concrete examples, explanations, or guides on how to "systematically" do this. Is this even possible? Or am I deluding myself, and strictness is the way to go? If so, has there been any work/research to address the issues pointed out in the last blog entry? And do most (experienced) Haskell users sacrifice cleanliness for speed, or speed for cleanliness? I feel that an understanding of how Haskell compilers work would help immensely in writing more performant Haskell code (or just better Haskell code in general), as well as understand what optimizations are reasonable to expect from the compiler. (I have a university class of background on the design/implementation of simple lexers/parsers/compilers/interpreters/code block graph analyses for imperative OO languages. However, i'd love to learn more about these things for a language like Haskell, including details on lazy evaluation, pattern-matching, overloading (why it's slow), currying, PFA, type classes, monads, etc. Preferrably something "lighter weight" than a bunch of research papers. Any pointers?) BTW, please also feel free to give me feedback/refinements on my code snippets. :) Thanks a ton for any enlightenment! Yang Aside: http://www.matthewmorgan.net/blog/archives/2004/10/07/haskell-performance. This post brings up a most reasonable point (venturing off onto tangent now) - space leaks are a much more serious concern than performance, and they're a difficult problem even for experienced Haskell users (http://www.haskell.org/pipermail/haskell-cafe/2003-December/005587.html)

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Profiling CAFs (re-post)
by Matthias Fischmann 11 Oct '06

11 Oct '06

Hi again, I posted a bunch of questions on profiling here a few days back, but couldn't tickle anybody to post a reply. Since I am not tired any more today, but still can't understand the documentation, or the output of the profiler, here it goes again: What qualifies as constant applicable form, and why is it not labelled in a more informative way? Why are there functions that inherit all of their (considerable) time and space consumption from elsewhere, but nothing in the list would allow for such a rich inheritage? As I said, I am happy to set up a wiki page if I learn anything that helps me improve the utility of profiling (the profiler and the rudimentary documentation are of great value as is). And i am happy to take pointers and go read articles if you tell me they help in the everyday work with ghc profiling. thanks / cheers, matthias

3 4

casting
by Thomas Conway 11 Oct '06

11 Oct '06

On 10/11/06, Bulat Ziganshin <bulat.ziganshin(a)gmail.com> wrote: > it's a sort of problem that bites me many times when i start to wrote > Streams library :) although you said that you discovered the > dictionaries mechanism, i propose you to read > http://haskell.org/haskellwiki/OOP_vs_type_classes page where you can > find something that you don't yet know Yes, I did actually know this stuff, it was just paged out, and I posted before thinking. :-) Something to do with having two small kids, one of whom gets up waaaaay too early in the morning (i.e. 5 - 5:30am). :-) In theory, however, there is no particular reason why a runtime type cast couldn't be implemented to allow downcasting, but it does require the runtime system to have a repository of instance declarations, or associate with static data for a type the classes of which it is an instance. It's been a few years, but if my recallection is correct, the Mercury runtime system does this. T.

1 0

Is Haskell a 5GL?
by Ch. A. Herrmann 11 Oct '06

11 Oct '06

Dear Haskellers, do you think that Haskell is a 3GL (third generation language) or a 5GL or that the hierarchy of programming language generations is useless? I see a problem if the potential of a language is evaluated according to the generation it belongs to. 1GLs: machine languages 2GLs: assembler languages 3GLs: programming based on algorithms (C, Java, ...) 4GSs: domain-specific languages (SQL,...) 5GLs: automatic problem solving (Mathematica, Prolog,...) and/or visual programming (Visual Basic,...) Haskell's laziness would in principle permit programming in a style akin to Prolog, e.g., for parser combinators. The question is: how strong is the influence of laziness in everyday Haskell programming? Cheers -- Christoph

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casting
by Thomas Conway 10 Oct '06

10 Oct '06

Hi All I'm having some difficulty with typeclasses. What I'm trying to do should be obvious, but it's still giving me trouble. I want to take a packaged item, and strengthen the constraints on its type. Rather than being just any type that is an instance of A, I want to do a runtime check and do something different if it is a type that is also an instance of B (which implies that it is an instance of A also). I've attached some code that attempts this, but ghci (er, 6.4.2 with -fglasgow-exts) complains: Compiling Main ( Cast.hs, interpreted ) Cast.hs:15:22: Ambiguous type variable `b' in the constraint: `B b' arising from use of `qux' at Cast.hs:15:22-24 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. So, can anyone suggest how I can achieve my goal? thanks, Tom

3 3

How would you replace a field in a CSV file?
by Pete Kazmier 10 Oct '06

10 Oct '06

The other day at work an opportunity arose where I was hoping to sneak some Haskell into the pipeline of tools used to process call detail records (CDRs). In the telecommunications industry, CDRs are used for billing. Each CDR is a single line record of 30 comma-separated values. Each line is approximately 240 characters in length. The task at hand is to replace field number 10 if a new value can be found in a hashmap which is keyed using the contents of the field. My colleague was going to write a C program (that's all he knows), but I whipped up a trivial python program instead. I was curious if a haskell version could be faster and more elegant , but I have not been able to beat my python version in either case. So, I'm curious as to how you would go about this task in Haskell. The input files are generally 300-400MB, and the hashmap will contain perhaps 20-30 items. For those that know python, here is a very simple implementation that happens to be very fast compared to my Haskell version and very short: for line in sys.stdin: fields = line.split(',') fields[9] = tgmap.get(fields[9], fields[9]) print ",".join(fields), For each line in standard input: - Splits the string on the comma: "field0,field1,...,field29" => ["field0", "field1", ..., "field29"] to obtain a list of strings. - Gets the value associated with the key of field9 from tgmap, if it does not exist, it returns a default value which is the original value. I.e., if it's not in the map, then don't replace the field. - Joins the list of fields with a comma to yield a string again which is printed out to standard output. The join method on the string is a bit odd: ",".join([1,2,3]) => "1,2,3" Here is my first Haskell attempt: import Data.ByteString.Lazy.Char8 as B hiding (map,foldr) import Data.List (map) import Data.Map as M hiding (map) -- This is just a placeholder until I actually populate the map tgmap = M.singleton (B.pack "Pete") (B.pack "Kazmier") main = B.interact $ B.unlines . map doline . B.lines where doline = B.join comma . mapIndex fixup . B.split ',' fixup i s = if i==9 then M.findWithDefault s s tgmap else s comma = B.pack "," -- f is supplied the index of the current element being processed mapIndex f xs = m f 0 xs where m f i [] = [] m f i (x:xs') = f i x : m f (i+1) xs' After talking with dons on #haskell, he cleaned my version up and produced this version which gets rid of 'if' statement and makes mapIndex stricter: import Data.ByteString.Lazy.Char8 as B hiding (map,foldr) import Data.List (map) import Data.Map as M hiding (map) -- This will be populated from a file dict = M.singleton (B.pack "Pete") (B.pack "Kazmier") main = B.interact $ B.unlines . map doline . B.lines where doline = B.join comma . mapIndex fixup . B.split ',' comma = B.singleton ',' fixup 3 s = M.findWithDefault s s dict fixup n s = s -- f is supplied the index of the current element being processed mapIndex :: (Int -> ByteString -> ByteString) -> [ByteString] -> [ByteString] mapIndex f xs = m xs 0 where m [] _ = [] m (x:xs') i = f i x : (m xs' $! i+1) That helped things a bit, but I must confess I don't understand how the strictness improved things as I had assumed things were going to be evaluated in a reasonable amount of time due to the printing of output. I thought IO was interlaced with the execution and thus I wasn't going to have to concern myself over laziness. In addition, the function is able to generate new elements of the list on demand so I thought it was a good citizen in the lazy world. Could anyone help explain? And then he came up with another version to avoid the 'unlines', but that did not that really speed things up significantly. So, with all that said, is there a better approach to this problem? Perhaps a more elegant Haskell solution? Thanks, Pete

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Re: [Haskell-cafe] casting
by Thomas Conway 10 Oct '06

10 Oct '06

Thanks Misha & Matthias. I now get what's going on. The mention of the word "dictionary" revealed it all. I've spent the last 7 years programming in C++, and had dynamic_cast<> firmly fixed in my head. I totally forgot that Fergus Henderson and I independently reinvented dictionary passing for the implementation of Mercury circa 1995. :-) I also realized that the concrete signatures from my actual application don't quite satisfy the requirements anyway, and that the problem evaporates when I get the signatures right.:-) T.

1 0

Announce: Monad Transformer Tutorial
by Martin Grabmueller 09 Oct '06

09 Oct '06

Hallo all, I hereby announce a small tutorial on using monad transformers. In contrast to others found on the web, it concentrates on using them, not on their implementation. I'd like to hear comments, suggestions, etc. about it! Both PDF and Literate Haskell source available here: http://uebb.cs.tu-berlin.de/~magr/pub/Transformers.en.html Have fun, Martin

3 3

Bizarre garbage collection behaviour
by Guillaume Theoret 09 Oct '06

09 Oct '06

I'm using Hugs98 for .NET and I'm running into some bizarre garbage collection issues. I hope I'm posting at the right spot. I didn't want to post this in Hugs bugs since I'm pretty new to Haskell and it's entirely possible I'm doing something a way I shouldn't. Back to the problem at hand. My script (link available at the bottom of this email) fetches a webpage through the .NET Net library and processes it. If I output the result of this processing (a list with the player's name, goals, assists and points) to the screen with putStrLn I can process about two dozen before I hit a .NET runtime exception (which is a different issue entirely). If I don't ouput the result to the screen (I delete 2 lines that only do putStrLn, Ln 148 & Ln 161) I get "ERROR - Garbage collection fails to reclaim sufficient space" after 2 players processed which seems completely counter-intuitive to me. Does someone have an idea as to what's going on here? The script (and its dependencies) are available here: http://smokinn.tengun.net/comp348/week6/ Thanks, Guillaume Theoret

1 0

Trying to serialize HList
by Matthias Fischmann 09 Oct '06

09 Oct '06

... and here is the code I am giving up on for today: Serialization of HLists. Questions below. {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} module Foo where import Char import List import Monad import Permutation import HListPrelude -- [1] http://web.engr.oregonstate.edu/~erwig/pfp/ -- Serializable is like Show, but -- (a) it carries explicit type information, and -- (b) it allows for serializing IORefs etc. class Serializable s where serialize :: s -> IO String deSerialize :: String -> IO s instance Serializable Int where serialize = return . ("Int::" ++) . show deSerialize s | isPrefixOf "Int::" s = return . (read :: String -> Int) . drop 5 $ s instance Serializable Char where serialize = return . ("Char::" ++) . show deSerialize s | isPrefixOf "Char::" s = return . (read :: String -> Char) . drop 6 $ s -- SList is a list of heterogenous serializable elements... class HList l => SList l instance SList HNil instance (Serializable s, SList ss) => SList (HCons s ss) -- ... so it should be possible to write instantiate Serializable, right?: instance (SList s, HMapOut Serialize s (IO String)) => Serializable s where serialize = liftM show . (sequence :: [IO String] -> IO [String]) . hMapOut Serialize deSerialize = error "Not yet. (I am not even done with serialize yet.)" -- Seems we need the trick from the paper that oleg pointed out to me earlier in this thread: data Serialize = Serialize instance (Serializable s) => Apply Serialize s (IO String) where apply _ = serialize -- Example: slist = HCons (1 :: Int) (HCons ('c' :: Char) HNil) test1 = serialize slist -- (This is where -fallow-overlapping-instances helps. There is a -- section in [1] on how to get rid of it, which I haven't read yet.) test2 :: IO (HCons Int (HCons Char HNil)) test2 = test1 >>= deSerialize Two questions: (1) Do you see any reasons why it should be impossible in principle to write deSerialize for the SList instance of Serializable? (I think the answer is "it's possible to write it, but you need to add quite some type information by hand".) (2) The problem with test2 is that I need to know its precise object-level type, ie which types occur at which positions in the SList. I am pretty sure this is a restriction I have to live with. Please tell me I am wrong. (-: (I think my application will make it possible for ghc to infer the type, which is fixed at compile time anyways, so it's not a severe restriction.) (3) (bonus question :) Who wants to write deSerialize for SLists for me? And another one: Why do I need to list the HMapOut instance in the context of the instance declaration of Serializable for SLists? (It's not a big deal, but I can't see why it can't be inferred automatically from the rest of the code.) Possibly back with another issue of my HList diary tomorrow. (Please tell me if you find this interesting or if you would like me to stop being so verbose.) cheers, Matthias

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collection monads
by tpledger＠ihug.co.nz 08 Oct '06

08 Oct '06

Matthias Fischmann wrote: > > Do you expect the contained type x to change during a > > sequence of monadic actions? e.g. would you ever use (>>=) > > at the type 'Permutation Int -> (Int -> Permutation Bool) -> > > Permutation Bool'? > > no, i don't need that. but aside from > the fact that > > > data Permutation k v = > > Permutation [(k, v)] > > instance (Ix k) => > > Monad (Permutation k) > > is redundant (i think of the permutation > as a function applicable to arbitrary > lists): how would that change anything? > my definition of return still doesn't > work. or how would you redefine > 'return'? Ah. Yes, my approach falls over because it lacks two things. #1: a distinguished value of the Ix-constrained type k, to pair off with return's argument. #2: a purpose. I don't have a clear idea of what a do-block in a permutation monad ought to mean. Whoops! <font color=red>:-]</font> Regards, Tom (crawling back under his rock)

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Haskell speaking Spanish
by Luis F. Araujo 08 Oct '06

08 Oct '06

Hello Haskell folks! I have recently started editing a page on the Haskell.org wiki site dedicated to our spanish-speaker community.[0] The main idea is to collect spanish information about Haskell. For example, tutorials, news, comments, etc. And that way to spread such a beautiful language and technology among our spanish-speakers. I also expect we could get some of the main Haskell wiki pages translated, at least the most popular pages.[1] After talking with Don Stewart on IRC; we have also decided it would be a nice idea to offer a spanish translation for the HWN (nice!), which i expect to post every week on this web page too. Please, feel free to start cooperating , and let me know if you got any doubt or suggestion about this new initiative. You are also invited to visit our irc channel #haskell.es on irc.freenode.net and say "hola" to us and idle for a while! Thanks! [0] http://haskell.org/haskellwiki/Haskell.es [1] http://haskell.org/haskellwiki/Special:Popularpages

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hsffig output produces errors from hsc2hs
by Soenke Hahn 08 Oct '06

08 Oct '06

Hi! I want to interface to JACK (http://jackaudio.org/) from a haskell programm and i tried to process jack/jack.h (the JACK API) with hsffig and its output with hsc2hs. I get several error messages from hsc2hs like this: JACK.hsc:198: error: syntax error before ‘}’ token JACK.hsc:198: error: void value not ignored as it ought to be The line in JACK.hsc: c_PTHREAD_COND_INITIALIZER = #const PTHREAD_COND_INITIALIZER The commands i used: gcc -E -dD /usr/include/jack/jack.h | hsffig | tee JACK.hsc hsc2hs -t template-hsffig.h JACK.hsc -I /usr/share/doc/hsffig i tried both version 0.1.2-2 from my distribution (ubuntu) and version 1.0 from the homepage. What am i doing wrong? Did i miss something essential? Soenke

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OOHaskell problems
by Jason Dagit 07 Oct '06

07 Oct '06

Hello, I wanted to try using OOHaskell as a library, but I've run into some problems I don't understand. I downloaded the copy from: http://homepages.cwi.nl/~ralf/OOHaskell/ In the HList subdirectory I created a .cabal file which exposes as many of the modules in HList as I could. I then installed the HList library using cabal. Back in the main OOHaskell directory I created a .cabal file for OOHaskell which depended on the newly installed HList library. The OOHaskell library exposes the modules: DeepNarrow New Nominal OOHaskell After I installed the OOHaskell library I ran: ghc --make -package OOHaskell -package HList SimpleIO.hs Chasing modules from: SimpleIO.hs Compiling Nominal ( ./Nominal.hs, ./Nominal.o ) Compiling New ( ./New.hs, ./New.o ) Compiling DeepNarrow ( ./DeepNarrow.hs, ./DeepNarrow.o ) Compiling OOHaskell ( ./OOHaskell.hs, ./OOHaskell.o ) Compiling SimpleIO ( SimpleIO.hs, SimpleIO.o ) SimpleIO.hs:44:11: No instance for (HasField (Proxy Field1) HNil v) arising from use of `foo' at SimpleIO.hs:44:11-13 Probable fix: add an instance declaration for (HasField (Proxy Field1) HNil v) In the definition of `testfoo': testfoo = foo ((field1 .=. True) .*. emptyRecord) SimpleIO.hs:116:7: No instance for (HasField (Proxy MoveX) HNil (a -> IO t)) arising from use of `#' at SimpleIO.hs:116:7 Probable fix: add an instance declaration for (HasField (Proxy MoveX) HNil (a -> IO t)) In the first argument of `($)', namely `p # moveX' In a 'do' expression: (p # moveX) $ 3 In the definition of `myFirstOOP': myFirstOOP = do p <- point (p # getX) >>= System.IO.print (p # moveX) $ 3 (p # getX) >>= System.IO.print SimpleIO.hs:124:19: No instance for (HasField (Proxy MutableX) HNil (IORef a)) arising from use of `#' at SimpleIO.hs:124:19 Probable fix: add an instance declaration for (HasField (Proxy MutableX) HNil (IORef a)) In the first argument of `writeIORef', namely `(p # mutableX)' In a 'do' expression: writeIORef (p # mutableX) 42 In the definition of `mySecondOOP': mySecondOOP = do p <- point writeIORef (p # mutableX) 42 (p # getX) >>= System.IO.print SimpleIO.hs:177:23: No instance for (HasField (Proxy GetX) HNil (IO a)) arising from use of `#' at SimpleIO.hs:177:23 Probable fix: add an instance declaration for (HasField (Proxy GetX) HNil (IO a)) In the second argument of `(>>=)', namely `(# getX)' In the first argument of `(>>=)', namely `localClass >>= ((# getX))' In the result of a 'do' expression: (localClass >>= ((# getX))) >>= System.IO.print SimpleIO.hs:225:8: No instance for (HasField (Proxy GetOffset) HNil (IO a)) arising from use of `#' at SimpleIO.hs:225:8 Probable fix: add an instance declaration for (HasField (Proxy GetOffset) HNil (IO a)) In the first argument of `(>>=)', namely `p # getOffset' In the result of a 'do' expression: (p # getOffset) >>= System.IO.print In the definition of `testPara': testPara = do p <- para_point 1 (p # getX) >>= System.IO.print (p # moveX) $ 2 (p # getX) >>= System.IO.print (p # getOffset) >>= System.IO.print To investigate this further, in the OOHaskell directory I typed: $ ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances -i./HList ShapesLub.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.4.2, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base-1.0 ... linking ... done. Compiling FakePrelude ( ./HList/FakePrelude.hs, interpreted ) Compiling HListPrelude ( ./HList/HListPrelude.hs, interpreted ) Compiling GhcExperiments ( ./HList/GhcExperiments.hs, interpreted ) Compiling HArray ( ./HList/HArray.hs, interpreted ) Compiling HZip ( ./HList/HZip.hs, interpreted ) Compiling HOccurs ( ./HList/HOccurs.hs, interpreted ) Compiling HTypeIndexed ( ./HList/HTypeIndexed.hs, interpreted ) Compiling Record ( ./HList/Record.hs, interpreted ) Compiling GhcRecord ( ./HList/GhcRecord.hs, interpreted ) Compiling Label4 ( ./HList/Label4.hs, interpreted ) Compiling New ( ./New.hs, interpreted ) Compiling TIP ( ./HList/TIP.hs, interpreted ) Compiling TIC ( ./HList/TIC.hs, interpreted ) Compiling GhcSyntax ( ./HList/GhcSyntax.hs, interpreted ) Compiling TypeCastGeneric1 ( ./HList/TypeCastGeneric1.hs, interpreted ) Compiling TypeEqBoolGeneric ( ./HList/TypeEqBoolGeneric.hs, interpreted ) Compiling TypeEqGeneric1 ( ./HList/TypeEqGeneric1.hs, interpreted ) Compiling Variant ( ./HList/Variant.hs, interpreted ) Compiling Nominal ( ./Nominal.hs, interpreted ) Compiling CommonMain ( ./HList/CommonMain.hs, interpreted ) Compiling DeepNarrow ( ./DeepNarrow.hs, interpreted ) Compiling OOHaskell ( ./OOHaskell.hs, interpreted ) Compiling Shapes ( ./Shapes.hs, interpreted ) Compiling ShapesLub ( ShapesLub.hs, interpreted ) Ok, modules loaded: ShapesLub, Shapes, OOHaskell, DeepNarrow, CommonMain, Nominal, Variant, TypeEqGeneric1, TypeEqBoolGeneric, TypeCastGeneric1, GhcSyntax, TIC, TIP, New, Label4, GhcRecord, Record, HTypeIndexed, HOccurs, HZip, HArray, GhcExperiments, HListPrelude, FakePrelude. *ShapesLub> main Drawing a Rectangle at:(10,20), width 5, height 6 Drawing a Rectangle at:(110,120), width 5, height 6 Drawing a Circle at:(15,25), radius 8 Drawing a Circle at:(115,125), radius 8 Drawing a Rectangle at:(0,0), width 30, height 15 *ShapesLub> So that seemed to work, but: $ ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances -package OOHaskell -package HList ShapesLub.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 6.4.2, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base-1.0 ... linking ... done. Loading package HList-0.1 ... linking ... done. Loading package OOHaskell-0.1 ... linking ... done. Skipping Nominal ( ./Nominal.hs, ./Nominal.o ) Skipping New ( ./New.hs, ./New.o ) Skipping DeepNarrow ( ./DeepNarrow.hs, ./DeepNarrow.o ) Skipping OOHaskell ( ./OOHaskell.hs, ./OOHaskell.o ) Compiling Shapes ( ./Shapes.hs, interpreted ) Compiling ShapesLub ( ShapesLub.hs, interpreted ) ShapesLub.hs:30:19: No instances for (HasField (Proxy MoveTo) HNil (a5 -> a6 -> IO t5), HasField (Proxy GetY) HNil (IO a6), HasField (Proxy GetX) HNil (IO a5), HasField (Proxy SetY) HNil (t4 -> t t1), HasField (Proxy SetX) HNil (t2 -> t t3), HasField (Proxy GetWidth) HNil (IO a7), HasField (Proxy GetHeight) HNil (IO a8), HExtract HNil (Proxy Draw) (IO ())) arising from use of `rectangle' at ShapesLub.hs:30:19-27 Probable fix: add an instance declaration for (HasField (Proxy MoveTo) HNil (a5 -> a6 -> IO t5), HasField (Proxy GetY) HNil (IO a6), HasField (Proxy GetX) HNil (IO a5), HasField (Proxy SetY) HNil (t4 -> t t1), HasField (Proxy SetX) HNil (t2 -> t t3), HasField (Proxy GetWidth) HNil (IO a7), HasField (Proxy GetHeight) HNil (IO a8), HExtract HNil (Proxy Draw) (IO ())) In the first argument of `mfix', namely `(rectangle (10 :: Int) (20 :: Int) 5 6)' In a 'do' expression: s1 <- mfix (rectangle (10 :: Int) (20 :: Int) 5 6) In the definition of `main': main = do s1 <- mfix (rectangle (10 :: Int) (20 :: Int) 5 6) s2 <- mfix (circle (15 :: Int) 25 8) let scribble = ... mapM_ (\ shape -> ...) scribble arec <- mfix (rectangle (0 :: Int) (0 :: Int) 15 15) (arec # setWidth) $ 30 arec # draw ShapesLub.hs:31:19: No instance for (HasField (Proxy GetRadius) HNil (IO a6)) arising from use of `circle' at ShapesLub.hs:31:19-24 Probable fix: add an instance declaration for (HasField (Proxy GetRadius) HNil (IO a6)) In the first argument of `mfix', namely `(circle (15 :: Int) 25 8)' In a 'do' expression: s2 <- mfix (circle (15 :: Int) 25 8) In the definition of `main': main = do s1 <- mfix (rectangle (10 :: Int) (20 :: Int) 5 6) s2 <- mfix (circle (15 :: Int) 25 8) let scribble = ... mapM_ (\ shape -> ...) scribble arec <- mfix (rectangle (0 :: Int) (0 :: Int) 15 15) (arec # setWidth) $ 30 arec # draw ShapesLub.hs:38:34: No instance for (HasField (Proxy RMoveTo) HNil (t -> t1 -> IO t2)) arising from use of `#' at ShapesLub.hs:38:34 Probable fix: add an instance declaration for (HasField (Proxy RMoveTo) HNil (t -> t1 -> IO t2)) In a 'do' expression: (shape # rMoveTo) 100 100 In a lambda abstraction: \ shape -> do shape # draw (shape # rMoveTo) 100 100 shape # draw In the first argument of `mapM_', namely `(\ shape -> do shape # draw (shape # rMoveTo) 100 100 shape # draw)' ShapesLub.hs:44:12: No instance for (HasField (Proxy SetWidth) HNil (a -> IO t)) arising from use of `#' at ShapesLub.hs:44:12 Probable fix: add an instance declaration for (HasField (Proxy SetWidth) HNil (a -> IO t)) In the first argument of `($)', namely `arec # setWidth' In a 'do' expression: (arec # setWidth) $ 30 In the definition of `main': main = do s1 <- mfix (rectangle (10 :: Int) (20 :: Int) 5 6) s2 <- mfix (circle (15 :: Int) 25 8) let scribble = ... mapM_ (\ shape -> ...) scribble arec <- mfix (rectangle (0 :: Int) (0 :: Int) 15 15) (arec # setWidth) $ 30 arec # draw ShapesLub.hs:46:12: No instance for (HasField (Proxy Draw) HNil (IO b)) arising from use of `#' at ShapesLub.hs:46:12 Probable fix: add an instance declaration for (HasField (Proxy Draw) HNil (IO b)) In the result of a 'do' expression: arec # draw In the definition of `main': main = do s1 <- mfix (rectangle (10 :: Int) (20 :: Int) 5 6) s2 <- mfix (circle (15 :: Int) 25 8) let scribble = ... mapM_ (\ shape -> ...) scribble arec <- mfix (rectangle (0 :: Int) (0 :: Int) 15 15) (arec # setWidth) $ 30 arec # draw Failed, modules loaded: Shapes, OOHaskell, DeepNarrow, New, Nominal. *Shapes> Inspecting HasField and Proxy in the working ghci says: *ShapesLub> :i HasField class HasField l r v | l r -> v where hLookupByLabel :: l -> r -> v -- Defined at ./HList/Record.hs:140:6 instance (HasField l x v, Nomination f) => HasField l (N f x) v -- Defined at ./Nominal.hs:49:0 instance HasField l r v => HasField l (Record r) v -- Defined at ./HList/Record.hs:143:0 instance (HEq l l' b, HasField' b l (HCons (l', v') r) v) => HasField l (HCons (l', v') r) v -- Defined at ./HList/Record.hs:149:0 *ShapesLub> :i Proxy data Proxy e -- Defined at ./HList/FakePrelude.hs:218:5 instance Show (Proxy e) -- Defined at ./HList/FakePrelude.hs:219:0 instance Typeable x => Typeable (Proxy x) -- Defined at ./HList/GhcRecord.hs:229:0 instance TypeEq x y b => HEq (Proxy x) (Proxy y) b -- Defined at ./HList/Label4.hs:27:0 instance (HType2HNat e l n, HTypes2HNats ps l ns) => HTypes2HNats (HCons (Proxy e) ps) l (HCons n ns) -- Defined at ./HList/HTypeIndexed.hs:90:0 instance Typeable x => ShowLabel (Proxy x) -- Defined at ./HList/Label4.hs:32:0 instance Fail (ProxyFound x) => HasNoProxies (HCons (Proxy x) l) -- Defined at ./HList/GhcRecord.hs:73:0 instance HTypeProxied l => HTypeProxied (HCons (Proxy e) l) -- Defined at ./HList/TIC.hs:68:0 instance HMaybied l l' => HMaybied (HCons (Proxy e) l) (HCons (Maybe e) l') -- Defined at ./HList/Variant.hs:53:0 While, the ouput in the non-working ghci sessions has equivalent output: *Shapes> :i HasField class HasField l r v | l r -> v where hLookupByLabel :: l -> r -> v -- Imported from Record instance (HasField l x v, Nomination f) => HasField l (N f x) v -- Imported from Nominal instance (HEq l l' b, HasField' b l (HCons (l', v') r) v) => HasField l (HCons (l', v') r) v -- Imported from Record instance HasField l r v => HasField l (Record r) v -- Imported from Record *Shapes> :i Proxy data Proxy e -- Imported from FakePrelude instance Show (Proxy e) -- Imported from FakePrelude instance Typeable x => Typeable (Proxy x) -- Imported from GhcRecord instance TypeEq x y b => HEq (Proxy x) (Proxy y) b -- Imported from Label4 instance Typeable x => ShowLabel (Proxy x) -- Imported from Label4 instance Fail (ProxyFound x) => HasNoProxies (HCons (Proxy x) l) -- Imported from GhcRecord instance HMaybied l l' => HMaybied (HCons (Proxy e) l) (HCons (Maybe e) l') -- Imported from Variant instance HTypeProxied l => HTypeProxied (HCons (Proxy e) l) -- Imported from TIC instance (HType2HNat e l n, HTypes2HNats ps l ns) => HTypes2HNats (HCons (Proxy e) ps) l (HCons n ns) -- Imported from HTypeIndexed I'm at a loss to figure out why the OOHaskell library I created does not behave the same as building the examples next to the HList source. Thanks, Jason

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