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how to debug?
by David Roundy 07 Oct '02

07 Oct '02

How does one debug in haskell? I have a function that I could swear should behave differently than it does, and after tracking down bugs for many hours, I'm wondering if there's any way to step through the evaluation of a haskell function? The other way I would be debugging in an imperative language would be to sprinkle printfs around. Is there any way to do something like that cleanly in haskell? The only thing I can think of would be to modify every function to accept an additional parameter, which seems like it's more likely to introduce bugs than remove them... For what it's worth I'm using ghc, and the only debugging options it seems to have are for debugging the compiler itself. I would be happy to install and use hugs for debugging if it has some nicer debugging mode. I have already isolated my bug within one function, but that function has somewhat funky recursion, and uses an array (which I'm none too familiar with in haskell), and there aren't any smaller parts that I can see to test. :( -- David Roundy http://civet.berkeley.edu/droundy/

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Re: how to debug?
by Zdenek Dvorak 06 Oct '02

06 Oct '02

Hello, >How does one debug in haskell? I have a function that I could swear should >behave differently than it does, and after tracking down bugs for many >hours, I'm wondering if there's any way to step through the evaluation of a >haskell function? > >The other way I would be debugging in an imperative language would be to >sprinkle printfs around. Is there any way to do something like that >cleanly in haskell? The only thing I can think of would be to modify every >function to accept an additional parameter, which seems like it's more >likely to introduce bugs than remove them... > >For what it's worth I'm using ghc, and the only debugging options it seems >to have are for debugging the compiler itself. I would be happy to install >and use hugs for debugging if it has some nicer debugging mode. > >I have already isolated my bug within one function, but that function has >somewhat funky recursion, and uses an array (which I'm none too familiar >with in haskell), and there aren't any smaller parts that I can see to >test. :( aside from (useful) general advices already given, just few tips that I found useful when I needed to debug: -- to print debug dumps, you may use 'trace' function from IOExts (but beware, it may affect behavior of your program if used incorrectly) -- other way (safer, but slower to learn and use) is to use Hood library -- if your program fails with some really informative error message of type '*** Exception: Maybe.fromJust: Nothing', compiling it with profiling (-prof -auto-all) and running with +RTS -xc will give you a stack dump (pretty useful, well hidden feature :-) Zdenek Dvorak _________________________________________________________________ Chat with friends online, try MSN Messenger: http://messenger.msn.com

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Re: how to debug?
by D.Wakeling＠exeter.ac.uk 06 Oct '02

06 Oct '02

David Roundy <droundy(a)jdj5.mit.edu> writes: > How does one debug in haskell? I have a function that I could swear should > behave differently than it does, and after tracking down bugs for many > hours, I'm wondering if there's any way to step through the evaluation of a > haskell function? The best place to start would be with the nhc98 compiler and the Hat tracing system developed by the Functional Programming Group at the University of York. See http://www.cs.york.ac.uk/fp/nhc98 David ------------------------------------------------------------------------------- Dr. David Wakeling, School of Engineering and Computer Science Harrison Building, University of Exeter, Exeter, Devon, EX4 4QF. unofficial web page: http://www.dcs.ex.ac.uk/~dwakelin -------------------------------------------------------------------------------

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Boxed versus Unboxed
by Shawn P. Garbett 03 Oct '02

03 Oct '02

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I've been reading on this list about boxed/unboxed structures. I'm not quite clear what this terminology means. Is there a good paper that covers this topic in detail, or a book reference? -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.7 (GNU/Linux) iEYEARECAAYFAj2a8OEACgkQDtpPjAQxZ6DUSgCeIRgAl3dpOPavJhrmwFxal5O1 Y8IAnRDzmdag9lyljlacKtE4aZMujfiG =h2Zn -----END PGP SIGNATURE-----

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Performance question
by ketil＠ii.uib.no 02 Oct '02

02 Oct '02

At the moment, I have a user defined data type on which I have defined some operations, instantiated (not derived) Eq and so on. However, for efficiency I'm storing the actual data in UArrays of Word8. In order for everything to work, I need functions 'toW8' and 'fromW8' to map between the data type and its representation in the array. I gather it is impossible to hide the 'Eq' instance of Word8 and/or redefine it, but could this be circumvented by instead of data Foo = F | G instance Eq Foo where F == _ = True _ == _ = False defining something like newtype Foo = F Word8 f, g, h :: Foo f = F 0 g = F 1 and using the same instance declaration? And will it still fit into a UArray? (It is of course possible that, while profiling indicates otherwise, the conversion functions have little impact in practice; i.e. the optimizer will do away with them.) -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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question about 'sort' algorithm
by David Roundy 02 Oct '02

02 Oct '02

I was wondering if anyone knows what sort of algorithm the 'sort' function in the List module actually uses? In "The Haskell 98 Library Report", they give an insertion sort implementation, but I find it hard to believe that the library actually uses insertion sort. Basically, I need O(n log n) sorting, and this is a bit awkward to accomplish efficiently with lists. I would have thought that this awkwardness should be dealt with in the standard library, but seeing an O(n^2) algorithm in the description troubles me. -- David Roundy http://civet.berkeley.edu/droundy/

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by Hayri DALDAL 30 Sep '02

30 Sep '02

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RE: Dealing with configuration data
by Simon Peyton-Jones 26 Sep '02

26 Sep '02

| Sorry, I should also mention implicit parameters, if you're willing to use | that extension. I don't like them, though, and my impression from SPJ is | that it's very unclear whether they will get into Haskell 2 or not... It's *linear* implicit parameters that are a weird beast. Ordinary implicit parameters are quite well behaved. Having said that, there is no Haskell 2 process at the moment... Simon

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Re: Pure File Reading (was: Dealing with configuration data)
by Yoann Padioleau 26 Sep '02

26 Sep '02

Lars Lundgren <d95lars(a)dtek.chalmers.se> writes: > On 26 Sep 2002, Yoann Padioleau wrote: > > > Koen Claessen <koen(a)cs.chalmers.se> writes: > > > > i find your idea very good. > > indeed for the library GetOpt, the argument of a program never change so it > > make sense to make this library without using IO monad, same for argv and for the enviroment. > > > for openFile it seems harder, it would require to have a lock on the file > > to be sure that no one modify the file, same for a webpage. > > > > No, not at all. All that is required is to make sure all calls to openFile > f returns the same string, i.e the contents of the file when it was read > the first time. yes you are right. but it requires to change the name of the function to have Once appended to it. so in the Prelude, we should have 2 version of the same kind of fonction, one pure such as readFileOnce with a cool type, and one such as readFile with an Io type. Sounds good idea to me, cos many programs require just to read a file one time. i dont know if practically it is useful. after all we can do all the IO stuff in the main, and then continue in a pure world. dont know if it is useful to read a file in the middle of a pure function. > > /Lars L > > > -- Yoann Padioleau, INSA de Rennes, France, Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

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Re: Pure File Reading (was: Dealing with configuration data)
by Yoann Padioleau 26 Sep '02

26 Sep '02

Koen Claessen <koen(a)cs.chalmers.se> writes: i find your idea very good. indeed for the library GetOpt, the argument of a program never change so it make sense to make this library without using IO monad, same for argv and for the enviroment. for openFile it seems harder, it would require to have a lock on the file to be sure that no one modify the file, same for a webpage. > Dear all, > > At the moment, a discussion on haskell-cafe is going on > about how to neatly program the fact that an entire program > depends on a number of parameters that are read in once at > the beginning of a program. > > The suggestion that many people came up with was using > unsafePerformIO in the beginning to read the file. Here is > my version of that: > > | data Configuration = ... -- config data > | > | getConfig :: Configuration > | getConfig = unsafePerformIO $ > | do ...read configuration from file... > | return configuration > | > | main = > | do doStuff > > It is quite disturbing that there is no other easy way to do > this than using unsafePerformIO (except for using implicit > parameters perhaps, but there are other reasons for not > using those). > > I have been thinking a little bit more about this and here > is what I found. > > Remember the Gofer days, when Gofer had a "function": > > openFile :: FilePath -> String > > This was of course a cheap and dirty way of implementing > things like the getConfig above, but it is impure. However, > one could imagine a functional version of this function: > > readFileOnce :: FilePath -> Maybe String > > This function will read the contents of the file (and return > Nothing if something went wrong), but it is memoized, so > that the second time you use this function you get the same > result. > > So, it is a pure function. (Admittedly, it is somewhat > unpredictable, but you will always get the same result for > the same arguments.) It is no more strange than GHC's pure > version of the getArgs function (I forgot what it was/is > called). > > How about space behavior, you say? Reading a file, and > memoizing the result means storing the whole contents of the > file in memory! > > The point is that the use of this function will typically > happen at the beginning of a program, when reading the > configuration file(s). When all this has happened, the > function readFileOnce, and its memo table, will be garabage > collected. (Of course there is no guarantee that all calls > to readFileOnce will be evaluated at the beginning of a > program, and it is not required, but when you do, there are > no space problems.) > > There could of course be pure "-Once" versions of other IO > operations. Here is a list of possibilities: > > - reading a file > - getting arguments > - getting environment variables > - downloading a webpage > - ... > > What do you think? > > Regards, > /Koen. > > -- > Koen Claessen > http://www.cs.chalmers.se/~koen > Chalmers University, Gothenburg, Sweden. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Yoann Padioleau, INSA de Rennes, France, Opinions expressed here are only mine. Je n'écris qu'à titre personnel. **____ Get Free. Be Smart. Simply use Linux and Free Software. ____**

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