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RE: ANNOUNCE: Release of Vital, an interactive visual programming environment for Haskell
by Tim Docker 13 Nov '03

13 Nov '03

As a "serious programmer", I'd be very happy to have a more graphical, more interactive programming experience as far as _output_ is concern. I'm happy to input textual expressions and definitions, but I'd like instant feedback and display of intermediate results as tables, graphs, trees, charts etc. Vital looks quite promising in the regard (though I've only browsed the website, and not used it). Spreadsheets are successful, I believe, because of the instant visual feedback they provide. An environment that worked in a similar way, but built upon a rigorous and high level language like haskell could well be a "killer app". Tim > -----Original Message----- > From: Graham Klyne [mailto:GK@ninebynine.org] > Sent: Thursday, November 13, 2003 10:03 AM > To: Marcin 'Qrczak' Kowalczyk > Cc: haskell-cafe(a)haskell.org > Subject: Re: ANNOUNCE: Release of Vital, an interactive visual > programming environment for Haskell > > > [Switching to haskell-cafe] > > For "serious programming", I entirely agree. > > But my view is that we are seeing some degree of > programmability entering > all sorts of everyday objects -- video recorders spring to > mind as an early > example -- and there's lots of work going on in the field of > "ubiquitous > computing". Many of these pervasive devices may be > fire-and-forget, but I > suspect many will not be. Graphical displays may be more > common than full > keyboards. So how is the user to be presented with options to enter > programming information? I don't have any final answers > here, but I do > have an intuition that for many users, where the > "programming" requirement > is a simple but flexible composition of existing functions, that a > graphical, self-documenting interface may be an appropriate > response to the > "video recorder programming hell" syndrome. > > Some of my thoughts about this came from considering issues > faced by a > friend of mine who has recently wired his new home for "total data" > (several kilometres of Cat5A cable in the loft!) -- it's all > very well > having all these intelligent devices around the home, but how > to actually > tell them what to do? Opening a door may signal that a light > should turned > on, or an alarm should be set off -- how to describe the > distinction? (Assuming the owner is not an experienced programmer.) > > Finally, as evidence for this view of user interfaces, I note > that for > tasks like computer system administration, graphical > interfaces have pretty > much taken over from the old command-line-and-text-file > approach. Even > Linux systems have graphical front-ends for most of the common > configuration, even though, for an experienced sysadmin, the > text-based > versions are generally quicker to set up and understand what's > happenning. In short, it's the occasional user, not the > full-time expert, > who may be better served by a non-textual approach. > > #g > -- > > At 23:56 12/11/03 +0100, Marcin 'Qrczak' Kowalczyk wrote: > >W li¶cie z ¶ro, 12-11-2003, godz. 11:06, Graham Klyne pisze: > > > > > I've sometimes thought that a functional language would > be the ideal > > > platform to usher in a purely graphical style of programming; > > > >I don't understand why so many people talk about graphical > programming, > >i.e. putting together functions, arguments, definitins etc. with the > >mouse instead of the keyboard, drawing arrows instead of naming etc. > > > >No wonder it didn't succeed. It would be much less convenient than > >typing text and less readable too. > > > >-- > > __("< Marcin Kowalczyk > > \__/ qrczak(a)knm.org.pl > > ^^ http://qrnik.knm.org.pl/~qrczak/ > > > >_______________________________________________ > >Haskell mailing list > >Haskell(a)haskell.org > >http://www.haskell.org/mailman/listinfo/haskell > > ------------ > Graham Klyne > For email: > http://www.ninebynine.org/#Contact > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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RE: Treating POSIX signals as exceptions?
by Simon Marlow 13 Nov '03

13 Nov '03

> There is a strong difference between asynchronous signals (like SIGIO, > SIGINT, etc..) and synchronous ones (SIGFPE, SIGSEG). I think only the > second type should be treated as exceptions, since they are tied to a > specific thread of execution. tying asynchronous signals to exceptions > just because they are posted to the program in the same way > seems wrong. Good point: the synchronous signals should be treated as synchronous exceptions. The question raised by the initial post was whether the asynchronous signals should be treated as asynchronous exceptions; indeed there are cases when this is useful: in GHC the SIGINT signal is converted into an asynchronous exception which terminates the compiler. It is necessary to use an exception so that the compiler can clean up before exiting. > especially when we have concurrency. registering a callback, or even > having a 'wait' like thing to blockingly wait for the next > asynchronous signal would be ideal. We already have the callback model, and it turns out you can implement pretty much anything else you want on top of it. There's no reason to restrict ourselves to just one API for signals. > we need some data type which contains the information in the > 'siginfo_t' > structure. then we can include this as a standard Exception > type for use with synchronous signals. Yes, we should do this too. Cheers, Simon

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RE: Why doesn't GHC do the following optimisation?
by Simon Peyton-Jones 13 Nov '03

13 Nov '03

It's a while since I paid serious attention to making sure that GHC's transformations are all working right. Things are usually more complicated than one might hope. In this case, * foldl is defined in the prelude using explicit recursion, as it happens * you've defined foldl' using foldr, and that has the potential to do list fusion with the [1..n] * but you've exported sum2, so the inlining doesn't happen. If you keep sum2 private module Main( main ) ... you'll see quite different code * there's an awkwardness with eta-expanding foldl, which I have never fully dealt with -- needs a kind of analysis. It's described in 3.2.3 of Gill's thesis (see also 4.4). http://www.cse.ogi.edu/~andy/ I wish it were all more predictable! Simon | -----Original Message----- | From: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of | andrew cooke | Sent: 12 November 2003 17:40 | To: haskell-cafe(a)haskell.org | Subject: Why doesn't GHC do the following optimisation? | | | Hi, | | This question is purely out of curiousity - I'm no expert on this. I | wrote the following code: | | sum1 :: [Int] -> Int | sum1 = foldl (+) 0 | | foldl' :: (a -> b -> a) -> a -> [b] -> a | foldl' f v l = foldr (\x -> \fld -> \v' -> fld $ f v' x) id l v | | sum2 :: [Int] -> Int | sum2 = foldl' (+) 0 | | main :: IO () | main = do n <- readLn; | print $ sum1 [1..n]; | print $ sum2 [1..n] | | and ran it through ghc with: ghc Folds.hs -ddump-simpl -O | | I was expecting the two calls to be optimized to something equivalent, or | even for the result of a single calculation to be printed twice. However, | peering at the Core output, it seems that the two sum functions exist | separately and have different structure (sum1 appears to have a simple | recursive definition, while sum2 involves two lamba expressions). | | I did all this because I read a paper introducing folds that showed how | you can express foldl in terms of foldr and giving the derivation. | Unfortunately I don't have the paper/derivation, but I believe my foldl' | is correct (that may be the problem, of course - that sum1 and sum2 are | not equivalent either because of a coding error, or because of some | subtlety in the types, although I carefully gave them explicit types to | try and avoid that). | | The derivation wasn't that complicated (althugh I find it simpler to | simply write the fold down). If it's not an error on my part, why doesn't | ghc do the same kind of transformation itself? Or am mistaken in | thinking that if derivation is easy then the revrese transformation as an | optimisation should also be easy? | | Thanks, | Andrew | | -- | http://www.acooke.org/andrew | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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Programming style (or: too clever by half?)
by Graham Klyne 13 Nov '03

13 Nov '03

I just spotted a possible idiom for something that seems to pop up from time to time: foldr const But I can't help feeling that this code is perversely obscure. Does it have any advantages over the more obvious form suggested below? [[ headOrSomething = foldr const headOrSomething1 _ (x:_) = x headOrSomething1 x [] = x h = headOrSomething -- h = headOrSomething1 test1 = h 0 [1,2,3] == 1 test2 = h 0 [] == 0 test3 = h 0 (repeat 42) == 42 ]] This suggests, e.g., an alternative coding of Maybe.listToMaybe: [[ altListToMaybe = foldr (const . Just) Nothing ]] Opinions? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Why doesn't GHC do the following optimisation?
by andrew cooke 12 Nov '03

12 Nov '03

Hi, This question is purely out of curiousity - I'm no expert on this. I wrote the following code: sum1 :: [Int] -> Int sum1 = foldl (+) 0 foldl' :: (a -> b -> a) -> a -> [b] -> a foldl' f v l = foldr (\x -> \fld -> \v' -> fld $ f v' x) id l v sum2 :: [Int] -> Int sum2 = foldl' (+) 0 main :: IO () main = do n <- readLn; print $ sum1 [1..n]; print $ sum2 [1..n] and ran it through ghc with: ghc Folds.hs -ddump-simpl -O I was expecting the two calls to be optimized to something equivalent, or even for the result of a single calculation to be printed twice. However, peering at the Core output, it seems that the two sum functions exist separately and have different structure (sum1 appears to have a simple recursive definition, while sum2 involves two lamba expressions). I did all this because I read a paper introducing folds that showed how you can express foldl in terms of foldr and giving the derivation. Unfortunately I don't have the paper/derivation, but I believe my foldl' is correct (that may be the problem, of course - that sum1 and sum2 are not equivalent either because of a coding error, or because of some subtlety in the types, although I carefully gave them explicit types to try and avoid that). The derivation wasn't that complicated (althugh I find it simpler to simply write the fold down). If it's not an error on my part, why doesn't ghc do the same kind of transformation itself? Or am mistaken in thinking that if derivation is easy then the revrese transformation as an optimisation should also be easy? Thanks, Andrew -- http://www.acooke.org/andrew

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Why no findM ? simple Cat revisited
by Ahn Ki-yung 12 Nov '03

12 Nov '03

Simple Cat (revisitied) \begin{code} import IO findM f [] = return Nothing findM f (x:xs) = do { v <- x; if f v then return (Just v) else findM f xs } isLeft (Left _) = True isLeft _ = False main = findM (isLeft) (hCat stdin) where hCat h = try (hGetLine h) : hCat h \end{code} This is my answer for the question of my own, which is posted a couple There are mapM, filterM in the Haskell 98 Standard Library. But why no findM there ? As you can see from simple cat, it seems quite useful. I think fildM should be added to the module Monad. -- Ahn Ki-yung

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Treating POSIX signals as exceptions?
by David Roundy 12 Nov '03

12 Nov '03

I was wondering why System.Posix.Signals is as it is, and whether it could be rewritten to use exceptions, which seems like the obvious way to handle signals. Of course, that requires asynchronous exceptions, but that doesn't seem like a severe problem. What I currently do is to use the following function withSig :: Signal -> IO a -> IO a -> IO a withSig s handler job = do id <- myThreadId installHandler s (Catch $ throwDynTo id s) Nothing job `catchDyn` catcher where catcher s' | s' == s = handler catcher s' = throwDyn s' which just catches a given signal and throws an exception, and then catches that exception with a given default handler. This has the nice effect that my 'job' can't be rudely interrupted by the user hitting ^C at just the wrong time (since I use this to catch sigINT). Thus, this makes bracket work in the presence of signals, allows me to use block to avoid being interrupted by a ^C when this could cause data corruption, etc. (Yes, I know that the above isn't very clean, since any other process that uses {catch,throw}Dyn on a CInt will mess things up, but creating a new Typeable instance seemed like too much trouble.) Is there any reason all of System.Posix.Signals couldn't be done in this manner? Have the RTS always add a handler for each signal, which just throws an exception. Off the top of my head, all we'd need is one function acceptSignals :: IO () -- tells RTS to send signal exceptions to this -- thread. We'd also need some sort of SignalException type (and of course, to define all the signals), and the rest would be done using the existing Control.Exception code. And, of course, default signal handlers would need to be converted to default exception handlers. Is there a problem with this idea? -- David Roundy http://www.abridgegame.org/darcs

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RE: Treating POSIX signals as exceptions?
by Simon Marlow 11 Nov '03

11 Nov '03

> I was wondering why System.Posix.Signals is as it is, and > whether it could > be rewritten to use exceptions, which seems like the obvious > way to handle > signals. Of course, that requires asynchronous exceptions, but that > doesn't seem like a severe problem. > > What I currently do is to use the following function > > withSig :: Signal -> IO a -> IO a -> IO a > withSig s handler job = do > id <- myThreadId > installHandler s (Catch $ throwDynTo id s) Nothing > job `catchDyn` catcher > where catcher s' | s' == s = handler > catcher s' = throwDyn s' Hmm, there is clearly a choice between having a signal spawn a new thread or raise an asynchronous exception, and it's not at all clear which is best. But, since we can implement either using the other one, it doesn't really matter too much which one is primitive. I've attached a more robust implementation of your withSig. This one: - doesn't suffer from race conditions around the point at which the signal handler is installed - it uses a datatype for the signal exception (Typeable can be derived these days) - it allows several threads to request notification of the same signal - it doesn't support nesting use of withSig with the same signal. (that could be added, but I'm not sure it's worth it). > Is there any reason all of System.Posix.Signals couldn't be > done in this > manner? Have the RTS always add a handler for each signal, which just > throws an exception. Off the top of my head, all we'd need > is one function > > acceptSignals :: IO () -- tells RTS to send signal exceptions to this > -- thread. You can implement this too using the module I attached. I guess we should provide this in the POSIX library. Would you like to play around with the API and submit it for inclusion? Cheers, Simon

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Value space transformations
by Graham Klyne 11 Nov '03

11 Nov '03

I just found myself writing some code like this: processFoo processBar = toFoo . processBar . fromFoo where fromFoo foo = ... toFoo bar = ... and noticed some similarity in style to certain kinds of transformation used in linear algebra, such as for diagonalization: D = T M inv(T) This got me to wondering if this is a common pattern in functional programming. There seems to be some similarity, for example, with the way that Monads are sometimes used, where operations are "lifted" into (?) a Monad. So my questions are: (a) is there a common functional programming pattern that corresponds to vector space transformations so that a function defined over one space can be used in another, and (b) if so, are there any not-too-heavy papers or articles discussing this pattern? #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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Is there an easier way to use guards in an expression...
by Graham Klyne 10 Nov '03

10 Nov '03

<aside> This is almost a reprise of a question that came up back in July [1]. At the time, Mark Jones posted a reference [2], but I'm not seeing how it is relevant to this issue of case expressions. [1] http://haskell.org/pipermail/haskell-cafe/2003-July/004708.html [2] ftp://ftp.research.bell-labs.com/dist/smlnj/papers/92-tr-aitken.ps </aside> The following code works, but it seems a bit clumsy: [[ mapXsdBoolean = DatatypeMap { -- mapL2V :: String -> Maybe Bool mapL2V = \s -> case s of s | matchT s -> Just True | matchF s -> Just False | otherwise -> Nothing -- mapV2L :: Bool -> Maybe String , mapV2L = Just . (\b -> if b then "true" else "false") } ]] The case expression seems rather an unwieldy way of getting guards into an expression. If this was an ordinary variable declaration rather than a field definition, I'd use a function definition with guards, but I can't see how to make that work with a lambda expression. Is there a neater way? And a supplementary question... do the prelude or standard libraries define any function like this: cond :: Bool -> a -> a -> a cond True a _ = a cond False _ b = b #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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determining if a int is a power
by ddekker6＠chello.nl 10 Nov '03

10 Nov '03

hello all, im new to haskell and have to do some excersises. Well i ran into this problem. I have to determin if a list of int's contains a int that is a power. So i have been trying using "any" with a helper function which return a Bool if the int is a power and applying that to the list. I think that this aproach should work. Anyway the problem im having is in the helper function. I have been trying this: isPower n = let powerF = floor(sqrt(n)) in if (powerF*PowerF == n) then True else False but this doesnt work. Prelude says that the definition of isPower needs a fractional int (if i remember correctly) anyway it looks like something is wrong with my type's? Could you guys help me out? Is there a simple standard function i overlooked. Or what should i change? Thanks in advance guys/gals

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Converting IO Int --> Int
by Tim Stitt 10 Nov '03

10 Nov '03

Can anyone tell me how to convert a value of type IO Int to a value of type Int. I wrote a simple script to read an integer from the user using IO and then the value is returned from the IO function using return value with a return type of IO Int. In the calling function I wish to do a comparison with the user value and a hard-coded integer. Unfortunately I keep getting type errors between the Int and the IO Int. I would be grateful for any assistance in this. Thanks in advance, Tim Stitt

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Beta Reduction, undefined vs Type Classes
by Jared Warren 10 Nov '03

10 Nov '03

Consider: > class Thing t where > thing :: t > > instance Thing Int where > thing = 0 > > instance Thing Char where > thing = 'a' Can someone please explain why > fst (1,thing) ...gets an 'ambiguous type variable' error, but > fst (1,undefined) ...doesn't? And is there anything I can change to make the former work as well? Even modifying fst doesn't work: > fst' :: Thing b => (a,b) -> a > fst' (~a,~b) = a -- ~ Jared Warren <warren(a)cs.queensu.ca> Computing Science, Queen's University

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RE: automaticly create the ana, cata, hylomorphisms
by Marta Isabel Oliveira 08 Nov '03

08 Nov '03

Ok, i read the page but i'm still stuck. I'm reading some papers about cata, ana and hylomorphisms but i need to know where to start putting it to code. So, 1. in order to have a pre-processor, i need to have a module with ALL data types? 2. from this module, how could i create the cata, ana,... i mean, a)how can i represent the result in haskell b) how can i create the cata(whatever) without knowing all of the possible functions that may be inserted in order to it's cata be created? thansk a lot, lesina.

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RE: automaticly create the ana, cata, hylomorphisms
by Marta Isabel Oliveira 07 Nov '03

07 Nov '03

Thanks and sorry. Beside new in Haskell i'm also new in this archive lists. So, i was not aware i had sent with sucess the mail to the first list. lesina > -----Original Message----- > From: Keith Wansbrough [SMTP:Keith.Wansbrough@cl.cam.ac.uk] > Sent: Friday, November 07, 2003 3:00 PM > To: Marta Isabel Oliveira > Cc: haskell(a)haskell.org; haskell-cafe(a)haskell.org > Subject: Re: automaticly create the ana, cata, hylomorphisms > > > Hello programmers, > > > > I have a problem to solve and don't have a clue where to start. > > This is a homework question. Please read > > http://www.haskell.org/hawiki/HomeworkHelp > > for some helpful hints. > > By the way, you shouldn't post the same message to multiple lists - you should choose the right one (haskell-cafe in this case), or if you must, cross-post one message to both. This means people who subscribe to both won't see your message twice. > > --KW 8-) > -- > Keith Wansbrough <kw217(a)cl.cam.ac.uk> > http://www.cl.cam.ac.uk/users/kw217/ > University of Cambridge Computer Laboratory. >

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Re: automaticly create the ana, cata, hylomorphisms
by Keith Wansbrough 07 Nov '03

07 Nov '03

> Hello programmers, > > I have a problem to solve and don't have a clue where to start. This is a homework question. Please read http://www.haskell.org/hawiki/HomeworkHelp for some helpful hints. By the way, you shouldn't post the same message to multiple lists - you should choose the right one (haskell-cafe in this case), or if you must, cross-post one message to both. This means people who subscribe to both won't see your message twice. --KW 8-) -- Keith Wansbrough <kw217(a)cl.cam.ac.uk> http://www.cl.cam.ac.uk/users/kw217/ University of Cambridge Computer Laboratory.

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Haskell pre-processor
by Marta Isabel Oliveira 07 Nov '03

07 Nov '03

Hello programmers, I have a problem to solve and don't have a clue where to start. I need to do a program in haskell that creates automaticly the ana, cata and hylomorphisms to arbitraty types. It's sort of a pre-processer. How can i start? I have the HaPar.hs that has uses Haskell parser to extract the type declarations from a file containing a Haskell module. I also have the TDec1.hs that has some dataTypes defined(TDec1, CDec1, Type) and some Show instances. Can anyone help me? I'm totally new to Haskell :(

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Another fold question
by Patty Fong 06 Nov '03

06 Nov '03

Having struglled over this for the better part of a day and only becoming more frustrated the more i try to understand it, i once again seek help :) I understand how basic folds work, i.e foldr replaces (:) with some parameter and [] by another i.e foldr (+) 0 [1,2,3] becomes 1+(2+(3+0)) I also understand how to write my own fold function. What i don't understand quite is how to use them. given this data type and this fold function i wrote: data Music = Note Pitch Octave Duration | Silence Duration | PlayerPar Music Music | PlayerSeq Music Music | Tempo (Ratio Int) Music data Pitch = Cf | C | Cs type Octave = Int type Duration = Ratio Int foldMusic :: (Pitch -> Octave -> Duration -> a) -> (Duration -> a) -> (a -> a -> a) -> (a -> a -> a) -> (Ratio Int -> a -> a) -> Music -> a foldMusic n _ _ _ _ (Note pitch octive duration) = n pitch octive duration foldMusic _ s _ _ _ (Silence duration) = s duration foldMusic n s p1 p2 t (PlayerPar partOne partTwo) = p1 (foldMusic n s p1 p2 t partOne)(foldMusic n s p1 p2 t partTwo) foldMusic n s p1 p2 t (PlayerPar partA partB) = p2 (foldMusic n s p1 p2 t partA)(foldMusic n s p1 p2 t partB) foldMusic n s p1 p2 t (Tempo rate part) = t rate (foldMusic n s p1 p2 t part) I understand that when i use the foldMusic function i need to pass it 5 parameters. given the type signiature, why can i pass (+) as a parameter for p1 but not for n, what determines what can be passed as a parameter, because they all have the return type a?? I attempted to create a function that utilises the foldMusic function that counts the number of notes: count_notes :: Music -> Integer count_notes = foldMusic (\_-> \_ -> \_ -> 1) (\_ -> 0) (+) (+) (\_ -> \_ -> 0) it appears to work, i think. Yet i'm still not certain of how it does so. This confuses me, Is there anyway to represent other fold functions in a tree like representation as foldr (+) 0 would appear as such? + 1 \ + 2 \ + 3 \ 0 Regards, confused Patrick _________________________________________________________________ Hot chart ringtones and polyphonics. Go to http://ninemsn.com.au/mobilemania/default.asp

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Another Newbie question :)
by Patty Fong 06 Nov '03

06 Nov '03

I seem to understand the basic idea behind the fold function, i think but do not seem to be able to write them myself. My understanding is that it works by replacing every constructor with a fold ? Could anyone please point me in the direction of a suitable resource of attempt to explain this to me :) Cheers, Paty _________________________________________________________________ Hot chart ringtones and polyphonics. Go to http://ninemsn.com.au/mobilemania/default.asp

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Trying to understand tuples and lists
by Karthik Kumar 05 Nov '03

05 Nov '03

Hi, I have a list of tuples ( characters and numbers ) . say, [('a', 1), ('b', 2)] I want to extract 'a' and 1 separately . (x,y) <- head [('a', 1), ('b', 2)] This doesnt work but my idea is to get x contain the value 'a' and y contain the value 1 as the first tuple in the list. Which is the right way of doing this. Thanks for your help. Cheers Karthik. __________________________________ Do you Yahoo!? Protect your identity with Yahoo! Mail AddressGuard http://antispam.yahoo.com/whatsnewfree

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Type errors in Haskell programming languages - Plz help
by Karthik Kumar 05 Nov '03

05 Nov '03

Hi, I was trying this Haskell program and was getting the following type error. The code is as follows - <-- Code starts --> entry :: [Char] -> [(Char,Int)] entry list = do t <- getGroups list mergeGroups t getGroups :: [Char] -> [(Char,Int)] mergeGroups :: [(Char,Int)] -> [(Char,Int)] <-- Code Ends --> The function getGroups and mergeGroups compile perfectly fine. The error i am getting is in the function entry . To paste the message, " *** Expression : mergeGroups t *** Term : t *** Type : (Char,Int) *** Does not match : [(Char,Int)] " Thanks in advance for helping me in this regard. Cheers Karthik. __________________________________ Do you Yahoo!? Protect your identity with Yahoo! Mail AddressGuard http://antispam.yahoo.com/whatsnewfree

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RE: Type tree traversals [Re: Modeling multiple inheritance]
by Simon Peyton-Jones 05 Nov '03

05 Nov '03

| We really should change GHC rather than keep trying to work around stuff | like this. GHC will be my light reading for winter break. Maybe so. For the benefit of those of us who have not followed the details of your work, could you summarise, as precisely as possible, just what language extension you propose, and how it would be useful? A kind of free-standing summary, not assuming the reader has already read the earlier messages. Simon

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Re: Data types basics
by Patty Fong 05 Nov '03

05 Nov '03

Another prompt reply! thanks Hal :) I think I understand this correctly now For my previous problem which was: <prolog> ::= (<assertion> ".")* <assertion> :: = <structure> | <rule> <rule> ::= <structure> ":-" <structure>("," <structure>)* <structure> ::= <name> [( <term> (, <term>)* )] <term> ::= <number> | <variable> | <structure> <variable> ::= <name> Does this appear to be correct: type Prolog = [Assertion] data Assertion = StrucAssert Structure | RuleAssert Rule data Rule = Rule Structure Structure [Structure] data Structure = Structure Name Term [Term] data Term = NumTerm Number | VarTerm Variable | StrucTerm Structure type Variable = Name type Name = String type Number = Int >From: Hal Daume III To: Patty Fong CC: Haskell Cafe , Hal Daume Subject: >Re: Data types basics Date: Tue, 4 Nov 2003 22:10:03 -0800 (PST) > >Hi again, > >On Wed, 5 Nov 2003, Patty Fong wrote: > > > Hi to anyone reading this. i'm still strugling a bit with data type > >declarations. > > The was i understand it is that if i delcare a new data >type: > > data myType = myType a | b | c > >This isn't entirely correct. The names of types have to begin with capital >letters, as do constructors (more later). So you would need this to be: > >data MyType = MyType A | B | C > >where A is an existing type. > >This type now has three constructors: > >MyType :: A -> MyType B :: MyType C :: MyType > >It's perhaps a bit easier to understand when the names are different. > >When we say: > >data Foo = Bar Int | Baz String > >this means that a "Foo" is of one of two forms (the "|" can be read as >disjunction). A value of type Foo is either of the form "Bar x" for some x >which is an Int or "Baz y" for some y which is a String. > >"Bar" and "Baz" are called "constructors" because they take arguments and >"construct" a Foo. So, in this case, > >Bar :: Int -> Foo Baz :: String -> Foo > >are the two constructors. > >Of course, you have have any number of constructors and each can have any >number of arguments. > >data Foo = Bar | Baz Int | Bazaa String Bool > >Now there are three constructors: > >Bar :: Foo Baz :: Int -> Foo Bazaa :: String -> Bool -> Foo > >they can be recursive: > >data Foo = Bar | Baz Int Foo > >Bar :: Foo Baz :: Int -> Foo -> Foo > >and can have "type variables", for instance: > >data Foo a = Bar | Baz a > >here, something of type "Foo a" is either of the form Bar or of the form >Baz x for some x which is of type a. This has constructors: > >Bar :: Foo a Baz :: a -> Foo a > >I hope this sheds some light on the issue... > > >-- >Hal Daume III | hdaume(a)isi.edu "Arrest this man, he talks in maths." | >www.isi.edu/~hdaume > _________________________________________________________________ Hot chart ringtones and polyphonics. Go to http://ninemsn.com.au/mobilemania/default.asp

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Data types basics
by Patty Fong 05 Nov '03

05 Nov '03

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Modeling multiple inheritance
by Brandon Michael Moore 05 Nov '03

05 Nov '03

I'm trying to build a nicer interface over the one generated by jvm-bridge. I'm using fancy type classes to remove the need to mangle method names. I would like methods to be automatcially inherited, following an inheritance hierarcy defined with another set of type classes. My basic classes look like this class HasFooMethod cls args result | cls args -> result where foo :: cls -> args -> result If I have classes A and B with foo methods like foo_JA_Jint :: ClassA -> Jint -> Bool foo_JB_Jboolean :: ClassB -> Bool -> Jint then I can make instances instance HasFooMethod ClassA Jint Bool instance HasFooMethod ClassB Bool Jint Now I can just use foo everywhere. I would like to avoid declaring an instance for every class though. In java methods are inherited from a superclass, and I would like to inherit methods automatically as well. In the bindings jvm-bridge generates a method is invoked with a function mangled after the highest ancestor that defined that particular overloading, so the implementation of HasFooMethod at a particular overloading is the same for any descendant. So I defined a class to model the inheritance relationships class SubType super sub | sub -> super where upCast :: sub -> super Now I can define a default instance of HasFooMethod: instance (HasFooMethod super args result, SubClass super sub) => HasFooMethod sub args result where foo sub args = foo (upCast sub) args This will propagate foo methods down the inheritance hierarcy. If a new class C is derived from A, I just need to say instance SubClass ClassA ClassC and ClassC gets a foo method. (In the actually code I piggy-back on a transitive subclass relation jvm-bridge defines that already includes an upcast method, so upCast has a default that should always be acceptable). The problem comes when interfaces are added to the mix. Interfaces are treated just like classes by jvm-bridge, and even though no implementation is inherited from instances in Java, the method accessors generated by jvm-bridge should be inherited. One problem is that the subclass relationship needs the functional dependency so that the default instance of HasFooMethod will respects the functional dependencies of HasFooMethod, so I can't declare subclass instances for multiple inheritance. On the other hand, if I don't use the functional dependency on HasFooMethod I end up needing to annotate most of the return values in a program. I run into similar problems trying to use numeric literals as arguments, because they are also overloaded. Does anyone know of clever solutions that would model multiple inheritance while preserving the functional dependencies (unsafe compiler flags are fine too), or ways to reduce the pain of overloading resolution without the functional dependency? One alternative is generating seperate HasFooMethod instances for every class in the system. The problem is that this would require alterating the bit of jvm-bridge that uses JNI to find information on classes, which currently only reports newly defined methods. JNI is black magic to me. Thanks Brandon

3 11

AW: AW: Heap profiling in GHC broken?
by Markus.Schnell＠infineon.com 04 Nov '03

04 Nov '03

> The canonical tutorial paper is in the Advanced Functional Programming > Summer Schools series: > > C. Runciman and N. Ro"jemo. Heap profiling for space efficiency. > In J. Launchbury, E. Meijer, and T. Sheard, editors, 2nd > Intl. School > on Advanced Functional Programming, pages 159-183, Olympia, WA, > August 1996. Springer LNCS Vol. 1129. Great! I knew only the more compiler-oriented Runciman/Röjemo papers. (Which were already helpful, but not very 'tutorial'.) Thanks, Markus

1 0

AW: Heap profiling in GHC broken?
by Markus.Schnell＠infineon.com 04 Nov '03

04 Nov '03

I have no idea how to set up cvs under windows to get things from a non-local repository, but anyway, profiling worked on the macintosh. I spent the whole weekend plus monday to find the space leak, but eventually I did. Heap Profiling was very helpful there. Unfortunately, space leaks are barely mentioned in the Haskell books/tutorials, but they are a real burden if you want to run programs for larger problems. A tutorial on this by one of the experts would be very welcome. :-) Thanks, Markus > -----Ursprüngliche Nachricht----- > Von: haskell-cafe-bounces(a)haskell.org > [mailto:haskell-cafe-bounces@haskell.org] Im Auftrag von David Roundy > Gesendet: Donnerstag, 30. Oktober 2003 13:48 > An: haskell-cafe(a)haskell.org > Betreff: Re: Heap profiling in GHC broken? > > > On Wed, Oct 29, 2003 at 05:43:23PM +0100, > Markus.Schnell(a)infineon.com wrote: > > When I tried to profile my program in search for space leaks, > > it core dumped on me (actually on Win2000/Cygwin). I'm > using ghc 6.0.1. > > Does anybody know what could be the problem? > > > > (To be more precise: any +RTS -h_ options make problems, > +RTS -p works > > fine. I get a message meaning the equivalent of "statement in > > '0x0058db43' points to memory in '0xfffffffc'. Could not read.") > > I believe this bug is fixed in CVS, so you could get it there > or wait for > 6.2. (I didn't fix it, I just reported it.) > -- > David Roundy > http://www.abridgegame.org/darcs > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

3 2

Data representation, maybe reflection, laziness
by Mark Carroll 04 Nov '03

04 Nov '03

People have been talking about transmitting Haskell values on the GHC users' list, and I wanted to ask some more general stuff, partly out of mild ignorance. Ralf Hinze and Simon Peyton-Jones wrote an interesting paper on generic programming and derivable type classes. It looked like maybe programmers would be able to write their own "deriving xml" stuff and whatever, which looked great because, if there's not already one out there, I'd love to derive some read/show analogue automatically for data in some encoding that's very efficient to write and parse (i.e. not XML (-:). I was also wondering how the ability to write "deriving" stuff related to what one might think of as being reflection - that is, for example, could a definition of deepSeq be derived that automatically knows how to recurse into and traverse data structures and make sure everything's evaluated? This leads me to the point that little of the code I write needs laziness, but many of my unpleasant performance surprises come from laziness. I do find it hard to figure out where to put strictness annotations to actually make things work - for instance, I think it's laziness things that are causing my uses of Control.Exception.evaluate to actually work more by trial and error. No doubt it'll all grow clearer in time. Maybe I need "laziness annotations" instead. (-: Still, I was wondering what current thinking and directions were with regard to any of the above! -- Mark

4 4

Doubt regarding Types
by Karthik Kumar 04 Nov '03

04 Nov '03

Hi All, I was trying to write go program in haskell - My code fragment looks like this - <---- > getBoardSize :: IO Bool -- TODO : What could be the type of getBoardSize getBoardSize = do c <- getLine validateBoardSize ( atoi c ) -- Convert a string to an integer. -- This works perfectly fine. atoi :: [Char] -> Int atoi (h : []) = if isDigit h then digitToInt h else 0 atoi (h : t) = if isDigit h then digitToInt h * ( 10 ^ length t) + atoi t else 0 -- validateBoardSize -- To validate the board size validateBoardSize :: Int -> Bool validateBoardSize d = (d == 9 || d == 13 || d == 19 ) <----> ERROR "test1.hs":21 - Type error in final generator *** Term : validateBoardSize (atoi c) *** Type : Bool *** Does not match : IO a Actually i guess there is something wrong with the type mentioned for getBoardSize function , but am clueless abt this. Thanks in advance for your help in this regard. Cheers Karthik __________________________________ Do you Yahoo!? Exclusive Video Premiere - Britney Spears http://launch.yahoo.com/promos/britneyspears/

3 4

Help ? :)
by Patty Fong 03 Nov '03

03 Nov '03

3 2

RE: fixed point
by Dean Herington 01 Nov '03

01 Nov '03

At 11:15 AM -0500 2003/10/31, Harris, Andrew wrote: > Hi - > I am trying to work out how the following function using "fix" is > evaluated. I am hoping someone could look at my step-by-step breakdown > of how I think evaluation works and see if I'm correct. My main > question is how the order of operation (fixity?) is understood in going > from step [3] to [4], if this is indeed how the evaluation would take > place. > > Any help is appreciated, > -andrew > > Here's the Haskell function: > > --- > fix f = f (fix f) > > wierdFunc2 x y z = if y - z > z then x (y-z) z > else y - z > > myRemainder = fix wierdFunc2 > --- > > Here's my "evaluation": You're pretty close. I've noted a couple of differences below. > myRemainder 12 5 -- [1] > = > fix wierdFunc2 12 5 -- [2] by substitution > = > wierdFunc2 fix wierdFunc2 12 5 -- [3] apply "fix" wierdfunc2 (fix wierdFunc2) 12 5 > = > wierdFunc2 myRemainder 12 5 -- [4] by substitution (?) No, equations "rewrite" only "from left to right". Skip the step above and the step below. > = > myRemainder 7 5 -- [5] apply "wierdFunc2" > = > fix wierdFunc2 7 5 -- [6] by substitution > = > wierdFunc2 fix wierdFunc2 7 5 -- [7] apply "fix" Similarly, the above needs (fix wierdFunc2) to be parenthesized. > = > wierdFunc2 myRemainder 7 5 -- [8] by substitution (?) Skip the above step. > = > 2 -- [9] apply "wierdFunc2" Dean

1 0

newbie question regarding type declarations
by gangadhar npk 01 Nov '03

01 Nov '03

hi, I am a newbie to Haskell. I am reading the tutorial by Hal Daume III. Regarding the function types which are an extension of lambda calculus, I was wondering if at all it is possible to write a type that would return two values as the output - something like a square function which would take a pair and return a pair. I tried this, but there are errors square :: (Num a, Num b) => (a ,b) square (x , y) = (x*x , y*y) How can I specify that the square functions output would be a pair ? thank you gangadhar

5 4

Monad @ Microsoft
by Josef Svenningsson 31 Oct '03

31 Oct '03

OK, this one is just too good not to post about it. Microsoft has developed a now command line interface. Guess what they call it? Read it here: http://slashdot.org/articles/03/10/31/1346201.shtml?tid=185&tid=190&tid=201 Well, it seems Simoj PJ is doing a good job introducting functional programming at MS although his methods are subtle in the extreme :-) /Josef

1 0

RE: fixed point
by Harris, Andrew 31 Oct '03

31 Oct '03

Hi - I am trying to work out how the following function using "fix" is evaluated. I am hoping someone could look at my step-by-step breakdown of how I think evaluation works and see if I'm correct. My main question is how the order of operation (fixity?) is understood in going from step [3] to [4], if this is indeed how the evaluation would take place. Any help is appreciated, -andrew Here's the Haskell function: --- fix f = f (fix f) wierdFunc2 x y z = if y - z > z then x (y-z) z else y - z myRemainder = fix wierdFunc2 --- Here's my "evaluation": myRemainder 12 5 -- [1] = fix wierdFunc2 12 5 -- [2] by substitution = wierdFunc2 fix wierdFunc2 12 5 -- [3] apply "fix" = wierdFunc2 myRemainder 12 5 -- [4] by substitution (?) = myRemainder 7 5 -- [5] apply "wierdFunc2" = fix wierdFunc2 7 5 -- [6] by substitution = wierdFunc2 fix wierdFunc2 7 5 -- [7] apply "fix" = wierdFunc2 myRemainder 7 5 -- [8] by substitution (?) = 2 -- [9] apply "wierdFunc2"

1 0

newbie question regarding type declarations
by gangadhar npk 31 Oct '03

31 Oct '03

hi, I am a newbie to Haskell. I am reading the tutorial by Hal Daume III. Regarding the function types which are an extension of lambda calculus, I was wondering if at all it is possible to write a type that would return two values as the output - something like a square function which would take a pair and return a pair. I tried this, but there are errors square :: (Num a, Num b) => (a ,b) square (x , y) = (x*x , y*y) How can I specify that the square functions output would be a pair ? thank you gangadhar

1 0

Heap profiling in GHC broken?
by Markus.Schnell＠infineon.com 30 Oct '03

30 Oct '03

Hi. When I tried to profile my program in search for space leaks, it core dumped on me (actually on Win2000/Cygwin). I'm using ghc 6.0.1. Does anybody know what could be the problem? (To be more precise: any +RTS -h_ options make problems, +RTS -p works fine. I get a message meaning the equivalent of "statement in '0x0058db43' points to memory in '0xfffffffc'. Could not read.") Cheers, Markus -- Markus Schnell

2 1

Haskell Asignement not working
by Rachel Reid 29 Oct '03

29 Oct '03

My code is --1)Define a function bLine to give a line of horizontal #s import Pictures sb, sw :: Picture bLine :: Int -> Picture bLine n ) |n==0 = ) |n>0 = `sideBySide` sb (bLine (n-1)) and when running Hugs, it says that there is a <unexpected symbol "#"> in line 9 which is bLine n ). There was a hash sign previously, but I have reloaded Hugs since then. can you help Thanks Rachel

2 1

fixed point
by Harris, Andrew 29 Oct '03

29 Oct '03

Hi - I am trying to do Exercise 9.9 in HSOE; and I've come up with an answer that works but I'm not sure if it answers the question properly. The problem is: The Question: ------------- Suppose we define a function "fix" as: fix f = f (fix f) Suppose further we have a recursive function: remainder :: Integer -> Integer -> Integer remainder a b = if a < b then a else remainder (a - b) b Rewrite this function using "fix" so that it's not recursive. My Answer: ---------- wierdFunc x y z = if y - z > z then ((\a b c -> a b c) x (y-z) z) else ((\a b c -> a b c) (\d e -> d) (y-z) z) myRemainder = fix wierdFunc My Question: ------------ Does the ((\a b c -> a b c) x (y-z) z) mean that I've not removed the recursion? I was assuming that I was returning a function that is to be evaluated and not actually doing any recursion. That's why I thought I answered the question. However, I have a headache now and would like another opinion. thanks, -andrew

7 8

Cast from and to CChar
by Christian Buschmann 28 Oct '03

28 Oct '03

Hi! I've got following problem. If I enter in ghci following line: Prelude Foreign.C> castCCharToChar $ castCharToCChar 'ü' I would expect that this returns 'ü', but it returns '\252'. Is this the correct behaviour? Or am I doing something wrong? Or are there any problems with language specific characters and CChar? thanks Christian B.

3 4

I/O multiplexing (repost)
by Peter Simons 27 Oct '03

27 Oct '03

[ I'm posting this article here again, because the general ] [ mailing list seems to be closed to non-members, and I'm ] [ reading/posting through gmane.org. Pardon me, if you see ] [ this on both lists, please. -peter ] Hi, I have a question concerning "manual" I/O multiplexing in Haskell, or specifically with GHC. I have written an interface to the C library ADNS, which performs asynchronous DNS queries. Everything is fine and dandy, but now comes the hard part: The function foreign import ccall adns_beforepoll :: State -- ADNS context -> Ptr Pollfd -- space for 'n' Pollfd -> Ptr CInt -- n -> Ptr CInt -- max timeout -> Ptr (Timeval) -- current time -> IO CInt generates an array of file descriptors, suitable for calling poll(2). The function foreign import ccall adns_afterpoll :: State -> Ptr Pollfd -> CInt -> Ptr (Timeval) -> IO CInt scans this array (after poll has been called), performs I/O on all readable/writable sockets, and processes any packets that have been received. Obviously, the library wants me to call poll myself, though. But since poll is not supported by Haskell's run-time system, the call would block _all_ running threads in my process, not just the thread doing the DNS resolving. I have considered the following options to solve this: 1. Run the DNS resolver in a forkOS thread. 2. Declare all imported functions to be thread-safe (what they are) and use forkIO. 3. Call poll(2) without blocking, then conjure some magic to decide when to call it again without busy polling. 4. Register the file descriptors with GHC's internal poll event loop. Option (1) seems to be straight-forward, but I don't want to rely on the non-standard forkOS function unless I really have to. The same goes for (4): This is probably a good solution, but it's not portable among compilers. (3) works fine, but is unsatisfactory because the resolver will have comparably large latency times. (I have been processing DNS queries every, say, second.) So option (2) seems quite attractive compared to the others ... But the question is: Would that even work? Any ideas, thoughts, recommendations? Peter

1 0

RE: fixed point
by Harris, Andrew 27 Oct '03

27 Oct '03

> > Notice that, (\x -> x) a reduces to a, so (\a b c -> a b c) x (y-z) z > reduces to x (y-z) z. You can therefore simplify your > function quite a > bit. > wierdFunc x y z = if y-z > z then x (y-z) z else (\d e -> d) (y-z) z > and you can still apply that lambda abstraction (beta-reduce) > wierdFunc x y z = if y-z > z then x (y-z) z else y-z > None of these (except, of course, fix and remainder) are recursive. A > recursive function is just one that calls itself. For wierdFunc to be > recursive, the identifier wierdFunc would have to occur in the > right-hand side of it's definition. > Thanks for your help. For some reason I didn't think "x (y - z) z" and "y - z" had the same type. I am still trying to understand how they do. Also, had a feeling the fix function was related to the "Y" combinator; it seems they're the same thing! http://en.wikipedia.org/wiki/Y_combinator thanks again, -andrew

2 1

replacing guile with haskell?
by David Roundy 25 Oct '03

25 Oct '03

I'm wondering what the possibilities are for replacing the use of guile with a haskell interpereter? I'd like to be able to embed a haskell interpereter (presumably hugs) withing my program, so that the input file could be a haskell program. The application is a numerical simulation code, for which the input is (among other things) a photonic crystal structure. Practically, this means that the input file needs to be a program (or the user needs to write a program to generate the input file--ugly). Currently my code works as a C++ library, but that isn't particularly convenient (except for development). A similar code (mpb--it's available in debian) by a coworker has the input format in scheme using guile. That's nice in many ways, except I can't stand scheme. So I'm wondering whether there might be some convenient way to accomplish the same goal with haskell. Currently, my best (and only) idea is to make the input file a script with #!/usr/bin/env runhugs at the top (and compiling an interface module with ffihugs). There are a couple of issues with this. The first is that I've heard that hugs isn't intended for numerical work (which is what I'm doing). I'm not sure if this will be a problem, since hugs won't be doing any of the real work anyways. The other is that according to the man page, it seems that hugs only supports ffi on x86, powerpc and sparc, which seems likely to be a show stopper. Since I'll need to run the code on supercomputers, I won't always have a choice of architectures, and at least support for POWER (maybe comes free with powerpc?) would be necesary--IBM SP machines are quite nice. In this regard, ghc seems worse than hugs (the thought of bootstrapping ghc on a supercomputer gives me the shivers), and nhc98 last time I looked didn't support 64 bit platforms. Is there a solution to these problems, or will I be stuck with thousands of parentheses in my input file? -- David Roundy http://www.abridgegame.org/darcs

9 17

random numbers
by Jacinta Mary FITZGERALD 24 Oct '03

24 Oct '03

I'm very much a beginning programmer, studying Haskell. I have two questions - Firstly, I am trying to generate random numbers in Haskell, but although I have found a 'random' library on www.zvon.org, I don't really know how to include library functions, and the documentation given doesn't really tell me the effective difference between all the functions provided in the library. In another interesting development, I couldn't find any mention of this library in haskell.org's list of libraries. Secondly, but perhaps more importantly, is there a more 'beginnerish' list that I should be addressing this to? I've been following discussions on this one and they don't seem to be at quite this level. Thanks Jac

4 3

RE: "hSetBuffering stdin NoBuffering" messes up terminal
by Simon Marlow 21 Oct '03

21 Oct '03

> > If I understand recent changes correctly, this is a > deliberate decision. The > > idea is that if you wanted to write a program like 'stty' > in Haskell, you'd > > be very disappointed if the terminal settings got switched > back the moment > > your Haskell program terminated. > > It's deliberate, but (AFAIK) not for those reasons. The idea is so > that "simple, stupid programs" do what the programmer expects (and > saving the list from lots of "my program doesn't get any input until > the user hits Return" questions). > > If you were writing "stty" in Haskell, you would presumably use the > PosixTTY functions directly. GHC 6.2 will have slightly better behaviour in this regard. If the terminal settings are changed as a result of hSetBuffering, then they will be restored on exit. This still allows stty-type programs to be written, as long as you don't also call hSetBuffering on the same Handle. Cheers, Simon

3 2

Strictness annotations
by Derek Elkins 21 Oct '03

21 Oct '03

I've wondered for a while why Haskell 98 didn't provide strictness annotations for functions a la Clean and no implementations provide it as an extension (actually I don't know about NHC, but I'm pretty sure neither GHC nor Hugs do). Clean's solution seems easier to read and write and seems like it would be trivial (though possibly tedious) to implement. So my questions are: What was the motivation for not providing them in Haskell 98? Are there any technical problems with adding them to Haskell now (or then)? If there aren't any issues, are strictness annotations on any implementors' wish-list to be added when there is sufficient demand or no more pressing matters? Googling the haskell archives didn't (immediately at least) turn up any answers to these questions, but did turn up a couple posts suggesting the desirability this. http://www.haskell.org/pipermail/haskell/2002-March/009176.html http://www.haskell.org/pipermail/haskell-cafe/2001-June/001995.html

3 2

tracing Haskell programs
by wvasconc＠csd.abdn.ac.uk 20 Oct '03

20 Oct '03

Hi folk Is there a way to follow the "execution" of a Haskell program? What I mean here is a way to see which function definition is being used at any moment, and how the execution control moves around a script file. If this is a preposterous question, just tell me nicely and I will go away! Oh, and if this has been discussed before, please tell me where and I will chase it up. Thanks! -- Wamberto Vasconcelos, PhD wvasconcelos(a)acm.org Department of Computing Science http://www.csd.abdn.ac.uk/~wvasconc University of Aberdeen, Aberdeen AB24 3UE, Scotland, UK Phone +44 (0)1224 272283 Fax +44 (0)1224 273422

3 2

"listProduct" -- is this a standard function?
by Graham Klyne 20 Oct '03

20 Oct '03

I've constructed a "listProduct" function that I think I've seen somewhere else... is it a standard function? If so, where is it defined? My code is: [[ -- |Given a list of lists, construct a new list of lists where -- each member of the new list is the same length as the original -- list, and each member corresponds to a different choice of -- one element from each of the original members in the -- corresponding position. Thus: -- -- listProduct [[a1,a2],[b1],[c1,c2]] = -- [ [a1,b1,c1], [a1,b1,c2], [a2,b1,c1], [a2,b1,c2] ] -- -- Note: The length of the resulting list is the product of -- lengths of the components of the original list. Thus, if -- any member of the original list is empty then so is the -- resulting list: -- -- listProduct [[a1,a2],[],[c1,c2]] = [] -- listProduct :: [[a]] -> [[a]] listProduct [] = [[]] listProduct (as:ass) = concat [ map (a:) (listProduct ass) | a <- as ] test1 = listProduct [["a1","a2"],["b1"],["c1","c2"]] test2 = listProduct [["a1","a2"],[],["c1","c2"]] ]] And a supplementary point. Following a comment made to me previously, I can replace this list comprehension (not having any guard expressions) with another map function, thus: [[ lp [] = [[]] lp (as:ass) = concatMap (\a -> (map (a:) (lp ass))) as ]] I think I should also be able to eliminate the lambda-abstraction, but I can't see how. I prefer the list comprehension, as I find that easier to read than the \-expression. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

4 6

Alex 1.x still available somewhere?
by Peter Simons 19 Oct '03

19 Oct '03

Hi, I'm playing with the BNF Converter and like it very much. But unfortunately, it generates code for Alex version 1, not 2. All I could find is the _new_ Alex version, so I wonder: Does anyone know an URL, where I can still get the old version? Or even better: Has anyone adapted the BNF Converter for the new Alex version already? Peter

2 1

"hSetBuffering stdin NoBuffering" messes up terminal
by David Roundy 18 Oct '03

18 Oct '03

On my terminal (aterm), calling hSetBuffering stdin NoBuffering within my program messes up the terminal settings somehow, so that backspace no longer works when I run my program (without the NoBuffering) a second time. The 'reset' command fixes the problem, so once again backspace works, but my program shouldn't be affecting the terminal state at all. A simple call to hSetBuffering stdin LineBuffering doesn't seem to fix the problem. Any ideas what might be going on here? It's really annoying to not be able to use backspace when entering data. -- David Roundy http://www.abridgegame.org

5 6

Old Messages
by Mark Espinoza 17 Oct '03

17 Oct '03

>> A couple of days ago I posted a question about the cgi library, >> which was not answered. >Hopefully, this repeat request will get you some answers (though I note >that >Graham Klyne did answer your question at length on 23/6/2003). Is there a place where I can search through the old messages to see if someone has already answered my question? Thanks Mark

2 1