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Doing exercises from "Haskell tutorial" ...
by Dmitry Astapov 02 Oct '01

02 Oct '01

It seems like an appropriate page for aske newbie questions, isnt it? I was reading through Haskell tutroial (http://www.di.uminho.pt/afp98/PAPERS/Tutorial.ps) and trying to do exercises from there. I'm stuck in the first exercise for "Classes" chapter (page 11). I need to define SetsAsLists as an instance of Set by supplying definitions for all Set methods, but definitions I wrote led me to adding additional constraints on "union" and "memeber" methods. Is it ok or it's possible to define "union" and "member" without using "==" ? Code follows: class Set s where empty :: s a isEmpty :: s a -> Bool singleton :: a -> s a union :: Eq a => s a -> s a -> s a member :: Eq a => a -> s a -> Bool choice :: s a -> (a, s a) data SetsAsLists a = SL [a] instance Set SetsAsLists where empty = SL [] isEmpty (SL []) = True isEmpty _ = False singleton x = SL [x] member x (SL ys) | isEmpty (SL ys) = False | otherwise = if x==head ys then True else member x (SL (tail ys)) union (SL []) (SL ys) = SL ys union (SL (x:xs)) (SL ys) | member x (SL ys) = union (SL xs) (SL (x:ys)) | otherwise = union (SL xs) (SL ys) -- Dmitry Astapov //ADEpt E-mail: adept(a)umc.com.ua GPG KeyID/fprint: F5D7639D/CA36 E6C4 815D 434D 0498 2B08 7867 4860 F5D7 639D

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update in-place
by Cagdas Ozgenc 02 Oct '01

02 Oct '01

> data Foo = Foo { a :: Int, b :: String } > > instance Show Foo where > show f = show (a f) ++ " " ++ show (b f) > > foo = Foo { a = 99, b = "green bottles\n" } > > main = do print foo > let bar = foo { a = 98 } > print bar The above code is from Haskell FAQ. I could not find the "data Foo = Foo }" notation in my Haskell, The Craft of Functional Programming book. What does it mean? Also > instance Show Foo where > show f = show (a f) ++ " " ++ show (b f) I do not understand show (a f) either. And same goes for "foo { a = 98 }" notation. Could you help me on this notation? Thanks

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Re: streching storage manager
by Marcin 'Qrczak' Kowalczyk 29 Sep '01

29 Sep '01

Sat, 29 Sep 2001 10:24:52 +0800 (GMT-8), Saswat Anand <saswatan(a)comp.nus.edu.sg> pisze: > As regard to Marcin's suggestion of using a list of compact arrays, > although elements can be accessed faster, there will be a lot if > redundancy since windows are overlapping. So consecutive arrays > will contain almost same data. I didn't mean to make them overlapping. Splitting into arrays is arbitrary. This can be made an abstract type. One possibility (perhaps another is needed): data CompactQueue a = CL [Array Int a] -- front [Array Int a] -- rear; becomes front (reversed) after front empty !Int -- block size, set during creation !Int -- offset in the head of front - this many elements -- logically don't exist !Int -- similarly, offset in the head of rear emptyQueue :: Int -> CompactQueue a -- Argument is block size. push :: CompactQueue a -> [a] -> CompactQueue a -- Add elements to the end. -- O(block_size) because it must copy one array. (!!) :: CompactQueue a -> Int -> a -- O(index/block_size). drop :: Int -> CompactQueue a -> CompactQueue a -- This can keep up to block_size elements too many in memory -- (in the head of front). Amortized O(n/block_size). When block_size is 1, it degenerates to a plain list or queue (implemented using one-element arrays). -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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streching storage manager
by Saswat Anand 29 Sep '01

29 Sep '01

Hi, I need some pointers and suggestion in the following problem: I have a very large time-series data. I need to perform some operation on that. At any instant of time, I only need a window-full of data, size of window being known. Starting from the first point, I slide the window towards right and operate on the data that reside in the current window. Currently, I read all of the data to arrays which takes a lot of space and hence GC takes up lot of time. After reading "Streching Storage Manager: Weak pointers, Stable names" paper, I had a feeling that I can do something better for my problem, especially weak pointers seem to be useful. But I do not really get the thread. What I would ideally like to do is: read all the data lazily into some data structure and as I slide the window towards right, I will invalidate the data that fall before the start point of current window. By invalidating, I mean I will somehow tell the GC to reclaim that space if necessary. Is there any way in which I can do this in Haskell, or there is a better(simpler) solution for this. Thanks, Saswat

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Re: streching storage manager
by Marcin 'Qrczak' Kowalczyk 28 Sep '01

28 Sep '01

Fri, 28 Sep 2001 20:28:29 +0900, Dylan Thurston <dpt(a)math.harvard.edu> pisze: > This sounds perfectly suited to Haskell's standard lazy lists. If you > only keep a pointer to the beginning of the data you need to work on, > then Haskell will automatically read in exactly as much data as you > use and GC it away after you are done with it. The downside is that > accessing elements within the window will take time O(window size). Perhaps window contents should be grouped in compact arrays (ghc provides some non-standard array variants more compact than Array). So in overall it's a list of arrays, with elements accessed as with something like l !! (i `div` blockSize) ! (i `mod` blockSize). -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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RE: Haskell <-> C/C++ comunication (sockets/pipes?)
by Simon Marlow 28 Sep '01

28 Sep '01

> Can you tell me what is the problem here? > Haskell app output is redirected to C++ input app (and > vice-versa) using > pipes. (used Erays shell script sugestion) > > When I try > Hsk: 1. get, 2.send > C++: 1. send, 2 get > > It works fine, but if I try it the other way around it locks. > C++ app won't get any input. > Seems to me like Hsk is not 'flushing' the output. You may need to set the buffering mode on the handle to LineBuffering, using IO.hSetBuffering. In GHC at least, the buffering mode is set to line buffering by default if and only if the handle is connected to a terminal. Cheers, Simon

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Re: Haskell <-> C/C++ comunication (sockets/pipes?)
by Marcin 'Qrczak' Kowalczyk 28 Sep '01

28 Sep '01

Thu, 27 Sep 2001 23:06:51 +0100, Jorge Adriano <jadrian(a)mat.uc.pt> pisze: > Seems to me like Hsk is not 'flushing' the output. I guess this is right (sorry, haven't checked). You can flush in two ways (using module IO): 1. At the beginning: hSetBuffering stdout LineBuffering or even hSetBuffering stdout NoBuffering (the latter slows down the I/O). 2. After each write to be flushed: hFlush stdout stdout is either line buffered or block buffered by default, depending on whether it's a terminal or not. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Re: Haskell <-> C/C++ comunication (sockets/pipes?)
by Jorge Adriano 28 Sep '01

28 Sep '01

Can you tell me what is the problem here? Haskell app output is redirected to C++ input app (and vice-versa) using pipes. (used Erays shell script sugestion) When I try Hsk: 1. get, 2.send C++: 1. send, 2 get It works fine, but if I try it the other way around it locks. C++ app won't get any input. Seems to me like Hsk is not 'flushing' the output. If this is the case, I can't really explain why it happens... if the output was not redirected I am pretty sure that, before trying to get input the output would be flushed (imagine something like putStrLn "please type an int:") So what is the issue here? Here is the code, And once again, thanks in advance. J.A. ------------------------------- Haskell app: send -> get ------------------------------- main :: IO() main = do --send hsnum putStrLn "6" -- get cnum and save num <- readLn :: IO(Int) writeFile "./reshs.txt" (show num) ---------------------------- C++ app: get -> send ---------------------------- #include <cstdio> #include <iostream> #include <fstream> nt main(){ // *get* hsnum and save int num; ofstream out("rescc.txt"); cin >> num; out << num; // *send* cnum cout << 10 << '\n' << flush; //it works without the flush return 0; }

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RE: GUI Library Task Force
by Simon Marlow 26 Sep '01

26 Sep '01

> Thats great. I don't disagree. Its just a matter of priority. > So how about this agenda? Careful: we're in danger of not getting *anything* done if we try to prioritise the tasks like this. I think it's far more important to just have one complete GUI system that everyone can use, even if it isn't perfect. I'm absolutely in favour of the current GUI effort. So, when the hierarchical libraries come on-line then we'll have to rename the modules of the GUI library to place them in the hierarchy. That's not a big deal, since we're going to rename all our other library modules too. > Addendum 1: Hierarchical Library Namespace The extension is virtually finalized and all the compilers implement it. However, there's the much bigger task of actually moving over to using the hierarchically organised libraries; this effort is ongoing. We intend to move GHC over for the next version, but leave some compatibility libraries in place. > Addendum 2: Concurrency Hasn't changed much in ages, and largely independent of the others (apart from module name differences with the library hierarchy). Consider GHC's implementation as a draft proposal for a report adendum. I plan to write up a proper proposal at some point (although if someone else were to volunteer, I wouldn't complain :-). > Addendum 3: FFI Almost finalized. Again, independent apart from module names. > Addendum 4: Exceptions Consider GHC's implementation a proposal, but omit the asynchronous exception support. Hugs (in CVS) implements nearly the same interface. > Addendum 4.5?: Proc Syntax for Arrows > Addendum 5: Library Interface Compatibility Guidelines The document we've been drafting for the new library organisation has the beginnings of a set of guidelines, and contributions are of course welcome. The point I'm making is that we can parallelise these tasks to a certain extent, and that doing so is essential in order for us avoid bottlenecks and to converge at all. I agree with many of the points you raise about Haskell needing these features in order to be an "industrial strength" language, but saying things like "we must wait for X so we can do Y properly" has historically not resulted in X appearing in a timely fashion, which in turn leads to no Y at all. Manuel has done a good job of specifying the job of the GUI task force so that it doesn't depend on any of these other tasks, and I think that's the right way to go. Cheers, Simon

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Haskell <-> C/C++ comunication (sockets/pipes?)
by Jorge Adriano 26 Sep '01

26 Sep '01

Hi all, I was thinking about making some GUIs in Qt/KDE for some haskell applications (compiled with ghc). My first idea was to simply run haskell applications with some args using the GUI and get the results from some file or something like that. Anyway someone told me that a better idea would be to use pipes to enable the GUI to comunicate with the application. I checked the ghc libs and there was a pipe function, but I would need named pipes here (right?) since the two processes are not related. I didn't found any function for that in the ghc libs... is there one? I was also told that another (better way) to do this would be to use sockets. I've worked a little bit with posix stuff (fork, pipes, etc) but I never worked with sockets. I was thinking about using named pipes if that's possible, anyway, Id like to know what you think. Is possible. Is sockets a good solution? better? worth the trouble of learning how to use them? Any better ideas? J.A. PS: Good luck to the GUI task force ;)

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RE: Joy and Concatenative Programming
by Frank Atanassow 25 Sep '01

25 Sep '01

[redirected to haskell-cafe] > I just found out about a functional programming language called Joy (see > http://www.latrobe.edu.au/philosophy/phimvt/joy.html) > > Joy differs from Haskell in that it has no variables. Instead, all > functions are postfix, taking a stack as their argument and returning a > stack as a result. Also, IIRC, Joy is untyped. Programming in Joy is like programming with categorical combinators, but ignoring the types. Indeed, a category with one object is just a monoid, and this is supposedly the theory Joy is founded on. But if you really think variables are a nuisance to programming, you can do it in Haskell too, by using list combinators and defining a reverse composition operator. Or just go program in CAML bytecode. It's pretty much the same thing. > Joy advocates contend that the elimination of variables and environments > improves on functional languages in much the same way the elimination of > state improved on imperative languages. I think the situations are qualitatively different. Saying that Joy programs "lack variables" is pretty close to saying that its free variable environments are linearly ordered. Yet a Joy programmer can reorder the arguments on the stack if he rewrites his program to take account of that fact. So, though the ordering is really irrelevant, every Joy program must pick one, and that can be understood as non-declarative, since it has nothing to do with the result of the program. FP languages factor out that arbitrary choice by treating free variable environments as (multi-)sets. (Of course, you lose that advantage when you package up your environment in a closure, because Haskell distinguishes between a -> b -> c and b -> a -> c, even though they are isomorphic.) --- Frank Atanassow, Information & Computing Sciences, Utrecht University Padualaan 14, PO Box 80.089, 3508TB Utrecht, The Netherlands Tel +31 (0)30 253-3261 Fax +31 (0)30 251-3791

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RE: gconsym
by Simon Peyton-Jones 24 Sep '01

24 Sep '01

I don't really understand the "regardless of.." part. But, yes, ":" is a qconop, with precedence and associativity infixr 5. Simon | -----Original Message----- | From: Ian Lynagh [mailto:igloo@earth.li] | Sent: 24 September 2001 13:34 | To: haskell-cafe(a)haskell.org | Subject: gconsym | | | | Just to make sure I am interpreting the report correctly, is | ":" meant to be a valid qconop regardless of the | precedence-level and associativity of the qconop? | | | Thanks | Ian | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe |

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gconsym
by Ian Lynagh 24 Sep '01

24 Sep '01

Just to make sure I am interpreting the report correctly, is ":" meant to be a valid qconop regardless of the precedence-level and associativity of the qconop? Thanks Ian

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RE: Socket library from hslibs
by Simon Marlow 21 Sep '01

21 Sep '01

Chris Webb writes: > For simple networking, I usually find that the simple Socket interface > from hslibs does everything I need, except that I always need > to import > sClose from SocketPrim as well, so I can close the sockets I open. > Tidying up some old code this morning, I was wondering: is the absence > of sClose in the Socket interface accidental, or should I be disposing > of listening sockets some other way? No, I think you're right. Socket should really export sClose too. Perhaps it should be called Socket.close, or closeSocket? Cheers, Simon

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Socket library from hslibs
by Chris Webb 21 Sep '01

21 Sep '01

For simple networking, I usually find that the simple Socket interface from hslibs does everything I need, except that I always need to import sClose from SocketPrim as well, so I can close the sockets I open. Tidying up some old code this morning, I was wondering: is the absence of sClose in the Socket interface accidental, or should I be disposing of listening sockets some other way? Cheers, Chris.

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Re: let/where
by John Hughes 20 Sep '01

20 Sep '01

The point of where is that it scopes over guards and multiple equations as well as right hand sides. f x | xsquared < 1000 = xsquared | otherwise = 0 where xsquared = x*x For the same reason, it has to be restricted to appear at the "top level" of a right hand side. Of course, we also need a construct to bind local variables inside an expression -- hence let, which therefore cannot scope over guards. Two kinds of scope, two binding constructions. We could have got away with one, if we had thrown out guards and relied just on if-then-else to test conditions instead. But guards are very expressive: some programs would have been a lot harder to write without them. Typical examples are when we just want to pick out one case in the first equation of a function, before lots of pattern matching: f x y | simplecase x y = simpleanswer f p1 p2 = ... f p3 p4 = ... ... There's no neat way to write this with if-then-else: you need to split f into two functions f x y = if simplecase x y then simpleanswer else f' x y f' p1 p2 = ... ... That's the reason Haskell has two binding constructions. John Hughes

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Re: let/where
by Norman Ramsey 20 Sep '01

20 Sep '01

> There was some debate about which to have - it's a matter of personal > preference and style. The problem is that if you have both let and where > in the expression syntax, then > > let a in b where c > > is ambiguous unless you make an arbitrary decision about the precedence, Was it not considered to forbid such ambiguous expressions, requiring explicit parentheses to resolve the ambiguity? Norman, who sometimes wishes `where' were a postfix `let', and not part of the declaration syntax (though he does not wish to give up scoping over multiple guards).

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let/where
by Mark Carroll 19 Sep '01

19 Sep '01

There seem to be a few situations where it's not clear to me when to use "let" and when "where". For instance, in this little example I was playing with to work out what syntax works, main = putStr (show (if maybe_index == Nothing then DP_Unknown else DP_Number index) ++ "\n") where maybe_index = maybe_read word (Just index) = maybe_index or... main = let maybe_index = maybe_read word (Just index) = maybe_index in putStr (show (if maybe_index == Nothing then DP_Unknown else DP_Number index) ++ "\n") Does anyone care? At the moment I use "where" so that at a first glance you get an overall idea of things, then you can read further for details if you like. I was disappointed to find that I don't seem to be able to write things like, main = let maybe_index = maybe_read word in putStr (show (if maybe_index == Nothing then DP_Unknown else DP_Number index) ++ "\n") where (Just index) = maybe_index BTW, is the above a sane way of getting the 'index' 'out of' the "Just"? I often seem to be using a "where (Just foo) = bar" type of idiom. (Of course, there is that special "let" stuff for "do" notation too which doesn't seem to use "in".) I hope all that was somewhat coherent, anyway, or at least sheds light on some of the confusion of newcomers to Haskell! -- Mark

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Subscribe
by John Miller 17 Sep '01

17 Sep '01

ProClarity Corporation John Miller Research & Development <mailto:jmiller@proclarity.com> jmiller(a)proclarity.com (208)344-1630 ext. 111

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Re: Multiple applications of a function
by Marcin 'Qrczak' Kowalczyk 16 Sep '01

16 Sep '01

Sun, 16 Sep 2001 10:52:07 -0400 (EDT), Mark Carroll <mark(a)chaos.x-philes.com> pisze: > To multiply apply a function, I'm currently using: > > multiplyApply f n a = (iterate f a) !! n > > ...is there a Prelude function I've missed that already does this? Unfortunately not. > Could I be doing this better? I think an explicit recursion would make it easier to compile efficiently, and it can give a better error message for n < 0. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Multiple applications of a function
by Mark Carroll 16 Sep '01

16 Sep '01

To multiply apply a function, I'm currently using: multiplyApply f n a = (iterate f a) !! n ...is there a Prelude function I've missed that already does this? Could I be doing this better? -- Mark

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Re: Application letters at the Haskell workshop: suggestion
by Ashley Yakeley 16 Sep '01

16 Sep '01

At 2001-09-15 08:31, Mark Carroll wrote: >AFAICS a simple way to get >out of this is to only have one exception type that carries no information >instead of different ones so we can't distinguish one exception from >another, but that's obviously not great. Isn't that what 'bottom' is? -- Ashley Yakeley, Seattle WA

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Re: controlling resource usage
by D. Tweed 11 Sep '01

11 Sep '01

On Fri, 7 Sep 2001, Richard Uhtenwoldt wrote: > Finally, are we all in agreement that it would be a great thing if > someone invented a Haskell-like language as good as Haskell at hiding > resource usage when resource usage is unimportant, but that made it > easier to control resource usage than Haskell does when resource usage > is important? Of course it's not just in functional languages that performance demands can require rewriting shorter, more comprehensible code in less obvious ways. (My favourite example of something so completely non-intuitive is the Intel example from within a Sieve of Eratosthenes program that for(i=0;i<lim/step;++i){ prime[i*step]=0; } can on some processor setups be slower than for(i=0;i<lim/step;++i){ if(prime[i*step]!=0){ prime[i*step]=0; } } due to cache coherency issues.) In that context, there was an old suggestion of Don Knuth's that program source code should in the idealized future not just contain the final, optimized code (with perhaps the original code in comments) but as a kind of set of layers each representing the same semantic program but with increasing orders of optimisation, and links between `things' annotated with a label placed by the programmer explaining why the transformation between an unoptimised version `thing' and an optimised `thing'. The nice bit about this would be that the development environment would be equipped with a subsystem that could almost always establish (possibly using the the label on the link) if the two things had the same semantic effect, and a naive system that could try and produce (maybe using brute force pattern modification) the changes to the optimised version corresponding to changes to the unoptimised version. The nice thing about this is that it would allow changes to the code (due to say changing requirements, or ideas for better algorithms) to be made first to the simple version and then the areas of the optimised version that needed changing would be automatically brought to the programmers attention. (The system for trying to change the optimised code automatically is just there to stop the system becoming really annoying every time you do something something like int i=1; _______\ int i=2; int j=2; / int j=1; to the unoptimised code. The reason I think this would be a good idea is because I don't really object that much to having to optimise code that's performance critical, I just dislike the way that I never manage to keep both the original and optimised version in sync and so end up having to maintain the optimised code directly. I suspect that such a system is still at least 25 years away from feasiblity though :-( ___cheers,_dave________________________________________________________ www.cs.bris.ac.uk/~tweed/pi.htm |tweed's law: however many computers email: tweed(a)cs.bris.ac.uk | you have, half your time is spent work tel: (0117) 954-5250 | waiting for compilations to finish.

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Reproducing lazy evaluation in Scheme
by David Barton 10 Sep '01

10 Sep '01

All right, I know this is a stupid question. I know the models of evaluation in call by name and call by value languages; however, I am kind of puzzled by trying to reproduce call by name in a call by value language using force and delay. If I put a delay around the right side of each equation and around each expression in a function call (*all* function calls?), and a force call around each variable reference, would that get me call by name semantics? Am I completely screwed up here? Is something more necessary? I'm trying to get the gumption up to try a proof by substitution of this idea, but I cringe at the formula manipulation....... Dave Barton <*> dlb(a)averstar.com )0( http://www.averstar.com/~dlb

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Pickling
by Mark Carroll 10 Sep '01

10 Sep '01

Am I right in thinking that there aren't currently any standard libraries for converting values to bitstreams and back, to be saved in files or squirted over network sockets or whatever to be resurrected later or elsewhere? I'd guess it would be hard to write, too - I've not noticed any introspection. How do people normally efficiently pass complex Haskell data structures to other co-operating Haskell processes? (I am thinking of something a bit like Java's serialisation.) -- Mark

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Algebraic types and `objects'
by S.J.Thompson 06 Sep '01

06 Sep '01

To follow up Cagdas Ozgenc's message of Wednesday, I have attached a series of files which show a gradual transformation (or refactoring) of an algebraic type of shapes into a Shape type, built using type classes and existential types, which more corresponds to an OO approach. Hope this is useful. Simon

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algebric types and db integration
by Cagdas Ozgenc 05 Sep '01

05 Sep '01

Hi, I am wondering how one would store instances of algebric types in a database. For example: data Shape = Circle Float | Rectangle Float Float | Triangle Float Float Float All three constructors are representing different schemas. Hence I won't be able to store them in a single table. I think algebric types seem to represent an inheritance hierarchy in terms of OO terms, Shape being the base class, and Circle, Rectangle, Triangle being its subclasses. Is this a correct way of looking at it? How do you suggest I conceptually map it to a relational database? Thanks

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Using Set: Hypergraph type
by Eray Ozkural 04 Sep '01

04 Sep '01

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Greetings all! I'm a new Haskell user. I've used Haskell for a number of small hacks and I can say that this is a brilliant language. I was just working on a hypergraph implementation in haskell. A hypergraph, for those not familiar with it, is a family of sets which are subsets of a vertex set X. I went straight along the mathematical definition, and I decided to try the Edison library that came along with my ghc-5.00.2 Without further ado, here is the code: type Hypergraph n = Set (Set n) - -- hgraph constructor - -- takes a list of edges hgraph edge_list = unionManySets (map mkSet edge_list) My trouble is that this does not seem to work well. Perhaps it's my lack of experience wih Edison, but the types do not seem to be right and the following expression fails to evaluate: Hypergraph> elementOf (mkSet [1]) (hgraph [ [1,2], [1] ]) I expect "True" in response to this query. Anyway, I think this is not ghc specific. (I enabled -fglasgow-exts, no difference. I get a lot of type errors...) What do you think is wrong with this code? It now seems to me that I may not be able to use Edison for my ADT, so how would you recommend me to go about coding this mathematical definition? All I need is to be able to define a set of sets in a convenient way; it also has to be halfway efficient. Should I implement this with lists, or is there an alternative set implementation which allows me to accomplish a decent hypergraph definition? [*] Regards, [*] I could also use a mix of arrays and lists it seems, similar to the edge list and pin list representations used in imperative code. But that does not give me the interface a set implementation would :) - -- Eray Ozkural (exa) <erayo(a)cs.bilkent.edu.tr> Comp. Sci. Dept., Bilkent University, Ankara www: http://www.cs.bilkent.edu.tr/~erayo GPG public key fingerprint: 360C 852F 88B0 A745 F31B EA0F 7C07 AE16 874D 539C -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.6 (GNU/Linux) Comment: For info see http://www.gnupg.org iD8DBQE7lKJQfAeuFodNU5wRAtyBAJ0SCRLAV5u4IBkt3B2VIYB04l9LNACeNL0Y GE9TPhrWoPY6rH58ZDSdweg= =IdNC -----END PGP SIGNATURE-----

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Re: Using Set: Hypergraph type
by Marcin 'Qrczak' Kowalczyk 04 Sep '01

04 Sep '01

Tue, 4 Sep 2001 21:12:41 +0300, Eray Ozkural <erayo(a)cs.bilkent.edu.tr> pisze: > Thanks for the suggestion, but isn't this a bit inefficient? Ordinarily one > would like to compare two elements of a set by reference, not by content. There is no such thing in Haskell. mkSet [mkSet [1, 2]] is indistinguishable from mkSet [mkSet [1, 2]] even if {1,2} sets were created independently. Whether representations of particular mkSet [1, 2] objects are shared is an implementation detail. Well, there are compiler-dependent unsafe functions which could detect sharing, but they are not normally used. Functional programming is about values, not about objects with identities. You could implement a "tagged object" type, like data Tagged a = Tagged Integer a give objects unique numbers as they are created, and make Eq and Ord instances which compare these numbers only, ignoring the real value. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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Re: Using Set: Hypergraph type
by Marcin 'Qrczak' Kowalczyk 04 Sep '01

04 Sep '01

Tue, 4 Sep 2001 20:28:55 +0300, Eray Ozkural <erayo(a)cs.bilkent.edu.tr> pisze: > Okay, I guess I got it now. The (types of) elements of a Set ought > to be instances of Ord. So I presume Edison supports only flat sets. You can add an inefficient implementation of Ord (Set a) yourself: instance Ord a => Ord (Set a) where compare s1 s2 = compare (toOrdList s1) (toOrdList s2) toOrdList seems to be exported only module Collection. If more than one module does this, they can't be used together. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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some problem question
by karan babuta 02 Sep '01

02 Sep '01

hi there i am having some problem with the haskell question. if u can help me in that will really be appreciated. thanks for help. MY PROBLEM IS WITH THE SEPERATING THE SHAPES IN A GRID waiting for a sooner reply.THANKS here is the question: A rectangular grid is list of list of Char, where all inner lists are of the same length ie. type Grid = [[Char]] . Inside this grid, white space will be denoted by a '.' and filled pixels will be denoted by '*'. For example, a grid containing a square may look like square :: Grid square = ["............", "............", "...****.....", "...****.....", "...****.....", "...****.....", "............" ] The shapes can be anywhere inside the grid and they can can be of any size > 2 x 2 . That is, the only restriction on the size of the shape is a minimum of 2 x 2 area on the grid. And in cases of non-square, the shape will at least occupy 4 pixels. You can assume that no shape given will be smaller than this and that the shapes given will distinctly fall into one of the categories and will not be ambiguous. There is no restriction on the size of the grid itself, but obviously, it must fully contain the shape(s). There can be either one or two shape(s) inside a grid, but if there are two shapes in the grid, then they are guarenteed to be at least 1 pixel apart, and that they can be separated by cutting the grid either horizontally or vertically. twoTriangles :: Grid twoTriangles = ["....................", "....................", "...........*********", ".....*......*******.", "....***......*****..", "...*****......***...", "..*******......*....", "...................."] Shapes The following shapes are allowed inside a grid : RECTANGLE - The very same rectangle as we all know it. It will not be rotated. TRIANGLE - If it looks like a triangle, then it's a triangle. Note that this means the length of the three edges can be different, and it can be rotated in any way. (You'll notice that this is a very ambiguous definition of a triangle. Within the restraints of this specification, including that (1) all shapes given will be valid. (2) there are only three shapes to be identified. and (3) definition of the other shapes are unambiguous and rigid. You should think about how you can identify the triangle using the simplist way you can think of.) DIAMOND - Any thing that has a shape like [".............", ".............", ".............", ".....*.......", "....***......", "...*****.....", "..*******....", "...*****.....", "....***......", ".....*.......", "............."] To make this easy, a diamond will have horizontal symmetry as well as vertical symmetry. Tasks You are to write the following functions: areaOfOneShape:: Grid -> Int For the shape in the grid, you should count the number of pixels it occupies. (ie, count the number of '*' for each shape), and return the count. recogniseOneShape:: Grid -> String The input Grid will be as describe above, with only ONE shape in the grid, and the output String should be the name of the shapes, eg, "RECTANGLE" Note: You MUST capitalise the name of the shapes in the string you returned, and you MUST spell the name of the shapes correctly The ONLY names of the shapes allowed are the ones described above in the Shapes section PROBLEM: separateShapes :: Grid -> [Grid] The input Grid will contain either one or two shapes. You are to separate the shapes into separate Grids, where each contains a whole shape respectively; ie, if there are two shapes in the original Grid, then you should return two Grids containing a shape each. If there is only one shape in the orignal Grid, then you should just return a list containing a single Grid. The Grids returned must also be rectangular. areaOfShapes :: Grid -> [Int] You should return the area of the shapes. If there is only one shape, then the output will just be a list with one Int. If there are two shapes, then the output will be a list of two Int. The order of the integer returned does not matter. You should (but you don't have to) write this function using functions already defined above. recogniseShapes :: Grid -> [String] Give a Grid containing one or two shapes, return a list of String of the names of the shapes. If there are two shapes, then you can return the name of the two shapes in any order you like. Eg, If there is a triangle and a diamond in the grid, then you should return ["TRIANGLE", "DIAMOND"] (the order of the names does not matter). If there is just a single shape, then you just return a list with a single string, eg, ["RECTANGLE"]. You should (but you don't have to) write this function using functions already defined above. ____________________________________________________________ Do You Yahoo!? Get your free @yahoo.co.uk address at http://mail.yahoo.co.uk or your free @yahoo.ie address at http://mail.yahoo.ie

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Re: hiding import, tuple instances
by Ashley Yakeley 01 Sep '01

01 Sep '01

At 2001-08-31 13:59, I wrote: >At 2001-08-31 09:27, Marcin 'Qrczak' Kowalczyk wrote: > >>> How do I define functions that work on all tuples? >> >>You can't, and this is unfortunate. > >You can make your own 'tuples' using cons = (,) and nil = (). Then you >can use multi-parameter classes to manipulate them. See, for instance, <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/truth-framework/Source/Type Calc/ListType.hs?rev=HEAD&content-type=text/vnd.viewcvs-markup> ListType> concatenate ('p',(3,(False,()))) ("Hello",(7,())) ('p',(3,(False,("Hello",(7,()))))) -- Ashley Yakeley, Seattle WA

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classes
by Cagdas Ozgenc 31 Aug '01

31 Aug '01

Hi, I am a newbie, so I might be making nonsense. In the following definiton: class Eq a where (==) :: a -> a -> Bool Why do we conlude that both of the "a" refer to the same type? Apparently using different types that both belonging to class Eq results in an error. For example 65 == 'a' It doesn't make much sense at first, however, how do you go by implementing multiple polymorphism? Secondly, why does Haskell distinguish between a Type and a Class? There seems to be no benefit of such approach. Thirdly, is it possible to define meta-classes in Haskell? Thanks

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Re: hiding import, tuple instances
by Ashley Yakeley 31 Aug '01

31 Aug '01

At 2001-08-31 09:27, Marcin 'Qrczak' Kowalczyk wrote: >> How do I define functions that work on all tuples? > >You can't, and this is unfortunate. You can make your own 'tuples' using cons = (,) and nil = (). Then you can use multi-parameter classes to manipulate them. -- Ashley Yakeley, Seattle WA

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Re: hiding import, tuple instances
by Marcin 'Qrczak' Kowalczyk 31 Aug '01

31 Aug '01

Fri, 31 Aug 2001 18:53:11 +0300, Cagdas Ozgenc <co19(a)cornell.edu> pisze: > 1) How can I hide an instance of a class imported from prelude? You can't. A given class/type pair must have at most one instance in the whole program. > How do I define functions that work on all tuples? You can't, and this is unfortunate. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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hiding import, tuple instances
by Cagdas Ozgenc 31 Aug '01

31 Aug '01

Hi, 1) How can I hide an instance of a class imported from prelude? For example I would like to redefine instance (Ord a, Ord b) => Ord (a,b) where (<=) (x,y) (z,w) | x < z = True | x == z = if y <= w then True else False | otherwise = False for educational purposes. 2) I could not find an entry for this instance in Prelude instead I found instance Ord () where compare () () = EQ Is this suppose to mean "for all tuples"? How do I define functions that work on all tuples? Thanks for taking time

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Job advert: Compiler designer/architect
by Benedict R. Gaster 31 Aug '01

31 Aug '01

The following job advert has been sent to both the GCC and Pro64 mailing lists and I thought maybe of interest to people on this mailing list. SuperH are not looking for Haskell developers but rather are keen to recruit people with experience in language design and implementation. (The Pro64 is a highly optimizing compiler initially designed by SGI and targeted for the MIPS and IA64 architectures. Recently the Pro64 was made open source and SuperH is in the process of porting this technology to a selection of their current and future microprocessors.) Advert: SuperH, Inc is a new company formed by STMicroelectronics and Hitachi in order to market and develop the SuperH series of microprocessors as an independent company. The SuperH series will be offered as licensable cores for synthesis into embedded applications. See www.superh.com for more information. SuperH is headquartered in San Jose CA with R&D centers in Bristol UK and Tokyo Japan. SuperH will be developing software systems based on Open Source tecnology, including Gnu tools (i.e., gcc, gdb, etc.) and Pro64, and is looking for a range of engineers who have interests and/or experience in the following skill areas: o Code generation and optimization. Software pipelining, register allocation. Specific experience with gcc or Pro64 would be an advantage. o JIT compilation and dynamic optimization. o Linux development. o Architecture design. o Benchmarking. o Debugging technologies including gdb. SuperH will be developing products based on Gnu and Linux technogy as well as carrying out research into new innovative architectures and advanced compiler design, including the Pro64 and dynamic optimization. The main location for these posts is Bristol, UK. SuperH will be offering a comprehensive benefits package as well as an emloyee stock option scheme and bonus schemes. Job Title: Compiler designer/architect. Qualifications. Good knowledge of compiler optimizations, experience with Gnu or Pro64 would be useful. In depth knowledge of C & C++. Java an advantage. Experience in embedded system design. Please send CVs to: benedict.gaster(a)st.com Benedict R. Gaster SuperH, Inc.

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help me!!!!
by Garry Hadiwibowo 30 Aug '01

30 Aug '01

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RE: Numeric literals
by Simon Marlow 29 Aug '01

29 Aug '01

> On Tue, 28 Aug 2001, Simon Marlow wrote: > (snip) > > In GHC, you can do this: > > > > import Exception > > > > do result <- catch (evaluate (read "foo" :: Int)) > > (\error -> ... ) > (snip) > > I do like that. Is it likely to become standard someday, do you think? > Errors like this should certainly be catchable in such a way; > I was sad > when I was first looking into this and found that 'catch' seemed to be > inextricably linked to IO. Well, we'd like it to become standard :-) The exception stuff is really very easy to implement - you just use the standard stack-unwinding implementation of throw, and catch has to push a special stack frame. GHC's asynchronous exceptions are slightly harder, but I don't think it was more than a day or two's work. As a bonus you get to use a more efficient implementation of the IO monad, because it doesn't have to deal explicitly with exceptions. And then IO becomes a straightforward instance of ST. I guess I should also document the Exception extension in a self contained way, similar to the way the FFI extension is documented. Cheers, Simon

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RE: Numeric literals
by Simon Marlow 29 Aug '01

29 Aug '01

> I also haven't yet worked out how to tell if a string is "read"able or > not, yet - if (read "45")::Integer or whatever gives an error > (e.g. if I'd > put "af" instead of "45"), it seems to be pretty uncatchable, > which is a > pain. How do I tell if an instance of reading will work, or > catch that it > didn't? In GHC, you can do this: import Exception do result <- catch (evaluate (read "foo" :: Int)) (\error -> ... ) but unfortunately read doesn't raise a useful exception (just error "Prelude.read: no parse"), so you can't filter it very easily. However I don't think that read is likely to raise any other exceptions. Cheers, Simon

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RE: Numeric literals
by Simon Marlow 29 Aug '01

29 Aug '01

> Today's other question has to do with numeric literals. How do I write > literals for interesting IEEE values, e.g. > > Prelude> x > Infinity > Prelude> :type x > Double > Prelude> (read "Infinity") :: Double > *** Exception: Ratio.%: zero denominator > Prelude> This looks like a bug in GHC, which has now been fixed for version 5.02. Note that Haskell leaves the behaviour of these exceptional conditions undefined. Hugs for example will give a program error on divide by zero, and NHC has even more interesting behaviour: Prelude> 1/0 :: Float 34.02823669209384634633746074317682114560000000 Cheers, Simon

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Re: Easy syntax question
by Mike Gunter 29 Aug '01

29 Aug '01

Or, if you need several cases, you can use something like: > funcGuard n | n `elem` [2,5,9] = 5 > | n `elem` [4,18] = 1 > | otherwise = 7 or, if you want an expression: > caseGuard n = case undefined of > _ | n `elem` [2,5,9] -> 5 > | n `elem` [4,18] -> 1 > | otherwise -> 7 mike Dean Herington <heringto(a)cs.unc.edu> writes: > Mark Carroll wrote: > > > I can write a function x :: Integer -> Integer that returns 5 if I give it > > 2, 5 or 9, or 7 otherwise. Say, > > > > x 2 = 5 > > x 5 = 5 > > x 9 = 5 > > x _ = 7 > > > > Generally, this is a question about where multiple cases lead to the same > > thing, maybe even in the middle of a function. (Like C's "case 1: case 2: > > case 3: foo; break;".) > > > > Does it get any better than this, though? I can't convince 'case' to do > > something like, > > > > case n of > > 2,5,9 -> 5 > > otherwise -> 7 > > > > Am I missing some syntax somewhere? I'm lost in the grammar in the Haskell > > report. > > > > -- Mark > > I would write: > > > if elem n [2,5,9] then 5 else 7 > > Dean

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RE: Prefix negation
by Simon Marlow 29 Aug '01

29 Aug '01

> (Moved to haskell-cafe) > > Syntacticly, -x parses as 0-x. That is, there is only one - > operator and the "0" is filled in when it is encountered in a unary > position. Thus > > -3 ^ 2 translates to 0 - 3 ^ 2 Slight correction: '-e' is in fact replaced by 'negate e', after fixity resolution, where negate is a method in class Num. The default definition of negate is 'negate x = 0 - x', but there's no requirement that it means this in general. The above example parses as '-(3^2)' because ^ has a higher precedence than unary '-' (8 vs. 7). Translation turns the expression into 'negate (3^2)'. Someone else will have to comment on *why* the precedences are ordered this way, though. Cheers, Simon

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Re: Prefix negation
by John Peterson 29 Aug '01

29 Aug '01

(Moved to haskell-cafe) Syntacticly, -x parses as 0-x. That is, there is only one - operator and the "0" is filled in when it is encountered in a unary position. Thus -3 ^ 2 translates to 0 - 3 ^ 2 There were many arguments on either side of this debate but I would have to dig deep into the archives to remember what they were. This is certainly a potential source of confusion! Anyway, experienced Haskellers always add parens around unary minus to avoid any ambiguity: (-3) ^ 2 John

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a shorter phrase than "algebraic data type"
by Richard 29 Aug '01

29 Aug '01

does anyone have a shorter name for a "algebraic data type" --the thing that is created by the declaration beginning with the keyword "data"? "ADT" is already in use, meaning "abstract data type", in many textbooks like Essentials of Programming Languages. the best I could come up with is "SPT", short for "sum-of-products" type.

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Re: ~ symbol
by Marcin 'Qrczak' Kowalczyk 29 Aug '01

29 Aug '01

Wed, 29 Aug 2001 12:06:37 +0300, Cagdas Ozgenc <co19(a)cornell.edu> pisze: > What is the use of ~ symbol in Haskell? For example in prelude > function unzip > > unzip = foldr (\(a,b) ~(as,bs) -> (a:as, b:bs)) ([], []) When a lambda abstraction is applied to arguments, objects passed as arguments are matched against patterns in the lambda. The pattern (a,b) causes evaluation of the top-level constructor of the matched value (the constructor must be (,) because of the type) and binds a and b to arguments of the constructor. Matching against the pattern ~(as,bs) succeeds without evaluation of the value. Names as and bs are bound to expressions which cause evaluation of the top-level constructor of the matched value and select appropriate components. So with \(a,b) the constructor is evaluated when the lambda is applied, before evaluation of its body. With \ ~(as,bs) it's evaluated when as or bs are evaluated inside the body. The effect is that laziness is improved: parts of the result are available before the input is consumed. The first argument of foldr should better be lazy in its second argument, so foldr can return something without waiting for the result of the recursive call on the tail of the list being traversed. With this definition unzip ((foo, bar):whatever) can return (foo:whatever', bar:whatever'') where evaluation of whatever' and whatever'' causes further traversal of whatever. It doesn't wait until 'unzip whatever' returns the pair but it returns an object whose components cause evaluation of 'unzip whatever'. There is no ~ on (a,b). This implies that unzip (spam:eggs) does evaluate spam to obtain a pair before returning anything. It delays only the recursive call of 'unzip eggs'. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK

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RE: String matching
by Simon Marlow 29 Aug '01

29 Aug '01

> I'm adding a search function to my www forum implemented in Haskell. > > Is there a Prelude/Library function that will efficiently > find a pattern > in a String? Nothing in modules PreludeList or List seem to > fit the bill. If you're using GHC, then there's a pretty good interface to the GNU regex library in module RegexString (see http://www.haskell.org/ghc/docs/latest/set/regexstring.html) Cheers, SImon

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String matching
by Andrew Rock 29 Aug '01

29 Aug '01

Hi Haskellers: I'm adding a search function to my www forum implemented in Haskell. Is there a Prelude/Library function that will efficiently find a pattern in a String? Nothing in modules PreludeList or List seem to fit the bill. Thanks, Rock. -- Andrew Rock -- arock(a)cit.gu.edu.au -- http://www.cit.gu.edu.au/~arock School of Computing and Information Technology Griffith University, Nathan, Brisbane, Queensland 4111, Australia

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~ symbol
by Cagdas Ozgenc 29 Aug '01

29 Aug '01

Hello, What is the use of ~ symbol in Haskell? For example in prelude function unzip unzip = foldr (\(a,b) ~(as,bs) -> (a:as, b:bs)) ([], []) It works fine without the tilda. Is the use of tilda specific to lambda expression? Thanks

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RE: Numeric literals
by Chris Angus 28 Aug '01

28 Aug '01

this is a bit ugly but would allow you to do if (x `isA` string) ... or (x `isA` int) ... int = int::Int string = string::String typeBind :: a -> ReadS a -> ReadS a typeBind _ x = x isA :: Read a => String -> a -> Bool isA s x = not (null (typeBind x (readsPrec 0) s)) -----Original Message----- From: Simon Marlow [mailto:simonmar@microsoft.com] Sent: 28 August 2001 15:53 To: Mark Carroll; haskell-cafe(a)haskell.org Subject: RE: Numeric literals > I also haven't yet worked out how to tell if a string is "read"able or > not, yet - if (read "45")::Integer or whatever gives an error > (e.g. if I'd > put "af" instead of "45"), it seems to be pretty uncatchable, > which is a > pain. How do I tell if an instance of reading will work, or > catch that it > didn't? In GHC, you can do this: import Exception do result <- catch (evaluate (read "foo" :: Int)) (\error -> ... ) but unfortunately read doesn't raise a useful exception (just error "Prelude.read: no parse"), so you can't filter it very easily. However I don't think that read is likely to raise any other exceptions. Cheers, Simon _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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