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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

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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

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"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

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Alex 1.x still available somewhere?
by Peter Simons 20 Oct '03

20 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

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"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

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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

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Re: "listProduct" -- is this a standard function?
by K. Fritz Ruehr 17 Oct '03

17 Oct '03

I don't know of a type-based search tool for Haskell libraries, but I vaguely recall someone pursuing this idea using type isomorphisms to provide more robust results. This actually came up at lunch the other day*, where someone pointed out that you'd want to allow not only for type isomorphisms (e.g., argument order), but also for functions which might fit your spec. when instantiated with (say) some default argument. I don't know if this latter idea was addressed in any of the "TI" work, eg Di Cosmo's book (see also Rittri, Thatte, and others). Note that much of the TI work mentions library search as a motivation--I thought that someone might actually have implemented something for Haskell, though. In any case, and until someone might fill in my vaque recall, my students have found the reference tool at zvon.org quite helpful: there is a type-based index of the standard libraries at: <http://zvon.org/other/haskell/Outputglobal/index.html> (but note that the class qualifiers are missing, so that sequence looks more general than it really is). They also have nice indices tied to the library structure, e.g., the Prelude index at: <http://zvon.org/other/haskell/Outputprelude/index.html> -- Fritz * Type isomorphisms don't often come up over lunch in my regular venue, but I am fortunate enough to be spending some sabbatical time with the PacSoft group at the Oregon Graduate Institute (highly recommended). PS: A quick googling suggests that perhaps I was remembering a paper by Rittri in JFP vol. 1, #1, which I don't have handy. Does anyone know if a practical tool was ever released? ----------------- Graham Klyne wrote: > At 22:04 15/10/03 -0500, Matt Harden wrote: >>On Wednesday 15 October 2003 11:07 am, Graham Klyne wrote: >> > 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? >> >>Yes. It's called "sequence". It's defined in the prelude. >>It works with arbitrary monads, not just lists. > > That's the one, thanks (both of you). > > I *thought* I'd seen this function before. > > I observe this seems to be a recurring problem for me... there are so many > very generic and useful data handling functions in Haskell, it's difficult > for a jobbing programmer (as opposed to a language expert) to be on top of > them all. And the very generic nature of these function means that it's > not so easy to index them (e.g. in the way that the Java Developers > Almanac > series does for Java). > > Am I overlooking any important developer resources? > > #g > > > ------------ > Graham Klyne > For email: > http://www.ninebynine.org/#Contact

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parsing a file line by line
by Anagha 17 Oct '03

17 Oct '03

Hi , I am a new bie to haskell. I need to read a file and get it contents line by line and then eventually word by word. I have written the code to read a file , but the returned value is all the lines in the file. How do I parse this whole text message ( contents of the file) into individual lines ( that is parsing / splitting the whole text to get me one line at a time. Also once I get the individual line, I o parse it word by word. I have a start on this thing. by using the line function in haskell, but I have not managed to write it successfully. Can anyone help me in this. Below is the code to read a file. fileFun :: IO () fileFun = do putStr "Please input a file name: " fname <- getLine contents <- readFile fname How do I split the data returned in contents( Contents contains all the text in the file, which is given as input) Thanks Anagha

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Cgi Library
by Mark Espinoza 16 Oct '03

16 Oct '03

Hello, I have ghc 5 and 6 installed on my redhat 7.3 system. I have downloaded and unpacked the cgi library, which wants to have hugs installed. Is there an easy way to make this work with ghc (including the examples, I think I'll need them), or should I just try to install hugs. Thank you. Sincerely, Mark

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Efficient Matrix Library
by Shawn P. Garbett 15 Oct '03

15 Oct '03

I'm looking for an efficient matrix library for Haskell. Suggestions? Shawn

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