- Haskell-Cafe - Haskell.org

Newbie Question about Error Handling
by Gerd M 25 Jul '05

25 Jul '05

Hello! I'm confused by the error handling in Haskell. I've written a program that uses a combined monad of type: type MyMonad a = ErrorT MyErrorType (StateT MyStateType IO) a (MyErrorType is an instance of Error) Therefore, to handle IO Errors inside of MyMonad I need to write: foo :: MyMonad a foo = do ... inp <- liftIO (getChar `catchError` (\e -> if isEOFError e then return '\0' else return '?') ) case inp of '\?' -> throwError ... .... `catchError` myErrorTypeHandler This of course is cumbersome and ugly and I'm sure there is a way to do better. Helpful advice is welcome! _________________________________________________________________ FREE pop-up blocking with the new MSN Toolbar - get it now! http://toolbar.msn.click-url.com/go/onm00200415ave/direct/01/

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Re: [Haskell-cafe] Functional Programming lecture on video
by Carsten Otto 25 Jul '05

25 Jul '05

The lecture excercises can be downloaded from following homepage. The lecture notes are not available, but the difference between the written word in the notes and the spoken word in the lecture is minimal. http://www-i2.informatik.rwth-aachen.de/lufgi2/fp05/ The last two lectures will be digitalized in the next few days. -- Carsten Otto carsten.otto(a)gmail.com www.c-otto.de

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Best way to build strings?
by Andy Gimblett 24 Jul '05

24 Jul '05

A small stylistic question: what's the "best" way to build strings containing other values? For example, I have: data Process = Stop | Prefix String Process | External Process Process instance Show Process where show Stop = "Stop" show (Prefix l p) = "(" ++ l ++ "->" ++ show p ++ ")" show (External p q) = "(" ++ show p ++ " [] " ++ show q ++ ")" but to me the extensive use of ++ is not particularly readable. I'm very fond of Python's interpolation approach, where we'd have something like the following for the External case: def __str__(self): return "(%s [] %s)" % (self.p, self.q) which to me seems clearer, or at least easier to work out roughly what the string's going to look like. (The %s does an implicit "convert to string", btw). Is there a facility like this in Haskell? Or something else I should be using, other than lots of ++ ? Thanks, -Andy -- Andy Gimblett Computer Science Department University of Wales Swansea http://www.cs.swan.ac.uk/~csandy/

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Lists vs. Monads
by Greg Buchholz 24 Jul '05

24 Jul '05

Here's a question for the Haskell community that I've been wrestling with lately. When we say "lists are monads" what does that mean? I can see one of two things. First the slightly superficial... A.) Lists can be made members of the Monads class, and you can define a couple of functions, "bind" and "return" that obey the Three Laws. ...or the more fundamental... B.) Monads somehow define lists. If you had a version of Haskell without recursive data types you wouldn't be at a loss, because you could always use monads to recreate lists. Maybe "bind" would replace the job of "cons" and "return" would replace "head" or somesuch. I'm of the mind that A.) is the right interpretation, but I've seen the "lists are monads" meme mentioned enough that I sometimes question it. In fact, I had a hard time articulating B.) because I don't know what the other possibilies are. In case it isn't clear enough, here's another analogy... A.) Insects are cold blooded animals. ...while that is 100% true, many other animals are cold blooded. It doesn't get at the essence of what it means to be an insect. Compare with a statment like... B.) Insects are six-legged animals with an exoskeleton. ...which tells us a lot more about the fundamental nature of insects. Thanks, Greg Buchholz

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Arbitrary files in a Cabal package: how to install
by Dimitry Golubovsky 24 Jul '05

24 Jul '05

I am trying to use Cabal for buiding HSFFIG as a package. Among other files to install I have the special template for hsc2hs which is a C include file. If I specify it in the Includes: clause nothing happens: it is not copied anywhere during installation. I need this file to be placed somewhere in the package instalation location, so when programs are compiled with my package could include that file. Of course I may specify additional step in the Makefile, but it would be more consistent to do this in Cabal. How can arbitrary files be declared parts of a package, and destination directories for them be specified? I am using the simplified (?) version of Cabal that Ross suggested whrn I asked in this list whether it would be possible to use it with older GHC. Dimitry Golubovsky Middletown, CT

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building Ghc with msvc ?
by Jan Scott 23 Jul '05

23 Jul '05

Hi, Has anyone ever tried to build Ghc with Microsofts c compiler (msvc) ? Thanks Jan

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ANN: MissingH 0.11.3, now supporting Windows
by John Goerzen 22 Jul '05

22 Jul '05

Hello, I have just released MissingH 0.11.3. The big news about this version is that it now supports Windows, in addition to the continuing support for Posix platforms. Downloads and info available from: http://quux.org/devel/missingh or gopher://quux.org/1/devel/missingh -- John

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How to parsing files
by yin 22 Jul '05

22 Jul '05

Hi, I need to parse a file in this format: float float float float float float Each row has 3-columns of floating procision number divided by white space. The number of lines is undefined. I use (lines (readFile "...")) to read the file. Thnks. Matej 'Yin' Gagyi

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Re: [Haskell] Re: A MonadPlusT with fair operations and pruning
by ajb＠spamcop.net 22 Jul '05

22 Jul '05

G'day all. (Moving the discussion to haskell-cafe.) Quoting oleg(a)pobox.com: > The last statement should probably be `return (otherState w,b')', > right? Yes, I think so. > mif (mif c t' e') t e > translates to > > ((X=1 ; X=2) *-> X=1 ; fail) *-> true; true. Using the predicate: test(X) :- ( ( ( X = 1 ; X = 2 ) *-> X = 1 ; fail ) *-> true ; true ). Testing it with SWI-Prolog gives me: ?- test(X). X = 1 ; No and: ?- X = 2, test(X). X = 2 ; No and: ?- X = 3, test(X). X = 3 ; No The problem is that soft cut isn't a logical construct unless the program is well-moded. Using Mercury with the above code plus the following declarations: :- pred test(int). :- mode test(in) is semidet. :- mode test(out) is nondet. gives one appropriate answer: the "in" mode succeeds on all integers, but the "out" mode doesn't compile since X isn't bound inside the predicate. Another appropriate answer is the NU-Prolog approach, which suspends negations until they are appropriately bound. I haven't tested it, but NU-Prolog _should_ do the equivalent of this: ?- test(X). X = 1 ; X = X ; No (Read this as "X can be either 1 or anything".) You can also get an appropriate answer if your Prolog supports constraint programming, and you prime X before calling test/1. I also haven't tested this. Now Haskell programs are always well-moded, so let's see if it's an issue here. The "in" mode looks something like this: test_in :: Int -> Logic () test_in x = mif (mif (guard (x == 1) `mplus` guard (x == 2)) (\_ -> guard (x == 1)) mfail ) (\_ -> mtrue) mtrue This succeeds for all values of x. The "out" mode looks like this: test_out :: Logic Int test_out x = mif (mif (return 1 `mplus` return 2) (\_ -> guard (x == 1)) mfail ) (\x -> return x) ??? What to replace "???" with is left as an exercise for the reader. By the way, the moral of this story is that despite both being declarative, functional programming is much, much simpler than logic programming because the dataflow is simpler. In summary, I think that whether the extra law is needed or not depends on how logical your logic variables are. > In any case, we > haven't encountered a need to simplify "mif (mif c t' e') t e" > algebraically. It is interesting to see when such a law may be > required. I think you need it if you're deriving using the John Hughes method. If you didn't need it, all the better for you. Cheers, Andrew Bromage

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RE: [Haskell-cafe] Re: FFI and callbacks
by Simon Marlow 21 Jul '05

21 Jul '05

On 20 July 2005 18:49, John Goerzen wrote: > On 2005-07-20, Simon Marlow <simonmar(a)microsoft.com> wrote: >> This paper might help shed some light: >> >> http://www.haskell.org/~simonmar/papers/conc-ffi.pdf > > Forgot to reply to this. *Very helpful* link, I had always wondered > what the bound thread functions in Control.Concurrent were for :-) > > So let me see if I understand this correctly. > > Let's say that I have: > > * A program that uses multiple lightweight Haskell threads (all > started with plain forkIO calls) > > * An event-driven C library, not not thread-aware, with a blocking > main loop > > * GHC 6.4 > > * My C calls all imported "safe". > > Now then, if I understand this correctly, that a call to the C main > loop will create a new bound OS thread, so it will not interrupt any > other forkIO'd threads in Haskell. Not necessarily a *bound* OS thread. A bound thread is only created by an in-call to Haskell. An out-call may happen in a separate OS thread if the foreign import is "safe", that is, another OS thread will continue to run the remaining Haskell threads while the call is in progress. > However, if one of my Haskell-based callbacks creates new threads with > forkIO, I could be in trouble; if they make any calls into C, a new > bound OS thread would be created for them, and this could wind up > causing trouble in C. I would probably need some sort of "global > MVar" to synchronize access into the C world. Bingo. This is why you need to make all your calls to the C library from a single thread. > I also have some follow-up questions after looking at the > Control.Concurrent API: > > 1. It seems that there is no function that says "block the current > thread until the thread given by ThreadId dies" You can do something like this with an exception handler and an MVar or TVar: do died <- atomically $ newTVar False forkIO (later (atomically $ writeTVar died True) $ ...) let wait = atomically $ do b <- readTVar died when (not b) retry where later = flip finally granted, it's hard to implement exactly what you were asking for. Another way to do it would be to put a finalizer on the ThreadId, but that would incur a delay until the GC discovered the thread was unreachable. Hmm, perhaps we should have threadIsAlive :: ThreadId -> STM Bool > 2. What is the preferred way to implement a simple lock? With an > MVar? TVars are the way to go, although MVars do perform slightly better at the moment (at least if you stick to the simple putMVar/takeMVar operations). Cheers, Simon

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RE: Re[2]: [Haskell-cafe] Re: FFI and callbacks
by Simon Marlow 21 Jul '05

21 Jul '05

On 21 July 2005 08:07, Bulat Ziganshin wrote: > Hello Simon, > > Thursday, July 21, 2005, 1:16:10 AM, you wrote: > >>> However, if one of my Haskell-based callbacks creates new threads >>> with forkIO, I could be in trouble; if they make any calls into C, >>> a new bound OS thread would be created for them, and this could >>> wind up causing trouble in C. I would probably need some sort of >>> "global MVar" to synchronize access into the C world. > >> Bingo. This is why you need to make all your calls to the C library >> from a single thread. > > you can either: > 1) made all calls from single thread > 2) put all calls in "withMVar lock", where `lock` is a global MVar You can't always do (2), because some libraries (eg. OpenGL) use per-thread state. So you really can only call into OpenGL from a single thread, and it must always be the same thread (this is why we introduced bound threads, as it happens). Cheers, Simon

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RE: [Haskell-cafe] Updating the Haskell Standard
by Simon Peyton-Jones 21 Jul '05

21 Jul '05

| I think an updated standard is overdue. I find it difficult anymore to | write any but the most trivial of programs using pure Haskell 98. Some | notable, and widely-used, features developed since then include: | | * Overlapping instances | * FFI | * Hierarchical namespace | * Undecidable instances | * All sorts of updates to the standard library In fact there's a well established way to express the results of such an exercise: an Addendum to the Report. Two of the things you mention here already are Addenda http://haskell.org/definition/ namely FFI and hierarchical namespaces. [Actually, for the latter, the Addendum seems to be stuck at 0.0, and doesn't have any names attached to it. So it's in a kind of limbo, but there's a draft at least.] These Addenda are useful for exactly the reasons you describe: to solidify and nail down the details of particular language extensions. How do Addenda get written? The Haskell community is a particularly informal one. There is no Haskell committee. Instead, someone gets up sufficient motivation to gather a bunch of people and write a document. It's a good idea to publish regular drafts, and ensure that any one interested can see and contribute, because ultimately the usefulness of the Addendum depends on its acceptance. In short, the way lies open. The only bottleneck is people willing to do the work. I think that many of the people who produced the existing Haskell Report feel that they've done their bit, and perhaps someone else can have a go. That's certainly what I feel -- doing the Haskell 98 book was a *lot* of work -- though I would gladly contribute to an effort driven by someone else. To my mind, for all the fun of overlapping instances, the single thing that would be most useful in this area would be more well-documented, well-engineered libraries. The last year or two have seen lots of really useful-looking new libraries coming along. But bringing all that under a single roof, rather as the Haskell Library Report did, with reasonable documentation and compatible conventions, would be a great service. It would also be Real Work. Simon

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RE: [Haskell-cafe] FFI and callbacks
by Simon Marlow 20 Jul '05

20 Jul '05

On 20 July 2005 16:23, John Goerzen wrote: > On Wed, Jul 20, 2005 at 04:10:38PM +0100, Simon Marlow wrote: >> On 20 July 2005 14:35, John Goerzen wrote: >>> These systems generally have some sort of an opaque main loop, >>> implemented in C. This loop would usually never return, or perhaps >>> only return once the UI is destroyed. >> >> Is the library thread-safe or not? I mean OS-thread safe. If it is, >> then you're home and dry: just compile your program with GHC's >> -threaded option, and any foreign calls that need to run >> concurrently with Haskell threads must be declared "safe" (which is >> the default in fact, so instead I should really advise you to mark >> all calls that don't need to run concurrently as "unsafe", because >> that's good for performance). > > That's half of the question. The other half is: what is the canonical > way of handling this mainloop that doesn't return from Haskell? Do we > just treat it that way in Haskell as well, or would some people forkIO > it? You could forkIO the main loop if you want; it shouldn't make any difference, except that GHC terminates the program when the main thread completes, so it's more convenient to have the main loop run in the main thread, so that the program terminates when the main loop returns. > Does any part of your answer vary depending on whether the Haskell > threads in question are forkIO threads or forkOS threads? I imagine > that it might. No, and I don't think you need forkOS here. This paper might help shed some light: http://www.haskell.org/~simonmar/papers/conc-ffi.pdf >> If the library isn't thread-safe, then you're in the same boat as Gtk >> and wxHaskell (I believe) where there are known problems with having >> a multithreaded Haskell GUI app. You can only have one OS thread >> (and hence only one Haskell thread) doing GUI operations, and that >> is the > > What about multiple forkIO threads? You should think of Haskell as having a one-to-one mapping between OS threads and Haskell threads. The only difference between forkIO and forkOS is that a forkIO thread can migrate from one OS thread to another, whereas a forkOS thread always runs on the same OS thread. This difference is therefore only visible when you make FFI calls, because there's no way in Haskell to tell what OS thread you're running on. In practice, GHC uses an optimised implementation to take advantage of the fact that forkIO threads are allowed to migrate from one OS thread to another, and it runs all the forkIO threads using a small pool of worker OS threads. The design of forkIO/forkOS and the FFI is carefully crafted to accommodate both GHC's optimised implementation and simpler implementations such as one-to-one or a giant lock. >> thread in the main loop. You have to somehow set up a communication >> between your other Haskell threads and the thread running the main >> loop - perhaps you can send requests of type (IO ()) down a channel >> to the main loop thread which wakes up occasionally to run them, for >> example. > > I imagine there could be exceptions to this.. For instance, perhaps a > given operation needs to do 5 things, perhaps simultaneously, before > displaying a result. It could, perhaps, fire off five threads and > just not display a result until all five have terminated, yes? Sure, but it's just a matter of converting direct GUI calls into RPCs to the GUI thread, using some appropriate RPC mechanism. Cheers, Simon

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Re: [Haskell-cafe] Best way to build strings?
by Frank-Andre Riess 20 Jul '05

20 Jul '05

Hello there, > I'm very fond of Python's interpolation approach, where we'd have > something like the following for the External case: > > def __str__(self): > return "(%s [] %s)" % (self.p, self.q) are you familiar with C's printf procedure? It's where that kind of notation comes from. You can use Text.Printf which provides the function printf :: PrintfType r => String -> r Thus you can write show (Prefix l p) = printf "(%s->%s)" l q show (External p q) = printf "(%s [] %s)" p q Alas, I couldn't test it, because the Text module seems to not be installed on my computer (sad thing not being the admin, isn't it?) Nevertheless, take a look at http://www.haskell.org/ghc/docs/latest/html/libraries/base/Text.Printf.html Regards, Frank-Andre Riess

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file line operation perhaps need loop
by Sun Yi Ming 20 Jul '05

20 Jul '05

Hello, I have two txt file,and i want to mix the two files line by line, e.g. $ cat url1.txt url1_1.line url1_2.line $ cat url2.txt url2_1.line url2_2.line and i want this file as result: $ cat aha.txt url1_1.line url2_1.line url1_2.line url2_2.line i first write this snippet of code: --- import System.IO mix :: [a] -> [a] -> [a] mix [] ys = ys mix xs [] = xs mix (x:xs) (y:ys) = [x,y] ++ mix xs ys f1 = do contents1 <- readFile "url1.txt" contents2 <- readFile "url2.txt" let urls1 = lines contents1 urls2 = lines contents2 urls = mix urls1 urls2 writeFile "aha.txt" (unlines urls) -- this works fine, but i think if the two file are very big, and the readFile will consume too many mem.so i need to read the file line by line but stunned by the loop in IO Monad: --- main = do h1 <- openFile "url1.txt" ReadMode h2 <- openFile "url2.txt" ReadMode line1 <- hGetLine h1 line2 <- hGetLine h2 print $ line1 : line2 : [] -- i don't howto do hClose h1 hClose h2 -- any ideas? thank you all. -- Sun Yi Ming

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RE: [Haskell-cafe] FFI and callbacks
by Bayley, Alistair 20 Jul '05

20 Jul '05

> From: John Goerzen [mailto:jgoerzen@complete.org] > > How would I make this sort of system play nice with Haskell threads? > > Also, the brief FFI docs that I've found don't really explain > callbacks > in FFI very well to me. Are there any other resources on > this anywhere? This might help: http://www.haskell.org/ghc/docs/latest/html/libraries/base/Foreign.Ptr.html# t%3AFunPtr ...and I can also show you some code which uses a simple callback. Or are you past that stage now, and more worried about interaction of callbacks and threads? Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

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FFI and callbacks
by John Goerzen 20 Jul '05

20 Jul '05

Hi, I'm looking at packaging an event-driven console widget set (CDK) for Haskell using FFI. I know that other event-driven widget sets have Haskell bindings, but I'm not quite sure how to make everything play nice with forkIO. These systems generally have some sort of an opaque main loop, implemented in C. This loop would usually never return, or perhaps only return once the UI is destroyed. How would I make this sort of system play nice with Haskell threads? Also, the brief FFI docs that I've found don't really explain callbacks in FFI very well to me. Are there any other resources on this anywhere? Thanks, -- John

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Word32 to Int converions
by yin 20 Jul '05

20 Jul '05

hello, how do I convert an Word32 (or WordXYZ) to Int, or Integer, or Float, ...? The Int conversion is the priority. Thanks. Matej 'Yin' Gagyi

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RE: [Haskell-cafe] Strict and non-strict vs eager and lazy, was Confused about Cyclic struture
by Simon Marlow 20 Jul '05

20 Jul '05

On 18 July 2005 15:19, Bayley, Alistair wrote: > Not a taker (yet - where can I find information about non-lazy > implementation of non-strict languages? From Google so far: > speculative evaluation (Eager Haskell), call-by-name vs call-by-need.) > > Wikipedia frustratingly hints that "other evaluation strategies are > possible", but that's all it says: > http://en.wikipedia.org/wiki/Non-strict_programming_language While most of Haskell admits other evaluation strategies, lazy I/O pretty much requires lazy evaluation for lazy input streams. If the list returned by getContents is evaluated mulitple times, you don't want to read more data from the input each time, hence previous evaluations must be shared. Cheers, Simon "down with lazy I/O" Marlow

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Haskell, SDL, OpenGL
by yin 20 Jul '05

20 Jul '05

Hello, I'm trying to write a 3D renderer in Haskell using SDL bindings (Lemmih) and OpenGL bindings. When I try to compile my work, ghc runs collect2: /usr/libexec/gcc/i686-pc-linux-gnu/3.4.3-20050110/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o a.out -u GHCziBase_Izh_static_info -u GHCziBase_Czh_static_info -u GHCziFloat_Fzh_static_info -u GHCziFloat_Dzh_static_info -u GHCziPtr_Ptr_static_info -u GHCziWord_Wzh_static_info -u GHCziInt_I8zh_static_info -u GHCziInt_I16zh_static_info -u GHCziInt_I32zh_static_info -u GHCziInt_I64zh_static_info -u GHCziWord_W8zh_static_info -u GHCziWord_W16zh_static_info -u GHCziWord_W32zh_static_info -u GHCziWord_W64zh_static_info -u GHCziStable_StablePtr_static_info -u GHCziBase_Izh_con_info -u GHCziBase_Czh_con_info -u GHCziFloat_Fzh_con_info -u GHCziFloat_Dzh_con_info -u GHCziPtr_Ptr_con_info -u GHCziPtr_FunPtr_con_info -u GHCziStable_StablePtr_con_info -u GHCziBase_False_closure -u GHCziBase_True_closure -u GHCziPack_unpackCString_closure -u GHCziIOBase_stackOverflow_closure -u GHCziIOBase_heapOverflow_closure -u GHCziIOBase_NonTermination_closure -u GHCziIOBase_BlockedOnDeadMVar_closure -u GHCziIOBase_BlockedIndefinitely_closure -u GHCziIOBase_Deadlock_closure -u GHCziWeak_runFinalizzerBatch_closure -u __stginit_Prelude /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../../crt1.o /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../../crti.o /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/crtbegin.o -L/home/yin/devel/third/sdlh/SDLstable/img/lib/SDL-0.2.0 -L/usr/lib/ghc-6.4 -L/usr/lib -L/usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110 -L/usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110 -L/usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../../../i686-pc-linux-gnu/lib -L/usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../.. Main.o -lHSSDL-0.2.0 \ /usr/lib \ -lSDL -lHSOpenGL -lHSOpenGL_cbits -lGLU -lGL -lm -lHSbase -lHSbase_cbits -lHSrts -lm -lgmp -ldl -lgcc -lgcc_eh -lc -lgcc -lgcc_eh /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/crtend.o /usr/lib/gcc/i686-pc-linux-gnu/3.4.3-20050110/../../../crtn.o Collect2 don't understand the /usr/lib parameter and fails with this message: /usr/bin/ld: /usr/lib: No such file: File format not recognized collect2: ld returned 1 exit status I just rerun collect2 by hand without pre /usr/lib parameter, and it works. What have I to do or where have to look in to find and resolve the problem? I use Gentoo 2005.0 and SDL binding from SDLStable brach installed by hand (I didn't use Portage). Is it the packages.conf file, where propably is the problem? Thanks for every answer... Matej 'Yin' Gagyi

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Named data type members
by yin 20 Jul '05

20 Jul '05

Hello, I've a data type: data SomeData = SomeData { int1 :: Int, int2 :: Int } class SomeClass where infix 1 `i_` i_ :: SomeData -> Int -> SomeData infix 1 `_i` _i :: SomeData -> Int -> SomeData instance SomeClass SomeData where (SomeData int1 int2) `i_` i = SomeData (int1 + i) int2 (SomeData int1 int2) `_i` i = SomeData int1 (int2 + i) 1. Possible optimalizations ...? 2. There are several data type members in 'SomeData' and I wish to access them in other way then "(SomeData m1 m2 m3 m4 m5 ... m100) `a` i = SomeData (m1 + i) m2 m3 ... m100" 3a. The 'SomeData' is something like c structure, which holds all application data. One instanciated variable is passed in more functions to each other: -- SomeData is now very complex initsomeData :: SomeData initsomeData = do var1 <- init... ... SomeData 0 0 0 1 1 True False (-1) var1 var2 ....... main = do p <- initAplication processEvents p -- First Function processEvents :: SomeData -> IO () processEvents p = do event <- pollEvent case event of Event1 -> do processEvents (SomeData v1 v2 v3 (v4*2) ... v100) Event2 -> do processEvents (SomeData v1 ... False ... v100) Event2' -> do processEvents (SomeData v1 ... True ... v100) EventQuit -> do return () NoMoreEvents -> do computeNext p _ -> processEvents p where (SomeData v1 v2 v3 v4 v5 ... v100) = p -- An time based function computeNextStep = do timeDelta <- getTimeDelta let i1' = compute1 i1 i2 i3 ... i50 i2' = compute2 i1 i2 i3 ... i50 ... if (condition) then -- need more computing computeNextStep (SomeData u1 u2 ... u50 i1 i2 ... i50) else do p' <- (SomeData u1 u2 ... u50 i1 i2 ... i50) draw p' processEvents p' where (SomeData u1 u2 ... u50 i1 i2 ... i50) = p -- ux - uninteresting (State variables, like left-key-down was last left-key related event: move left) -- ix - interesting for computing -- where x = 1, 2 ... 50 ... ... 3b. every funtion needs only a part of the data... can I build for every fintion a class with defined operators to manipulate with the data and then: * pass the variable in func1 known as class A to func2, which is infering a class B? Thank you all... Matej 'Yin' Gagyi

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How to variables
by yin 20 Jul '05

20 Jul '05

Hello all! I'm doing a 3D simulation. Now I need something like variables in imperative languages. My mainLoop check for new events and renders scene. To use input for controling camera position I need variables. An equivalent code in C: void main_loop() { int loop_num = 0; bool run = 1; SDLEvent e; while(run) { while(SDL_PollEvent(&e)) { if(e.type == SDL_KeyDown) { if(... == SDLK_Left) camera_pos_x--; } else if ... ... } drawScene(); loop_num++; } } How to implement camera_pos_x, y and z as variables, which chage values in run? This is only simplified example, but it's important to implement it in as most imperative form as possible. Thank fo all answers. Matej 'Yin' Gagyi

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RE: [Haskell-cafe] Proposal: deriving ShallowEq?
by Ralf Lammel 20 Jul '05

20 Jul '05

Ben, thanks for your clever questions. Let's look at a little GHC 6.4 demo. {-# OPTIONS -fglasgow-exts #-} import Data.Generics main = do print $ toConstr () == toConstr () print $ toConstr (5::Int) == toConstr () print $ toConstr (\(x::Int) -> x) == toConstr () print $ id `shallowEq` (id::Int -> Int) shallowEq :: Data a => a -> a -> Bool shallowEq x y = toConstr x == toConstr y This gives: *Main> main True False *** Exception: toConstr Some observations: a) We need to say what sort of Num this 5 is because otherwise instance selection will not know how to obtain the constructor for 5. In this sense, SYB is not different from normal type-class-based programming. Well, modulo default declarations; http://www.haskell.org/tutorial/numbers.html So this works (because of a default declaration) *Main> show 5 == show () False This one doesn't because there is no default declaration: *Main> toConstr 5 == toConstr () <interactive>:1:0: Ambiguous type variable `a' in the constraints: ... b) You are right. An independently polymorhic shallowEq is more expressive since it allows us to compare the top-level constructors of different specializations of the same type, e.g., Just Char vs. Just String. shallowEq' :: (Data a, Data b) => a -> b -> Bool shallowEq' x y = toConstr x == toConstr y You also spotted the issue that shallowEq' cannot operate on data of types a and b where either of these is a type scheme. The reason for that is that the instances of Data (and Typeable) involve constraints for the types of the children. As long as these children types are not fixed, toConstr cannot be computed since instance selection cannot be completed. In general, this is a good idea because of the types of gfoldl (and the gmap derivates), but for exercises like shallow equality this is in the way. I don't have a good solution to offer. (I don't see either that this is a show stopper in practice.) We may think of splitting up the API as to define several more precise classes ... c) Just for the record, SYB's toConstr throws for function types. Ralf > -----Original Message----- > From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe- > bounces(a)haskell.org] On Behalf Of Ben Lippmeier > Sent: Tuesday, July 19, 2005 8:56 PM > Cc: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Proposal: deriving ShallowEq? > > Ralf Lammel wrote: > > As Bulat points out, the GHC primitive dataToTag# > > indeed nicely solves the problem. Ben, just for > > completeness' sake; with SYB, you get such reflective > > information too (and others): > > > > shallowEq :: Data a => a -> a -> Bool > > shallowEq x y = toConstr x == toConstr y > > > > (dataToTag# returns Int, while toConstr comprises other things like the > > constructor name.) > > > > Ralf, > Yes, I ended up using the "propper" SYB approach instead, though I have > noticed that the reflection data types Constr and DataRep make no > mention of type variables or functions. > > For example, this works fine: > > getTag (Just 5) ==# getTag (Just{}) > > getTag (Just (\x -> x)) ==# getTag (Just{}) > > But this does not > > toConstr (Just 5) == toConstr (Just{}) > Ambiguous type variables. > > > toConstr (Just (\x -> x)) == toConstr (Just{}) > No instance for Data (t -> t) > > I appreciate the reasons why this is so, though I think it's interesting > to see the practical consequences. > > A toConstr version of shallowEq works ok so long as you provide a type > signature to constrain both arguments to be the same type, and one of > them is always fully constructed - which is fine for me at the moment. > > Ben. > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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what does ' mean?
by Logesh Pillay 20 Jul '05

20 Jul '05

Dear list A v. newbie question. I found the following code on the web /-- Andrew Bromage / /-- If you're doing permutations, then I suppose you want this too: //-- (subLists might be a more suggestive name) /combinations *:: [a] -> [[a]] *combinations [] = [[]] combinations (x:xs) = combinations xs ++ [ x:xs' | xs' <- combinations xs ] What does ' (apostrophe) mean? I see no reference to it in Simon Thompson's book. Thanks Logesh

7 7

Named data type members
by yin 20 Jul '05

20 Jul '05

Hello, I've a data type: data SomeData = SomeData { int1 :: Int, int2 :: Int } class SomeClass where infix 1 `i_` i_ :: SomeData -> Int -> SomeData infix 1 `_i` _i :: SomeData -> Int -> SomeData instance SomeClass SomeData where (SomeData int1 int2) `i_` i = SomeData (int1 + i) int2 (SomeData int1 int2) `_i` i = SomeData int1 (int2 + i) 1. Possible optimalizations ...? 2. There are several data type members in 'SomeData' and I wish to access them in other way then "(SomeData m1 m2 m3 m4 m5 ... m100) `a` i = SomeData (m1 + i) m2 m3 ... m100" 3a. The 'SomeData' is something like c structure, which holds all application data. One instanciated variable is passed in more functions to each other: -- SomeData is now very complex initsomeData :: SomeData initsomeData = do var1 <- init... ... SomeData 0 0 0 1 1 True False (-1) var1 var2 ....... main = do p <- initAplication processEvents p -- First Function processEvents :: SomeData -> IO () processEvents p = do event <- pollEvent case event of Event1 -> do processEvents (SomeData v1 v2 v3 (v4*2) ... v100) Event2 -> do processEvents (SomeData v1 ... False ... v100) Event2' -> do processEvents (SomeData v1 ... True ... v100) EventQuit -> do return () NoMoreEvents -> do computeNext p _ -> processEvents p where (SomeData v1 v2 v3 v4 v5 ... v100) = p -- An time based function computeNextStep = do timeDelta <- getTimeDelta let i1' = compute1 i1 i2 i3 ... i50 i2' = compute2 i1 i2 i3 ... i50 ... if (condition) then -- need more computing computeNextStep (SomeData u1 u2 ... u50 i1 i2 ... i50) else do p' <- (SomeData u1 u2 ... u50 i1 i2 ... i50) draw p' processEvents p' where (SomeData u1 u2 ... u50 i1 i2 ... i50) = p -- ux - uninteresting (State variables, like left-key-down was last left-key related event: move left) -- ix - interesting for computing -- where x = 1, 2 ... 50 ... ... 3b. every funtion needs only a part of the data... can I build for every fintion a class with defined operators to manipulate with the data and then: * pass the variable in func1 known as class A to func2, which is infering a class B? Thank you all... Matej 'Yin' Gagyi

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RE: [Haskell-cafe] Proposal: deriving ShallowEq?
by Ralf Lammel 20 Jul '05

20 Jul '05

As Bulat points out, the GHC primitive dataToTag# indeed nicely solves the problem. Ben, just for completeness' sake; with SYB, you get such reflective information too (and others): shallowEq :: Data a => a -> a -> Bool shallowEq x y = toConstr x == toConstr y (dataToTag# returns Int, while toConstr comprises other things like the constructor name.) Regards, Ralf > -----Original Message----- > From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe- > bounces(a)haskell.org] On Behalf Of Bulat Ziganshin > Sent: Tuesday, July 19, 2005 1:18 AM > To: Ben Lippmeier > Cc: haskell-cafe(a)haskell.org > Subject: Re: [Haskell-cafe] Proposal: deriving ShallowEq? > > Hello Ben, > > Tuesday, July 19, 2005, 11:01:32 AM, you wrote: > BL> I often find it useful to determine whether two objects are using the > BL> same constructor, without worrying about the constructors' arguments. > > BL> There is way to hack together a partial implementation of the > ShallowEq > BL> class within GHC, but it leaves much to be desired: > > BL> > instance Show a => ShallowEq a where > BL> > (=@=) a b > BL> > = (head $ words $ show a) == (head $ words $ show b) > > reading GHC sources is always very interesting :) > > that is from GHC/Base.hs : > > %********************************************************* > %* * > \subsection{@getTag@} > %* * > %********************************************************* > > Returns the 'tag' of a constructor application; this function is used > by the deriving code for Eq, Ord and Enum. > > The primitive dataToTag# requires an evaluated constructor application > as its argument, so we provide getTag as a wrapper that performs the > evaluation before calling dataToTag#. We could have dataToTag# > evaluate its argument, but we prefer to do it this way because (a) > dataToTag# can be an inline primop if it doesn't need to do any > evaluation, and (b) we want to expose the evaluation to the > simplifier, because it might be possible to eliminate the evaluation > in the case when the argument is already known to be evaluated. > > \begin{code} > {-# INLINE getTag #-} > getTag :: a -> Int# > getTag x = x `seq` dataToTag# x > \end{code} > > > > > -- > Best regards, > Bulat mailto:bulatz@HotPOP.com > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Error with Float
by Dinh Tien Tuan Anh 19 Jul '05

19 Jul '05

So there's no way to get exact stream that represents a fraction, such as: .5 = .10000000000000000.... .2 = .00110011001100110011............ ??? >From: Udo Stenzel <u.stenzel(a)web.de> >To: Dinh Tien Tuan Anh <tuananhbirm(a)hotmail.com> >Subject: Re: [Haskell-cafe] Error with Float >Date: Tue, 19 Jul 2005 20:51:14 +0200 > > > Value is: 0.800000000000001 > >What did you expect? Floating point numbers are inexact by their very >nature. Since 0.8, like many other numbers, is not representable >exactly in binary, (10 * 0.8 - 8) will certainly not be zero. > >Take a course in numerical mathematics some time. It helps. > > >Udo. >-- >HTML needs a <rant> tag. -- Alan Cox ><< signature.asc >> _________________________________________________________________ It's fast, it's easy and it's free. Get MSN Messenger 7.0 today! http://messenger.msn.co.uk

1 0

Proposal: deriving ShallowEq?
by Ben Lippmeier 19 Jul '05

19 Jul '05

Hello, I often find it useful to determine whether two objects are using the same constructor, without worrying about the constructors' arguments. An example, using some arbitrary data type "Thingo": > class ShallowEq a where > shallowEq :: a -> a -> Bool > data Thingo a b > = TOne a > | TTwo a b Int Char Float > | TThree Int Char b b > (TOne 23) `shallowEq` TOne{} True > (TThree 5 'c' True False) `shallowEq` TTwo{} False -- Having some sort of generic shallowEq operator reduces the need for a host of predicates such as: (this one from Data.Maybe) > isJust x > = case x of > Just {} -> True > _ -> False .. which is an approach that is obviously going to be tedious when the size of the data type becomes large. -- There is way to hack together a partial implementation of the ShallowEq class within GHC, but it leaves much to be desired: > instance Show a => ShallowEq a where > (=@=) a b > = (head $ words $ show a) == (head $ words $ show b) Notice that in the example the term "TTwo{}" is a partially constructed record. The implementation relies on laziniess to avoid trying to show the missing fields (which would fail). -- Questions: 1) Does anyone know a better/existing way to implement ShallowEq that doesn't involve enumerating all the constructors in the data type? 2) If not, can anyone think of reasons why it wouldn't be a good idea for GHC to derive ShallowEq (by expanding said enumeration)?

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RE: [Haskell-cafe] Strict and non-strict vs eager and lazy, was C onfused about Cyclic struture
by Bayley, Alistair 19 Jul '05

19 Jul '05

> From: Jerzy Karczmarczuk [mailto:karczma@info.unicaen.fr] > > Bernard Pope wrote: > > >I'll be a little bit pedantic here. Haskell, the language definition, > >does not prescribe lazy evaluation. It says that the language is > >non-strict. Lazy evaluation is an implementation technique which > >satisfies non-strict semantics, but it is not the only > technique which > >does this. > > > > > This pedantry is renewed periodically. > > It is a pity that nobody ever writes anything about that other > methods of implementation of non-strictness, nor about the > languages which use those methods. > > I believe it might do some good to people who learn functional > programming in general, and Haskell in particular. > Any takers? Not a taker (yet - where can I find information about non-lazy implementation of non-strict languages? From Google so far: speculative evaluation (Eager Haskell), call-by-name vs call-by-need.) Wikipedia frustratingly hints that "other evaluation strategies are possible", but that's all it says: http://en.wikipedia.org/wiki/Non-strict_programming_language Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

4 3

lazy Graphics, was - How to variables
by Jake Luck 19 Jul '05

19 Jul '05

>> I need to write functions fast and efective. Math, heuristic, metadata >> and expert systems are better in haskell. If I could use haskel from C, >> I would do it. The problem are optimalizations, which are a critical >> change in algorithm. Other (and me too) won't understand my concepts. >> The speed and usability of Haskell is a good argument to use and learn it. > > I can buy that. Well, one can actually call into Haskell from C. It is less > commonly done, but very possible. Skim through the FFI addendum; you can > export static functions (foreign export) or arbitrary thunks (with the > confusingly named foreign import "wrapper"). If you are real adventurous, > you can tie directly into the GHC API from the C side as well (although I'm > not sure I can seriously recommend this method). > > Having said that, if you feel that Haskell has sufficient advantages to > warrant its use, I don't think you lose much by writing your main loops etc. > in Haskell as well, and I would recommend you go with the labeled record > technique to contain your program state. I too am working on a Haskell 3d renderer. However, my motivation is mostly inspired by Conal Elliott's Tbag and Vertigo projects. In my opinion, geometry, expressions and most 3d technologies fit naturally with the functional approach, like you said ("math, heuristic, metadata and expert systems"). Haskell's lazy execution is where I see opportunity for parallel optimization. Think about all the computed pixels and polygons that we throw away in our culling and clipping. Now if we can structure the system inside out .... (do you see what I am getting at?) Having written several keyframe system, I feel that Fran's frameless timing approach is ingenious (Hudak's School of Expression book has an excellent section describing its motivation). Though I am not without reservation in all of this, paying close attention to the graphics world, where things are progressively moving towards programmable hardware. IMO, Vertigo is a good example of a practical application using Haskell. I think that the division between where does functional stops and where imperative starts will be a key deciding factor in the performance feasability as well as adaptability for haskell software in the real graphics world. In other words, 1 million IORefs make me feel quite unsettled. Anyone care to comment on the feasability of those hypothesis? jake

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RE: [Haskell-cafe] Strict and non-strict vs eager and lazy, was C onfused about Cyclic struture
by Bayley, Alistair 19 Jul '05

19 Jul '05

> From: Bernard Pope [mailto:bjpop@cs.mu.OZ.AU] > > I should have mentioned this paper: > > @article{Tremblay01, > author= {G. Tremblay}, > title= {Lenient evaluation is neither strict nor lazy}, > journal= {Computer Languages}, > volume= {26}, > number= {1}, > pages= {43--66}, > year= {2001}, > } > > (however I think he says that Haskell is lazy!) Thanks. Do you have a link to a free (beer) version? I don't have an ACM subscription. I found this related paper, which was useful: How Much Non-strictness do Lenient Programs Require? http://www.cs.ucsb.edu/~schauser/papers/95-fpca.ps (unrelated) BTW, I found this, which sounds similar to the STM in GHC 6.4. Again, does anyone have a link to a free version? P. Tinker and M. Katz. Parallel execution of sequential Scheme with ParaTran. http://portal.acm.org/citation.cfm?doid=62678.62682 Alistair. ----------------------------------------- ***************************************************************** Confidentiality Note: The information contained in this message, and any attachments, may contain confidential and/or privileged material. It is intended solely for the person(s) or entity to which it is addressed. Any review, retransmission, dissemination, or taking of any action in reliance upon this information by persons or entities other than the intended recipient(s) is prohibited. If you received this in error, please contact the sender and delete the material from any computer. *****************************************************************

2 1

hawiki Haskell SDL openGl example works strange
by yin 18 Jul '05

18 Jul '05

Hello, again, I tried to run the example from http://haskell.org/hawiki/HaskellOpenGl (the SDL version), but the scene is shown, only when I resize the window. I tried to insert the drawGLScreen before mainloop call, but it didn't worked. Do you some idea how to resolve this little problem? Thanks Matej 'Yin' Gagyi

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RE: Unicode in GHC 6.2.2 and 6.4.x (was: Re: [Haskell-cafe] Unicode.hs)
by Simon Marlow 18 Jul '05

18 Jul '05

On 17 July 2005 04:42, Dimitry Golubovsky wrote: > Dear List Subscribers, > > Simon Marlow wrote: >> On 30 June 2005 14:36, Dimitry Golubovsky wrote: >> >> >>> It is in CVS now, and I believe will be in 6.4.1 >> >> >> Not planned for 6.4.1, but definitely in 6.6. >> > > I have put those files that work for me in GHC 6.2.2 (Unicode support) > for download. Please read the Wiki page: > > http://haskell.org/hawiki/GhcUnicode > > for instructions. > > Any feedback will be appreciated. I believe, the code will work with > GHC > 6.4.x as well. Please let me know if there are any problems. I should mention that 6.4.1 will contain Dimitry's updated Unicode character-class support, albeit without the changes to library APIs that went along with it. The upshot is that all platforms should have consistent behaviour for the isXXX functions from Data.Char in 6.4.1. Cheers, Simon

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RE: [Haskell-cafe] Re: Module.T naming style
by Simon Marlow 18 Jul '05

18 Jul '05

On 15 July 2005 16:13, Malcolm Wallace wrote: >> Hand-writing .hi files is how GHC used to work (up to version 0.29, >> IIRC). It's not entirely satisfactory because you can get into >> situations where you have to 'make' several times to get to a fixed >> point, and you can construct examples that never reach a fixed point. > > I believe, provided the hand-written boot file gives accurate type > signatures in the first place, there is no (correctness) need to > compile any file a second time. At least, this is the case in nhc98 > where the interface file records only type signatures. However, > certainly if you /do/ re-compile, often the /sequence/ of signatures > in the .hi file never reaches a fixpoint. Yes - actually I was thinking of the case where you modify something in a mutually-recursive group and recompile. It's tempting (especially for me, since I'm lazy) to just type 'make;make;make;make' rather than modify any .hi files by hand. So the .hi files don't initially contain correct information, but you hope they converge. It's possible by writing wrong type signatures in .hi files to diverge instead of converge. I can't remember the particular example right now; IIRC it was due to Lennart. Cheers, Simon

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RE: [Haskell-cafe] Re: Module.T naming style
by Simon Marlow 18 Jul '05

18 Jul '05

On 15 July 2005 11:53, Malcolm Wallace wrote: > "Simon Marlow" <simonmar(a)microsoft.com> writes: > >> I hope you weren't including GHC in "most compilers" :-) GHC's >> implementation of mutually recursive modules is conformant with >> Haskell 98 (see Section 5.7). > > It depends on how you interpret this phrase: > "may require that imported modules contain additional information" > If you count writing a separate .hs-boot file as being the additional > information that is somehow "contained" within the module, then I > suppose nhc98 is also conformant with the report. True, I considered mentioning this detail, but decided against it - the fact that the type signatures aren't actually in the same file isn't really important. We follow the spirit of the specification. You guys should really document that nhc98 support mut. rec. modules too. > The only difference with nhc98 is that you hand-write a .hi file, > rather than a .hs-boot file, but the syntax is very similar to plain > Haskell. Also, the hand-written .hi file will be replaced with a > machine-generated one after compilation, but that is only a problem > if the user inadvertently removes all the .hi files. Hand-writing .hi files is how GHC used to work (up to version 0.29, IIRC). It's not entirely satisfactory because you can get into situations where you have to 'make' several times to get to a fixed point, and you can construct examples that never reach a fixed point. And 'make' complains about recursive dependencies a lot. What does hmake do? It used to be Really Fun(TM) when GHC was built like this... you could improve GHC's performance by simply typing 'make' a couple more times to let the inter-module optimisations propagate a bit further :) Cheers, Simon

4 3

IO platform for testing
by yin 18 Jul '05

18 Jul '05

Hello all, first I have to apologise for my English. So, I don't fully understand haskells IO, but I need some testing 'main' function: ./bundle01 <cmd> <arg1> <arg2> ... I more apoarches, but no one worked. Now I tried this: m01_mod :: Int -> Int -> Int b01_mod a b = a mod b main = do (cmd:ss) <- getArgs putStrLn ( if (cmd == "mod") then do show (b01_mod (read (ss!!1)) (read (ss!!2))) else do "unknown command!") Please, send me somethung like this (I need to understand it). I need runing more <cmd>s and with various argument counts and types. All functions "in a bundle" should be in one file. Thanks to you all :) Matej 'Yin' Gagyi

4 4

flushing trace debug output before getting other output
by Terrence Brannon 18 Jul '05

18 Jul '05

I'm working on exercise 6.16 in Thompson's "Haskell; the Craft of Functional Programming" My goal for the moment is to call printPicture to see my results: type Picture = [[Char]] printPicture :: Picture -> IO () printPicture = putStr . concat . map (++"\n") And everything is encouraging. The return type of my function syncs up: *Main> :t superimposeImage imgA imgB superimposeImage imgA imgB :: [[Char]] Now, the only issue is that I have put a number of trace statements in my program whose output is *interspersed* with the return data. The return data is simply lists of characters, where each character is "." or "#" *Main> printPicture $ superimposeImage imgA imgB siPointLogic i1 i2 0 15 .siPointLogic i1 i2 1 15 .siPointLogic i1 i2 2 15 .siPointLogic i1 i2 3 15 .siPointLogic i1 i2 4 15 .siPointLogic i1 i2 5 15 .siPointLogic i1 i2 6 15 .siPointLogic i1 i2 7 15 .siPointLogic i1 i2 8 15 .siPointLogic i1 i2 9 15 .siPointLogic i1 i2 10 15 .siPointLogic i1 i2 11 15 When I remove my various trace statements, I get what I want: *Main> printPicture $ superimposeImage imgA imgB ................ ................ ................ ................ ...........##... So the question is, how can I leave the trace statements in the code and yet get the printPicture output after all the debugging output has output? It only makes sense for it to work that way. The trace output occurs during the program and in particular, during the program *prior* to the call to printPicture. Therefore all of the trace output should have finished *prior* to printPicture outputting it's rows. -- Carter's Compass: I know I'm on the right track when, by deleting something, I'm adding functionality.

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Re: [Haskell-cafe] IO platform for testing
by Frank-Andre Riess 17 Jul '05

17 Jul '05

Hi there, > There are some other, minor problems -- for indexing you need ss!!0, the > name m01_mod, and you may need an 'import' to make getArgs visible. yup, and you have to get rid of the two "do"s in the application of putStrLn. Regards, Frank-Andre Riess

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RE: [Haskell-cafe] Unicode.hs (was: RE: Re[2]: ANNOUNCE: GHC surveyresults)
by Simon Marlow 17 Jul '05

17 Jul '05

On 30 June 2005 14:36, Dimitry Golubovsky wrote: > It is in CVS now, and I believe will be in 6.4.1 Not planned for 6.4.1, but definitely in 6.6. Cheers, Simon

2 1

O'Haskell
by Monique Louise 16 Jul '05

16 Jul '05

Is there any project for extending GHC or any other Haskell compiler with support to O'Haskell / any object-oriented Haskell extesion? That could be very useful for improving Haskell interoperability with OO languages. Thanks in advance, Monique Monteiro -- ________________________________ Monique Louise B.Monteiro Msc Student in Computer Science Center of Informatics Federal University of Pernambuco

1 0

weired
by wenduan 16 Jul '05

16 Jul '05

The following function which converts a number represents a sum of money in pence didn't work as expected and the result didn't make any sense to me: penceToString :: Price -> String penceToString p = let str = show p len = length str in if len ==1 then "0.0" ++ str else if len ==2 then "0." ++ str else (take (len-2) str) ++ "." ++ (drop (len - 2) str ) *Main> penceToString 234566666667899999999786 "-6710990.94" ?!?! -- X.W.D

2 1

Problem linking against Data.Graph
by Andy Gimblett 16 Jul '05

16 Jul '05

Hi all, I'm something of a Haskell newbie, and this is my first post to this list. I'm trying to do some very basic things with graphs, but can't get started with Data.Graph. In particular, linking fails - I'm wondering if something's wrong with my install. (For the record: ghc-6.4 compiled/installed fresh today on gentoo linux - it _looks_ OK to my Haskell-untrained eyes). Here follows a small example which I believe should compile just fine: ---- Code begins "Main.hs" module Main where import Data.Graph.Inductive import System.IO main :: IO () main = putStrLn "Hello, world!" ---- Code ends As you can see, this basically does nothing! Unfortunately, here's what happens: [csandy@cspcag spa_haskell] ghc -o Main Main.hs Main.o(.text+0x145): In function `__stginit_Main_': : undefined reference to `__stginit_DataziGraphziInductive_' collect2: ld returned 1 exit status I get similar errors whatever I try to do with Data.Graph. Trying to compile ghc-6.4/libraries/fgl/test/test.hs (from the source tree) gives me multiple errors of the above type (though not identical). If anyone can shed any light on this, or tell me what stupid thing I'm doing wrong, I'd really appreciate it. Many thanks, -Andy -- Andy Gimblett Computer Science Department University of Wales Swansea http://www.cs.swan.ac.uk/~csandy/

4 4

weird
by wenduan 15 Jul '05

15 Jul '05

The following function which converts a number represents a sum of money in pence didn't work as expected and the result didn't make any sense to me: penceToString :: Price -> String penceToString p = let str = show p len = length str in if len ==1 then "0.0" ++ str else if len ==2 then "0." ++ str else (take (len-2) str) ++ "." ++ (drop (len - 2) str ) *Main> penceToString 234566666667899999999786 "-6710990.94" ?!?! -- X.W.D

1 0

Functional dependencies and type inference
by Einar Karttunen 15 Jul '05

15 Jul '05

Hello I am having problems with GHC infering functional dependencies related types in a too conservative fashion. > class Imp2 a b | a -> b > instance Imp2 (Foo a) (Wrap a) > > > newtype Wrap a = Wrap { unWrap :: a } > data Foo a = Foo > data Proxy (cxt :: * -> *) > > foo :: Imp2 (ctx c) d => Proxy ctx -> (forall a b. (Imp2 (ctx a) b) => a -> b) -> c -> d > foo p f x = f x The type of "foo (undefined :: Proxy Foo)" is inferred as "forall c. (forall a b. (Imp2 (Foo a) b) => a -> b) -> c -> Wrap c" which shows the outmost functional dependence is working fine. ctx is carried to the inner Imp2. However "foo (undefined :: Proxy Foo) Wrap" will fail complaining that Couldn't match the rigid variable `b' against `Wrap a' `b' is bound by the polymorphic type `forall a b. (Imp2 (ctx a) b) => a -> b' at <interactive>:1:0-32 Expected type: a -> b Inferred type: a -> Wrap a In the second argument of `foo', namely `Wrap' My guess is that GHC cannot see that the functional dependency guarantees that there are no instances which make the inferred type invalid. Any solutions to this problem? - Einar Karttunen

1 0

RE: [Haskell-cafe] Re: Module.T naming style
by Simon Marlow 15 Jul '05

15 Jul '05

On 15 July 2005 09:48, Henning Thielemann wrote: > On Thu, 14 Jul 2005, Andrew Pimlott wrote: >> 2) enshrining one-type-per-module in the naming convention is not IMO >> justified, and may prove limiting; > > Other languages like Modula-3 and Oberon do it with great success. The > limit in Haskell is that most compilers don't conform to the Haskell > 98 report which allows mutually recursive modules. But I think the > compilers should allow them instead of forcing users to put many type > and class definitions into one module. I hope you weren't including GHC in "most compilers" :-) GHC's implementation of mutually recursive modules is conformant with Haskell 98 (see Section 5.7). Cheers, Simon

3 2

RE: [Haskell-cafe] Module.T naming style
by Simon Marlow 15 Jul '05

15 Jul '05

On 15 July 2005 11:24, Henning Thielemann wrote: > On Fri, 15 Jul 2005, Simon Marlow wrote: > >> On 15 July 2005 09:48, Henning Thielemann wrote: >> >>> The limit in Haskell is that most compilers don't conform to the >>> Haskell 98 report which allows mutually recursive modules. But I >>> think the compilers should allow them instead of forcing users to >>> put many type and class definitions into one module. >> >> I hope you weren't including GHC in "most compilers" :-) GHC's >> implementation of mutually recursive modules is conformant with >> Haskell 98 (see Section 5.7). > > But there are unresolved problems with class definitions. > http://www.haskell.org/pipermail/haskell-cafe/2005-May/009873.html > > Is this covered by the statement "Modules may be mutually recursive."? > http://www.haskell.org/onlinereport/modules.html Ok, mututal recursion between classes in separate modules isn't currently supported (I'd forgotten about that). But it's a bit harsh to say that because of this missing feature GHC doesn't support mutually recursive modules - we've been using mutually recursive modules in GHC itself for nearly a decade! Cheers, Simon

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Re: Coin changing algorithm
by Okasaki, C. DR EECS 15 Jul '05

15 Jul '05

ChrisK wrote: > During a single evaluation the recursive calls never "collide", so > unless this overlap is optimized, then the subproblem memoizing won't do > anything... Actually, the recursive calls can collide. For example, if you are trying to make 87 cents with 7 coins, you can make the first 80 cents with four coins using either 50-10-10-10 or 20-20-20-20. Either way, you'll end up making a recursive call on 7 cents and 3 coins. Admittedly, however, with this choice of coin denominations, you don't get very many such collisions... -- Chris

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Re: Coin changing algorithm
by Okasaki, C. DR EECS 14 Jul '05

14 Jul '05

This is a classic dynamic programming problem. Dynamic programming is easy to do in Haskell using recursive arrays. Instead of using recursive arrays directly, however, I'll add a few helper functions that make this kind of problem easier. > import Array > > tabulate :: (Ix a) => (a,a) -> (a -> b) -> Array a b > tabulate bounds f = array bounds [(i,f i) | i <- range bounds] > > dp :: (Ix a) => (a,a) -> ((a->b) -> a -> b) -> a -> b > dp bounds f = (memo!) where > memo = tabulate bounds (f (memo!)) 'tabulate' just turns a function into an array. 'dp' is a memoizing Y combinator. That is, you give the same kind of almost recursive function you would give to the Y combinator, and it "ties the knot" for you, but also interposes an array to hold the intermediate results. (What do I mean by an "almost recursive function"? I mean that instead of writing a recursive function "f = ...f...", you instead write f to take what will end up being the recursive function as its first argument. Then, whenever the f would normally call itself, it instead calls this argument, as in "f rec = ...rec...".) Now, here is the change algorithm, using esentially the same recursion as Radu, except that I introduce an 'i' parameter, that is the index of the largest denomination that you are allowed to use. (Using this parameter makes it easier to store the results in an array than passing around a list of the available coins.) > change m n = dp bounds f (m,n,8) where > bounds = ((0,0,0), (m,n,8)) > coins = listArray (1,8) [1,2,5,10,20,50,100,200] > f rec (m,n,i) > | m == 0 = [[]] > | n == 0 || i == 0 = [] > | c > m = rec (m,n,i-1) > | otherwise = map (c:) (rec (m-c,n-1,i)) ++ rec (m,n,i-1) > where c = coins!i In the f function, m is the amount of change to make, n is the number of coins that can be used, and i is the highest kind of coin that you are allowed to use. You can do most traditional dynamic programming problems this way. Enjoy, Chris

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Re: Coin changing algorithm
by Yitzchak Gale 14 Jul '05

14 Jul '05

Actually, something along the lines of Dinh's attempted solution to the original partition problem is a very nice solution to the coin changing problem: Make change for the amount a, using at most k of the coins cs. > coins _ 0 _ = [[]] > coins _ _ 0 = [] > coins cs a k = [h:s | t <- init (tails cs), > let h = head t, > h <= a, > s <- coins t (a-h) (k-1)] (The functions "init" and "tails" are from the Data.List module.) Note that Dinh is already using a monad for his solution, via Haskell's special syntax for the List monad. Here it is translated into regular monad syntax: > coinsM _ 0 k = return [] > coinsM _ _ 0 = [] > coinsM cs a k = do > t <- init (tails cs) > let h = head t > unless (h <= a) [] > s <- coinsM t (a-h) (k-1) > return (h:s) (The function "unless" is from the Control.Monad module.) -Yitz

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a simple function
by wenduan 14 Jul '05

14 Jul '05

Anyone please tell me what is wrong with the function: isEmpty ::[a]->Bool isEmpty xs | xs == [] = True |otherwise =False When I tried to load it into the interpreter,it says the following: Could not deduce (Eq a) from the context () arising from use of `==' at mylab2.hs:16 Probable fix: Add (Eq a) to the type signature(s) for `isEmpty' In a pattern guard for the definition of `isEmpty': xs == [] In the definition of `isEmpty': isEmpty xs | xs == [] = True -- X.W.D

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